When choosing new windows, the chances are that you’ll want to increase the energy efficiency of your home.
The overall efficiency of a building is limited by the weakest point, so if the walls are poor, the energy efficiency will be limited by the walls rather than the windows. For example, there can be a lot of difference between a Victorian villa and a modern built insulated house.
If you have uninsulated cavity walls or poor loft insulation, it’s a good time to deal with this when you’re installing new windows as you’ll get far greater energy efficiency all round. That way you will get better value from the energy efficiency of your newly installed windows and doors.
Mythbuster – Triple glazing
There’s a perception that adding an extra pane of glass to make triple glazing will give a big improvement on heat retention and noise reduction. However, this will not necessarily be the case. Our tip is to ask for evidence of the thermal and noise reduction performance that has been measured for the whole window or door design.
How windows and doors are rated
Building Regulations require energy efficiency for windows of either a U-Value of 1.6 W/m²K or a Window Energy Rating (WER) of C. But what are the differences between U Values and WERs?
U-Values
Windows and doors are part of the insulation of your home, but they will still transfer heat. This transfer of heat is called thermal transmittance, which is measured in Watts (W).
To compare the thermal transmittance of different products, the amount of heat that is transferred per square meter is calculated i.e. the heat loss in Watts divided by the surface area (m²), with the temperature being expressed in degrees Kelvin – W/m²K. This value is called the U-Value of the product.
You want less transfer of heat for better insulation, so a lower U-Value means better performance:
- 1.6 W/m²K = Building regulations compliant window
- 1.2 W/m²K = Window with high thermal performance
- 0.8 W/m²K = Window for use in Passive House (extremely low energy usage) or equivalent building
For doors, Building Regulations require a U-Value of 1.8 W/m²K or better.
Certification: Energy Efficiency
Window energy ratings
Heat is not only lost through thermal transmittance, but also through air leakage.
You can also have positive gains of heat, through heating by the sun – if you stand by a window that is in direct sunlight, you can normally feel this effect, which is called solar gain.
Window Energy Ratings take the thermal transmittance, air leakage and solar gain into account, giving an overall energy efficiency value.
Just like the label on a fridge or washing machine, the rainbow colours show the Window Energy Rating from the worst, G, to the best, A++.
There is a Door Energy Ratings scheme (DERs), but unlike windows, only U-values are included in the Building Regulations.
Window Energy Rating: 218.6 x Window Solar Factor – 68.5 x (Window U value + Air leakage)
Mythbuster
An A-rated window is not always the best to fit. This is because the WER calculation includes solar gain, which only works when the sun is shining on the window. If your window or door is not in the line of the sun, only the U-Value and the air leakage is important.
