Loft insulation advice - PIR over boards?

tunatuna

Hi hoping for some advice on adding insulation in our loft. There's sporadic rockwool installed between joists I would say 3-4 inches deep and then the loft is fully boarded out. Two options I'm considering:

We don't really use the loft for storage so I am thinking of installing 75mm PIR sheets directly on top of the boarding and foil taping all the joints. Positives of this are fast and relatively easy installation. Downside is not being able to walk on it. Could put some boards on top for storage of some stuff i.e. Xmas decorations etc. 

Option 2 would be to raise the boards, put the PIR across the joists and then re-fit the boards on top. This would be a much bigger job but the advantage would be being able to walk on top. 

Any thoughts or suggestions? Also, is it worth installing 100mm thick PIR as opposed to 75mm? 

ComicGeek

I would remove the boards in the areas where you don't need storage/access, and just install 200mm loft roll across the joists. Limit the board area to just what you need for storage immediately around the access hatch, and either install PIR sheets between joists in this area or just stuff with loft roll.

You can't put PIR across the joists and just refit boards on top, the PIR won't take the weight - you would need to use loft legs, and then you might as well use cheaper loft roll rather than PIR anyway. 

WIAWSNB

Hi TT.

I sense you are hoping to avoid some hard work I feel your pain, man. 

Insulating your loft well is really important. If you want proof, then stand on a chair and get yer 'ead close to the ceiling when your CH is on - woooo, it's warm up there, and you really don't want to lose this.

The good news is that loft insulation is usually the cheapest and easiest to install, but - darn - yours has been covered with flooring. 

You reckon the current boarding is placed on the original joists, so the insulation underneath is only around 4" deep? Are you sure it's even that? Is it nicely stuffed under there? If so, then your sandwich will be pretty effective, but really you should be going to full current recommendations. 

I wouldn't bother with rigid insulation here - the only place I'd put this on a ceiling is on the underside, in the rooms. That way it would vapour-seal the ceiling, as well as being super-effective, but that's a lot of hassle. 

The potential problem with adding rigid insulation to the upper side of the ceiling is that is might, I don't know, prevent any moisture that comes up from the house from being evaporated away. 

So, if you can bear it - and you should - please do this the right way; remove the flooring, add stilts, add the recommended amount of insulation, and relay the boards.

The really good news is that loft flooring is great - even if you aren't all that bovvered, future buyers will love it! And it's already there - you don't even have to have it shoved up through a wee hatch. 

So, remove half the boards, stack them on the rest, add the stilts, add the insulation, refit these boards, and repeat for the rest. 

Wear a dust mask. Ideally overalls too. 

And make absolutely certain you do not block off the eaves where the ventilation comes through. 

(Just checked my local Fb Marketplace - a cove is giving away nearly 30 bin bags full of loft insulation. That sort of thing would be ideal, as it'll do the same job as new stuff, but you won't even see it...)