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Housing - Renovation: vapour barriers and eps backed plasterboard

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    Hi

    I am planning on insulating my loft with eps backed plasterboard over the rafters. Does anyone know where the vapour barrier goes in this case as it clearly can't go on the warm side of the insulation?

    Thanks in advance for advice!!
  2.  
    Hi Aaron, that's a tricky one. I am guessing that laminated boards are not supposed to be used in that application, as I cannot see a way of incorporating a vapour barrier. Can you use EPS sheets, then barrier than plasterboard? You could tack the insulation with large head galvanised nails, then tack your barrier, then use plenty of long drywall screws to fix the plasterboards.
  3.  
    It may vary from one manuf to another, but the manufacturer of the eps/plasterboard laminate we used to use 20+ years ago certified that the eps, being closed-cell, was in itself a vapour barrier. I am under the impression that the eps board I most often use now also claims the same quality. XPS apparently does not.
  4.  
    Ah, good, I wasn't wrong (breathes sigh of relief). The manuf of the eps board I use says on their web-site: "The closed cell structure of the foam provides integral vapour control." (No advertising!).

    Nick

    On a vaguely related point, the adhesion of the insulant to the palsterboard on some of the products, especially the XPS ones, is atrocious. Bearing in mind the standard form of fixing on walls, with adhesive only, I think some people may find their rooms getting smaller....
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    As an additional measure that I have used in the past, is making sure all joins are taped (paper tape) and skim coated then use waterproof undercoat paint. A first line of defence!
  6.  
    Good point Jeff. This crosses over with the thred on PassivHaus. A vapour barrier is only as good as its joints. I sometimes coat the meeting faces of the eps with silicone mastic and then push tight, but nothing's guaranteed. Foil-backed pl'bd, used when dry-lining with, say, sheep's wool, is only as good as the tape which covers the back of the joint, if you want to be that fiddly, or the joint tape which covers the front, but then that's not on the vapour barrier, it's on the (porous) pl'bd. I think what lets us 'get away with it' mostly is the fact that, till recently, we've probably painted it with a vinyl 'paint-on plasticbag'.
  7.  
    Hi Nick, I wondered whether EPS would be classed as a vapour barrier, its just that I have not seen aminated boards used in this application. The method I mentioned would overcome the problem with the joints, and would thereby help enormously with the reduction of air leakage and the associated heat loss.
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      CommentAuthorfostertom
    • CommentTimeAug 22nd 2007
     
    No, it's easy, just order the vapour-check grade of whatever you're having - a layer of polythene's laminated between the plasterboard and the insulation.
    • CommentAuthorMike George
    • CommentTimeAug 22nd 2007 edited
     
    Posted By: fostertomNo, it's easy, just order the vapour-check grade of whatever you're having - a layer of polythene's laminated between the plasterboard and the insulation.


    Yes, but there are joins, and the air will leak through them, especially in an existing building where rafters have a howling gale above them. Also rafters are often not parallel and/or on an even plane, making airtight joints difficult. Laminated boards are also more rigid, exacerbating the fixing issue. My suggestion works well, I have done it in my own house, only using PUR in place of the EPS. I filled any gaps with expanding PUR foam before adding a vapour barrier.

    I have no heating in my loft-floor are approx 60m2. The insulation levels are less than average at around 0.4W/m2K, but the air tightness is verrrry goood :bigsmile:
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      CommentAuthorfostertom
    • CommentTimeAug 22nd 2007
     
    I agree, Mike George, laminates are obsolete - we need to do lots of clever things nowadays requiring access to all the layers. As you probably know by now, I'd do it a different way from the start!
  11.  
    Mike, did you fill between the rafters aswell.

    I'm thinking now to fill between the rafters with 50mm eps, then v.barrier. then eps backed p.board, perhaps 30mm. Going by the rule that if insulation is placed on the warm side of insulation then it must be a thinner layer/less resistant.

    Does anyone know the ideal thickness of eps I should spec for the p.board backing if theres 50mm betwee the rafters to avoid interestitial condensation?
    • CommentAuthortony
    • CommentTimeAug 22nd 2007
     
    Aaron, Why do you want to insulate at rafter level? Is it an illegal loft room? If not then insulate at ceiling level with quilt.
    • CommentAuthorSaint
    • CommentTimeAug 23rd 2007
     
    Not sure if I'm grasping this detail but if you're laying over the rafters, I'd go for PUR (good thermal) or XPS (high moisture resistance and compressive) over the rafters, battens, breather membrane, tiling battens, tiles and that's it. Finish off inside, taping joints and cut and fit plasterboard between rafters. Couple of points, EPS is not a vapour barrier, it is at best a vapour check or "control". Its about 95% closed cell compared with 99% for XPS. Closed cell content determines its vapour permeability, the higher the better. If that is the detail then not so sure why you're so keen on having a vapour barrier. Any vapour that gets through this build up is vented away through the membrane into the ventilated void.
  14.  
    Hi Saint, the barrier I used in my loft was to prevent ventilation heat losses, not interstital condensation.
    • CommentAuthorSaint
    • CommentTimeAug 23rd 2007
     
    Hi Mike, I figured that and you'd be doing a "pukka" job. Squirty foam is a good idea
  16.  
    Arron, Yes I put 30mm between the rafters as that was all I could use while keeping a 50mm airspace for ventilation above. I would have a chat with the Building Inspector as he may relax the u-value requirement if there are height constraints. The thickness you would need will in any case depend upon the k-value of the insulation. Aerogel will be the thinnest but expensive, then PUR, then EPS/XPS
  17.  
    Thanks Mike and Saint

    I will talk to BCO about the points you've raised

    Saint, why no requirement for a vapour barrier in this scenario? Are they not necessary in any heated space (close to the heat), to prevent draughts and also interstitial condensation?
    • CommentAuthorSaint
    • CommentTimeAug 24th 2007
     
    Aaron, I fear I may have misunderstood your initial installation description. When you said "over" the rafters i took it as being "OVER" the rafters i.e. on the topside. For that of course the tiling would have to be off. I think I was having a "senior" or at least "middle aged" moment, so I apologise for any confusion.
    What I now think you mean is fixed to the underside of the rafters, i.e. the inner face which of course would also be "over" them. In which case as you can't guarantee the effectiveness of the existing ventilation beyond the newly fixed laminate, I'd go for a vapour checked tapered edge insulated plasterboard that includes a foil between the plasterboard and insulation, then tape and skim the joints. Have a look at the website in the attached link. It shows a variety of options plus a list of suitable products

    http://www.sheffins.co.uk/findex.htm?pa_guide/loft_solutions.pdf#

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