Thermal efficiency is an important consideration for any new window or door. It can make the difference between a warm and welcoming property and somewhere uncomfortable and cold.
There are also various legal restrictions surrounding thermal efficiency, with a push to keep heating costs down and improve the efficiency of properties in the UK.
Document L
Document L Building Regulations require windows and doors to comply with energy performance guidelines. This does not apply for listed buildings or where the window or door is being repaired rather than replaced (including installing new sashes or door leaves into the existing frames).
Assuming that the windows and doors comply with Document L, you should receive a certificate after the work has been completed – although it is important to check that your installer is registered with FENSA as otherwise you will need to pay for a certificate.
We are registered with FENSA so all Doc L windows and doors that we install will receive a FENSA certificate on completion.
Obtain a quote for new Doc L windows or doors by completing our contact form.
U-values and thermal efficiency
U-values measure how quickly the heat from the inside passes through to the outside, with lower U-values having better insulation. The design, along with the materials used, will make a difference in the U-value of the specified window or door.
You may be restricted on the required U-value, and this will be different depending on whether the project is a new build or renovation. U-value requirements for new builds also vary depending on the percentage of the window or door that is glazed.
For renovation projects, a minimal U-value of 1.4 W/m2K is normally required. For new builds, where over 60% of the window or door is glazed, a U-value of 1.2 W/m2K is required. For new builds where 60% or less of the window or door is glazed, a U-value of 1.6 W/m2K is required (source).
Our U-values using Argon gas start from 1.1W/m2K, although please note that the achievable U-value will depending on your requirements. We can also use FINEO glazing, when required.
Air leakage
Air leakage is the measurement of the air that passes through a window or door, representing how easily heat is lost. The lower ratings represent the least amount of air passing through.
Unlike other ratings, air leakage can also be improved by draught-proofing existing windows. In fact, Historic Environment in Scotland found that draught-proofing sash and casement windows can reduce air leakage by up to 80% (source).
Air leakage ratings (also known as the L factor) in replacement windows and doors are measured on the assumption that they will be securely sealed and draught-proofed on installation, so it is important to ensure that the installation is also of high-quality to meet these ratings.
To confirm the air leakage value for your windows or doors, please speak to your sales surveyor.
Solar factor
G values, also known as solar factor, measure the solar heat energy that passes through the glass. The higher the value, the higher the solar gain. Different types of glass will provide different G values.
Depending on the location of the windows or doors, solar gain can be an advantage or disadvantage. A higher solar gain will help heat the room in the winter but can create problems with overheating on hot days. It is therefore worth considering the direction that the windows and doors face in the property when deciding on the ideal G value.
Similar to air leakage and U-values, the solar factor can vary depending on the design on the window or door. If you require the windows or doors to meet a specific G value, we therefore recommend that this is discussed with the sales surveyor who will work with you to try to achieve this.
Specifying thermally efficient timber windows and doors
At The Sash Window Workshop, we have over 30 years of experience working with homeowner and commercial customers to provide energy-efficient timber windows and doors for renovations, extensions, and new build projects. Our sales team will work with you to ensure that the new joinery meets your specifications.
To obtain a free, no-obligation quotation for your project, call us today on 01344 868 668 or send any specification documents to info@sashwindow.com.
