U-factor
U-factor is the rate of heat transfer through a whole assembly such as a window or wall; a lower U-factor means better insulating performance.
U-factor is the rate of heat transfer through a whole assembly such as a window or wall; a lower U-factor means better insulating performance.
Definition
U-factor measures how readily heat passes through an assembly, per unit of area and temperature difference. It is widely used for windows, doors and skylights, and also for walls and roofs in energy compliance.
A plain example
Compare two windows on a rating label. The one with the lower U-factor lets less heat through, all else equal. For a wall, a designer can compute an overall U-factor that includes the framing, insulation and surface films.
Why it matters
U-factor lets you judge the assembly as installed, not just a single material. Energy codes commonly set maximum U-factors for fenestration, so it often appears in submittals and compliance reports.
Common confusion
U-factor and R-value are related but not interchangeable: one is flow, the other is resistance. A lower U-factor is better, while a higher R-value is better, which causes plenty of reading mistakes. U-factor is also separate from solar heat gain, which has its own rating.
On a project
Look for the U-factor on the rating label or product data, and match it against the maximum in the energy code or specification. Compare products using the same rating method.
Quick answers
- Is a lower or higher U-factor better? Lower. It means less heat flows through the assembly.
- Is U-factor the same as R-value? No. They are inverse ideas: U describes flow through an assembly and R describes resistance of a layer.
- Where do I find a window U-factor? On the manufacturer data or the rating label, issued under a recognised rating program.
Related terms
Always check the project drawings and specification, and confirm requirements with the engineer of record and the local authority.
Questions people ask
Is a lower or higher U-factor better?
Lower. It means less heat flows through the assembly.
Is U-factor the same as R-value?
No. They are inverse ideas: U describes flow through an assembly and R describes resistance of a layer.
Where do I find a window U-factor?
On the manufacturer data or the rating label, issued under a recognised rating program.
Sources and further reading
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