Insulation above plasterboard ceiling (second floor)

[shadyron]

Hi there,

I'm re-plasterboarding a ceiling. Directly above this is the roof. The house design is somewhat odd, in that the roof comes down about six inches from where the plasterboard will be. As such, I'm not sure what the building regs are on if I can put roll insulation here. Any thoughts? The plasterboard that came down had none in that space, which makes me think that I can't (isn't there something about no insulation in the eaves?)

Thanks!

 

[shadyron]

Scratch that... it looks like (having just found somewhere else) that as long as there's 50mm between the end of the wall and where the insulation starts (to allow air flow) that I'm ok.

I think.

 

[noseall]

Building reg's require you to fit in excess of 270mm thick glass fibre type insulation to loft ceilings.

This thickness of insulation is usually impossible to achieve in the depth of a raked ceiling and is the reason for a heck of a lot condensation related problems i.e. black spot mould etc.

You are better off using foam board insulation, i.e. Kingspan, Celotex, Quintherm etc.

Reg's are currently asking for 100mm between rafters with another 50mm across the rafters including a 50mm air gap above the insulation. This 50mm airgap can be difficult to achieve with most standard depth (75mm - 100mm) rafters, so the solution is to fix a thickening batten to the underside of the rafter.

 

[shadyron]

Bloody hell. That's going to lose me a massive chunk of the room!

Thanks for the advice, though - I'd have serious underestimated that.

 

[noseall]

Bloody hell. That's going to lose me a massive chunk of the room!

You could probably get away with less and not notice any difference. However, with the extremely cold winter temperatures we are having lately the consequences of reducing the thickness may be ugly black spot mould forming at this junction.

 

[shadyron]

I've gone with putting 25mm Celotex between rafters and then 52.5mm (40/12.5) insulation backed plasterboard under that. Reason being is I've then got the requisite air gap above the Celotex, then a reasonable level of insulation on the plasterboard.

Down side is this is well below the building regs - I've got 65mm plus 12.5 of plasterboard... and need 270

What happens if it doesn't meet building regs (which it clearly won't)?

I can't see how I can get anywhere near 270 in there without bringing the ceiling down by a considerable margin.

 

[noseall]

I've gone with putting 25mm Celotex between rafters and then 52.5mm (40/12.5) insulation backed plasterboard under that. Reason being is I've then got the requisite air gap above the Celotex, then a reasonable level of insulation on the plasterboard.

Down side is this is well below the building regs - I've got 65mm plus 12.5 of plasterboard... and need 270

What happens if it doesn't meet building regs (which it clearly won't)?

I can't see how I can get anywhere near 270 in there without bringing the ceiling down by a considerable margin.

270mm refers to glass wool type insulation only.

150mm thickness for foam board (celotex) type inso'.

 

[shadyron]

Ooooooooh.

That makes things easier.

Thanks. Still difficult, but more doable now.

 

[shadyron]

Whilst I imagine that none of you really care that much, having thought about it for all of thirty minutes, I've decided the best way to do this is use the 25mm I've already got between rafters, have a 100mm board going across the rafters (ie between rafters and insulation backed plasterboard) then the 52.5mm board under that, and just suck it up on the loss of height.

I believe this gives a grand total of 25mm + 40mm + 100mm = 165mm which is in excess of the 150 required. Thanks to noseall for pointing this out as I'd totally missed it.

I thank you all so much for your patience with me

Thanks!

 

[noseall]

Sorted.