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improving dormer knee wall insulation

  • 13-05-2013 11:57am
    #1
    ravendude
    Registered Users, Registered Users 2 Posts: 589 ✭✭✭


    Have put a post in "Construction and Planning", - but realize now it might be better here as this is a DIY job I intend to do (Mod, feel free to remove the other one).

    So we have currently got 75mm of fibreglass on the backs of the knee walls, and greater than the current regulations on the crawl space floor (over the ceiling below, a foot or more deep). So the floor is OK, - knee wall could def be improved. It's not bad, but could be better. I think a large part of the problem is wind blowing warmth out of it.

    So, thinking of 2 possible options:

    1/ Covering the knee walls with thermal foil (Like this: www.diy.com/nav/build/insulation/constructional-insulation/B-and-Q-Value-Aluminium-Foil-Loft-Insulation-12848829?skuId=13359535) . Here, I'd staple thermal foil loosely over the existing fiberglass on the knee. The main beneift here would be protecting the existing FG insulation from the wind.

    2/ Adding an additional layer on top of current fibreglass. Was thinking of tacking on a 200mm layer of Knauf Spaceblanket (http://www.diy.com/nav/build/insulation/loft-insulation/space_blankets/Space-Blanket-T-200mm-9273743?skuId=9283443) over it. This would really beef it up.- bringing it to double the thickness. As far as I know, the spaceblanket stuff is breathable. - at least it has perforated holes in the light plastic lining. I'm not sure though, the breeze might still be a problem, - though it would be good and thick.

    Number 2 would be quite a bit more expensive - But probably much more effective. I don't mind spending the extra if it will be better in terms of outcome in the long run.

    Any thoughts? My main concern with both of the above would be condensation. getting into the stud walls.
    The stud/knee wall timbers would no longer be exposed directly to the air. The knauf stuff I believe though should be breathable, and the I could staple on the foil wrap loosely


    I'm even thinking if doing both options in the long term!
    Any thoughts, experiences or advice?


Comments

  • #2
    dathi
    Registered Users, Registered Users 2 Posts: 1,422 ✭✭✭dathi


    ah the joys of dormers probably neither of the above the first one is just bubble rap by another name . as you have no air tight membrane behind the plasterboard any moisture traveling through your wall will condense on the plastic which over time could give rise to problems . a better solution would be to use wood fiber insulation boards (gutex) over the studs on the attic side this would beef up your insulation and help with your wind issues . BUT! there is always a but, unless you also upgrade the insulation in the sloping ceiling part of the roof this now becomes the weak link when it comes to heat loss , with the flat ceiling and knee wall well insulated the majority of the heatloss will be through the sloped ceiling, thus cooling that part of the roof leading to condensation and even mold on the sloping ceiling.l ike i said at start ah the joys of dormers! it either all or nothing


  • #3
    ravendude
    Registered Users, Registered Users 2 Posts: 589 ✭✭✭ravendude


    OK, that was my worry with number 1 alright, so I think its discounted, - I don't think I'd be

    I think doing a full on job with gutex board would be the ideal thing alright. Its probably a non runner for me at the moment due to restricted access, and I'm not really in a position to do a bigger job on it at the moment.

    Would there be any benefit from number 2? The space blanket knauf wool stuff is covered in plastic, but it is perforated, so hopefully should be OK condensation wise. This is an easy enough job for me to do and not too expensive (I should be able to pick it up on special offer over the summer).
    While I understand its obviously not airtight still, - but it would add another 200mm on top of the 75mm that's there, so should surely help?
    The perforated plastic cover on the Knauf stuff might help from the breeze a bit as well. Presumably the thicker it is the better.

    Agree about the slopes, - this would still be the weak part. There is 50mm of kingspan board in it though, so it could be worse (slightly at least!).
    I'm resigned to the fact I'll have to live with the slopes as a weak point for some time, - but at least i'll try and cover off everything else as best as I can in a practical way.

    So, is number 2 worth doing at all? - or a complete waste of time.

    My dormer actually isn't too bad now, - certainly not freezing anyway, since I sealed up the crawl space doors and a gaping hole from the crawl space out to the porch roof. Would like to improve it insofar as practical though.


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