Do the Building Regs Require Me to Use a Big Window Manufacturer Over a Local Joiner?

26 January 2022
by Alex Nissim

I want timber sash windows in my new build. If my windows are going to be made by a local joiner, who is going to use DG units with a U-value of 1.1, how do I get the Uw value of the whole windows?
Does each window have to reach 1.6 or 1.4 Uw to comply with Build Regs?

I am a little confused because this guy makes beautiful windows, but does compliance with Building Regs mean I must go to one of the big manufacturers?

Another confusing thing is PAS24. All of the windows will be lockable and have restrictors, and each lower sash will have toughened glass. Once again does this mean I am pushed to the big suppliers? If so, it is very upsetting that my local guy will miss out!


2 Answers

  1. Anamika Talwaria says:

    Hi Alex,

    Thanks for your question. I have passed this on to our expert team and will aim to get an answer over to you ASAP.

    Anamika Talwaria (Build It features editor)

  2. Nigel Griffiths says:

    It is normally no problem getting new timber sash windows to comply with the Regulations. Building Regs set out limiting U-values (which are higher than the values you cited) but what you need to attain depends on the whole dwelling TER – size of windows, etc. See Criterion 2. (2.31 and following).

    Joiners can build more or less what you want and there are plenty of good new sash windows around which have passed Regs.

    The calculation procedure is set out in SAP – not sure whether a more recent version has been published. See excerpt below.

    3.2 Window U-values
    The U-value for a window should be that for the whole window opening, including the window frame.
    Measurements of thermal transmittance in the case of doors and windows should be made according to BS EN ISO 12567-1. Alternatively, U-values of windows and doors may be calculated using BS EN ISO 10077-1 or BS EN ISO 10077-2. In the case of roof windows, unless the measurement or calculation has been done for the actual inclination of the roof window, adjustments as given in Notes 1 and 2 to Table 6e should be applied.
    Table 6e gives values that can be used in the absence of test data or calculated values. Use a value from Table 6e which corresponds most closely to the description of the actual window; interpolation should not be used in this table. The table provides default values for windows corresponding to the generic descriptions given in the table. Measured or specifically calculated values can be better than those in the table because of better frame performance, improved spacer bars and other factors.
    The effective window or roof window U-value to be used in worksheet (27) and (27a) takes account of the assumed use of curtains; it is calculated using the formula:
    Uw,effective 1 (3) U1 0.04
    where Uw is window U-value calculated or measured without curtains.

    In terms of restrictors etc, this tends to apply to social housing but I wasn’t aware that it applied to private development. Toughened glass make perfect sense – and no reasons why your joiner can’t use this – we just get the glass sent off to be toughened!

    Absolutely we need to encourage the small manufacturers – and many joiners are FENSA registered for window manufacture.

    Nigel Griffiths (Build It sustainability expert)

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