Hello, I'm renovating a 1980s property that has 150mm stone, a cavity without insulation and an internal leaf comprising of 100mm Thermalite blocks. I recall using these back in the day, they were lightweight and could be cut with a handsaw. They were sold as having insulation qualities and I guess that is so, but the mortar joints would be a cold spot.

I'm wondering if they will help though, I'm adding insulated plasterboard but space is an issue, it's a tiny house, but I'd like to work out final u values.

Any thoughts anyone? I can't see them mentioned on the online u value calculators.

Posted By: djhAccording tohttps://www.forterra.co.uk/wp-content/uploads/2021/10/Thermalite-brochure-web.pdf" rel="nofollow" >https://www.forterra.co.uk/wp-content/uploads/2021/10/Thermalite-brochure-web.pdfthere are different types of blocks with different conductivities. They're all apparently marked by different scratch marks! Who knewhttps:///newforum/extensions/Vanillacons/smilies/standard/bigsmile.gif" alt="" title="" >

Insulating the cavity would seem like its worth doing.

the lesser the number of scratch squiggle lines the stronger the block and the higher the conductivity.
Ranging from about 8.7N 0.2 w/mk conductivity 2 scratch lines to 2.9N 0.11 conductivity 6 scratch lines.

Depending on your location and exposure to driving rain I would be very cautious about filling the cavity with insulation unless you constructed an external cavity with rainscreen cladding.

Is there a specific concern with aerated block and fullfil insulated cavity?? As far as I know it was the standard build for decades

Building regs have a fairly comprehensive section on which regions need a partial fill cavity (in terms of driving rain) and which are ok for full fill, also depending on waterproofness of outside surface, width of cavity etc.

Ubakus is great for its intended job (U values and heat loss) but the underlying code it's based on is not reliable for anything to do with moisture. It doesn't deal with penetrating rain, drainage planes, seasonal cycles, hygroscopic mortar, etc. IME it will flag up just about any insulated cavity is a 'condensation risk' yet millions of homes have them.

Right tool for the right job is Wufi, and even that is totally dependent on the quality of input data (of course)!

Yeah I second what Will says about Ubakus's moisture indications. They are hardly ever useful/relevant. We do badly a need a free WUFI-equivalent tool though. I don't think it's _that_ hard to write one if one just knew which papers the analysis was based on. But I don't...

@richy. Whatever you do, don't use the combined insulated plasterboard - it's impossible to seal the joints properly and prevent moisture ingress that you didn't want. Use separate insulation and plasterboard products, which is cheaper too (see many threads on here). You must have room for 50mm PIR, except maybe on staircases/toilet wall. You will only do this once so have a good think about how much you will really miss 50 or 60mm?

50mm of PIR gets you to U=0.29, (60 is 0.26) which is respectable under the circumstances. Everyone should be aiming for 50kWh/m2/yr, and this may well be enough for that if you also put in 3G windows and treat roof +floor. Do your sums with SAP, HEM or PHPP (better) to get an idea of the tradeoffs.

As everyone said, EWI is always best for thermals and moisture, but if your sandstone look is more important than the climate (*wry smiley*), IWI will have to do.