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Green Building Bible, Fourth Edition
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    • CommentAuthorSprocket
    • CommentTimeJun 16th 2020 edited
    I have a situation a bit like Peter's recent post although in this case we are upgrading a flat roof (currently just single ply membrane over 9mm ply on 200mm joists with 25mm PUR board between).

    Initial calculations on the planned new roof suggest a condensation risk.
    I am wondering about the best way to avoid this and how "safe" I need to be.

    We will have, from top to bottom

    a torch-on 2-layer membrane
    100mm of Foamglas Readyblock (λ 0.036)
    9mm ply
    200mm roof joists with additional insulation between
    12mm plasterboard

    Taking as an example 22 C inside at 60% gives a dew point of about 6 degrees.

    My original spec put as much sheep wool (λ 0.035) in as I could get... ie. 180mm ish of sheep-wool insulation between joists. This gets me an overall average U for the roof of 0.13 (0.12 between and 0.22 at joists) BUT this insulates well enough to put the lower surface of the ply between joists at 6.2 C. At -5 outside this falls to 5.1 C below the ply.

    Adding a VCL on the warm side would be tricky and feels wrong. It would create a sealed airspace. And anyway we have quite a few downlights (thought they are pretty airtight) to fit into the plasterboard.

    So presumably the sensible solution is just to use less insulation between the rafters? Just decreasing to 100mm fleece seems to give U=0.17 (average for 0.16 and 0.22) and raises the temperature below the ply to 7.4 at -5 outside (or 9.6 at -1 outside).

    My questions:-

    Is reducing the insulation (never thought I would say that in here) between joists the way to go?

    Does the simple numerical approach really sensibly represent what happens with sheep-wool insulation given that it probably allows a little air movement and it has humidity stabilising properties? Does sheep-wool give me some sort of extra leeway?

    What sort of temperatures in and out should we really be considering for these dew-point calculations? Do we need to consider rare extremes or are averages enough given that the building will smooth things out a bit? Is my 7.4 degrees C below the ply a safe enough margin when the dew point is 6?

    • CommentAuthortony
    • CommentTimeJun 16th 2020
    I don’t like the 9mm ply , I would always want to see a vcl on the warm side

    -3C for condensation risk - u value not sufficient for me
    • CommentAuthorIan1961
    • CommentTimeJun 16th 2020
    It needs a VCL.
    • CommentTimeJun 16th 2020
    Posted By: SprocketAdding a VCL on the warm side would be tricky and feels wrong. It would create a sealed airspace.

    Indeed, which is why the regs require ventilation above the insulation, I think. So the membrane and its support is cold and in free air.
    • CommentAuthorvivienz
    • CommentTimeJun 17th 2020
    We have a warm roof build up on parts of our ground floor. The ground floor rooms extend beyond the upper floor, forming balconies to the rooms above. The balcony floor is a rubber epdm finish and below this is a sandwich of timber firrings, then the warm roof build up. The firrings create the ventilation gap between the rubber roof and the warm roof below.
    • CommentAuthorSprocket
    • CommentTimeJun 17th 2020
    OK, I've tweaked it a bit. I've managed to increase insulation above the boards to 150mm over most of the roof. This has reduced the upstand at edges a little (upstand is a bit variable because of the falls in the roof) but I think it'll still be OK.

    Overall U is now 0.14 and that will have to do... I definitely can't get any more insulation on top but with the fleece between rafters reduced to 100mm almost everything is now well above dew point even down to -5.
    The rest of the house is only moderately insulated and airtight so I won't benefit much from a lot more insulation here.

    The membrane is torch-on; sealed to the top of the insulation. The membrane and insulation are airtight. I can't get anything below the ply because the joists are in the way. I don't particularly like the 9mm ply either but removing it would be a pain and I need to keep it to support the insulation and with the existing furring to provide the existing roof falls. I'd probably be looking at a complete roof off and on again to do a different approach :-/
    Sprocket, out of interest, was there a reason you didn't go for a higher performance insulation board above the ply deck? That would improve your overall u value and also keep the ply warmer. You lay the VCL on top of the deck so the ply stays in the 'inside the building' envelope.

    Many brands available, just one example here:
    • CommentAuthorSprocket
    • CommentTimeJun 19th 2020 edited
    Lots of small reasons. We have used Foamglas elsewhere on this house and others before and I really like the simplicity of it on a complex roof design... completely impermeable and waterproof, not eaten by animals, does not compress like PUR foam boards, easily bonded to itself and membranes, and leadwork (we have lots of lead junctions) etc.

    But in this case we mainly chose it for this roof initially because after plans went wrong a couple of years ago fixing a badly leaking single ply membrane roof (and I was not able to get a GRP or liquid membrane roof like I wanted - nobody would do it) I opted for a torch-on system and was impressed how well it works with Foamglas. Does not easily get bypassed either as the membrane and insulation ends up one big sheet - no gaps for air or water to sit in.

    We did use Kingspan boards on our last warm roof in 2014 but that was a very different and much more simple... a fairly standard double pitch roof design with clay tiles over. That previous build was also a full-on eco new-build whereas this is a rennovation (have to keep much of existing roof).
    Thanks, interesting material!

    Is it possible to lay say 50mm foamglass on top of 100mm PIR?
    • CommentAuthortony
    • CommentTimeJun 19th 2020
    Yes, you can lay concrete on it, it can take the weight, won’t like being walked on so much when on top of pir as the foam glass could start to rock and dig in at the edges
    • CommentAuthorSprocket
    • CommentTimeJun 20th 2020
    Oh! Interesting idea.

    Yes, compressive strength of PIR is pretty decent if you can spread the load. I can imagine though that stepping on a corner the PIR might give enough to cause the foamglas above it to crack (foamglas does not give much and might snap) but some of the fibre-faced boards might do OK in that sort of setup.
    • CommentAuthorrevor
    • CommentTimeJun 22nd 2020
    Could you not introduce ventilation by having holes in the joist above the insulation and ventilation grills in the barge boards and eaves. I had 2 relatives who had problems with new built flat roof extensions and condensation in the void below the roof boarding over a short period (approx. 3 years) caused the boards to rot. The solution on reroofing was to provide ventilation as described.
    • CommentAuthortony
    • CommentTimeJun 22nd 2020
    Please could you share more details on this, I am very interested as it used to happen all the time to chipboard and black chipboard decks but I haven’t seen problems for lots of years though I hate flat roofs so probably have missed a lot of problems.

    I have a suspected mechanism that would be made worse by ventilation!
    Revor, what you are describing is a 'cold roof’ where the boarding is on the cold side of the insulation and needs adequate ventilation. These cold roofs have effectively been banned in newbuild Scottish Building Standards, I had understood the same down in England but haven't checked.

    If the new built roofs and the replacements were recently built, shouldn't they have confirmed to legal standards IE have been built as warm roofs with the insulation located above the deck boards?

    Edit to add: looks like cold roofs are still legal in England if ventilated, so the replacement roofs would be legal in England but not Scotland, but the original roofs were not legal in either.
    • CommentAuthorrevor
    • CommentTimeJun 24th 2020
    My examples were from 40 years ago England ( my cousin's house) new build roof collapsed walking on it after approx 5 years Reroofed ventilation added (as per my suggestion in earlier post). Okay after that at least 15 years when property sold. 2"nd example Wales (sister) new build after 3 years roof leaked boards rotted was not built to spec. Reroofed ventilated. Interesting point on both houses had building guarantees but because flat roof are classed as cladding they are not covered. So both had to cough up.
    How much rework is author of post wants to do. How much original is being kept. If roof is fine not leaking an inside job would be easier cheaper and not dependent on weather. With 200 mm to play with I would consider taking ceiling down insulate to leave 50 mm gap holes in joists in that space for cross ventilation, and eaves ventilation for the run.
    150 mm PIR will give you a very good u value about 0.12. Overlay with 25 mm insulation to avoid cold bridging through joist and plasterboard out with duplex board to give a vapour barrier. I would not cut holes in ceiling.
    surface mount light fittings.

    If going to do a warm roof then I would strip back re-board with something stronger than 9 mm (min 18 mm) and follow a prescribed method as detailed by insulation manufacturer. Kingspan have very good data sheets for warm roof there is even a tapered board designed to give you a built in fall.(Thermataper). there are guidance notes on overlaying an existing roof also.

    End of day how much do you want to spend.
    • CommentAuthorSprocket
    • CommentTimeJun 30th 2020
    Yes, the 9mm ply still has me a little worried.The method above is as recommended by insulation manufacturer... it's just the limited insulation space above (because of upstand) that was causing concern, and then mainly because of too much insulation below.
    Now it's 150mm of insulation (and doubling as VCL) the dew point problem has gone away.
    I do wonder if I should replace the 9mm ply with 18mm. It's 70m2 though and has been in place for 24 years without any problems.
    • CommentAuthortony
    • CommentTimeJul 1st 2020
    It is all OK then 🙂
    • CommentAuthorSprocket
    • CommentTimeJul 1st 2020 edited
    Hmm.... Thanks for raising it folks but now I'm thinking 9mm ply is probably OK too.
    I'm no structural engineer but playing with online timber sag calculators I get less than 1mm of movement for 9mm ply over 400mm joist span with 100kg load. Foamglas is a bit rigid and brittle but I think it will cope fine with that.
    I really was not looking forward to pulling up all that ply and removing all the nails from joists whilst trying to keep the furring undamaged.

    Oh, and I have swapped the 100mm fleece between joists for a wood fibre product with supposedly almost identical breathable humidity buffering properties (Gutex Thermoflex in our case but there are others). It was going to be pretty difficult to keep the fleece in place against the ply at the top of the joists but this wood fibre insulation seems to be intended for just this sort of thing.

    I think I'm fairly happy now... just needed somewhere to go to talk this through :-)
    • CommentTimeJul 1st 2020
    Posted By: SprocketI really was not looking forward to pulling up all that ply and removing all the nails from joists whilst trying to keep the furring undamaged.

    You wouldn't need to take up the existing ply if it was in good condition. Just glue/screw another layer of 9 mm over the top.
    Thin ply can be very stiff when supported in 2 directions (so with noggins inbetween the rafters).

    FWIW I have a sailing dinghy built in 1957 out of 5mm ply which is still plenty stiff enough to stand on.

    I think people go for thicker ply if there's doubt about the quality of the plywood, or if there might be point loadings like ladder legs, or impacts like dropped tools. Thick ply is easier to fix, doesn't need noggins and can tolerate patches of damage.

    If your 9mm roof has been ok this far, it should be fine when protected with thick insulation.

    Edit to add:. Two 9mm sheets on top of each other are twice as stiff as a single 9mm. An 18mm sheet is 8 times stiffer than a 9mm.

    If you did decide to overlay the existing ply, then you could glue the new sheets onto the old, so they act as a single 18mm composite. (Or just replace with thicker sheets if that's less work.)
    • CommentTimeJul 1st 2020
    Posted By: WillInAberdeenIf your 9mm roof has been ok this far, it should be fine when protected with thick insulation.

    I don't think the question is about protecting the ply with insulation; it is rather about whether the ply is stiff enough to protect the insulation. Foamglas is brittle and may break if loaded between point supports.

    If the foamglas is glued down to the ply, then the two will form a stressed-skin structure that will be more resistant to bending and thus possible fracture of the foamglas core.
    lay say 50mm foamglass on top of 100mm PIR
    and the PIR will compress enough to absorb millimetre high spots, if the foamglass cares about them. But the ply can only sag if the insulation is in firm contact with it and pushing it down, so the foamglass will never find itself not in contact with the layers beneath and so point supported.

    All the layers should definitely be glued to each other to resist wind uplifting.
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