Radiant Barrier under a flat concrete roof, need advice

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Radiant Barrier under a flat concrete roof, need advice

Postby windjammer » Thu Feb 25, 2016 6:17 pm

So I am having a house build with a flat concrete roof and was wondering if putting in a radiant barrier would be a good idea. In the past I have build houses with a sloped roof and always put the aluminium foil under the roof battens which helped a lot in keeping the heat out.
I still have a few rolls of the thin foil left but if there is a better material for flat roofs i won't mind buying that.
I saw a hotbox in homepro to test radiant barriers and the PE (polyethylene?) seemed to work best. I also seen the thick rockwool with aluminiumfoil on the side, not sure if that would work.
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Re: Radiant Barrier under a flat concrete roof, need advice

Postby Sometimewoodworker » Thu Feb 25, 2016 8:32 pm

windjammer wrote:So I am having a house build with a flat concrete roof and was wondering if putting in a radiant barrier would be a good idea. In the past I have build houses with a sloped roof and always put the aluminium foil under the roof battens which helped a lot in keeping the heat out.
I still have a few rolls of the thin foil left but if there is a better material for flat roofs i won't mind buying that.
I saw a hotbox in homepro to test radiant barriers and the PE (polyethylene?) seemed to work best. I also seen the thick rockwool with aluminiumfoil on the side, not sure if that would work.


I can't give any advice on the material that is best to use as I haven't researched that case, however you need to make sure that the shiniest face (if you have 2 silver faces) is down assuming that you are attaching the barrier to the roof as the barrier works by being a very poor radiator (hence the name "radiant barrier")

If they had a PU version that could have been better.

A thing you have to be careful of is, will the beasties like to chew or nest in it. I know they don't like PU.
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Re: Radiant Barrier under a flat concrete roof, need advice

Postby schuimpge » Thu Feb 25, 2016 9:21 pm

Under a flat concrete roof..

1. Paint the outside/topside high-gloss white.
2. Spray PU (Poly Urethane) foam under it.

Critters hate PU foam as SWW said, and it's about the best isolator you could have.

Done both the above under a cement-tile roof old style shophouse I'm living and it made a hell of a difference..

Cheers,
Luc

Also makes it a lot easier to identify my house on Google Earth... :mrgreen:
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Re: Radiant Barrier under a flat concrete roof, need advice

Postby Andyfteeze » Sun Jul 17, 2016 4:55 pm

I understand your delema. I too will build with a slab roof and have thought long and hard about it. I have decided that in my case its a multifaceted approach.
On the underside, i will spray foam. The topside will also double as an al fresco area so i rumaged through many ideas. Here is my ultimate solution.
Cement planks will do the form work. A layer of high density closed cell foam on top , about 25-30mm. 200-250mm foamed concrete on top. One layer of K10 water proofing with a fiberglass mesH on top,fly screen is cheap but very effective in giving the membrane high strength, two more layers of K10, finally a rubber based external Glue and tiles.
Isolating the concrete from direct sun exposure is the trick. Just using a slab as a roof is no better than cement roof tiles.
Just treating the underside is not the best approach, but if you have no choice, don't skimp. I would use fiberglass bats with a high R rating. You also need all that heat to have an exit, preferably not back into the room.
Lots of luck.
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Re: Radiant Barrier under a flat concrete roof, need advice

Postby Andyfteeze » Sun Jul 24, 2016 10:06 am

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Re: Radiant Barrier under a flat concrete roof, need advice

Postby Sometimewoodworker » Sun Jul 24, 2016 10:42 am

Andyfteeze wrote:Here is another method
http://www.foamular.com.au/polystyrene- ... insulation

Which will provide endless fun for the local rats who love polystyrene, possibly other wildlife as well.

If Fred were still alive he would give chapter and verse.
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Re: Radiant Barrier under a flat concrete roof, need advice

Postby Andyfteeze » Sun Jul 24, 2016 1:01 pm

Yes rodents are an issue but only if they get to it. I have the notion you can seal it in so they can't get to it.
The idea i have in mind is to place a foam layer no closer than 20cm from the edge under the foamed slab.
The slab itself has no foam, only air bubbles. The gap between the cement planks needs to be filled with non shrink cement grout - a very very strong bit of kit. I admit its not a straightforward solution, but it will work for me.
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