Warm Flat Roof Question

RaymoS2

Due to height restrictions I am putting a Warm Flat Roof using the the full infil of the rafters/joists with absolutely no air space or ventilation as I don't have the option to put insulation over the first layer of OSB3 followed by another cover of OSB3 then the fibreglass.

My question is that when you do the warm flat roof this way what is the best insulation to use in my 9" rafters (higher at one end almost 12" due to furrings) and secondly can you still use down-lighters as you will be introducing air into the ceiling through them although it won't make it into the roof due to the insulation above them been compressed so tightly.

BryanF

Is this a new extension or a re-roof?

RaymoS2

BryanF wrote: »

Is this a new extension or a re-roof?

New roof on new extension.

BryanF

So you need 0.2w/m2k (u-value) insulation or better factoring for the timber %

What has you arch/Eng specified?

Vapour barrier inside rafters, 9inch mineral wool between the joists, 50mm GUtex or similar over joists, breathable felt over joist then firing with min inch at low end, vented at each end, smart ply and roofing.

You can get air tightness covers for spots, even better if you can install a service cavity. Which could be insulated with mineral wool.

RaymoS2

My engineer is saying that I should use Metac insulation which is a lot more expensive than most mineral wool, no vents at all as it is to be completely airtight to prevent condensation forming as its a complete joist infil to create a warm flat roof. Nobody I have spoken to has mentioned the 50mm Gutex at any point so thanks for that, will investigate.

BryanF

You need to vent it.

RaymoS2

Do you mind me asking why you think it must be vented when the idea of a warm flat roof is that there is no air circulating, even when I search all warm flat roof designs you can see there is no venting at all. The usual 50mm air spacing above the insulation of the cold flat roof is no longer there with a complete infil?

BryanF

You're insulating between the joists, you should vent above. If you were insulating above the joists (over vapour barrier or taped ply) you wouldn't need to vent.

Edit: it doesn't have to be gutex, you are obliged to reduce thermal bridging, and the best way to do that is add (even) a thin layer of insulation above the rafters

Metac is cheaper than mineral wool

RaymoS2

There is 2 types of warm flat roof and I am open to correction. One is where you place insualtion above joists and the other is where you completely fill all air space with no ventilation to prevent condensation. Am I incorrect?

RaymoS2

Please see example of what I am intending to do attached.

sydthebeat

RaymoS2 wrote: »

There is 2 types of warm flat roof and I am open to correction. One is where you place insualtion above joists and the other is where you completely fill all air space with no ventilation to prevent condensation. Am I incorrect?

no you are not correct.

a "warm roof" is a roof construction where the condensation point is external to the structural timbers.

The simplest way to achieve this is to put adequate insulation above the structural timber so that the condensation point is on the cold face of this insulation.

I have seen some felt certifications which suggest they are suitable for a "hybrid" type where there is full fill within the structural timbers.
However, they recommend that the only way to be sure of no interstitial condensation is to have a hygroscopic assessment done on the construction detail.

on your attached drawing.... the only place youll find "no air able to condense" is within a vacuum. This construction make up is very naive, and very risky in my opinion.

have a read here:
http://www.boards.ie/vbulletin/showthread.php?p=68250588

BryanF

RaymoS2 wrote: »

My engineer is saying that I should use Metac insulation which is a lot more expensive than most mineral wool, no vents at all as it is to be completely airtight to prevent condensation forming as its a complete joist infil to create a warm flat roof. Nobody I have spoken to....

RaymoS2 wrote: »

I can't seem to find tables that show max span for 7 x 2 at specifically 250 / 300 centres........might need to find an engineer.

it's hard to believe an engineer has been engaged, based on the posts, statements and questions asked to date

Op would you be willing to PM me the name of you engineer and the website you're referring to?