U value is a formula that measures the rate of heat transmission through building materials and is shown as W/m2. The amount of heat lost in watts (W) per square metre (m2) of said building material. The lower the number the better the performance.
Slimline CF77 Sliding Glass Door with 44mm Triple Glazed 0.6 W/m2K
When talking about U values it’s important to understand the 3 different measurements that you are likely to encounter when browsing for information.
- Ug = thermal transmittance of the glazing itself (or the ‘centre-pane’ U-Value)
- Uf = Thermal transmittance of the frame itself
- Uw value = thermal transmittance of the whole window – including glazing and frame. (This is the key number unless you are buying separate frames or glass)
- ‘Edge’ or PSI value is a calculation of the spacer/edge effect of a unit. It doesn’t improve the Ug, but does improve the Uw. (
- Look out for Sliding Glass Door companies that state U values in what could be described as misleading in an attempt to make their Sliding Glass Doors Uw values appear better than they are.
When talking about Sliding Glass Door U values please understand that there are multiple factors that go into calculating the Sliding Glass Door System U value.
The factors that can change the U value of a Sliding Glass Door are:
- Size of the Sliding Glass Door
- • Number of panels in a Sliding Glass Door
- Opening configuration of a Sliding Glass Door
- Does the Sliding Glass Doors open inwards or outwards (some systems use different profiles for each.)
So when you see an advertisement stating Sliding Glass Doors at a certain U value you should look for the actual size, number of panels and configuration to which it refers.
An example might look like this: * 4200mm x 2100mm, 4:3:1 Opening Out using 1.2 Ug value Glazing = Overall Frame and Glass Uw Value of 1.77 W/m2K
Lets break this down so that you clearly understand which part of the Sliding Glass Door is being measured
* 4200mm x 2100mm – width and height in mm
4:3:1 – Opening configuration (this means a total of 4 panels, 3 moving left, 1 moving right)
Opening Out – Opens away from the dwelling.
Using 1.2 Ug value Glazing – The actual U value of the glass unit only.
Uw Value of 1.77 W/m2K – Overall Frame and Glass U value.
We can offer various ranges of products to suit your U Value requirements ranging from the base model that uses a 28mm Double Glazed Unit of Ug 1.2 W/m2K that easily surpasses both categories of document L building regulations. Uw Value of 1.77 W/m2K
Next level is a 36mm Triple Glazed Unit of Ug 0.8 W/m2K that changes the above Sliding Glass Doors Uw value to 1.39 W/m2K (when using our CF77 Slimline Vent)
Our Top of the range offering is a 44mm Triple Glazed Unit of Ug 0.6 W/m2K that changes the above Sliding Glass Doors Uw value to 1.20 W/m2K (when using our CF77 Slimline Vent)
So hopefully now you can better understand all about Sliding Glass Doors U Values and what can be achieved by increasing the performance of the Glass Unit.