Vapour control membrane

Discussion in 'Builders' Talk' started by acm718, Nov 22, 2006.

  1. acm718

    acm718 New Member

    Hi, quick question if I may - on a newly constructed dormer, insulated with kingspan, should a plastic sheeting type membrane be used between the foil backed insulation and plasterboard on the timber frame walls and roof. We have kingspan insulation between the rafters (with 2" airgap between it and the felt/flat roof areas), fully insulated 100mm timberframe walls (with vapour permeable membrane on the outside) and an additional FULL layer of kingspan 'underdrawn' everywhere from floor to roof and back down again - all joints and any damage in the top foil layer sealed with aluminium foil tape, creating a vapour control barrier. I've sealed the foil onto the edge of the upvc window frames, round all lighting cables exiting the ceiling void, as per manufacturers instructions.
    Reason I'm asking, is the architects plans show a plastic membrane, stapled (creating holes!) before boarding out, but when pushed, architect says it's not really necessary, as the insulation's foil backing acts as a membrane, and a builder-type bloke has just laughed at me and stated he'd never bother with a membrane, never seen one used, and wouldn't even be doing what I am with the joint sealing tape, just plasterboard straight over, joints and all, water ingress never having been a problem. All my plasterboard screws will pierce any membrane, whether plastic or foil anyway - fair point I thought.

    Who's right, in the real world, plastic membrane or not? and is this something a building control officer would pick up on - I'm tempted to leave a room unplastered with a couple of boards off for the final inspection, so bco can see what's underneath!
    Ta!!
     
  2. hallbeck

    hallbeck New Member

    Ask building control! If its on the drawings and they have been stamped then the bco is perfectly within his rights to insist on it.

    Are you not due a pre - plaster inspection? In case you didn't know - that means pre plasterboard!
     
  3. acm718

    acm718 New Member

    Yep, had inspection and an ok before insulation and first fix elecs/plumbing were done, next inspection is final 'ready-to-move-in' visit.
    Reason for posting was really to try to find out how important the vapour barrier is - I'm doing my best with the ally tape, sealing everything in sight, but having just been told builders just board it out without any decent barrier in place I wanted to know if I was wasting my time being so picky. Also been laughed at for getting plasterboard screws instead of nails - I know I'm right on that one!
    Thanks for the reply - call to BCO next step tomorrow!
     
  4. hallbeck

    hallbeck New Member

    Membrane is a good idea but they are a swine to fit and as you say you them punch holes in them!

    You are 100% right on the plasterboard screws - with an auto feed driver they are way faster than nails too
     
  5. acm718

    acm718 New Member

    I did look at autofeed screwdrivers on ebay, but they don't take the range of sizes I'm using - I've got 50mm screws for the stud walls (yes, miles too big I know), 75mm for the dormer walls and ceilings (thru 25mm kingspan as well as 12.5 p/board), and some 125mm ones for the slopey bit of the roof which has 75mm of kingspan under the rafters. I'm going to go roughly for 100mm centres, and it'll all be done with my trusty dewalt cordless and a special drywall screwdriver bit. Fun, fun, fun!
     
  6. yorkshireboy

    yorkshireboy Member

    As you say the kingspan acts as a vapour barrier,I have done a lot of dormers and never used one
    In my view you are asking for condensation.Where is all this supposed vapour going to go if you totally seal the room.
    Also I may be wrong but I don,t think it is in the regs that you must have a vapour barrier
     
  7. trustmark

    trustmark New Member

    Vapor barrier is intended to stop moisture getting into and through insulation.
    The water vapor in your home has to go some where if it passes through walls or cealing it will condensate once the temperature is cold enough if you are lucky it will happen in the cavity or under the roof tiles but generally it will occur on the cold side of the insulation or even within the insulation and is called interstitial condensation.

    Applying a Vapor barrier Warm side of the insulation prevents this and gives you control over where the water vapour goes ie through your extractor fans.

    An additional advantage of vapour barriers is they help to prevent air leakage from your property a big cause of heat losses and an major issue in part L and one that will soon cause problems for a lot of builders sticking religiously to Methods of construction they have always used.
    tyvec make a vapour barrier a bit like their roof felt which is non tear and easier to use than visquine but it will cost you
     
  8. mudhut

    mudhut New Member

    i suspect that the kingspan instructions will ok your method (fully taped joints) as a vapour barrier. adding a seperate vapour barrier on top will be a waste of time. if you really want to know, surf the web for kingspan bba or bre certification, this is the final word as far as bc are concerned.

    sounds like you have a good handle on it, long live the thinking builder/diyer.
     
  9. acm718

    acm718 New Member

    Well, the word from bco is I don't need a separate membrane, as long as I do what I'm doing with the foil tape on any joints or punctures in the kingspan. I really am trying to do this right, all my insulation is friction fit between joists/rafters/framework, and if there were any gaps, I've foamed them, and am confident that with the 'underdrawn' full insulation layer and tape there should be a full barrier. The construction has 6 to 8" deep areas of wooden framing at top and bottom of the dormer walls - am also taping this so there's foil from floor to ceiling, but it strikes me that the area under the floor, the height of the joists, will not be sealed, so really an air tight construction is impossible using conventional timber construction methods (unless the permeable barrier on the outside of the upvc clad walls counts as air tight?), made even more difficult on an extension to an existing 40 year old house - someone building entirely from scratch might stand a chance of getting it right.
    As far as vapour laden air removal goes, I've fitted a heat recovery system, which runs 24-7, feeding warmed fresh air into the bedrooms and living areas, extraction ducts in the bathrooms and eventually the kitchen, with an extract boost pull switch next to both showers - seems to work so far in the downstairs rooms - no dripping window cills or misted up glazing, even in the frosts we've had so far this winter. Downstairs bathroom mirror and window are clear in a few minutes - most impressed!
    Thankyou again, it's good to get opinions from the professionals.
     
  10. yorkshireboy

    yorkshireboy Member

    In my view the new part l air leakage regs are a joke.
    Very few people have the air recovery systems that acm has so condensation inside will be caused.
    Who in their right mind wants to live in a visqueen sealed box with no ventilation.Also,trustmark,what is the point of insulating and sealing every mm of a room and then sticking an extractor fan in it??
    In my view as long as there is adequate ventilation in the roof a vapour barrier is not needed
     
  11. trustmark

    trustmark New Member

    yorkshireboy I might off used your handel myself once and I certainly dont want to live in a visqueen box.
    but the OP asked about using an additional vp.
    Using one saves on the hassel of taping joints etc
    The whole point op extraction is to get rid of moisture in a controled way
    once upon a time before I was a yorkshire boy it whent up the chimney then when we blocked them up or moved to gas and people moped it up off their window cills.
    After a while we got double glazeing and loft insulation and houses got a lot warmer and were therfore abel to suport more moisture in the atmospher which rose up into the loft condensated and started to cause all sorts of problems.
    now if your clever and fit a taped visqueen membrain across all your upper cealings there is no need to ventilate the roof space.
     

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