Loft Insulation for storage

[davidph]

Hi,

We would like to board out our loft for storage, but it's currently got a huge chunk of rockwool covering all of the joists. I'm aware you can get risers for the joists, but we're a bit limited in height so I've started looking at replacing all the rockwool with celotex, so I can just board over at the joist level.

I'd like to try and comply with the current building regs, so wondered if it's acceptable to put 100mm of celotex in between the floor joists and then do a suitable thickness in between the rafters as well? The celotex calculator seems to suggest I'd need a full 200mm in one chunk to get to a U of 0.16, but also suggests that if I was to go over the joists in a separate layer I could get away with 50mm extra, so 150mm in total.

Elsewhere I've seen the total requirement being 175mm for celotex.

Is it acceptable to split the insulation up like we'd like?

Cheers,
David

 

[motorbiking]

Realistically - how much height are you going to gain? You're probably talking £600 worth of celotex. My calcs say you need 140mm - sealed

 

[davidph]

I don't really want to use the risers and the alternative is to take off the rolls of rockwool covering the joists and then just leave behind the stuff in between.

Then we'd be nowhere close to having the correct amount - at least with the celotex we'd have a decent level of insulation - with 100mm in the floor I think it would be around £400, plus extra for the rafters.

 

[flameport]

Is it acceptable to split the insulation up like we'd like?

Acceptable for building regulations - clearly not, as there won't be enough insulation.

Acceptable for you - no, since you are spending £100s to degrade the insulation significantly, which will result in higher heating costs every year from now until you move out. There is also the non-trivial task of removing the existing insulation and installing the new.

 

[davidph]

OK - I'm getting the impression that the consensus is that this isn't a good idea.

What I'm not understanding is why. The rockwool is fairly old and, even if it was 270mm (which is the requirement for 0.16W/m2K) when it was laid, it's unlikely to be that now.

If 140mm of celotex gives the equivalent U rating of 270mm of rockwool, then 100mm of celotex would be the equivalent of about 190mm of rockwall, which is probably about what is there now. So I'm not going to be any worse off, thermally at least. I can then board out and get a decent, usable storage space, without the issue of risers which have a limited load rating.

If I was to then put another 100mm of celotex in the roofing rafters, then the house would have the total equivalent of 380mm of fibreglass in the roof, which is significantly more than it currently has.