Insulation for extension flat roof

TheMilkyPirate · 24-03-2023 06:55PM #1

Renovating a house and got a new roof put on all three extensions. New joists ply wood and two layers of felt. Just wondering what insulation to put in between the joists? Our slabber recommended Metac but it is just too expensive and out of our price range.

No pictures so just a couple of screenshot from a video to show what it's like.

DOCARCH · 25-03-2023 05:49AM

If you put insulation between the joists, the flat roof becomes (what's called) a 'cold roof' and the roof structure over the insulation (under the ply deck) needs to be ventilated. Has any allowance been made for ventilation?

With regard to the cost of insulation, I would have thought not many cheaper options than Metac?

Bricriu · 25-03-2023 06:36AM

Off-topic: felted rooves are rubbish, and will leak eventually. A waste of time and money.

Get money somewhere and put slates on it.

DOCARCH · 25-03-2023 07:47AM

https://www.boards.ie/discussion/comment/120398681#Comment_120398681

Probably not very helpful for the OP at this stage!

TheMilkyPirate · 25-03-2023 08:42AM

https://www.boards.ie/discussion/comment/120398663#Comment_120398663

Yeah a 50mm gap between the insulation and timber deck and a vapour barrier between joists and plasterboard is what I've gathered? I presume the likes of Knauf ekoroll or earthwool wouldn't be suitable?

What thickness Metac is recommended?

https://www.boards.ie/discussion/comment/120398681#Comment_120398681

Replacing the roof on all 3 extensions was a 5 figure sum we had not budgeted for so cost was a massive factor. The lads who done our roof work are very well respected and the man who installed the torch on has been doing it for 35 years and his work is top notch. Not ideal compared to pitch roof and slates we know but a well done and looked after felt roof can last 15-20 years + and this one is definitely well done.

Biker1 · 25-03-2023 09:14AM

https://www.boards.ie/discussion/comment/120398853#Comment_120398853

Once you are sure you can cross ventilate the roof you are then left with a defined depth to the bottom of the joists. That is as much insulation as you can get in that area and will have to use insulated plasterboard underneath to the required thickness to achieve a maximum U-value of 0.16W/m2k.

TheMilkyPirate · 25-03-2023 09:54AM

https://www.boards.ie/discussion/comment/120398922#Comment_120398922

So basically whatever space is left the the bottom of the joists after the 50mm gap?

muffler · 25-03-2023 10:15AM

https://www.boards.ie/discussion/comment/120399008#Comment_120399008

That's basically it. Depth of joist less 50mm = depth of insulation.

muffler · 25-03-2023 10:18AM

https://www.boards.ie/discussion/comment/120398922#Comment_120398922

Agreed. Just to add if insulated plaster board is used then it shouldn't reduce the floor to ceiling height to less than 2.40 metres.

TheMilkyPirate · 25-03-2023 03:51PM

Perfect thanks lads what should I use for the vapour barrier beneath the insulation?

hydrus21 · 25-03-2023 06:38PM

I could be wrong here but i think there is no minium ceiling height in a attic conversion apart from the stairwell.

muffler · 25-03-2023 08:09PM

https://www.boards.ie/discussion/comment/120400412#Comment_120400412

I dont think anyone mentioned an attic.

Even if that were the case minimum ceiling heights apply for all habitable rooms there.

hydrus21 · 26-03-2023 06:58AM

https://www.boards.ie/discussion/comment/120400717#Comment_120400717

Different to uk regs. so.

muffler · 26-03-2023 10:10AM

https://www.boards.ie/discussion/comment/120401671#Comment_120401671

I'm not familiar with UK regs as they don't apply here in Ireland.

Insulation for extension flat roof

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