Loft insulating boards

Any variation of celotex PIR insulation?

One question though - do you have breathable roofing membrane, because if not, you will have to check that you have adequate ventilation for the roof timbers.

Thanks for the suggestion - I'll look into that.

Actually, I'm thinking of this for two properties. One is our early victorian cottage, which will definitely not have this problem, as the slates are not felted (probably should be?).

The other is my daughter's fairly recent (maybe 20 years old) terraced house, where there could be a problem, though I think there is a gap at the eaves.

It ought not be an issue with insulation on the floor joists. This tends to be more of a problem if insulating the underside of the rafters.

Any rigid PIR foam board... Celotex, Kingspan (both quite pricey), Ecotherm, Rubberoid and quite a number of other makers. Look for a local seconds place or try one of the online seconds suppliers. They are often a third of the price of the big name ones.

That's fine. No membrane is OK.

The BCO told me to either have a 25mm gap under breathable membrane or a

I assumed that was what the OP was doing???

Nope, he said it was to go over the jpists

No, nothing as complicated as that. Just laying boards on the attic floor joists.

Oh - you're good then...

At least as far as moisure goes.

How much load do you think you'll put on the new insulation? Overboarding with ply or chip?

Celotex (etc) is very string in compression if it sitting on a flat surface. Over rafters, it will see much more concentrated loads wher eit is bearing on the rafter.

OK, thanks for the warning. I'll tell my daughter to take care.

They will still take a fairly substantial load... 2" wide joists several meters long give plenty of area. I found when doing warm deck roofs with the foam under the felt and top layer of ply, even screwing it down hard every 12", and then walking on it was not enough to make anything more than a tiny deformation into the foam.