Ceiling sanity check

ShiresV2

I have a ceiling in an upstairs bedroom that needs replacing.

I'm considering pulling the existing ceiling down, then affixing 20mm PIR board to the joists / trusses, then facing that with 1/2" foil backed plasterboard, as the house has a low pitched roof and there is not much scope for additional insulation in the attic. The floor-ceiling distance is 8ft and a tiny bit, nothing unusual.

Does it sound like a reasonable plan? No obvious gotchas around building regs or whatnot?

Thanks!

BryanF

re current regulations: see table 5 page 29 http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,27316,en.pdf

50mm of a 0.023w/mk PIR type insulation will bring you just above 0.35wm2k U-value required under the current, but its really not that much insulation to keep you warm.. whats the rest of your roof build-up?

ShiresV2

BryanF wrote: »

re current regulations: see table 5 page 29 http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,27316,en.pdf

50mm of a 0.023w/mk PIR type insulation will bring you just above 0.35wm2k U-value required under the current, but its really not that much insulation to keep you warm..

Thanks.

BryanF wrote: »

whats the rest of your roof build-up?

Bottom of the trusses is made of 2x8s and the gap between is filled to the top with fibreglass roll, so about 200mm. Above this no other insulating material, it's a T&G chipboard floored storage area.

BryanF

well scrap what I've said above so.

BryanF

when you say storage i'm presuming unheated attic space? consider swamping the PIR with a wood fibre board or similar material. while not as good thermally and costing a little more, you may find it better acoustically, from a health perspective, similar thermally to the existing insulation and can be taped to achieve air-tightness. you can then screw the plaster board over it or depending on the chosen product skim and paint

sinnerboy

ShiresV2 wrote: »

I have a ceiling in an upstairs bedroom that needs replacing.

I'm considering pulling the existing ceiling down, then affixing 20mm PIR board to the joists / trusses, then facing that with 1/2" foil backed plasterboard, as the house has a low pitched roof and there is not much scope for additional insulation in the attic. The floor-ceiling distance is 8ft and a tiny bit, nothing unusual.

Does it sound like a reasonable plan? No obvious gotchas around building regs or whatnot?

Thanks!

You must have a min floor to ceiling height of 2400mm.

200mm of fibreglass is not too shabby - yes 300mm is current min but if you can't fit more in above or below ....

ShiresV2

On checking now it's actually 2x6 for the truss so 150mm of fibreglass.. Apologies.

BryanF wrote: »

when you say storage i'm presuming unheated attic space? consider swamping the PIR with a wood fibre board or similar material. while not as good thermally and costing a little more, you may find it better acoustically, from a health perspective, similar thermally to the existing insulation and can be taped to achieve air-tightness. you can then screw the plaster board over it or depending on the chosen product skim and paint

Yep, it's an unheated, vented attic space. The 2nd floor to attic interface is reasonably airtight for the age of the building. I had chosen PIR as I am led to believe it has somewhat better fire resistance and contains fewer (and different) nasties than EPS/XPS, but some of the wood fibre products I have found sound very interesting indeed. Thanks for the lead!

sinnerboy wrote: »

You must have a min floor to ceiling height of 2400mm.

200mm of fibreglass is not too shabby - yes 300mm is current min but if you can't fit more in above or below ....

Had suspected as much re: the height requirment.. That gives me a little to play with, I will see what can be done.

Thanks again.

ShiresV2

I thought I would reply for the benefit of anyone else doing something similar at home.

Firstly apologies for the confusion above.

BryanF wrote: »

consider swamping the PIR with a wood fibre board or similar material. while not as good thermally and costing a little more, you may find it better acoustically, from a health perspective, similar thermally to the existing insulation and can be taped to achieve air-tightness. you can then screw the plaster board over it

Thanks for the suggestion, I went for 40mm wood fibre board under the rafters as part of the build up.

I had a play with the stuff and it does have a couple of "cosy" properties in that it has a bit of thermal mass and as you say it's acoustically very good -- the room has quite a nice feel to it now and I'm very pleased with that.

On the downside the particular product I chose is a little dusty, so I couldn't see how it could be taped / made air impermeable without a seperate layer, and it's somewhat more expensive in comparison to man made alternatives.

With the whole build-up I think I managed to achieve 0.16w/m2k overall. Thanks again for the advice.

BryanF

dusty? pm me the type, glad it worked for you - i maybe would have looked at some form of caulking for air-tightness if that's was the case