What Is A Window 'U Factor'?
The U factor, or U value, measures the thermal resistance of a product, or in other words how well a product prevents heat loss. This rate of heat loss is expressed in terms of the U factor, which for windows generally falls between 0.20 and 1.20 BTU/ft2 hour °F. The lower the U factor, the better the thermal performance.
When glazed with insulating glass units having a centre of glass U factor (or Ucog) of 0.249 BTU/ft2 hour °F, the U factors of the various Crittall ranges are as follows:
| NFRC window type AA (residential) 36" x 60" |
NFRC window BB (commercial) 48" x 72" |
|
|---|---|---|
|
Homelight |
Vent - 0.40 BTU/ft2 hour °F Fixed Light - 0.37 BTU/ft2 hour °F |
Vent - 0.37 BTU/ft2 hour
°F Fixed Light - 0.33 BTU/ft2 hour °F |
| Corporate W20 | Vent - 0.40 BTU/ft2 hour °F Fixed Light - 0.31 BTU/ft2 hour °F |
Vent - 0.37 BTU/ft2 hour
°F Fixed Light - 0.29 BTU/ft2 hour °F |
| Corporate 2000 | Vent - 0.42 BTU/ft2 hour °F Fixed Light - 0.35 BTU/ft2 hour °F |
Vent - 0.38 BTU/ft2 hour
°F Fixed Light - 0.33 BTU/ft2 hour °F |
| Berkeley | Vent - 0.38 BTU/ft2 hour °F Fixed Light - 0.33 BTU/ft2 hour °F |
Vent - 0.35 BTU/ft2 hour
°F Fixed Light - 0.31 BTU/ft2 hour °F |
It should be borne in mind that a whole-window U factor depends on the following: frame size, frame configuration (i.e. opening or fixed), whether true divided lights are required and the glass type. Generally speaking, the larger the window and the greater the glass to steel ratio, the better the whole-window U factor.
We would be pleased to advise you of the whole-window U factors for your particular project. Please contact us with details of the project name and location together with window range, size, configuration and glass Ucog factor.
What is the 'R value'?
The R value is the reciprocal of the U factor (i.e. R = 1/U) and
measures thermal resistance. The R value measure is more often used
to calculate the overall U factor of a complete structure.
Individual components such as windows usually have their thermal
performance expressed as U factors.
![[logo] - BSI FM 37264](/gfx/logo_bsi.gif){kind=logo}
![[logo] - FENSA registered company](/gfx/logo_fensa.gif){kind=logo}