Garage conversion / wall insulation - nervous about damp
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Garage conversion / wall insulation - nervous about damp

Post by pete247 » Mon Sep 05, 2022 10:14 pm

Hi, this is my first post, although I have referred to many DIY Doctor posts over the years. I now have something I could do with some specific advice on....

Following a house fire that started in our integral garage, we are about to convert the garage into a new kitchen. The house was built early 2000s with twin skin cavity wall construction. The existing cavity insulation generally appears to be a thin layer of thermal foil face bubble wrap type insulation fitted to the outer face of the inner block skin.

The inner skin of blockwork to the garage (and the two internal block walls) have been condemned due to the fire damage, therefore we will be replacing the inner skin of the external walls to the garage. We registered the project with building control before 15/6/22, so working to the old Part L. Now, because we are removing and re-building the entire inner skin for the conversion, I still need to confirm with building control if the u-value we need to achieve is for a "new thermal element" (0.28W/m2K), or for an upgrade of a "retained thermal element" (0.55W/m2K).

Either way, I am paranoid about the risk of damp penetration from adding insulation to the cavity - workmanship is fundamental when installing cavity insulation and I fear the buildability of this part of the conversion (i.e. reconstruction of the inner skin with the outer brickwork left in-situ) could prove to be an issue for correct insulation installation and damp penetration. I know the most common way to combat this in a regular garage conversion is to insulate internally, but I really wanted to avoid eating into the available floor area.

So if I am to stick with only insulation in the cavity, I can do this with:

a) To achieve 0.28 - 100mm full fill mineral wool batts, plus 100mm Aircrete High Strength block inner skin.


b) To achieve 0.55 - 50mm PIR board with 50mm clear cavity, plus 100mm medium dense block inner skin.

I am wondering if it is sensible to continue down this path; how easy will it be to install cavity insulation with the outer brick skin left in-situ? Are there any additional "belt and braces" measures I can add to provide meaningful safeguards to damp and give me peace of mind? I'm thinking a membrane of some sort somewhere in the construction...?

Any advice will be very much welcomed. Thanks in advance!

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Re: Garage conversion / wall insulation - nervous about damp

Post by stoneyboy » Tue Sep 06, 2022 8:51 pm

Hi pete247,
Suggest you go for option a but make sure the bottom of the cavities are well below the concrete floor level.
Regards S

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