Minimum thickness of cavity wall insulation?

16 Jul 2007
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United Kingdom
Hi, I'm building a small kitchen extension with two external walls. The walls will be cavity wall built with 100 mm celcon solar blocks or equivalent. The outer skin will be rendered and pebbledashed the inner plastered. Can you tell me the minimum cavity size and the minimum thickness of insulation using celotex or kingspan rigid insulation to meet the appropriate U values for building regs? Cheers to all!!!!
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using lightweight 100mm thick blocks internally, i.e. thermalite, celcon etc, building reg's expect a 100mm full fill cavity, using dritherm cavity batts.
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Minimum U-value is 0.30W/m²K. 35mm Celotex will achieve that with 2x100mm skins Celcon Solar blocks. If you use different blocks you must make sure the thermal value remains the same otherwise the thickness of insulation may change.

Any new windows must be better than 1.8W/m²K and the total area of windows must be 25% or less. If not the only way to comply is to calculate a SAP.

Also, any new heating must comply, all new light fittings must comply (1/3rd of fittings must be low energy) and you must include measures to avoid thermal bridging and excessive air leakage.

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