Insulating Roof what thickness

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j-a-m-e-s_7
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Insulating Roof what thickness

Post by j-a-m-e-s_7 » Tue Sep 25, 2012 1:07 pm

Hi, going to be having out roof replaced and work will be starting in 1-2 weeks. I want to insulate the whole roof with celotex inbetween then rafters. The timbers are 75x50mm its a victorian end terraced house. What thickness would you reccomend. I have asked the roofer what membrane he will be using and he said it will be tyvek supro. Does there need to be an air gap left if so how much? The property has a main roof and a wing, so there will be a fair amount of insulation required?

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Pooneil
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Re: Insulating Roof what thickness

Post by Pooneil » Tue Sep 25, 2012 3:40 pm

Do you mind me asking why you are insulating between the rafters rather than regular insulation on the ceiling joists? The latter would be much cheaper, easier and less likely to compromise the roof design.
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Re: Insulating Roof what thickness

Post by j-a-m-e-s_7 » Tue Sep 25, 2012 4:45 pm

Hi thanks for your reply, no not at all mate. The wing part of the house all rooms have been finished and the only chance to get any insulation in there is when the roof is off as there is little or non at all in most places. Not a fan of the roll type stuff and from my inspection into the roof space there is very old and dirty fiberglass type stuff in there so i feel the rigid baord types insulation between the rafters such as celotex would be my best option. Regarding the main roof a couple fo velux windows are being fitted and the floor has been taken out and leveled as it was previously on two levels. New joists have been put in (8x2") with 22mm p5 chipboard with insulation etc. The attic will only be used for storage with the possibilty of properly converting it in the future at some point (but not for a few years). I would like to state than in no way am i planning to use the attic as a habbitable space/bedroom etc. I just want to insulate it and board it out with plasterboard so it dust/dirt free. I have equipment for home servers up there as house has been wired for av so node 0 rack etc everything runs from there. I was thinking about either 50-75mm between the rafters with 25mm screw and taped over the top and then plasterboard over that. I understand i may need to batten out to gain extra depth as current rafters are only 3x2 and i assume an air gap is required even tho it is a breathable membrane. Heres a pic of soemthing i want to create. https://lh5.googleusercontent.com/_BWC4 ... G_5085.JPG
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Re: Insulating Roof what thickness

Post by Pooneil » Tue Sep 25, 2012 7:41 pm

Tyvek supro is a breathable membrane and it's my understanding that you therefore don't need the air gap. If the floor has already been insulated and boarded I assume there is no 'cold roof' ventilation now anyway, otherwise you'll be getting draughts.

Personally I would fill the whole depth with 75mm while you're at it, and as you suggest another 25mm under the rafters, taping the joints, to minimise cold bridging. I think you need a vapour barrier in there somewhere (before the PB I think), but I'd be guessing as to where; your roofer or someone else on here should know.

I'd still use loft roll in the wing (100mm to fill the joists and 175mm perpendicular)... if you're like us, that's where the servants sleep so it doesn't matter if it's a bit dusty. :-)
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Re: Insulating Roof what thickness

Post by Perry525 » Sat Sep 29, 2012 7:01 pm

Are you going to live in the roof?
If you are not, then the insulation must be between the ceiling joists.
If you cut to a tight push, (or thump) fit any of the closed cell foam boards will do fine.
Fill the whole space if you can.
Any holes can be filled with canned foam.
You must ensure that there are no holes through the ceiling into the roof.
Gypsum plaster is air tight.

You will use a lot less insulation doing the joists in the ceiling, you can look at Quinn who are cheaper.
Or polystyrene from Wicks or other builders.
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