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    • CommentAuthorMarkyP
    • CommentTimeJan 4th 2017 edited
     
    I am building a flat roof which is soon to be insulated and covered and will include a large roof light (1200 x 2000). I have secured a good deal on a fixed 3G roof unit with manufactuer incorporated trims and insulated upstand. However, there remains a need for a structural upstand and following standard details, this will create a cold bridge. I have come up with the following design to reduce this (pic included)

    extend warm roof PIR over the structural opening timbers, fix PIR to deck with tube/mechanical fixings. create a further upstand on top of PIR with compac foam high type insulation. Fix compac foam to deck with long recessed fixings and washers through PIR and foam close holes. Sit roof light on compac foam, bed with silicone or expanding gasket and then screw fix per manufaturer's detail into compac foam.

    I have noted other discussions where DIY roof light is suggested, however I'm really up against it for time now and lack the inclination as I'm up to my neck in DIY on other parts of the project.
      IMG_0118.JPG
    • CommentAuthortony
    • CommentTimeJan 4th 2017 edited
     
    I would like to see as much insulation outside the upstand as there is on the roof with no thermal bridges which is why I get my own 3g unit larger than the hole and sit it on, angle trim forms a drip on three sides and the stepped unit on the low side drips clear of upstand.
    • CommentAuthorMarkyP
    • CommentTimeJan 4th 2017
     
    the rooflight will be visible from above, I'd not be happy with my efforts at DIY mitred trims stuck on, and seeing a 200mm+ wide upstand under the glazing would I think look clumsy. Fine if not viewed from above but this will be very much in sight of several other windows. I guess it may be possible to black out that section of glazing but it would look very heavy if going with an upstand + same thickness again of insulation as warm roof.

    I realise DIY would be much cheaper but I've decided to buy one and be done with it, and am trying to work out how to make the best of it thermally. The manufacturer and architect detail shows a timber upstand from the joists up to the manufacturer's upstand/frame. It seemed like a horrid cold bridge to me, hence my design above. But then I did wonder how much it really matters when there's a great big panel of glass there anyway which, even in 3G, is thermally speaking pretty rubbish.
    • CommentAuthordjh
    • CommentTimeJan 4th 2017
     
    Posted By: MarkyPThe manufacturer and architect detail shows a timber upstand from the joists up to the manufacturer's upstand/frame. It seemed like a horrid cold bridge to me, hence my design above. But then I did wonder how much it really matters when there's a great big panel of glass there anyway which, even in 3G, is thermally speaking pretty rubbish.

    Even with thermally broken (PH) windows, it is the frame that is the weak point, in association with the glass spacer. The glazing itself is always the best performing part.

    I presume you've worked out the compression of the PIR with all that weight on top of its edge, and are happy that its well within the load limit?
    • CommentAuthorMarkyP
    • CommentTimeJan 10th 2017 edited
     
    Posted By: djh

    I presume you've worked out the compression of the PIR with all that weight on top of its edge, and are happy that its well within the load limit?


    I confess I havent yet. I dont understand fully understand how to work this out. The warm roof insulation is rated as "Compressive strength at 10% compression 150 kPa".

    does that mean that it takes 150kPa of pressure to cause a 10% compression of the material? I asked google convert 150kPa to kg/cm2 and it says 1.52kg/cm2. So have I correctly concluded that this means the insulation can take 1.52kg of my roof window per cm2 of insulation under load?
    • CommentAuthorgyrogear
    • CommentTimeJan 10th 2017 edited
     
    so how much does the window unit weigh ?
    "large roof light (1200 x 2000)"
    VERY MUCH, I'd say (at a guess, not far short of 120 kg ?)

    (my "slope-roof" neighbour installed one about the same size - one of the installers got the jeepy creepies and asked to be taken off the job !) They had to call in a bloke off of another job !).

    You'll need to calculate the area of the top of the XPS upstand, in square cms...

    gg
    • CommentAuthorEd Davies
    • CommentTimeJan 10th 2017
     
    10% compression is a lot, isn't it? I'd have thought you'd want to go for about a tenth of that, or so.
    • CommentAuthorMarkyP
    • CommentTimeJan 11th 2017
     
    so it looks like I'm OK then.

    area of upstand taking load of the window is 4800cm2. Window mass is tbc by supplier but GG's 120kg seems a good estimate. So I have just 0.025kg/cm2 which leaves a reassuring amount in hand against the 1.52kg/cm2 stated on the insulation datasheet.
  1.  
    I'm in exactly the same situation but instead of compac foam I will be using standard timber (easier at the stage I'm at) with an insulation angle fillet going on the sides and joining with the insulated upstand for a few millis. Not ideal, true, but hopefully good enough.

    I hope your calculations are sound! Please do let us know once you've proceeded with this arrangement.
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