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Will you have a 50mm ventilated air space above the insulation ?
Even if he has a modern membrane it's non needed as they are now breathable. However leaving a 10mm gap to drape the membrane would prevent the need for counter battening which would otherwise be necessary to allow runoff.Not needed - he has no membrane at all.
Even with a breathable membrane you still need to leave an air gap over the insulation. Let's all have a look in Bldg. Regs Approved Documents and NHBCEven if he has a modern membrane it's non needed as they are now breathable. However leaving a 10mm gap to drape the membrane would prevent the need for counter battening which would otherwise be necessary to allow runoff.
Ok, but i don't see your quote from them. And anyway that's not what the membrane manufacturer says.Even with a breathable membrane you still need to leave an air gap over the insulation. Let's all have a look in Bldg. Regs Approved Documents and NHBC
Standards and see what they say .
I didn't say I was quoting from them, I said let's all have a look etc, but have you looked at BS5250 code re condensation in buildings ( free download from DLSCRIB ) and BRE Report BR262 thermal insulation avoiding the risks. There is also a school of thought that you should not rely solely on breathable membranes for ventilation, NHBC Standards Extra 48 asks for additional ventilation to breathable membranes. I could refer you to insulation manufacturers' technical data sheets where they specify a 25mm drape space above the insulation.Ok, but i don't see your quote from them.
If you are putting Kingspan between the rafters, and then insulated plasterboard below, ensure that it is the Kingspan (yellow foam) insulated type and not just polystyrene-faced -plasterboard.
With the latter, you will get condensation forming within the rafter space as it does not have an integral vapour barrier, which the Kingspan-type does.
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