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Housing - New Build: PIR as VCL and airtightness

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    • CommentAuthorward32
    • CommentTimeNov 9th 2016
     
    I have read some posts and threads and the consensus seems to be that foil faced PIR will be an adequate VCL and airtightness layer if properly taped and sealed. However, I have some doubt after reading the BS 5250-code of practice for condensation in buildings.

    My warm roof build up will be:
    1. Concrete tiles on 50mm ventilated batten space.
    2. Wood fibre sarking @22mm - vapour permeable @0.55MN.s/g
    3. Omnifit stud between rafters @200mm
    4. 70mm PIRfixed to inner face of rafters, taped/sealed at all joins
    5. plasterboard stuck and screwed to PIR and rafters.

    PIR datasheets gives vapour resistance at about 100MN.s/g and BS5250 says you should have a minimum of 250MN.s/g.

    Everything above the PIR has good vapour permeability so moisture escaping up should dissipate!

    Question is will a well jointed layer of PIR work as a good vapour control and airtightness layer in this situation?
    • CommentAuthorMarkyP
    • CommentTimeNov 9th 2016
     
    I've been mulling over the same question. I have OSB sarking where you have wood fibre, I will have full fill mineral wool between rafters and then PIR under. I think the general view is that you want vapour to get out of the build up faster than it can get in. In your case, foil backed PIR taped would seem to satisfy that test adequately. I would think a VCL would be overkill if you can joint and tape your PIR neatly. Even if a few joints fail, the build up you have is nicely vapour open and you have a decent air space under your tiles. Perhaps you could VCL as a belt a braces job in any bathrooms.
    • CommentAuthorDarylP
    • CommentTimeNov 9th 2016
     
    ward32, is the PIR foil-faced? if so....
    ...and if you can ensure all joints are glued/sealed, AND use aluminium foil tape securely fixed to cover joints, you will be fine.
    We have done just this before, and BC were totally happy..:smile:
    • CommentAuthorMarkyP
    • CommentTimeNov 9th 2016
     
    what did you use to glue seal the joints, DarylP?
    • CommentAuthorDarylP
    • CommentTimeNov 9th 2016
     
    hybrid polysuplhide 310mm carts where the gaps were less than 5mm, otherwise foamed in Soudal foam.
    Cheers:smile:
    • CommentAuthorward32
    • CommentTimeNov 9th 2016
     
    " I think the general view is that you want vapour to get out of the build up faster than it can get in. "

    is a good way of looking at the problem. But having looked at the BS again it says that a warm roof with a top covering (underlay) with >0.25MN.s/g should be ventilated behind the underlay. This seems excessive but would be negated if I use a VCL with >250MN.s/g. - plastic!

    Its my house and I only want to do this once so a VCL might be the best option.
    • CommentAuthorMarkyP
    • CommentTimeNov 9th 2016
     
    you mention a 50mm batten space, is that batten and counter batten? I'm draping my breather membrane over the counter batten, then batten over for tiling. So I have a space between my OSB sarking and the breather. Is your membrane direct on the wood fibre?
    • CommentAuthorringi
    • CommentTimeNov 9th 2016
     
    Personally I would leave a small gaps between the PIR boards, and between them and the walls, then fill this gap with expanding foam, before taping over it. I trust 70mm of expanding foam to give a better airtightness layer then a plastic sheet that will have nails/screw holes in it.

    However I don't think adding a plastic sheet as well below the PIR boards would do any harm.
    •  
      CommentAuthordjh
    • CommentTimeNov 9th 2016
     
    Posted By: ward32But having looked at the BS again it says that a warm roof with a top covering (underlay) with >0.25MN.s/g should be ventilated behind the underlay.

    I would tend to rely on a BBA or similar for the specific underlay rather than a BS. Quite apart from its unfortunately confusing acronym, you can't sue a BS if it's wrong.
    • CommentAuthorward32
    • CommentTimeNov 9th 2016
     
    Well, BBA for the PIR boards repeats what BS5250 says in that they will be suitable if well sealed so long as a vapour permeable (Low Resistance) underlay is used. But the space under the underlay must be ventilated if the underlay is High Resistance (>0.25MN.s/g). Well, my roof is in place with wood fibre sarking with a vapour permeability of 0.55MN.s/g and I cannot ventilate behind it so I might add the extra VCL over the PIR to be sure; its not very expensive! Thoughts?

    Incidentally this dilemma has arisen because I am changing the roof build up by replacing EPS under rafters with the PIR. BC originally approved my plans which stated a VCL which I though was more necessary with EPS as its more vapour permeable.
    • CommentAuthorward32
    • CommentTimeNov 9th 2016
     
    MarkyP

    MY 50mm ventilated batten space does not have another breather membrane, instead relying on the Uditop wood fibre sarking, expensive but good vapour permeability. Its also waterproof as several weeks of rain have tested.
    • CommentAuthorringi
    • CommentTimeNov 9th 2016
     
    VCL will not cost more then £30 including the tape. For example WICKES GREEN POLYTHENE VAPOUR BARRIER 2.5 X 20M. But you must come up with a way to seal it to the walls, otherwise it is pointless.
    • CommentAuthorMarkyP
    • CommentTimeNov 9th 2016
     
    wood fibre sarking - nice and vapour open and also perhaps having some hygroscopic qualities and therefore some buffering capability as well? Seems low risk for condensation but as ringi says, plastic sheet is peanuts. But I dont know it will add much more to a well taped and foamed foil backed PIR layer other than perhaps a bit of peace of mind (which isn't without value). I have OSB sarking which has joints sealed (air tight) but should be somewhat vapour open. I considered a VCL but it's a loft room and I dont fancy my chances of getting a VCL sealed well enough right down into the eaves to make it worth the effort, so likely to rley on PIR taped and foamed, and any vapour getting out through the OSB.

    another safe guard might be a sheepswool based insulation between rafters. I keep meaning to look into this as it would add some humidity buffering capability to my buildup. I remember reading about moth problems with sheepswool insulation but havent had time to read up on whether that remains an issue. There are, I think, sheepswool rolls with in the order of 0.035W/MK so up there with the best mineral/glass rolls.
    • CommentAuthorward32
    • CommentTimeNov 9th 2016
     
    I will be starting the roof insulation this weekend and will be meticulous with my detailing. I have an airtight membrane from behind the floor joists to connect to the VCL at eaves and will plaster the VCL in to the walls where necessary with lathe over. I'm hoping to get high airtightness!
    • CommentAuthorringi
    • CommentTimeNov 9th 2016
     
    Posted By: MarkyP
    another safe guard might be a sheepswool based insulation between rafters. I keep meaning to look into this as it would add some humidity buffering capability to my buildup.


    I think this is pointless even one coat of a normal paint on the plasterboard will stop the sheepswool doing any useful buffering.
    •  
      CommentAuthordjh
    • CommentTimeNov 9th 2016
     
    Posted By: MarkyPanother safe guard might be a sheepswool based insulation between rafters. I keep meaning to look into this as it would add some humidity buffering capability to my buildup.

    According to the Tim Padfield paper I linked to in the humidity buffering thread, wool insulation isn't terribly effective as a humidity buffer.
    • CommentAuthorMarkyP
    • CommentTimeNov 9th 2016
     
    interesting, I'll have a read.
    • CommentAuthorMarkyP
    • CommentTimeNov 9th 2016
     
    Posted By: ringi
    Posted By: MarkyP
    another safe guard might be a sheepswool based insulation between rafters. I keep meaning to look into this as it would add some humidity buffering capability to my buildup.


    I think this is pointless even one coat of a normal paint on the plasterboard will stop the sheepswool doing any useful buffering.


    could you explain why? (for the sake of discussion let's assume sheeps wool could in principle act as a buffer)
    • CommentAuthorDarylP
    • CommentTimeNov 9th 2016
     
    The paint (vinyl?) acts a very good VCL.....
    • CommentAuthorEd Davies
    • CommentTimeNov 9th 2016
     
    So the wool wouldn't help buffer the indoor RH but might help buffer water vapour which might otherwise condense on your timbers.
    • CommentAuthorMarkyP
    • CommentTimeNov 9th 2016
     
    Posted By: Ed DaviesSo the wool wouldn't help buffer the indoor RH but might help buffer water vapour which might otherwise condense on your timbers.


    yes, that was idea behind the sheepswool - buffering the rafter zone which will be sandwiched between PIR under rafter and OSB sarking.
    • CommentAuthorringi
    • CommentTimeNov 9th 2016
     
    But the wood is a good buffer on its own. Personally if you want something better then standard insulation under OSB with air gap above, I would go for a sharking board that breaths more. The coldest part of the buildup in winter will be the sharking board. (Even just use planks of wood with 5mm gaps between them instead of the OSB.)

    (Note I am assuming the UK and that AC is not in use.)
    • CommentAuthorringi
    • CommentTimeNov 9th 2016
     
    If I every had to insulate a roof again, I could consider fixing thick counter batons and getting a quote for Icynene Spray Foam along with their calcs to see if building control would above it without any VCL. (With a breathable/vented makeup above it.)
    • CommentAuthorEd Davies
    • CommentTimeNov 10th 2016
     
    sharking batons

    But, yes, sarking boards (treated 150x18 softwood) is probably the cheapest and most robust vapour-open solution. It's what I'll be using in place of the ply the structural engineer originally specified.

    Not sure about 5 mm gaps, though; just butting them up and relying on the general wonkiness and shrinkage is sufficient I believe though I might well leave 1 mm or so gaps anyway. I'll be using sarking boards round the edge of the craw space too and BC only want a 1.5 mm gap to ventilate the space (1500 mm² per linear metre) so 5 mm seems a lot every 150 mm on the roof.
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2016
     
    Posted By: ringiI could consider fixing thick counter batons and getting a quote for Icynene Spray Foam along with their calcs

    How does Icynene mix with insects? Or to put it another way, how would you keep insects out of the Icynene? As far as I can tell, the outer parts of roofs are pretty much a zoo.
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2016
     
    Posted By: Ed Daviessharking batons

    But, yes, sarking boards (treated 150x18 softwood) is probably the cheapest and most robust vapour-open solution. It's what I'll be using in place of the ply the structural engineer originally specified.

    Not sure about 5 mm gaps, though; just butting them up and relying on the general wonkiness and shrinkage is sufficient I believe though I might well leave 1 mm or so gaps anyway. I'll be using sarking boards round the edge of the craw space too and BC only want a 1.5 mm gap to ventilate the space (1500 mm² per linear metre) so 5 mm seems a lot every 150 mm on the roof.

    I seem to remember reading about 2 mm gaps between sarking boards but I don't remember where. Sarking boards don't have the same racking resistance as sheets of ply, so I hope that wasn't needed.
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2016
     
    Posted By: DarylPThe paint (vinyl?) acts a very good VCL.....

    Although some paints are vapour open. Many water-based paints, like the clay paint I used. Silicate paints, Dulux Supermatt etc etc.
    • CommentAuthorringi
    • CommentTimeNov 10th 2016
     
    Posted By: djhHow does Icynene mix with insects? Or to put it another way, how would you keep insects out of the Icynene? As far as I can tell, the outer parts of roofs are pretty much a zoo.


    Given how long Icynene have been in use for, I think we would have heard by now if insects where a big problem it.

    You could tape your breathable felt and finish it carefully at the walls etc so insets cannot get in, likewise if you are using a sarking board. There are also inserts that can be used when tiling a roof to keep insects out.
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2016
     
    Posted By: ringiGiven how long Icynene have been in use for, I think we would have heard by now if insects where a big problem it.

    Perhaps I misunderstood but I thought you were proposing to use the Icynene above the sarking.

    You could tape your breathable felt and finish it carefully at the walls etc so insets cannot get in

    Breathable felt, tapes etc won't keep insects out.
    • CommentAuthorringi
    • CommentTimeNov 10th 2016
     
    Posted By: djhPerhaps I misunderstood but I thought you were proposing to use the Icynene above the sarking.


    No, put cross battens (say 2by3) below the joists to increase the space for insulation without creating cold bridges, then get all the space between the joists and battens filled with Icynene. Then just fix plasterboard to the battens.

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