Calculate the accurate u-value of wood windows and doors

Whole Window u-values
Conforms to Part 1 of BS EN ISO 10077

We provide a service for small bespoke manufacturers who are required to supply information about u-values to building control. We do not just supply a u-value taken from the centre point of the glass or the centre of a window panel. This is a whole window u-value calculator giving the combined u value of the glazing, the frame and the spacer.

No restriction on manufacturer

Our calculator does not restrict you to using products from any one manufacturer. We can calculate the u value of single, double or triple glazed windows built with softwood, hardwood or accoya frames.

We can calculate the overall u-value where other types of frame are used if you can provide the u-value of that frame.

No Membership or Registration Required

We do not require you to sign up for any sort of membership to use our uvalue calculator. It costs just £35 for a calculation and for that we will send you a three page pdf certificate (usually the same day) using the information you submitted to us.

Why you require U Values - An explanation

From 1st October 2010, window manufacturers needed to show that their windows complied with the energy efficiency requirements in the latest 2010 revision of Part L of the Building Regulations and the Approved Documents L1A (new dwellings) and L1B (existing dwellings).

Obtaining a precise u-value for a manufactured window can be extremely expensive especially for smaller manufacturers or architects with bespoke designs.

Architects and specifiers are now increasingly asking for the thermal transmittance values of windows, so that they can calculate the SAP ratings of the buildings where the windows are to be fitted. Clients require that value for completion certificates. You need to comply with Part 1 of BS EN ISO 10077 for building regulation approval. U-value calculator can provide this information at a very reasonable cost.

Whole Window U Values - How to achieve a low u-value

What affects the uvalue of a door or window ? Heat loss through the timber sections, through the glass units and through the junction of the glass and the timber (i.e. the spacer bar).

Normally glass will have a lower thermal conductivity than timber elements so the higher the glass to timber ratio, the lower the overall u-value should be. Insulating the timber elements (e.g. panels) can also improve the overall u-value.

The Frame

The choice of wood you use for the frame is important for achieving a low uvalue. The type of wood, Hardwood, Softwood or Accoya determines the thermal conductivity. That thermal conductivity is used to determine the uvalue of the frame. Accoya is the best followed by Softwood and then Hardwood.

The Glass

For double or triple glazed units it is generally true that the higher the ratio of glass to frame means the lower the u-value. To achieve a low u-value it is best to use glass units with a low uvalue. We hold u-values for some units but details should be readily available from your glass manufacturer.

The Spacer Bar

Heat loss via the spacer bar can have a significant impact on the overall u-value of the window. The lower the psi value of the spacer bar, the lower the u-value of the window. The standard aluminium spacer is not the best way to achieve a low u-value and more efficient bars such as a swiss spacer or thermobar are available.

Simple 3 Step Process

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