Windows contribute a wide range of functions to any building, from the aesthetic to the purely practical – natural lighting, a visual connection to the outside, ventilation, sunlight, sound insulation, and so forth. In a Passivhaus or other ultra-low-energy design, windows play an important role in the overall performance of the building, both in terms of comfort and energy in use. This is why the measured thermal performance of the windows is part of the required standard: it’s not simply an optional extra. Even with high-performance windows, the heat/energy losses through them will be around five times more than losses through the equivalent area of walls, roof and floor. Furthermore, passive solar gain through the windows makes a significant contribution to the building’s overall heat gains. Optimising window performance is therefore key to ultra-low-energy design.

Development of high-performance windows

Window design has already developed from single-glazed to double-glazed units. Originally the gap between the two panes was filled with air, but now this is commonly filled with argon, as it has a lower thermal conductivity than air. Leakage of the argon has been a problem with regard to retaining long-term performance, and, while lower leakage levels are now achieved, performance will drop as the argon dissipates (see Chapter 7, page 99). With currently available products, most of the gas may be lost within the first ten years of the window’s life, although product performance is improving all the time. Other gases (krypton and xenon) are also sometimes used; these are significantly more expensive but further improve thermal performance. Krypton and xenon are relatively rare, and utilising them for windows is not encouraged by the PHI. This resistance to their use is also related to the common use of thinner spacers between the panes of glass (10-12mm), which means that once the gas has leaked, performance drops much more markedly.

In addition to filling the gap with gas, one of the surfaces of the glass is now commonly treated with what is termed a ‘low-emissivity’ or ‘low-e’ coating – an extremely thin metal-oxide layer. All materials reflect or absorb radiant energy, but to very different degrees. Black materials, for example, have high thermal emissivity – they will absorb high levels of radiant energy and therefore become very hot in the sun. Materials with low thermal emissivity have high reflectivity. The effect of the low-emissivity layer is to reflect long-wave radiant heat from indoors back into the house, thus keeping the house warmer in winter. Short-wave radiation from the sun is still able to pass through the coating (although it will also reflect this back to the outside to a small degree). There are hard and soft low-e coatings; the hard coating being more durable but less efficient. A standard triple-glazed window unit will be argon-filled and have two low-e coatings, on the inside surface of the inner and outer panes, which are thereby protected from scratching, etc.

In future it may be that the glass panes will be separated by a vacuum, using nearly-transparent pillars or spacers. This will improve performance even further but is not yet commercially viable. One of the challenges will be to retain the vacuum over the window’s design lifetime.

Many low-energy windows open inwards, which is unfamiliar in the UK. This is partly owing to the need to insulate around the frame externally, which makes opening outwards problematic. Inward opening can make it somewhat impractical to display items on your window sill, although this problem is avoided with tilt-and-turn windows, which also offer security and rain protection advantages when tilted (see photo above). Some manufacturers do offer low-energy externally opening windows.

A tilt-and-turn inward-opening Passivhaussuitable window. With the window open in tilt mode, the window ledge can still be used.

U-values

The thermal performance of a window is often given by a single U-value. You can ask your supplier for this value, even if it does not appear on the ’advertising’ packaging. In the UK it is still common to appreciate only that there is a performance difference between single-, double- or triple-glazed windows (see Table 11.1 below). However, double- or triple-glazed units can perform very differently, with a wide range of U-values. The U-value is a function of the thermal conductivities of the different materials making up the window and their thicknesses: the lower the U-value, the better the thermal performance. Manufacturers may quote the U-value as calculated through the centre of the window pane, since this gives the most favourable results. In fact, the thermal performance of a window is affected by a much wider variety of factors, which are discussed here.

Table 11.1 Typical domestic window U-values in the UK

Typical window performance U-value: >1.5W/m²K

The worst U-value you can achieve with a construction is approximately 5.0W/m²K (watts per square metre per degree kelvin). This is for a single-glazed window – which is why changing from single to double glazing makes such a huge difference to energy performance and comfort levels! The poor performance of windows means that the internal surface temperature of the glass will normally be the lowest in the house (older readers will probably have memories of scraping ice off the inside of window panes on cold winter days). The lower the internal surface temperature of the glass becomes, the more uncomfortable it is for the building’s occupants, because it creates an asymmetry with other, warmer wall surfaces. We feel the contrasting radiant temperatures from these different surfaces and don’t like it! This is one reason why radiators are commonly placed under windows – to compensate for this effect. If surface temperature variations are greater than 4°C, we will feel the difference and choose not to sit next to windows during cold periods, effectively reducing the usable floor area of our homes. Furthermore, the cold surface of the glass cools the air adjacent to it and the air then sinks down, spreading out across the floor and setting up air rotation within the room. This can have the further effect of reducing temperatures at lower levels (around your feet) and increasing temperatures higher up (at the ceiling and around your head). If this temperature stratification becomes greater than 2°C, again it starts to feel uncomfortable.

The current (2010) Building Regulations in England and Wales stipulate a maximum U-value of 2.0W/m²K for windows in new dwellings.

Passivhaus window performance U-value: <0.80W/m²K

A Passivhaus-certified window must achieve a whole-window U-value (see below) of 0.80W/m²K (in a cool-temperate climate). Part of the reason for this particular performance level is to ensure that the internal surface temperature of the window is high enough to keep variations of surface temperature within any room below 4°C. This removes the need for a heat source adjacent to the window, and also means you can remain comfortable wherever you choose to sit, effectively maximising usable floor area. This surface temperature of the internal glass will be high enough to ensure that you don’t get temperature stratification in the room, which again is likely to boost comfort levels.

Different climatic regions will require different target window U-values and different glazing types to meet the Passivhaus standard. Full details of these targets, relating to seven different climate regions (from ‘arctic’ to ‘extremely hot, often humid’!), are published by the PHI,¹ along with examples of cities within each region. In a warm climate (e.g. Hawaii), double-glazed units are specified, whereas in a cold climate (e.g. Anchorage), a quadruple-glazed low-e window is required.

Calculation of the window U-value

Although in the UK you may still be quoted just a single U-value for a given window, taken through the centre of the pane, manufacturers here are now beginning to quote a more complex whole-window U-value, Uw (see box overleaf). However, this differs from the whole-window installed U-value that is calculated by the PHPP (see page 174). The three different factors taken into account for this are as follows.

1. Differences between the window frame and the window pane

In reality, every window is made up of two very different ‘constructions’: the window pane and the window frame. The single U-value calculation can therefore be very misleading, especially if it is based on the window pane only, since this will perform much better than the frame. The window frame will generally be the weakest point in the performance of a building’s thermal envelope. The construction and design of the frame is thus a key focus point for manufacturers of high-performance, triple-glazed windows – commonly the frame will be made of multilayered bonded wood, with some insulation. This could be hard or soft polyurethane such as purenit^® (hard), extruded polystyrene, cork or cellulose. The whole-window calculation takes into account the following:

•  The U-value as calculated through the centre of the window pane (Ug).

•  The U-value as calculated through the window frame (Uf).

(The above two values are adjusted to take into account the proportion of glass to frame.)

•  The window spacer psi-value (explained on page 174).

The whole-window U-value will therefore change according to the proportion of frame to glass. Since the frame is the weaker component, a larger window will perform more efficiently than a small window. Additionally, a window frame designed to be openable will perform worse than a fixed, i.e. non-openable, window frame. So, again, the straightforward conclusion is that large, fixed glazing will perform best of all (as well as being cheaper!). This simple understanding can be very helpful at the design stage – installing a fixed window on a north-facing façade is a good trick to minimise heat loss while still maximising views and light. Not all windows need to be openable, as long as there is one in each room.

Section through a typical Passivhaus suitable triple glazed window, in this case with an aluminium finish externally and wood internally.

The U-value of the window frame in Passivhaussuitable windows can vary by around 0.13W/m²K, and it has been shown that this could make a difference of more than 2.5kWh/m².a to the building’s annual heat demand. On a certified project, this could easily make the difference between a pass and a fail. On a non-certified project, this may not be as critical as the cost savings that might otherwise be achieved – a slightly less efficient window frame can still perform as required. For Passivhaus Certification of a window, the frame U-value should be below 0.80W/m²K, and only a few manufacturers to date have achieved this.

2. The effect of the window spacer

The window spacer is the small edge piece that keeps the panes of glass apart. In effect, each spacer acts as a mini thermal bridge. In the UK, these spacers are mostly made of aluminium – a rather effective conductor! The fact that the spacer performs so poorly is one reason you tend to get condensation forming at the glass perimeter of standard windows, where the internal surface temperature ends up being the coldest, and this then encourages mould growth. You will never have a solid aluminium spacer in a high-performance window, but there are several alternatives that will be suitable, generally referred to as ‘warm-edge’ spacers. Examples include a thin-walled stainless steel ‘U’ channel, which is normally filled with a highly insulating plastic desiccant and has a separate moisture-proof butyl seal, or thermoplastic spacers, which are made from a semi-rigid plastic material with a desiccant entrained within them (the best-performing products to date). You can boost window performance by specifying the most efficient spacer, for obvious reasons. Durability may be an issue with some of these solutions, in terms of ensuring that the spacer does not allow any undue leakage of the argon gas, although windows can now achieve extremely low leakage levels. Also, new and better spacers will become available as the market for high-performance windows grows.

Silver-coloured warm-edge spacers in a Passivhaus-suitable window.

The psi-value for the spacer is multiplied by the length of the spacer around the perimeter of each window to further adjust the final U-value calculation. All these calculations are done within the PHPP (see Chapter 7).

3. The installation of the window within the wall construction

It is now understood that the manner in which the window is installed in the wall will have a significant impact on its final performance. Investment in an expensive high-performance window could be wasted if the window is then not installed appropriately. A poorly installed window will create a significant linear thermal bridge (see Chapter 8) around the four sides of its frame, which will effectively increase the U-value of the component, worsening its performance. In fact, an extremely poor installation (similar to how windows are commonly and currently installed in the UK) will badly compromise your annual space heating demand on an ultra-low-energy build. For this reason Passivhaus sets a second window U-value requirement – a whole-window installed U-value (Uw, installed) of less than 0.85W/m²K. This is calculated within the PHPP.

A linear thermal bridge is calculated in W/mK, which is referred to as the psi-value (ψ). The psi-value for a construction detail is normally measured using a calculation package such as THERM (see Chapter 8, page 120). This value is then multiplied by the length of the thermal bridge to ascertain the final effect on the thermal performance of any particular window installation. In order to make this calculation, the construction detail for how the window is to be installed in the wall is critical, and (as we saw in Chapter 7) the psi-value of the installation detail (ψ_installation) needs to be entered into the PHPP (along with other values relating to the performance of the window itself). The designer’s aim is to detail the junction around the perimeter of the windows to try to achieve a thermal-bridge-free solution. Within the IBO book (see Resources) there is a range of illustrated construction details. These are rather diagrammatic and also use some materials that are more familiar in Austria and Germany than in the UK; however, they can be extremely useful in developing appropriate and effective new-build solutions. The psi-value is pre-calculated for each of these details, which is ideal for the PHPP and at least gives a guide figure to follow at the initial design stage. There are two psi-values given – one for the head and sides of the window installation, and a separate one for the sill detail. We will look at sill detail later in this chapter, but suffice to say this is more difficult to detail for good performance.

With a really good window installation detail, you will improve the overall performance of the window. To achieve this, you will have thoroughly wrapped the window frame with the wall insulation (see ‘Effective installation of windows’, page 176). There are also one or two verified window details (with a PHI certificate), where manufacturers and installers have collaborated to optimise installation solutions and provide the psi-value for you – hopefully this trend will continue.

It will not be time-efficient to individually model numerous psi-values for a given project, so the best approach is to check against other similar details and be on the cautious side when determining a psi-value to be entered into the PHPP (perhaps discuss with the Passivhaus Designer or Certifier). It is only if you want a certified building and you are near the ‘magic’ 15kWh/m².a space heat demand that every little extra improvement might be needed. Then your Certifier may wish to see a full calculation for your window installation detail. In reality, for most designers it is an understanding of the principles of a good installation that is essential. Optimising the installation includes looking at reducing the time, effort and materials involved.

Calculating the whole-window installed U-value

Figure 11.1 opposite shows the formula for calculating the whole-window installed U-value. A calculated example is given here, based on the window elevation in Figure 11.2. U-values for the window glass (e.g. 0.70W/m²K) and frame (e.g. 0.80W/m²K) should be provided by manufacturers of Passivhaus-suitable windows, along with the psi-value for the warm-edge spacer to be used (e.g. 0.03W/mK – you must aim for a value below 0.05W/mK).

The psi-value for your installation depends on how good your construction detail is for building into your wall. If you use a suggested detail from a supplier or one illustrated in the IBO book, then a figure may well be given there, or you need to add a conservative figure of 0.04W/mK, or calculate for your specific detail using a modelling package such as THERM. With a good detail, your psi-value may be 0.004W/mK, but for this example let’s say your value is 0.20W/mK, so your installation detail is far from ideal. The units of areas and lengths are metres and metres squared.

Figure 11.1 U-value for a whole-window installation.

So for the window in Figure 11.2, the whole-window installed U-value is:

= [(0.7 x 1) + (0.8 x 0.44) + (0.03 x 4) + (0.2 x 4.8)] / 1.44

= (0.7 + 0.35 + 0.12 + 0.96) / 1.44

= 2.13/1.44

= 1.48W/m²K

Figure 11.2 Example window elevation.

This is not a good outcome for a Passivhaussuitable window installation, where you are aiming for a value of below 0.85W/m²K. If we assume the installation psi-value was 0.004W/mK (a good install detail) the revised U-value calculation becomes:

= (0.7 + 0.35 + 0.12 + [0.004 x 4.8]) / 1.44

= 0.83W/m²K

The importance of a good installation is clear!

Solar gain

A Passivhaus gets its name partly from the ‘passive’ energy it will gain from the sun during the winter months. Capturing the winter sun has always been part of the low-energy approach, and exploiting any ‘free’ energy makes absolute sense. Passive solar gain will then make up part of the total energy balance (total energy losses balanced against total energy gains – see Chapter 6, page 76). For this particular function in an ultra-low-energy build, the glass needs to meet particular criteria. Firstly, the glass U-value (Ug-value) should ideally be <0.75W/m²K. Secondly, the glass should ideally have a ‘g-value’ (see below) of more than 0.5. In essence, the glass should minimise heat loss to the outside while allowing some sun radiation admission.

Solar transmittance and the g-value

The g-value is the fraction of the heat from the sun that enters through window glazing and is expressed as a fraction between 0 and 1. A low g-value means that the window transmits less of the sun’s heat. A g-value of 0.5 is sometimes expressed as 50 per cent. In the USA, this figure is referred to as the solar heat gain coefficient (glazing), or ‘SHGC-glazing’. Note that low-emissivity (low-e) coatings and argon filling both lower the g-value slightly.

In the northern hemisphere, south-facing façades will capture the most energy, while east-facing will capture the morning sun and west-facing the evening sun. For Passivhaus energy modelling, the g-value of all the windows is inserted into the PHPP, which uses an equation (including adjusting for building orientation, latitude, window orientation and shading) to calculate the windows’ contribution to the overall energy balance. The PHPP does not overestimate what the passive solar gain from windows will be, which is another reason why it gives realistic energy ‘in use’ predictions. The window g-values are entered in the WinType worksheet within the PHPP.

In hotter climates, where wintertime solar gain is not needed and internal overheating is a potential issue, you will want a low g-value; a value below 0.35 would be preferable. The PHPP recommendation of a g-value of more than 50 per cent suits a cool-temperate climate, but will not always be appropriate.

Effective installation of windows

The four basic principles of a good installation are to make sure that:

•  the window sits in the same line as the insulation, not staggered to it (the window forms part of the continuous insulation layer around the building)

•  you wrap the outside of your window frame with at least 60-70mm of insulation

•  your airtightness layer transfers from the wall to the window with an effective transition detail (normally using a tape on to the window frame)

•  the window is wind-tight along outside edges and waterproof against driving rain.

Understanding these principles, which are not complex, will enable you to work out your own effective installation solution, which should be considered early in the design process.

The fact that in an ultra-low-energy build the frames are normally wrapped with external insulation means that only a small depth of the perimeter frame is visible on completion, allowing an almost frameless appearance. Most manufacturers offer timber window frames with an aluminium external facing for low maintenance. Some cut this metal facing right back (to, say, only 15mm or 20mm depth), exposing the insulation/wood frame (pictured below). This means that after fitting there is minimal aluminium facing embedded in the wall insulation. This is an effective low-energy strategy as otherwise the aluminium will act as a rather effective conductor and make the installation perform less well; it also saves on high-embodied energy material.

The following examples – typical window installation details for retrofit masonry and new-build timber-frame constructions – illustrate the principles of a good window installation.

Window in which the aluminium covering to the external frame is minimised. It is designed so that the wooden ‘uncovered’ frame and part of the aluminium is wrapped by the wall insulation.

Masonry wall with external insulation

This detail would be fairly common on a retrofit project, and of course, as we saw in Chapter 10, it is preferable to insulate externally rather than internally, for the following reasons:

•  It avoids any potential moisture problems generated by internal insulation.

•  It avoids losing internal floor area.

•  The thermal mass of the masonry wall ends up on the warm side of the construction, which might help with summer cooling.

Make sure you pre-plaster (‘parge’) your masonry window reveals, prior to fitting windows, with a gypsum undercoat plaster such as Thistle Hardwall^®. Block, brick and mortar are not airtight.

The window has to be fitted on the outside of the masonry wall so that it sits within the insulation zone. This involves structurally supporting the window with either steel angle brackets or a combination of metal straps/ties and a timber batten. If the window is conventionally sized, you should not need to resort to steel; the timber and straps should be sufficient. The result will be deep window reveals internally.

Window installation in a masonry wall.

For airtightness you may tape on to the window frame, either on the internal face or on the frame sides (see Chapter 9, page 140). If taping on to the window frame face, aim to keep the tape width to about 10mm (there are tapes with multiple release papers for ease of application), otherwise you might have difficulty covering up the tape at the finishing stage; be sure to leave enough frame visible to allow your openable windows to swing inwards. The tape must then be embedded within the plaster coats.

If you are undertaking a retrofit project and improving the building fabric over a period of time, it would be worth considering installing new windows in the correct position with a temporary cover frame, ready for future installation of external wall insulation.

Supported window on a solid masonry wall that is to be externally insulated, at a Passivhaus retrofit in South Ealing.

Lightweight timber ‘I’ wall

An ‘I’ wall refers to a timber-frame wall made up of I-shaped timber columns, i.e. two small section timbers joined together by a relatively thin web of timber. These are commonly used as the structural frame for walls and roofs.

In this case, the window sits within the wall in line with the insulation. If the main wall is insulated with a blown-in insulate such as recycled waste paper (cellulose), then the frame wrapping will need to be a more rigid material, such as a woodfibre compressed board. (Insulations such as recycled newspaper have a loose, ‘fluffy’ appearance and are often blown into voids through flexible pipes, the advantage being that the material tends to fill every nook and cranny and there is no waste.)

Airtightness can be relatively straightforward – in this example, it would involve taping from the window frame on to an internal oriented strand board (OSB) lining.

Window installation in a new timber wall.

Window sills

Windows do not usually come with integral sills, and these sometimes have to be sourced and ordered separately. One or two window manufacturers offer sills (see www.passivhaushandbook.com for details) as an additional service, which keeps responsibility and coordination to one supplier. There are pre-insulated hollow metal sills available from Europe, and your window supplier should be able to supply brochures and contact details for these products.

We found that getting information on these products in English was problematic, so in the Totnes Passivhaus we used a locally made standard-profile metal sill and simply ensured that the wall insulation ran right up to the underside of the sill profile. This proved to be as effective as a purpose-made sill, although it required some care and attention to detail on-site.

Standard-profile sill used within external insulation.

To avoid ingress of wind-driven rain, you need to fill any gaps around the perimeter of your sill with an appropriate waterproof sealant or with weather-tight flashing tapes. There is always some potential for water to be drawn in under the sill, so allow for any water to drain or dry to the outside.

Doors

Most Passivhaus window manufacturers supply a range of Passivhaus-suitable doors. Currently, Passivhaus-certified main entrance doors are sourced from only three European manufacturers, but are often supplied through the window manufacturers as well. The supply and installation issues are the same as with windows. The designs for front entrance doors tend to be uncompromisingly contemporary, and the doors themselves are surprisingly heavy (make sure you have fixed them securely in place before you slam the door shut!)

Passivhaus-certified doors are not cheap, although there is no requirement to use them – you can have a certified project without one. If budgets won’t stretch to a Passivhaus-certified entrance door, there are also balcony doors, tilt-and-slide doors, patio doors, etc., based on standard windows, i.e. they are in effect fully opening windows. If the head, sides and sill are the same as a window, they can then be certified components. These might require you to step over the sill, as it may not be designed to stand on. Where you need the sill to be weight-bearing, you can usually insert a threshold detail – either a thermally broken glass-reinforced plastic threshold (insulated) or an ultra-low (i.e. shallow), thermally broken rigid PVC threshold for wheelchair access. The window U-value will need to be adjusted to reflect the final threshold detail. Some manufacturers supply units with integral load-bearing insulated sills (pictured above).

A tilt-and-slide Passivhaus-suitable door.

Insulated sill under a sliding door unit, at the South Ealing Passivhaus retrofit.

If the installation is not dimensionally accurate (i.e. not skewed), you may encounter airtightness issues with opening units and seals.

Roof lights

If introducing roof lights, it is important to first consider the positives and negatives. One problem with roof lights is that they tend to sit proud of the main construction, breaking the good-installation rule number one (the window should sit in the same line as the insulation, not staggered to it) and making it difficult to meet rule number two (wrap the outside of your window frame with insulation). The centre pane U-value may be excellent but the frame U-value is unlikely to be great, and installation psi-values will almost certainly be poor. There is also an overheating consideration – roof lights do let in more light than vertical windows and therefore also more heat energy. However, if opened on hot days they could form part of a natural ventilation strategy (e.g. located at the top of a stairwell – a passive stack effect), contributing to summer cooling. There are FAKRO^® and VELUX^® triple-glazed low-energy versions, some even with external shading devices to deal with the overheating risk.

Of course, not every detail in a Passivhaus build needs to be thermal-bridge-free, and you can choose to compensate by over-performance elsewhere. Generally we would currently advise designing out the need for roof lights, or keeping them to a minimum. However, improved products are already beginning to ease this constraint.

Strategies to address the risk of summer overheating

In an ultra-low-energy build it is unlikely that you will be cold in winter; the challenge is to ensure you do not overheat in summer. To meet the Passivhaus standard, you are limited to overheating (above 25°C internal temperature) for 10 per cent of hours in a year. This would actually represent an uncomfortable amount of time, so we would suggest you adopt an even more stringent target, perhaps 5 per cent maximum, and of course aim for as close to 0 per cent as practicable. The Passivhaus aim is to minimise the ‘cooling’ season, just as you aim to minimise the ‘heating’ season. In a cool-temperate climate such as in the UK, it is possible to eliminate the ‘cooling’ season altogether (in terms of the need for any mechanical cooling systems), and to manage the overheating risk through solar shading and your passive ventilation strategy (see Chapter 12). The design and layout of the windows is critical in addressing this successfully, as they determine how much summer solar gain you will have and should also contribute to your summer ventilation strategy.

Solar shading

The key to a successful design is to optimise winter solar gain while also avoiding too much solar gain in summer. The depth of the window within the wall (the reveal depth) will affect how much solar gain is achieved – the deeper your reveal, the less solar gain. If your window is set too deep in the wall, you may get very little winter solar gain – although you can consider splaying the side reveals. The position of the window in the wall is likely also to affect how optimised your installation psi-value is. (Ideally you should ensure that the wall insulation overlaps your window/door frames by 60-70mm. Usually, because windows tend to open inwards, this will have to occur on the outside of the frame.) These two factors need to be balanced against each other. If modelling in the PHPP, you can play with different depths to see the effect on the overall energy balance, which is very useful.

For those considering a Certified Passivhaus, the PHPP will adjust the effective shading of a window to reflect whether you choose window shading solutions that rely on manual handling or not – only a fixed or electronic shading device will be effective 100 per cent of the time. Even if you are not aiming for certification, it is best to assume that manually operated shading devices will not be 100-per-cent effective.

A variety of strategies for providing solar shading in summer are described overleaf – decide on which you prefer at an early stage and design solutions in! Large east- or west-facing windows are likely to be the worst culprits for overheating risk: this is because the sun is low in the sky morning and evening, and will therefore shine more directly through the windows. Large windows in small rooms will also increase the risk of overheating. It’s important to be aware that internal blinds have a small value in reducing overheating (say, 10-20 per cent).

External window blinds

There are several options available for external blinds, which are all very standard in European countries but unfamiliar in the UK. The aesthetic is therefore a little alien and some solutions are likely to be problematic in conservation areas. Blinds can be manual or electrically operated and/or linked to a sensor. If linked to a sensor the blind may go up and down many times during a cloudy/sunny/windy day! The basic Options are as follows.

•  A separate external box, with side runners/ guides for the blind, which is fixed on to the finished wall above the window. The larger the window, the larger the box to contain it. If externally insulating, consider how the weight might be supported without creating a point thermal bridge.

•  A separate external box that can be butted up to the outside of the window frame and built into the wall – the blind can then run within the window reveal, which is a neater solution. Some window manufacturers offer particular models that integrate well with their window frames.

•  A double-glazed unit with an external third pane of glass, which conceals an integral blind (pictured opposite). The U-value of such a window will be compromised but the aesthetic result is extremely neat and may be worth considering in some locations, where the overheating risk must be addressed but aesthetics are also essential. We have used these in the Totnes Passivhaus and they do look very smart and discreet. However, the gap for the blind can accumulate flies and increases cleaning!

Internal and external views of windows at Denby Dale. Large glazing areas must be managed carefully for overheating – here large external electronic blinds are used. Images: Green Building Store

Section through double-glazed window with third outer pane (Internorm^®), allowing external shading integral blind to be located in the gap.

The external blind boxes that are built into the wall should come with integral insulation to minimise thermal bridging. This will only mitigate the effect and the box will almost certainly increase your installation psi-value (i.e. worsen it). With a box fixed on the outside of the wall, there should be no thermal bridge issues, only visual.

Overhanging roof eaves

It is possible to design extra-deep eaves that will allow in low winter sun but cut out high summer sun. Having extra-deep eaves does create a particular ‘look’, again familiar elsewhere in Europe but not common in the UK. This is a solution for south-facing windows, not those facing east or west. Planners will need to consider this as part of a necessary design shift as we move to more energy-efficient buildings.

Independent canopy or veranda

A ground-level canopy or veranda to around 1m depth could provide good summer shading, although this should be optimised for your exact orientation and climate. The effectiveness of any canopy will be determined by its height above the window, as well as by the depth of the window reveal in your design. A framed veranda with green vegetation is another solution if the vegetation is verdant in summer only. Any canopy/veranda should ideally be structurally independent from the main house in order to avoid thermal bridging of the insulation layer.

Trees and other structures

Trees in the right location, especially those shedding their leaves in winter, could be a good solution. A wall or adjacent building could also provide some shade. Approach shading solutions creatively: using planting can be quite effective (and inexpensive).

Choosing the best strategy

Our advice would be to design in fixed shading solutions, such as overhanging eaves and canopies – these can have an aesthetic value as well as serving a useful function. Most importantly, remove overheating risks by placing glazing intelligently and utilise the power of the PHPP to test this, otherwise solutions could look tacked on or require overly complex detailing.

The PHPP includes a Shading-S worksheet (see Chapter 7, page 107), which requires data input for each window orientation and any shading (trees/buildings, etc.; window reveals; canopies/eaves). There is an average shading reduction factor of 75 per cent, which is the PHPP default value before you fill in the sheet, but this may be significantly different from your actual shading factor. Don’t rely on this default to predict your solar heat gains. The rigours of the window and shading worksheets in PHPP are a necessary, if perhaps tiresome, part of the design process, as overheating is a genuine risk. In fact these sections of the PHPP are some of the most input-intense, which reflects their relative importance.

Summer ventilation

As discussed in Chapter 12, a Passivhaus adopts a ‘mixed-mode’ ventilation strategy. This means you are normally adopting passive strategies for summer cooling using windows and any roof lights. Simply using tilted windows on opposite sides of a building is the simplest method, usually with airflow from low to high level. When designing for effective summer ventilation, also make sure your solution is rain-, child- and burglar-proof!

There is a selection for your night ventilation strategy (‘flushing’) in the Summer worksheet in the PHPP. The mechanical ventilation with heat recovery (MVHR) would be set to ‘summer bypass’ mode or switched off in summer, which means the heat exchanger (or heat recovery function) is not being used and fresh air is delivered direct to your rooms instead. This would normally be the case when external temperatures range from 15°C to 25°C. It should be noted that the summer bypass mode is not a cooling system and should not be relied on to act as such. In Germany the general approach has been that for any significant number of hours where the temperature rises above 25°C, you would start to use the heat exchanger again (this will minimise the air temperature being brought in) and then keep the windows closed during the day and open them at night for cooling.

Alternatively, you could install a ground source heat exchanger (GSHX), which will pre-cool the incoming air. A typical system (brine-to-air) uses pipes filled with brine that are laid in the ground, where at depths of more than 1m the temperature remains static through the year – this ‘coolth’, or coolness, is then transferred from the brine to the incoming air prior to it entering the air handling unit of the MVHR system. Air-to-air GSHX systems also exist, but are now becoming less common in Europe, as condensation can form in the underground pipes, and if condensate cannot drain away adequately there are potential health issues. If you do decide to install a GSHX system, we would suggest not using an air-to-air system for this reason. GSHX systems were often installed in early Passivhaus homes, but they are far less common now for milder climates, where the benefits-versus-costs balance is less clear (although if you can install the pipework yourself, the economics change). A brine-to-air heat exchanger also works effectively as frost protection in winter (see Chapter 12, page 198). If the climate is more extreme (hot or cold), then the use of a system such as a GSHX becomes increasingly beneficial to avoid the use of high-power cooling devices, which would undermine the Passivhaus low-energy approach. In a maritime climate (i.e. less extreme) such as in the UK, sensible window design and appropriate shading should eliminate the need for any mechanical cooling.

In a more complex project – say, a block of flats – it may be necessary to look separately at some of the flat units, as overheating risks could vary significantly from flat to flat. This might require more expert input, not just the PHPP modelling.

The thermal mass of the building

The risk of overheating is influenced, to some degree, by whether the build is lightweight (timber) or of more massive construction (brick/ block/concrete). The thermal mass of a body of material refers to its ability to absorb, store and subsequently release heat (due to its specific heat capacity and its mass); heavyweight construction materials, such as brick and stone, have a high thermal mass, while lightweight materials such as timber do not. Thermal mass is useful when the heat transfer between the material and the interior air roughly matches the daily (24-hour) heating and cooling cycle of the building (often referred to as the ‘diurnal temperature cycle’). The material will then absorb heat during the day and release heat during the night, and this will effectively dampen the internal temperature variation, helping to stabilise it. Some materials (e.g. steel) can absorb heat very effectively but will release it too quickly, so you need materials with moderate thermal conductivity but a high specific heat capacity. Data needs to be input into the PHPP Summer worksheet to reflect this (see Chapter 7, page 107).

You don’t need huge thick masonry walls to achieve effective thermal mass; something like a 25mm clay plaster can be sufficient to help dampen temperature swings between day and night. We would suggest that some provision is made for thermal mass in the design; the ground-floor construction can be a good option. Larger buildings will generally have a greater need for thermal mass than single houses.

Ordering windows

Finding someone well informed who can talk you through the various options is not easy at present, unless you can speak German fluently! It is worth spending time selecting your preferred manufacturer early on in the design stage and then optimising the specifics of the window (glass type, width, g-value, etc.), as this can save a lot of money. The PHPP will help with this, since you can see the immediate effect of tweaking elements of your design – 20mm extra on your external insulation, for example, will probably be much cheaper than cranking up your window specification. Remember that order periods can be quite long for doors and windows – typically 8-12 weeks.

There are UK and Irish companies beginning to supply Passivhaus-suitable windows and doors, although generally these have been sourced from the Continent (one or two are assembling in Wales and Ireland). It is hoped that products will soon also be designed and made in the UK.

The construction phase

Installing windows and handling such expensive items on-site does demand extra care and attention. Triple-glazed windows (and insulated doors) can be quite heavy and, being costly, need careful protection so may need lifting equipment to install. Some are not necessarily individually pre-wrapped for delivery to site and you may want to stipulate this requirement when you place your order. As you will already have appreciated, fitting is not the same as a standard window installation and this needs to be properly understood by the main contractor, so that the installation sequencing runs in a practical and efficient manner. The windows and doors are both critical for the final thermal performance and the airtightness of the design; therefore the window subcontract becomes a package of work that needs early consideration at the planning and construction phases.

Window subcontractors

Think carefully about your strategy for window installation and seriously consider retaining interest in selection of subcontractors if they are to be used rather than the main contractor. There will be very few UK window installers that have any experience in installing for a Passivhaus build; even those recommended by your window supplier may not be that experienced! Some manufacturers will be able to supply and fit or may offer an advisory service, which would involve a qualified technician attending your site to demonstrate how a window should be fitted – a minimal one-day service. This is a very effective method of training your fitters (who may be more local to you); with reasonable competence they can then replicate the demonstration when fitting the remaining windows/ doors. The alternative is to negotiate that one or two of your selected installers attend a training course (preferably on offer from the manufacturer). These courses are much needed and well worth an installer’s investment. They could vary from two to three days to two weeks, depending on the manufacturer. Some also include training on repairs and maintenance, which might well be the best option on multiunit schemes where a longer-term interest is to be retained, e.g. registered social landlords (RSLs); those trained can then train others on-site and ensure that quality is maintained.

It will take time for an established network of trained installers to develop across the UK, but in the meantime do not let your window installation just ‘happen’ – retain control.

Future developments in windows

Low-energy windows have to date had rather chunky frames, due to the additional insulation and airtight seals. Newer windows are already slimmer, and this trend is bound to continue as the technology develops and becomes more familiar. As noted at the beginning of this chapter, we would also expect progress in the development of outward-opening designs.

It is also true that low-energy windows generally do not meet more historic aesthetic requirements, especially as larger single panes are always going to be more energy efficient. Small window panes, with numerous window bars or mullions, are the common aesthetic in many conservation settings, and, while they generally perform poorly, we do seem to like the effect and our nostalgia proves to be rather strong! One or two window manufacturers are beginning to respond to this by designing windows that are aesthetically suitable for many conservation settings while still delivering the level of energy performance needed in an ultra-low-energy building (see bottom photo on page 77). Whether culturally we need to ‘move on’ is worth asking (the original reasons we had small panes of glass are long gone), but the reality is that there will always be a demand for replicating the historic look. Of course it can be only a ‘dummy’ effect, but even this will affect window performance to some degree – in particular reducing solar gain. If you want to go down this avenue, you will need to carefully assess the effect of compromised performance.

Window costs

At present, high-performance windows are expensive, but costs are already reducing. Such windows are essential and integral to the Passivhaus or an ultra-low-energy approach, in terms of both energy and comfort/health. The quality of these windows is extremely high and they must be seen as a long-term investment – in the UK, windows are generally replaced more frequently than is desirable, but a Passivhaus window will last a lifetime. See Chapter 2 for a discussion of Passivhaus economics.

Conservation triple-glazed ‘sash’ window at Princedale Road, London. Images: Princedale EcoHaus Ltd