U-Value Calculator for Timber Windows and Doors
Whole-window and whole-door U-value calculations
We provide whole-window and whole-door U-value calculations for timber and wooden-framed windows and doors.
Our calculations use the dimensions, frame construction and glazing information supplied for the particular window or door. The result is a whole-element U-value, taking account of the frame as well as the glazing.
We specialise in bespoke timber windows and doors, including hardwood, softwood and Accoya® frames, with double or triple glazing.
A detailed PDF U-value calculation certificate with a unique serial number is provided for each calculation.
No Membership / No Subscription
You do not need to become a member or subscribe to use our service.
You can order one calculation or multiple calculations as required. There is no minimum number of calculations.
Our standard charge is £35 per certificate, with discounts available where several calculations are required.
What We Calculate
Our service is designed primarily for bespoke timber-framed windows and doors.
Suitable for
We are not tied to any particular window manufacturer or glazing supplier.
The calculation is based on the information supplied for the individual window or door and follows the calculation principles of BS EN ISO 10077-1.
A Whole-Window U-Value — Not Just the Glass
The U-value of the glass is only one part of the thermal performance of a finished window or door.
The frame, glazing area, glazing edge and dimensions also affect the overall result.
Our calculation therefore determines the whole-window or whole-door U-value, using the information supplied for the particular product being assessed.
This gives you a U-value for the complete window or door, rather than simply quoting the centre-pane U-value of the glass.
Scope of the standard calculation
Our calculation is intended for straightforward window and door configurations where the frame and glazing arrangement can be represented from the information requested on our submission form.
Some additional construction features can affect the thermal performance of a window or door and may require a more detailed calculation. These can include projecting sills or thresholds, frame extenders, dummy sashes, additional mullions or transoms, coupling components and other non-standard frame components.
If your window or door incorporates features of this type, please contact us before ordering a calculation so that we can confirm whether the particular configuration is within the scope of our service.
How our U Value Calculator Works
The calculator evaluates frame material, glazing configuration, spacer type, and dimensions to provide an accurate U-value. Timber frames are handled specially to reflect real-world performance, unlike generic calculators.
Timber vs Aluminium vs PVC
Timber (Wooden Frames):
Timber is naturally insulating and can improve thermal performance. However, U-values for timber windows and doors vary depending on species, frame thickness, profile geometry, and glazing interface. Timber frames require a specialist U-value calculator to ensure accurate results and regulatory compliance.
Aluminium Frames:
Aluminium conducts heat efficiently, so frames without thermal breaks can reduce energy efficiency. Modern aluminium windows use thermal breaks and multi-chamber frames. Accurate calculations must account for these differences in the frame details.
PVC (uPVC) Frames:
PVC frames have predictable thermal performance due to a multi-chamber construction. While easier to model than timber or aluminium, accurate results still require precise data from the frame and glazing.
Why Material Choice Matters:
A reliable U-value calculator models each frame material correctly. Treating timber, aluminium, and PVC the same can produce inaccurate results. Our calculator focuses on timber windows and doors, where material choice and frame construction can significantly affect U-values.
When a U-Value Certificate Is Required
A U-value certificate provides formal evidence of thermal performance, and is often required for:
Indicative calculations are fine for early-stage planning, but final approval often requires a certified calculation carried out according to the standards specified in BS EN ISO 10077.
Who Needs a U-Value Certificate
Certification is typically required by professionals involved in design, manufacture, or installation of windows and doors:
Certification is particularly critical for timber windows and doors, where material and construction details significantly influence thermal performance.