Understanding insulation values
About U
The technical name for which we use the shorthand ‘U-Value’ is Thermal Transmittance.
The U-value of a building component like a wall, roof or window, measures the amount of energy (heat) lost through a square metre (m2) of that material for every degree (K) difference in temperature between the inside and the outside.
Before we start looking at what that means, let’s sort out the units we use to define it.
- Energy flows along in watts (which is a measure of energy in ‘joules’ flowing over a period of time in ‘seconds’ ).
- Temperature difference is measured in degrees Kelvin – which practically speaking equates to degrees Celsius to the rest of us.
Example of how U-values work:
The U-value of a single sheet of glass as found in a traditional window pane is 6.0W/m2K – which means that for every degree of temperature difference between the outside and the inside, a square metre of the glazing would lose 6 watts. So for example, if the temperature difference on a typical cold day was 15 degrees, then the amount of heat loss would be 15×6 = 90 watts per square metre. That’s a lot of heat!
By comparison, the U-value of a modern piece of triple-glazing can be as low as 0.7W/m2K – 15x.07 = 10.5 watts per square meter, which is considerably better.
The importance U values in glazing
Modern building designs often incorporate large glazed windows and doors, and that presents a challenge when it comes to maintaining good levels of insulation, both to stay warm in cold weather and cool in hot weather.
Even with modern insulated units, glass windows and doors are much less insulating than even poorly insulated wall systems. Even a 100mm stud wall with 50mm Rockwool with a U value of 0.35 is vastly more insulating than a Class A double glazed window with a typical U value of 1.4, in fact 4 times so.
Lets consider a typical 3m wide x 2.2m tall bifold door which is an area of 6.6sqm on a cold uk winter day at 2 degrees, with an comfortable inside temperature of 20 degrees.
New build doors and bifolds need to meet a minimum U value of 1.8, so the heat loss through the window = 1.8 x 6.6 (area in sqm) x 18 (temp difference in degrees) = 214w
Typical GartenHaus triple glazed thermally broken windows and doors have a U value of 1.0, so the equivalent heat loss would be 1 x 6.6 x 18 = 118w
Almost half the heat loss
Comparison with wall heat loss
An equivalent 3m width of external wall (175mm SIP and reflective membrane), 2.2m high, area = 6.6sqm
U value 0.12, heat loss 0.12 x 6.6 x 18 = 14w which is a trivial loss per m2 compared to glazing.
Why we consider the building as a whole.
It is clear from the above that even high quality glazing in windows and doors loses more heat than walls, but one of the joys of garden rooms is the view. However, by using super insulated SIPs for our walls, roofs and floors we can compensate for large glazed areas and still keep high insulation values for the building as whole.
Our typical 1 U triple glazed 3m wide, fold and slide, thermally broken aluminium framed units cost about £200 more than a uPVC double glazed alternative, we think that’s money well spent.