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Which energy efficient windows are best for an eco-home?

Which energy efficient windows are best for an eco-home?

You want energy efficient windows, but do you need double, secondary or triple glazing, Low-E coating, PVC or wooden window frames? And what has solar gain got to do with it?

David Thorpe brings some transparency to the mystifying range of choices concerning windows and sustainable building.

A little light on the subject of windows…

We need glazing to provide both light and heat for a low energy building, to bring our fuel bills down and enhance our comfort.

We’re going to look at the glazing, the frame and the opening separately and find out what is the best performing window for the price – as well as other ways to cut costs. We will also see how pergolas, shutters, curtains and light shelves can be used to moderate the amount of light and heat permitted to enter or leave the building, and finally look at installation issues. Much of what we find will also apply to exterior doors.

Solar gain

The heating effect of the sun in a building is called solar gain. It varies with the strength of the sun, its angle and the effectiveness of the glazing to transmit or reflect its energy. Energy efficient window design aims to maximise solar gain within the building in the winter (to reduce space heating demand) and to control it in summer (to minimise cooling requirements and glare).

Windows’ size, position and shading can be used to optimise solar gain, while the composition and coating on each face of the glazing can be manipulated to optimise the greenhouse effect.

Windows admit heat and daylight differently at different times of the year and day. This partly depends on the angle of incidence: the angle at which the sunlight strikes the surface of the glass. If this is within 20 degrees of the perpendicular (straight on), the light will mostly pass through it; at over 35 degrees, most of the energy will bounce off.

Quantifying solar gain

The amount of solar energy transmitted through a window is quantified by a coefficient that is called, in Europe, the ‘G-value’, and in North America the ‘solar heat gain coefficient’ (SHGC). G-values and SHGC values range from 0 to 1 (a higher value indicates more solar gain). It is specified on the energy label of a new window.

The coefficient values are the sum of the primary solar transmittance (T-value: the proportion of the total solar insulation entering through the glazing) plus the secondary transmittance (the proportion absorbed in the window or shading device).

For south-facing windows in temperate or cooler climates like ours, a G value of 0.76 or greater is recommended. This can be achieved with hard-coat low-E (see below) or clear glazing. Large west-facing windows should be coated with low-E to a G value less than 0.6 to prevent overheating.

Energy labels: the best energy efficient windows are ‘A’ rated

Modern windows are rated by national bodies and come with a declaratory energy label. In the UK this body is the British Fenestration Rating Council (BFRC). Choose windows with the highest possible rating on the label. ‘C’ is the minimum level for an eco-home (which the Energy Saving Trust says has a payback of five or six years). The most energy efficient windows are ‘A’ rated. The label displays the following information:

the rating level: C
the energy rating: -14kWh/(m2K)/yr (= a loss of three kilowatt hours per square metre per year)
the U-value (a measure of the insulation value of the window): 1.7W/(m2K)
the effective heat loss due to air penetration as L: 0.10W/(m2K)
the G value: 0.5.
The best glazing for energy efficient windows

Triple glazing is the best, if you can afford it, for long-term energy saving, and necessary to achieve an ‘A’ rating.

Try also for windows that have an inert gas (argon, krypton or xenon) sealed between the panes. The gas minimises the conduction of heat back out of the building, better than air does; xenon is the most effective, but argon is cheapest.

Ideally, the gap between the panes should be 16mm or more if they contain no gas, but can be less if they are filled with gas.

It goes without saying that you should ensure that the seals are perfect! I have had some windows made that leak air into these gaps; you can tell because condensation forms inside the windows.

It is possible to specify windows made with up to 33 per cent of recycled glass.

Low-E coatings

A special coating that reflects infrared radiation should be specified for the inside surface of one pane. It’s called ‘low-E’ coating (for low emissivity). In colder climates, the low-E is applied to the interior, inner-most surface to deflect heat back into the home, helping to reduce heating costs. In warmer climates, low-E is applied to the interior, outer-most surface to deflect heat out of the home, reducing cooling costs.

Other coatings

A self-cleaning coating can be added to the exterior face of a pane, useful for a conservatory roof or skylight. These use ultraviolet light to decompose organic materials and rain to wash them off.

A huge range of further coatings is available from manufacturers: e.g. to permit only 6 per cent of light to enter the building, or 8 per cent of heat. Individual panes specifically designed for eco-homes in higher latitudes have extra-clear outer layers, letting up to 80 per cent of light and 71 per cent of the sun’s heat in, although the sum of the three panes in a low-emittance coated triple-glazed unit would reduce this to about 65 per cent.

The best frames for energy efficient windows

Frames for energy efficient windows should ideally be made of sustainably-sourced timber, preferably hardwood or treated soft wood. Try to avoid PVC because of its inflexibility, liability to crack and the risk of dioxin pollution with attendant health risks when it is sent to landfill, if it is not recycled at the end of its life (how can you know what will happen?).

Basic aluminium or other metal frames are definitely to be avoided because the metal will conduct heat out of the building.

There should be insulation between the inner and outer frames (see Fig. 5 above), to avoid thermal bridging, which is the conduction of heat from the inside to the outside, or, conversely of cold from the outside inwards.

There can be metal cladding or flashing on the outside of the frame, particularly the sill, to provide protection from the elements. Beware that when fitted any flashing does not constitute a thermal bridge.

It is hard to find a local carpenter who can make this type of window, and they are usually sourced from specialist suppliers (see below).

The amount of the window opening taken up by the frame should be minimised to maximise the amount of light admittance. For example, avoid the use of grids or muntins if at all possible.

Sash windows are on the whole not recommended because of the large area taken up by the frames, and the difficulty of draughtproofing them. However, if building regulations stipulate their use, as in a listed building, it is possible to get very well-performing ones (see Fig. 7), but it is crucial that they are fitted correctly.

The positioning of the glazing within the frame is important. It can be positioned nearer to the inside or outside edge of the frame, or centred equally. The ideal is to have it centred within the frame when seen in cross-section. Positioning it near to the exterior will reduce its performance.

Window positioning

If this is a refurbishment you’re not going to have much control over where windows go; someone else has already done this! However, if you’re working on an extension or newbuild, strive to position the windows on the south, east or west facing side. This can be combined with a pattern of room use as in Fig. 8. Have only small windows or no windows on the north side (in the northern hemisphere).

Figures 9 and 10 are photographs of a Passivhaus construction in Wales called Larch House. Note how it is designed this way, with large windows on the south facing side, the front, and small windows on the back.

You wouldn’t think it, but research by energy specialist Bronwyn Barry has also shown that positioning windows in the centre of a wall in a room can reduce heat demand by up to 22%.

In south-facing walls, there’s an optimum ratio for energy efficiency (heat gain vs. heat loss) of window glazing-to-wall area that is 0.3 (with no shading) to 0.45 (with an overhang).


Use compression seals all around the frame.

Figure 11 shows the seals in a Passivhaus French window which are used in my own studio. By having three sets of seals all around the door, airtightness is guaranteed.

Passivhaus windows

If the building is being renovated or built to Passivhaus or Carbonlite standards, the U-value is calculated differently. It includes not only the U-value for the whole of the window including the frame (as in the energy label above), but also the thermal bridge, which is dependent on the installation detail of the window or door in the external wall. Windows in Passivhaus buildings must have a whole window U-value of 0.8 W/m2K. (Figure 20 at the end of this article shows how this is calculated).

The pictures below (Figures 12, 13 and 14) show more details of the Passivhaus-standard windows which I bought for my studio. Having shopped around, I found that for the same guaranteed performance, buying these Optiwin Austrian-made windows was the cheapest option. Their representative in this country, whom I found helpful, is Touchwood Homes.

These highly energy efficient windows are tilt and turn and can be made to any design specification.

Reducing the cost

You can bring the cost down by reducing the number of openings, mullions and by keeping the windows to a standard size.

A taller, more narrow window with a single opening, which fills the whole window, will be cheaper and perform better than one, for example, with a transom at the top and split into two panes below. It will also have a lower U-value.

Secondary glazing

If you can’t afford replacement windows, and you have single glazing, you can install secondary glazing at a lower price (See Fig. 15). This will be custom-made by a local carpenter to fit the reveal and fitted using compression seals. It should be easily removable to facilitate access to cleaning and exit through the window in case of emergency.

Shutters and curtains

Shutters, both inside and out, and curtains, can be used both to keep heat within a building and prevent drafts around windows and doors, and, if closed in the daytime, prevent overheating in the summer (See Fig 16). This is common practice in southern Europe. They can be used whether or not you’re replacing windows.

Overhangs and pergolas

You can have too much of a good thing! A dwelling with too much equator- or west-facing glass can result in excessive winter, spring, or autumn day heating, too much glare and excessive heat at certain times of the year. Although the sun is at the same altitude six weeks before and after the solstice, the heating and cooling requirements before and after the solstice are significantly different.

Overhangs or pergolas above windows are a common form of shading device (see Fig. 17). They should be sized relative to the latitude, location and window size, so that the amount of light admitted through the window at different times of the year prevents overheating in summer and helps solar gain in winter.

Using natural light

An eco-home will try to minimise the use of artificial lighting by making the most of natural light. There are several tricks that can help to reduce lighting costs by 30-50 per cent while avoiding the risk of overheating, such as:
• by painting the reveal, lintel, window sill and opposite walls a light colour to reflect more light
• it may be possible to slant the sides of reveal to increase the amount of light entering the room (see Fig. 18)
• by positioning mirrors in the reveal and opposite the window to reflect more light into the room
• by adding light shelves to reflect more like deeper into the room.

Light shelves (Fig. 19) have high-reflectance surfaces that reflect daylight deep into a room from horizontal overhangs above eye-level, onto the ceiling and up to four times the distance between the floor and the top of the window into a room. Usually they consist of an extruded aluminium chassis system and aluminium composite panel surfaces.

Fitting windows and doors

Correct fitting is important to preserve airtightness in the building, and prevent draughts and thermal bridging.

If the building uses external insulation, this should overlap slightly the outside of the window frames.

If the building uses internal insulation, the insulation in the reveal should meet up with the wall insulation. When this is not possible, spray foam can be used to fill the gap around the window frame once it is in place. Take great care not to over-fill as this can cause damage (it expands!). Where space inside the reveal is at a premium, thinner slab insulation can be used, or the old plaster can be hacked off and replaced with new plaster with which has been mixed granular insulation such as perlite, vermiculite or polystyrene balls, or a hygroscopic plaster such as lime.

With cavity walls, proprietary pre-insulated cavity closers are available that prevent thermal bridging. These avoid the need for separate strips of insulation within the cavity or for internal insulated linings that partially mask the frame. This allows much greater versatility in positioning window and door frames within the depth of the reveal.

Lintels and sills must be insulated on the inside or outside, to prevent thermal bridging, as they may cross from the interior to the exterior. For detailing see the Energy Saving Trust’s Enhanced Construction Details (ECDs).

Safety first

With all glazing, allow for safe cleaning and escape through the window in case of fire.

Modern windows are beautiful, highly technical constructions. In my humble opinion it’s worth shelling out for the best that you can possibly buy. You won’t regret installing the most energy efficient windows. Just remember: people in high-performing glass houses shouldn’t have high bills! Here’s looking at you…

© David Thorpe, Manager of Green Deal Advice and author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency

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