A: In Scotland, the Section 6 (Energy) standards in the Scottish Building Standards Technical Handbook set the U-value requirements:
-
Windows and doors are typically required to be 1.4 W/m²K or better for compliance
-
Energy performance alternatives such as Window Energy Ratings (WER) may also be accepted depending on the situation
Scotland’s energy standards tend to be stricter than the basic backstop values in England.
A: In England, the requirements are set by Approved Document L (Part L) of the Building Regulations:
-
New build dwellings: limiting U-value for windows and doors is typically 1.6 W/m²K or better — with many designers targeting ~1.2 W/m²K for compliance in energy models
-
Replacement windows and doors (existing dwellings): often expected to be 1.4 W/m²K or better, or a suitable Window Energy Rating (WER) alternative
These values are whole element values (including frame, glass, spacer, etc.) and are used to demonstrate compliance with Part L energy targets.
A: U-values show how much heat is lost through a building element. A lower U-value means better insulation and less heat loss. U-values are used by Building Control and energy assessors to check compliance with energy performance standards and Building Regulations. Typical whole-window U-values now expected in the UK are significantly better than older products.
A: The U-value of a wooden frame depends on the thermal resistance (R-value) of the wood and its thickness.
Different species and types of timber have different insulating properties:
-
Accoya® tends to have higher natural insulation
-
Softwood generally performs better than hardwood
-
Hardwood will usually have a slightly higher U-value
In practice, thicker sections of timber with higher R-values give lower overall U-values. The exact performance depends on the frame design and glazing details.