Friday, 24 November 2017

Energy performance for windows

A proportion of heat loss from our homes escapes through our windows - this is often quoted as 25% but it could be even more if the rest of the house is well insulated. How do you know when you buy a window that it is energy efficient? There is a rating scheme in common use, defined by the BFRC (British Fenestration Rating Council) and it combines several performance factors. Somewhere like the Green Building Store, that really cares about efficiency,  specifies each factor separately. You may not want the same kind of window everywhere, depending on the direction the window is facing and how much it is shaded.

The BFRC certificates look similar to EU energy certificates for TVs, fridges and such like, even though they are nothing to do with the EU.  The rating scheme (which goes up to A++) is based on a combination of three performance factors:
  • Conductive heat loss i.e. window U-value
  • Thermal transmittance, a measure of how much of the sun's heat energy will pass through as solar gain (this is different from transmittance of visible light).
  • Air tightness.

Unfortunately they do not publish the details of how these measures are combined to give a rating. (All the EU standards are described in detail on the Eur Lex website.)

An 'A' rated window gains more heat than it loses -  depending on orientation, climate and other factors.
It seems the BFRC rating is based on the overall balance of heat gain and loss. I found this statement on the Anglian Windows website
Our uPVC windows achieve an A rating as standard - meaning that they gain more of the suns[sic] free energy than they lose through thermal transmittance.
In practice this may well be true on average through the year, but probably not all the time in all cases. Solar gains depend on location (latitude),  climate (cloudiness) and orientation - north facing windows get less - and shading factors as well as the windows. The heat loss on the other side of the balance depends on the temperature you keep in the house as well as external temperature. So behind this simple statement there are assumptions on location, orientation, shading, internal temperatures and climate. This chart shows differences in solar gains with different orientations and in different parts of the country, assuming no shading. The highest gains are actually for East or West facing windows because they get a lot of direct sunshine during the morning or evening. You get more gains when the sun is low and shining straight into the window.

Variation of solar heat energy on a window surface by direction (left) and region (right). Solar energy dips in summer for South facing windows because when the sun is very high the light is at a grazing angle to the vertical window. These graphs are based on the solar gain calculations in SAP [1]

Solar gains in summer are not necessarily helpful.
Solar gains are not always a good thing. Gains in summer are likely to be irrelevant to your energy bills and can cause an overheating problem, though you can fix excessive solar gain in summer with shading and/or blinds.

The best windows have very low conductive losses with reasonable gains.
By and large, high heat gains are linked to high heat losses: windows that keep the heat in also tend to keep solar gains out. This chart shows transmittance and U-values for some different kinds of window. The 'ultra' window is a very high performance window from Green Building Store[2]; it has very low U-value but good transmittance too.

Thermal transmittance and U-value for different kinds of window. Data is from SAP [1] (double and triple glazing is air filled with 12mm gap), except for the Ultra window from Green Building Store [2]. This is the Passivhaus certified option with insulated wooden frames.

In this chart, I have estimated the heat gains/loss balance through the year for a North facing window with no shading. I have assumed all the windows are completely draught proof and the internal temperature is an average of 17°C (it will be warmer than this during the day when you are heating but it can get cold when the heating is off so this is a plausible average for a well insulated house). Only the Ultra window gives net gains in winter, even with a North facing window. I have spared you the charts, but if you have a South facing window, the triple glazing is also a net gain throughout the year and the double glazing only loses a tiny bit.

Estimated gains/loss balance for the four different types of window. Assumes mean internal temperature is 17°C. Data from SAP [1] and Green building Store [2] as before.

You usually want to minimise heat loss, but on the South side gains can be useful.
You may not want the same kind or windows all around your house. For example if it is heavily shaded or north facing you are better off looking for a low U-value for minimum heat loss. Also for East/West facing windows you might prefer low U again to minimise gains. For a South facing window gains are likely to be useful.

If you really care about efficiency, you need to know the U-values.
If you go to a website that cares about energy efficiency, such as Green Building Store, you will see that they quote U-values, transmittance and air tightness separately. Also U-values are quoted for the glazing and for the frame. The overall U-value is a combination of these and the shape of the frame affects the ratio of frame to glazing. For the best windows the U-value for the glazing is better than the frame even with insulated wooden frames.

[1] Standard Assessment Procedure for Energy Rating of Dwellings (BRE) 2016
[2] Ultra windows (Green Building Store)


  1. What did you use to calculate the daily gains/loses?

    1. This was all as per SAP. For heat loss, I used mean monthly temperatures for East Anglia. Heat loss from the window is simply (Ti-Te)*U, where Ti > Te. For heat gain, I used the SAP calculation which is solar flux * transmittance * incidence factor (0.9) * frame factor (0.8 for most windows). All converted to kWh/day. I ignored air tightness, as I said. Do the results surprise you?

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