Demystifying U-Values: A Guide to Window and Door Energy Efficiency

03 October 2023

If you're manufacturing timber windows and doors, understanding U-values is vitally important, when ensuring compliance with building regulations. With higher energy costs and a general push to improve the efficiency of houses in the UK through government policy, the energy efficiency of homes has never been more important. U-values are the key metric for assessing the energy efficiency of the windows and doors, so it's good practice to understand these values, especially if you're manufacturing windows and doors.

What are U-Values?

U-values are the measure of thermal transmittance through combined elements that make up a building component (in this case windows and doors) measured in units of W/m².K (Watts per metre squared Kelvin)
U-values can be improved as a direct result of improved insulation, either through retrofitted insulation, draught proofing, or choice of materials for window and door manufacturing.
The lower the U-value for a window or door, the better insulated it is. Resulting in a more thermally efficient product and home in general.

Why U-Values Matter for Windows and Doors

Windows

All areas in the UK require U-values for windows to demonstrate compliance with building regulations.
Windows can be a huge source of heat loss. Large windows will have a larger surface of glass, where most of the heat will be lost through a window.
The lower the U-value you can achieve for your windows, the more thermally efficient that window will be.

The glazing choice for windows is going to have the greatest impact on the U-value of your windows. So it's good to know what values these choices have. Below are a few examples;

U-Value Thickness

Single glazing 5.8 W/m².K 4mm
Thin Double Glazing 1.9 W/m².K 12mm
Double Glazing 1.0 W/m².K 24mm
Triple Glazing 0.7 W/m².K 44mm

There are also some specialist glazing options available on the market, as shown below. (For more information and costs we suggest contacting these suppliers directly)

U-Value Thickness

Fineo Vacuum glazing 0.7 W/m².K 7.7mm
C.U.in Glazing 0.4 W/m².K Various

It's important to understand that to comply with current building regulations, windows in new dwellings must not have a U-value of more than 1.6 W/m².K and 1.4 W/m².K for existing dwellings.

Timber choices can also have an impact the U-value of windows, with Accoya outperforming other soft and hardwoods. Buildcheck, a BFRC (British Fenestration Rating Council) approved assessment body, has proven that a window’s U Value can be significantly improved merely by changing the frame substrate from traditional hardwood or softwood to Accoya wood. (As shown on page 17 of this document by International Timber)

Doors

All areas in the UK require U-values for doors to demonstrate compliance with building regulations.
Doors (including glazed doors) in new dwellings must not have a U-value of more than 1.6 W/m².K and 1.4 W/m².K for existing dwellings.
Front and rear entrance doors perform a key role in the energy efficiency and security of a home.
Doors that are poorly fitted or aren't draught sealed, can provide a route for heat to escape, leading to a less energy efficient home. Poorly insulated doors can lead to drafts and energy waste and may not meet building regulations.
Whether you are using Hardwood, Softwood or Accoya is critical as it determines the density and hence the thermal conductivity of the frame material. Lower density softwood materials are better insulators and therefore have the edge over the more dense hardwoods. (ref: BWF)
An alternative to using soft or hardwood in the manufacturing process is external door blanks. We supply OH Panels, available here or by calling our sales team. These door blanks have a U-value of 1.28w/m².K (58mm thick doors) so well within building regulations.

U-Values and Energy Savings

Ensuring the U-values of your windows and doors are as low as possible, can have a significant impact on energy costs. Potentially saving hundreds of pounds a year on energy bills.

Tips for Improving U-Values

The best way to improve U-values when manufacturing timber windows or doors is to ensure your customers are aware of the glazing options available. If energy efficiency is important, then perhaps the extra cost, of say triple or specialist glazing is worth it.
If you're repairing windows or doors and replacing the glazing isn't an option, the best thing to do is to replace any weather sealing that may have deteriorated over time. Ensuring all possible gaps are covered, reducing any drafts.

The key point to remember is your choice of materials is going to have the biggest impact on the U-value of your windows and doors. Some of these materials will come at a higher cost, but savings can be made in the long term from energy efficiencies. In the case of door blanks, for example, you can actually save money by reducing labour costs, in comparison to manufacturing a door from scratch in raw timber. Certainly something to consider when manufacturing windows and doors.

For more information on U-values please visit the BWF. Information on building regulations can be found in the Approved Document L, Conservation of fuel and power Volume 1: Dwellings - 2021 edition incorporating 2023 amendments - for use in England

*All the informtion in this blog post has used references from reputable sources. Please do let us know if you have further information, so that Joiners and other trades are even more informed.