U–value for Widnows and Doors

According to  Building Regulations Part L we have to comply with min U-value requirements for Windows or Doors but different values are considered for New Dwellings and Existing Dwellings.

Approved Document L1A ‘Conservation of fuel and power in new dwellings’  2013 edition implements :

Windows and glazed doors    – 1.4 W/m2K

Semi- glazed doors                  – 1,2 W/m2K

The revision to Part L1a took effect from 6th April 2014.  This new specification has been strengthened to deliver a 6% improvement across new home builds relative to the previous 2010 Part L1a specification.

The limiting fabric parameters for glazing and window performance remain the same as 2010 at 2.0 W/m2K to allow for design flexibility, however the ‘Model Design’ specified in Part L1a recommends a whole window U-value of 1.4 W/m2K with a g value up to 63% (g = 0,63) to avoid overheating.  Other glass performances can be used, however the Design Model recommends a notional specification that will achieve the new performance requirement.

u-value timber glazing L1A


Approved Document L1B ‘Conservation of fuel and power in existing dwellings’  2013 edition (Replacement & Refurbishment Residential) implements :

Windows and glazed doors    – 1.6 W/m2K

Semi- glazed doors                  – 1,8 W/m2K

Part L1b refers to the replacement of glazed windows and doors in existing residential buildings.

The minimum required performance is either a whole window U-value of 1.6 W/m2K or a Window Energy Rating band C (Window Energy Ratings can be issued by BFRC, Certass or BSI), there are special circumstances where a centre pane U value of 1.2 W/m2K can be used instead, but this is only for hard to treat area’s where the required performance values are not an option.


u value building regulations



How to calculate Uw (u-value) of timber window and make sure it meats building regulation criteria?

The method of calculating coefficient of heat penetration of timber window – Uw with single glazing is presented below, where the influence of the connection of the frame with the pane is taken into account.




u value formula for calculations - building regulation

Uw – coefficient of heat penetration of window [W/m2K]

Af – surface of timber frame

Uf – coefficient of heat penetration of frame – timber [W/m2K]

Ag – glazed surface  [m2]

Ug – coefficient of heat penetration of pane [W/m2K]

Lg – circumference [m] (edge of glass)

Ψ – Linear heat penetration of window [W/mK]



WoodAndHome provide all calculations for all timber windows and doors we are manufacturing. Alternatively we can provide above details for our clients to check  U-value themselves.