BR specific to warm roof designs?

d000hg · 13 Nov 2019

In my other thread (https://www.diynot.com/diy/threads/does-this-roof-need-a-dpm-or-not.533146/) I talked about an 'ice cream sandwich' design for an insulated, felt shingled roof on our cabin, which I believe is not especially common in the UK but reasonably well known:

The extra frame adds quite a lot of material and labour costs and I was interested to read this alternative solution on the blog of a guy who constructs cabins (scroll down past the section on floors to Insulating the Roof of your log cabin)
https://www.tuin.co.uk/blog/insulating-a-log-cabin-floor-and-roof/

In short for thinner insulation (up to 50mm PIR) he endorses nailing shingles through the PIR into the roof boards with longer clout nails, relying on the compression strength of the PIR. There are a bunch of photos and better description in the link.

Our SBEM guy has currently got our roof insulation down to 75mm and is investigating if we can get it to 50 while adding extra elsewhere because the recommendation is you do not follow this design beyond 50mm. My builder agrees with that assessment.

I'm happy to trust my builder if this is practical (though thoughts welcome) But my main question is whether there any specific BR this breaches? For instance they don't recommend a VLC here - I assume because the lack of external wooden sheath allows warm air to escape rather than get trapped(?) and I am not sure if BR requires one, or makes other very specific requirements here. Or is this within the scope of me deciding how to do things (the SE had no issue but then that doesn't mean much probably).

tony1851 · 13 Nov 2019

With a warm roof you generally need a vcl immediately under the PIR, otherwise moisture vapour will find its way up between the joints and condense on the cold side of the insulation.

d000hg · 13 Nov 2019

But here the cold side is the inside of the shingles, so the air could pass through... And would condensation inside your shingles be a problem anyway if you have foil covered PIR? I suppose you could still fit a VPL here.

I've seen people online (these forums included) claim that properly taped PIR is a VPL or at least they convinced building control of that. I can't comment if that's a load of tish!

I would agree this isn't an optimal solution for a few reasons (nails act as thermal bridges, puncture VPL) but it seems in real life an adequate one, especially for a building with a shorter expected life... If it's legal.

tony1851 · 13 Nov 2019

The first cold surface the vapour would reach would be the outer foil finish of the insulation and it would condense on that. If the shingles are nailed directly on top of the PIR, there would be very little air space to provide effective ventilation to get rid of the vapour.

Imagine a cold, clear, still night in winter with the temperature a few degrees below 0; any vapour getting through the inevitable gaps in the PIR boards will condense on the cold outer foil surface of the boards, probably turning to thin strips of ice. When the sun comes up, the ice will melt and you get browns stains of rivulets down the inside of the ceiling when it reaches the next joints down the slope.

If an extra, thinner layer of PIR can be added below the main thickness, and the joints sealed with aluminium tape before plasterboarding, that would prevent vapour getting through.

d000hg · 13 Nov 2019

Thanks for that. So should it be taken as read that even using tape you can't make the cracks between PIR boards air or waterproof?
And you're suggesting overlapping two thin layers of PIR so these cracks don't go all the way through?

I'd never really thought about it that way, but is this something BC would care about or my own stupid decision to make?

So even with clout nails though it, seems a VLC would be a good idea

Notch7 · 14 Nov 2019

Ive always wondered if holes punched ttrough VCL allows water vapour through.

I ask because in every construction situation Ive come across, any VCL gets punctured with holes

This would be my concern with fitting shingles with clout nails through the PIR.
I think its poor practice because thats a lot of thermal bridging given the number of nails.

Cant the shingles be nailed directly to 18mm OSB board or structural ply?

^woody^ · 14 Nov 2019

tony1851 said:
need a vcl immediately under the PIR

Doesn't all PIR come with one already glued on it?

Notch7 · 14 Nov 2019

^woody^ said:
Doesn't all PIR come with one already glued on it?

I had always assumed the foil face once taped was acceptable.

In a warm roof it isnt possible to tape the underside, hence on a flat roof a vcl maybe put down first.

I tend to think taping the top face is ok, as long as all gaps are foamed, then that should still work as a vapour barrier.
Id be interested to know what the pros think in regards to warm roofs.

d000hg · 14 Nov 2019

Notch7 said:
Ive always wondered if holes punched ttrough VCL allows water vapour through.

I ask because in every construction situation Ive come across, any VCL gets punctured with holes

I've seen mixed comments on this, and don't feel remotely qualified to answer - if a foil-taped PIR constitutes a VCL. Whether it's one of those things that in theory should be fine but people prefer a belt&braces approach to avoid any risk. You'd be peed off if you had to go remedy it later!

This would be my concern with fitting shingles with clout nails through the PIR.
I think its poor practice because thats a lot of thermal bridging given the number of nails.

In reality I don't care about bridging that much but I was wondering. I don't know the exact size of a shingle but I would guess .15m^2. And you affix each one with 2(?) nails, which I think are maybe 3.5mm diameter. Their area in m^2 is (.0035/2)^2.Pi.2 per tile = .000077m^2. In ballpark terms, I make that .05% of your roof surface is metal. My roof area is approx 100m^2 so that gives equivalent of .05m^2 (1300 nails ish). Of course the nails don't come all the way through the roof so there's a bit of wood. I can't tell if this is significant though.

Cant the shingles be nailed directly to 18mm OSB board or structural ply?

As in just put a layer of OSB on top of the PIR all the way across, screwed through the OSB into the sub-roof/rafters? Someone (maybe you?) mentioned that in the other thread - not to build a frame in the insulation layer but just lay a board on top relying on the strength of the PIR.

Notch7 · 14 Nov 2019

d000hg said:
mentioned that in the other thread - not to build a frame in the insulation layer but just lay a board on top relying on the strength of the PIR

the normal way to fit insulation in a flat roof is to cover the roof fully, no gaps, then lay on the 18mm OSB board and screw through with long screws -180mm are needed for 120mm insulation.

you dont need a frame within the insulation layer -it would only be detrimental to thermal qualities

d000hg · 14 Nov 2019

That's interesting. Now my roof isn't flat but it is only 15 degree pitch. This would certainly make things much easier!

Leofric · 16 Nov 2019

Notch7 said:
and screw through with long screws

helical fixings

ps how many threads are there on this particular roof

seems to be going round in circles

d000hg · 16 Nov 2019

Well I asked about BR for roofs in the BR section. And then I asked about roofing design in the roofing section. People that know about regs don't necessarily know how to build things, and people that know how to build things DEFINITELY don't always know about regs

I am still not sure if we got a clear answer if a VCL is a requirement or not, and it taped PIR counts as a VCL. Do you know?

Leofric · 17 Nov 2019

d000hg said:
I am still not sure if we got a clear answer if a VCL is a requirement or not, and it taped PIR counts as a VCL. Do you know?

Yes , a vapour barrier is required on the warm side of the insulation.

^woody^ · 18 Nov 2019

Leofric said:
Yes , a vapour barrier is required on the warm side of the insulation.

^woody^ said:
Doesn't all PIR come with one already glued on it?

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BR specific to warm roof designs?

Leofric

Leofric

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