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Target U=Values

Discussion in 'Building Regulations and Planning Permission' started by gbr9282r, 25 Jul 2020.

  1. gbr9282r

    gbr9282r

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    Hi all, I am having a single storey extension (25m2 ish) built at the rear of the property, with a warm flat roof. Plans done (but some issues have arisen (see my other post in building on steel beam problems!). Doing via a building notice, rather than full plans... I have to resolve this by monday am, or have a bricky sat doing nothing all day again... or worse, the BCO refusing to sign off...

    So the official plans drawn have a target value on various new thermal elements of the build - eg, the new flat roof, designed to achieve 0.18, etc etc... i note there is a new version of the thermal reg's in 2020.... Does table two apply (attached) or is it the ones in the A version of the document for new builds?... They are different... but the one for existing homes cites the one for new homes... If so, I need to go back to the SE and get a new design of roof (well thicker insulation as 120mm thermaroof won't achieve the values in L1A, but does achieve the values in table 2 in L1B)

    Another example the new regs are saying a new cavity wall should be 0.18 in L1A, but table two (attached from L1B) says 0.28... The reason i ask is there is a change to design to avoid a complex steel issue, and without falling out with the SE. we need to consuctuct a small section of new solid wall to provide a base for a less complex steel arrangement. Do I need to bring this new solid wall to 0.18 or 0.28.. there is a big difference in the amount of insulation i need to find!!
     

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  3. tony1851

    tony1851

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    The table you quoted is the one you use - it's from L1B (existing dwellings).

    When dealing with domestic extensions, the main thermal consideration is the area of new glazing to be incorporated, as per sq m. glazing looses more heat than wall and roof. The amount of glazing (including roof lights) you are normally allowed is 25% of the floor area of the extension, plus the areas of any windows or doors covered over or which no longer exist because of the extension.

    Your architect/plan drawer should have taken thermal insulation into account as it is critical in the design; or have they just written "walls/floor/roof to have minimum U-values of X/Y/Z" as notes on the plans without indicating how these are to be achieved?
     
  4. DevilDamo

    DevilDamo

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    As above, L1B is for works to existing dwellings, i.e. extensions and L1A is for works to new build dwellings. At present, the regulations for new build dwellings are those that are subject to change and not those to existing dwellings.
     
  5. charliegolf

    charliegolf

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    Blimey, is that current? I built my house in 1995 with walls at 0.29. If so, things have really moved on apace eh?

    PS, does U value degrade over time for any reason other than the envelope being breached? IE, do the materials degrade?
     
  6. tony1851

    tony1851

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    PIR degrades over time, but very slowly.
     
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  7. Notch7

    Notch7

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    120mm PIR on warm roof should get to 0.18

    That's what I've always done and had passed.
     
  8. charliegolf

    charliegolf

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    What's PIR?

    Edit: Got it!
     
    Last edited: 26 Jul 2020
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