50mm insulation ok for a 80mm bathroom stud wall? (rock wool slabs)

[JohnD]

Hi,

I've seen online people recommending tightly packing in insulation and others saying leaving a little gap especially on the shower/wet walls.

Stud walls only 80mm so have bought 50mm but now wondering whether I should have got 100mm and either jam or in or trim it

1) for thermal insulation, if you pack 100mm into a 50mm gap, you have 50mm of insulation, not 100mm

2) you seem to be talking about insulation between two heated rooms, so you mean sound insulation,

3) sound insulation is mostly proportional to the weight/density of the barrier, hence the batts are much more solid. You must have no gaps or thin bits. Two layers of plasterboard will add to the sound barrier.

4) interstitial condensation can occur in porous materials if there is not a vapour barrier on the warm/steamy side to prevent the water vapour penetrating

 

[Ivor Windybottom]

It's already been stated that it's going to be tanked. I'd be worried about waterproofing both sides of the backer board, IMO better off without a vapour barrier under it too as then you could be trapping anything that ever ends up there between two waterproof layers.

But I probably wouldn't be using that insulation in a shower anyway. Better off with something waterproof, accepting that steam or water will eventually find its way in. The insulation may all end up like a bath sponge.