When it comes to the building envelope, windows are the thermal weak point.
Let me put this into a bit of perspective. In most Australian climate zones, as defined in the National Construction Code, the minimum ‘Deemed To Satisfy’ R-value for an external wall is R2.8. A Typical aluminium framed, single glazed window has an R-value of R0.15. That’s roughly 5% of the wall’s R-value. Even an aluminium framed, double glazed window at R0.24 is only about 15% of the wall’s R-value. I will put this in a list below just to emphasise the point.
|Aluminium Single Glazed window:||R0.15||U6.5|
|Aluminium Double Glazed window:||R0.24||U4.12|
(remember that an R-value is the inverse of the U-value. 1/R=U)
Unfortunately the bad news doesn’t end there………it’s called air infiltration. Most people, (I hope) will be familiar with the SHGC (Solar Heat Gain Coefficient) and Uw (conductance) figures when choosing windows. If you look hard enough, somewhere tucked away to the right of these figures is the AI (Air Infiltration) figure. The maximum allowance for Air infiltration for Australian windows is 5 litres per second per square metre, tested at 75 Pascals. If you pay attention to this figure you might get a shock to see how much your ‘thermally advanced’ windows leak air.
As I mentioned in a previous post, no amount of insulation will keep a drafty house warm. Likewise, a low U-value will not make a leaky window thermally efficient.
When we consider the thermal barrier of our building envelope, windows are well worth thinking about. A large percentage of glass equates to a large percentage of [effectively] uninsulated wall. I have assessed many houses with a window/wall ratio of over 50%. That’s like only insulating half of the walls. A simple UAΔT heat load will highlight this hole in the thermal fabric.
So, is the answer to have no windows? Of course not. We want and need windows. Windows have a tough job, they need to do all that a wall does as well as let us see through them, provide natural light, provide ventilation, and even permit some solar heat, (if designed for the purpose); no wonder they’re expensive.
As much as walls are cheaper and are better insulators than windows, we don’t want to live in a cave with no windows. Carefully considered windows are a very important part of design.
The purpose of this post is just to emphasise the insulation properties of windows and to hopefully promote some careful consideration in this regard when choosing windows within the context of house design. When choosing a window to perform well in the building envelope it is important to look at the U-value, the SHGC value, and the AI value.