Insulate flat roof

MegamanBoo

Hi, I'm doing up an old house. We're making decent progress while trying to keep costs down. Unfortunately the kitchen is in a flat roofed extension which wasn't built to any great standard.

I haven't cut back the existing plasterboard yet but I'm pretty sure there's no insulation whatsoever in there. Just flat roof, then joist, then plasterboard.

What's the best way to insulate this roof. I have a lot of rockwool insulation I could use but the roof is low and if i were to put up 200-300mm it might take too much from the height.

The insulated plasterboard looks good, is it tricky to put up?

Do I need some kind of moisture membrane underneath?

stiofan85

MegamanBoo wrote: »

Hi, I'm doing up an old house. We're making decent progress while trying to keep costs down. Unfortunately the kitchen is in a flat roofed extension which wasn't built to any great standard.

I haven't cut back the existing plasterboard yet but I'm pretty sure there's no insulation whatsoever in there. Just flat roof, then joist, then plasterboard.

What's the best way to insulate this roof. I have a lot of rockwool insulation I could use but the roof is low and if i were to put up 200-300mm it might take too much from the height.

The insulated plasterboard looks good, is it tricky to put up?

Do I need some kind of moisture membrane underneath?

Think you'll definitely need a vapour barrier. My understanding is vapour will still get through light fixtures and any plasterboard screws. In the roof structure you'll need to establish some form of cross ventilation to take this away.

I did the same 18 months ago with rockwool and a very very large plastic sheet. Has been great for keeping the room warm. Roof is always iced over in cold weather so no heat leaking through.

My roof has cross ventilation above the insulation, but I don't think it's very effective. I've no means of checking if there is condensation up there. Probably is and will rot the timber eventually but the roof is pretty poorly made but I didn't have the money at the time to replace it. It was a case of stretch to replace it now, or do it in a few years time anyway but get the benefit of some warmth in the meantime and probably pay for a better replacement down the line.

I'm conscious of the vapour problem so keep the room well vented, don't dry clothes there, open windows when cooking etc...

Good luck!

Calahonda52

http://www.environ.ie/search/archived/current/category/housing/type/publications?query=acceptable%20construction%20details

https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice

There are home insurance issues with flat roofs that are too big as % of total roof.

listermint

You'll need a vapour barrier and also ventilation space in the roof so account for that in your 200-300mm

whizbang

Unless your roofing material is top spec in immaculate condition, it would be far better and a lot easier insulating the outside, and recovering.