Loft insulation and boarding

[DVader]

Currently I have 4"x2" joists with 18" centres, and filled with 4" of insulation. There are two brick walls directly underneath the loft that the loft floor rests on. Longest unsupported span is 3m.

I wish to board the loft for light storage only but add some extra insulation.

Plan: Lay 50mm insulation boards at 90 degrees to the joists. Where there are cables crossing joists, scallop out the underside of the insulation boards to leave a gap so as not to crush the cable.

Lay and screw loft boards over the insulation boards (and mark where there are cables).

Should use foil backed boards or just plain?

Is this ok? I know the recommendation is 270mm of insulation, but that would impact on my space, as the roof is fairly shallow.

Thanks for any advice.

 

[John D v2.0]

Where there are cables crossing joists, scallop out the underside of the insulation boards to leave a gap so as not to crush the cable.

Do you have a shower cable there or just lighting? Lighting will be fine if it's on a 6A circuit, but shower will need redirecting.
If your insulation is polystyrene you'll need to protect the cable insulation from touching it. Polythene would be enough ie thick bin bag.

Should use foil backed boards or just plain?

Either is fine, foil backed would provide a vapour control layer but not really in the right place. I'd recommend sealing any gaps in your ceiling especially in the bathroom though.

I know the recommendation is 270mm o

The recommendation is for the u value, so it depends how good the boards are.
Glass wool would need 270mm to meet current regs, is your board white polystyrene or grey, or celotex?
If celotex you could get away with about 75-80mm on top but polystyrene would need 160ish. As long as you're happy it's ok, most heat is lost per square metre through the windows, but next the roof, and the roof is a big area. After that your walls lose loads. So no point going crazy in one area.
Good luck!

 

[DVader]

Thanks John. That's me sorted.

Cables: The shower cable runs right next to hatch so it won't be covered. The rest is lighting. So that's great for me.

I hadn't thought of using the celotex, but having had a read, that's the way forward.

 

[DVader]

One more thing. Will the foil covering cause any condensation problems?

 

[John D v2.0]

One more thing. Will the foil covering cause any condensation problems?

So there's your issue, condensation is a probability/risk thing.
So the general theory is to put the vapour impermeable layers to The warm side and the relatively permeable layers outside.
Then ventilate and drain any cold cavities as appropriate.
In your case your bathrooms would be the highest risk rooms so ideally you'd want to block any vapour at ceiling level.
Since that's tricky on a retro fit I'd recommend making sure your bathroom extraction is up to scratch, sealing all ceiling gaps and cracks with caulk, and leaving it at that.
Since the celotex is more insulating than the wool, the timber should be in a warmish area so the hope is it wouldn't be a worst case.