DIY Doctor

Insulation problem and small air gap - please help

Postby alldalights » Fri Jan 08, 2021 12:26 pm

I have a loft extension with a pitched roof. The builder has been really good to date, but i noticed when installing the insulation board, he's not left 2 inches as a void before the roofing felt (it's more like 2 cms). This is a concern for a couple of reasons. Is this realistically enough of a gap to allow the air to move freely (there are vents in the bottom between the joist ends)?

Even if this is ok...... there is now a 50mm gap between the front side of the 100mm insulation and the front of the joists. I know his plan is to use a thinner Kingspan style insulation (i think 25mm) and lay that in the other direction.

Option 1 - remove all the insulation and bring it forward by 50mm to allow the correct gap at the back to ventilate, plus remove the gap so it is inline with the joists at the front. I'm not sure this is possible as he's fitted them really tightly and any gaps expanding foam filled. Accepting this, the only real concern would be if the 2cms ventilation would be enough. (As I guess I can buy more 50mm rigid board and then have 150 cm total)

Option 2 - except the stuff that's been put in has to stay. So it's a case of fitting either rotwool to fill the gap or 50mm board. Then fit the 25mm board at 90 deg, then plasterboard etc etc.

Option 3 - Except the existing stuff has to stay, fit the 25mm board... and hope the 50mm airgap doesn't create a condensation problem?

Any help gratefully received!!

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Simply Build It

Postby stoneyboy » Sat Jan 09, 2021 10:46 pm

Hi alldalights
Assuming breathable slaters felt has been fitted there should not be any problems. Fill the inside gap with more insulation, put a layer of insulation with foil over the whole inside - do tape the joins to form an effective DPM.
Regards S
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Postby alldalights » Sun Jan 10, 2021 12:04 am

Thanks S,
Many thanks for your reply. Yes he’s fitted a branded breathable membrane, so all good.

Feeling reassured so thanks again!
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