inadequate loft insulation?

[masamune]

I appreciate this has been discussed to death, since every case is slightly different it may help posting my details out here for help...

I have a 1930's semi that came with boarded loft and sandwiched rockwool between the under the boards in the loft. The depth of the joists is 100mm and the rockwool appears to be the same thickness. The roof is low pitch and has overhang eaves with angled ceilings near the walls below. There is extra rockwool on top of the boards near the eaves. This roughly gives 200~300mm rockwool thickness in total. The house also has mineral wool injected inside the cavities (that white fluff that falls out during building work

This is just to give a general picture of the house insulation, pics attached





Right below those pics, I have a small box bedroom facing north which is about 2 degrees colder than the rest of the house. First year it had some condensation issues close to the eaves (to be fair it was around 2oC outside). I had a spare roll of rockwool in the garage so I topped up the areas near the eaves and also stuffed it down the overhang rafters. This has helped. However the overall heat performance of the room is still poor. Is there anything more I can do?

I am thinking to take out the wool from the boards above the room and fit PIR panels, probably 50~75mm allowing a gap of 25mm with battens for any lighting wiring. Then put the boards back and add the spare rockwool on top. Unfortunately the joists are not deep enough for thicker PIR. The loft is used for storage, there is no felt under the roof tiles, also no issues with the roof in general. Any potential condensation issues with this idea?

I am also not sure if swapping the single panel radiator in the room with a double panel would necessarily improve things, as I would only pump more heat into the room that will dissipate at night when heating is off.

 

[masamune]

Also to add that plumber and electrician messed the fiberglass wool up while carrying out work. This left the the insulation scattered but I noticed too late after going up there again.

I am thinking to put insulation panels under all the loft boards now (not rafters) so I want to understand what would be best for this depth of joists without causing any condensation problems.

 

[endecotp]

I am thinking to take out the wool from the boards above the room and fit PIR panels, probably 50~75mm allowing a gap of 25mm with battens for any lighting wiring. Then put the boards back and add the spare rockwool on top.

Keep wool between the joists as it is easiest to avoid leaving gaps with wool. Wool is also better as acoustic insulation.
Then PIR panels over the joists. Then chipboard or ply over that, for storage.

 

[masamune]

Then PIR panels over the joists. Then chipboard or ply over that, for storage.

Thanks, wouldn't that mean that I have to raise the loft floor overall?

 

[endecotp]

Yes.

 

[masamune]

So would you nail/screw the PIR on the joists? What sort of thickness would you go for on the PIR?

thanks

 

[endecotp]

You can get special fixings; google for "insulation board fixings".
The thickness is up to you. I'd use lots.

 

[masamune]

Thanks again!

What board type would be most appropriate for the loft floor, would it be the same thing as the rafters?

Also, what would you do above a bathroom for instance, as I imagine these would condense big time. Perhaps just wool there?

 

[Doggit]

It's possible that the cold room faces north, and that would explain why it's colder. As to the a bigger radiator, can you find if it's sized properly first. If you get a larger rad, as you say, it'll warm the room up, but the heat will still dissipate when it goes off.

The current standard for loft insulation, is 100mm between the joists, and then 170mm laid over them to stop any cold bridging. You can get loft legs that raise the loft flooring sufficiently to get the 170mm in, but if you want a simple solution, then 70mm PIR between the joists, and then add a 100x50mm piece on top of the joists, and then 100mm loft insulation on top of the PIR.

If you were to continue with the insulation between the joist, and then put the PIR on top, then you can just use screws to keep it in place temporarily, whilst you put the boards on top of it, and then crew through both with long wood screws.

 

[masamune]

I found this while looking around online, it looks great to use for building the raised level...

http://loftzone.co.uk/products-storefloor.html