Loft Insulation

rockabaloo

I'm thinking of putting loft insulation in the house I'm in at the minute. There are 100mm joists and I was thinking of putting in 150mm mineral wool between joists and then 150mm mineral wool on top of that at right angles.

Will the second layer of mineral wool compress the first layer because the joists aren't deep enough to support its weight?

Would I be better putting 100mm between joists and then 200mm on top of it?

I'd rather just use the 150mm rolls if I could because I can get them at a good price.

gman2k

Rule of insulation - Don't compress insulation - it's the air trappped within the insulating material that acts as the insulant.
If you have a 100mm gap, then get 100mm insulation.

rockabaloo

I thought that might well be the case. Thanks for the info.

gman2k

It's ok to lay other insulation on top of the first layer (which is between the joists) at right angles. Just don't squash!

ravendude

My understanding is that basically the reduction in performance is down to how much you compress. So, if you compress 150mm to 100mm you basically get the performance of 100mm not 150mm. (Someone correct me if I'm wrong).

If 150mm is cheaper than 100mm it may make sense to just get the 150mm but don't expect it to perform as such if you compress it down.

gman2k

ravendude wrote: »

My understanding is that basically the reduction in performance is down to how much you compress. So, if you compress 150mm to 100mm you basically get the performance of 100mm not 150mm. (Someone correct me if I'm wrong).

If 150mm is cheaper than 100mm it may make sense to just get the 150mm but don't expect it to perform as such if you compress it down.

Yes, you are wrong. 150 squashed to 100 will not equate 100, as there will be less air trapped.
So, it makes no sense and would be money wasted.