chrisb

What next for my new flat roof?

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I'm approaching the point where I need to crack on with covering my new extension flat roof (approx 5m x 5.5m) deck. It's currently 18mm T&G OSB over firrings over posijoists. The room underneath is a ground floor kitchen / dining/ living open space, with a study / playroom on the side.

 

GRP is going to be the waterproof covering, done by my builder.

 

So the remaining questions are insulation (type, thickness & supplier) and VCL +/- another deck, with or without pre-laminated sheets.

140mm PIR or greater seems to be sensible. I think any old PIR will do, foil faced is unnecessary? I have seen references to T&G PIR and PIR with a fabric coating that GRP can stick directly to, but the GRP systems seem to demand a timber deck underneath, not PIR, so not sure how useful these boards really are?

I've seen some plywood/PIR laminates, which could go under the GRP, but not sure how these work out cost-wise yet, and I think the ply is only 12mm where the GRP systems demand 18mm, so not sure if they're an option yet either.

 

I'm thinking that pre-laminated sheets may be better given the season. Suppliers on the web seem hard to come by though.

 

All advice welcome!

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I have a simalar type roof to you

I used the 18 layer multi foil on top of ply then the OSB three 

I didn’t want a cold roof as I have seen so many with condensation problems 

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Moving this thread along, I now have a watertight roof (well I will do when the rooflights go in when the air temp is above zero for a day). The builder laid membrane on top of structural OSB3 deck, 150mm PIR, membrane, 18mm ply, GRP.

 

The fascias and soffits have been done, along with guttering. I now need to insulate the resultant space behind it, and would like to hear any advice please.

The wall is 120mm PIR inside 140mm TF, cavity with brick outer skin.

A significant portion of the part to be insulated is above bifolds (4.5m).

Pictures attached to illustrate far better than words can.

 

I've thought about cutting PIR to fit in there and foaming it in to form a vertical barrier above the wall insulation. This is probably doable, likely to need quite a bit of foam to get around the metal webs.

Alternatively, I could lay mineral wool in there which is probably my preference as I have significant stock, and is far less effort compared to the PIR option. I'm not sure if there would be any issue with just filling the whole space behind the fascia, on top of the soffit, back to over the top of the wall insulation? This seems to me to be the most sensible way to eliminate any wall-roof cold bridge, but I might be missing something. Tiredness and stress of the build is increasingly affecting concentration and logical thought :-(

 

Any thoughts appreciated!

above doors.jpg

side wall.jpg

above doors closeup.jpg

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I have the exact same question on my mind right now and for ease am inclined towards the same solution of mineral wool...but i'm bugged by the idea that it might be effectively creating a hybrid roof at that point, with insulation both above and below the osb deck.....so maybe foil backed insulation is needed to mitigate condensation.  This article and its subsequent comments seem to support my concern; http://blog.celotex.co.uk/technical/flat-roof-refurbishment-the-perils-of-the-hybrid-construction/

 

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I would Rockwool the lot - but watch how much you put in the ceiling. Can you get a condensation risk analysis done by your architect or online? You are right that too much rockwool below the PIR might create a hybrid roof. We put 50mm Rockwool inbetween the joists which have 140mm PIR above them. 

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A great case of 'you can't get there from here'. To maintain the envelope and avoid a lot of the cold bridging, shouldn't the facia and soffit areas have been insulated... so as to maintain the warm roof skin, rather than reverting to cold roof internal insulation in those areas. But on the basis of 'we are where we are' the key detail will be the right membranes and sealing to try to limit condensation in that soffit box.

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Yes, I think in an ideal world, the soffit box would have been insulated, but I'm not even sure how this would have been achieved - there's no way we could have added an extra 150mm on the fascia and another 150mm under the soffit - the projections would be unacceptably large.

 

I'll see what the architect has to say.

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If you want to add any further insulation underneath your completed roof it will be critically important that you install a complete VCL layer on the warmest side. Any area where the VCL is compromised will lead to a major condensation risk - the warm moist air will get through any gaps in the VCL and condense on the OSB underside of your completed roof build-up.

 

EDIT

I did a quick (Glaser type not WUFI) condensation risk analysis and using the software's default temperature and humidity settings (internal 20 degC and RH 50% and external -5 degC and 80%) it showed that you 'should' be okay adding up to 100mm of mineral wool below your completed roof build up without an extra VCL but anything more than that produces a risk of condensation. If I then alter the values of the internal temperature and RH to 22 degC and 60% RH to make them more onerous (eg above a bathroom) then it shows damaging levels of condensation.

Edited by Ian
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very very useful responses for me, thank you all.

 

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