Mike1

Why do you say "leading with the the outer leaf first"? My builders are leading with the inner leaf and I see the Green Building Store did the same.

Sounds like small print dreamed up to avoid any claims

Wondering if I can resurrect this thread. Single storey extension. Builder has generally been organised and precise. However, walls up, waiting for the long lintel for the glass on the rear elevation, and (after a lovely period of dry weather) it has been raining for 2-3 days. Cavity insulation batts in the walls must be completed wet now. I know the fibres don't aborb water but they feel sopping wet if you put your hand on them. Not obviously compacted, though.

This must be normal practice for builders. If mine didn't cover it I can't believe it's a well-known requirement. But surely it will take months of warm, dry summer weather to get anywhere near drying these out. I'm hoping (a) the blockwork on the inner leaf will be waterproof enough to stop all that damp affecting the plasterboard and decor, (b) if the U-value of the wall is badly affected it will be okay again by next autumn.

Can anyone reassure me that they've experienced walls like this and it didn't lead to damp on the inside?
Can anyone give me U-values for wet cavity wall batts (compared to dry ones) so I know what my definitive level of unhappiness should be atm?
Can anyone reassure me in the long run it makes little difference if new walls go up with wet or dry insulation because they all get wet/dry out with normal annual weather cycles?
Thanks.

Oddly enough I was pondering the same on Sunday when passing a site (?hip-to-gable conversion?). It was lagging it down and the completely unprotected glass-fibre fill will have been soaked. This was full fill so no vents; no air-stream. Yes it *probably* will dry out in the end but it is not good practice!!

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If the house is warmer than outdoors then moisture in the form of water vapour will move out through the wall and waft away. I reckon allow 25mm per month to dry out in summer.

Posted By: do_discimusSuperglass - I think... It seems ridiculous that builders let insulation get soaked as fairly common practice.

Clear bad practice. And, unsurprisingly, the Superglass BBA certificate says:

"3.3 Partially completed walls should be protected from inclement weather (eg wind, rain or snow) and covered at the end of the day's work."

I wonder what other corners are being cut?

That's encouraging, Tony.

Our main end elevation on the house faces south - catches all the rain obviously - and had cavity wall insulation added before we moved in. A few years ago it had water coming through because it needed repointing and I guess some combination of wall ties and insulation was allowing the rain to cross the cavity. But opening up a new window in roughly the same spot last week, the insulation was more-or-less dry. So that fits, too.

Think we've got the walls well covered now with planks and polythene. Taking this off for dry days. I'm thinking it will dry a bit even at this time of year with the sun on it and the top of the cavity open.