Basic U-Values

Basic U-value calculations using BBA Software

The BWF is able to offer members a basic U-value calculation to enable them to obtain the figure they need to comply with Building Regulations and the Construction Products Regulation (CE Marking) at a highly reduced cost.

What will I get from BWF?

We are able to carry out a basic U-value calculation based on Part 1 of the standard “BS EN ISO 10077 Thermal performance of windows, doors and shutters.  Calculation of thermal transmittance. General”. You will need to be a BWF member to access this service

Whether or not the window or doorset itself complies with the requirements of Building Regulations depends on the U-value it actually achieves. The BWF technical team can advise you on how to comply with the requirements of Part L.  We will liaise with you if necessary to check aspects of this data and then calculate the U-value using the BBA software, and issue you with a certificate in accordance with that standard, which can be used as formal support when you supply U-value information to potential customers or Building Control Bodies (BCB).

A U-value calculated by this method will be sufficient to satisfy the requirements of Building Regulations and/or CE Marking declarations (see below), but is not as precise as a full U-value simulation.  It cannot be used as the basis for obtaining a full BFRC Window Energy Rating (WER), nor could it be submitted as if it were a WER.  A U-value for an energy rating needs to be simulated to Part 2 of BS EN ISO 10077.  The BWF offers other Toolkit + packages which can help you obtain full U-value simulations or Window Energy Ratings.

For all windows for which we have calculated the U-Value, we will register the window design with us if you wish to provide U-values to clients in the future. This is particularly useful if you later want to change the design or specification, for example, the timber species, section size, glazing unit or other data and require a new calculation to be performed.  We will maintain the database of windows registered with us, and make it available to FENSA and other inspection bodies.

Can I use this on my Declaration of Performance for CE Marking?

U-values are considered ‘essential characteristics’ for the sake of CE Marking, and will need to be included as part of your Declaration of Performance (DoP).

If you are a micro-enterprise* this can be calculated using any available calculation tool that follows ISO 10077-1 or -2.  In the case of all other windows and doors manufacturers a certificate will need to be accredited via a notified body. BWF will be able to provide U-values calculations through a notified body upon request. For further information, please contact our technical helpline.

*A micro-enterprise is defined as an enterprise which employs fewer than 10 persons and whose annual turnover and/or annual balance sheet total does not exceed EUR 2 million.

What is involved?

The basic U-value service is for BWF Members only. If you are a member, then you can log in and visit to our Toolkit+ publications section where can then choose the appropriate form to download and complete from:
Flush and stormproof casement windows
Vertical sliding sash windows – spiral balance and weights & pulleys

Both casement and sliding sash windows will be modelled on the BR 443 standard design. In the case of dwellings, the BR 443 model window will represent all the windows using a standard window 1.48 m high by 1.23 m wide, with a central vertical divider, and one opening light and one fixed, or directly glazed, light.

We are also able to carry out U-value calculations for doors, which you can find in the same location

‘For further information, please contact

What information will you need to provide?

Contributing factors to the U-value of a window or door is the heat loss through the timber elements, through the glass unit and through the junction between the glass and the timber (i.e. the spacer bar).  Generally the glass will have the lower thermal conductivity than the timber elements and hence the higher the glass to timber ratio, the lower the u-value.  Insulating the timber elements (e.g. panels) will improve the timber elements contribution.

The software used by the BWF is based on standard (BR433 model) doorset.  Window sizes and styles are assumed to be representative of a product range (i.e. a single U-value can be assumed for an entire product range, regardless of the actual window or doorset supplied).  It is vital that you complete all of the sections related to the type of window or door that you wish to obtain a U-value for or a calculation cannot be completed (some data for typical components is provided in this appendix to be helpful, .

Contribution of the Frame
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.

For the purposes of this calculation, Panel and Insulation details relate only to model doors.  Details required should be readily available from the supplier, typical examples are: Celotex, Kingspan, Proctor Group product, Timbmet engineered panels.

Contribution of the Glass
As a rule of thumb, in the case of double or triple glazed units, the higher the ratio of glazing to frame means the lower the u-value of the window (all other things being equal).  Required details should be readily available from your glass manufacturer, the detail required from some of the commonly used units is listed below:


Saint Gobain :

Pilkington :

Contribution of the Spacer Bar
Heat loss via the spacer bar and so selection can have a significant impact on the overall u-value of the window.  Typically the lower the psi value of the spacer bar, the lower the u-value of the window.  Typical examples of commonly  used spacer bars and their respective linear thermal transmittance details are provided below:

Spacer Bar Type Name (psi value (W/mK)
Swiss spacer U     (0.031)
Swiss spacer V      0.032

Thermoseal For double glazing  
Thermobar 3 mm hotmelt    (0.027)
Thermobar 3 mm polysulphide   (0.031)

Thermoseal For triple glazing 
Thermobar 3 mm hotmelt    (0.025)
Thermobar 3 mm polysulphide   (0.029)

Edgetech for Double Glazing 
Super Spacer Triseal Standard / T Spacer Premium Plus (0.033)

Edgetech for Triple Glazing 
Super Spacer Triseal Standard / T Spacer Premium Plus (0.031)

Please note that it is vital if you are using this data to check it and confirm that it relates to the precise Spacer Bar that you are using

Further information on spacer bars and links to datasheets is available via: