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Housing - New Build: Another ply window box question

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  1.  
    • CommentAuthorwoodgnome
    • CommentTimeApr 9th 2014
     
    I had intended to do a Denby Dale type of ply box for my windows. However im struggling to see how i can get my internal and external lintel heights the same. If i keep the lintels the same height there will be an 18mm gap between the outer lintel and the head of the window. Build is block with 300mm cavity. Am i missing something obvious. ? I think Tony's method might be simpler...
    Any ideas?
    Ta.
  2.  
    If you're using separate inner leaf & outer leaf lintels there is no need for the two to line-up.

    The outer leaf opening height is set by the height of the window & the extent to which the lintel overlaps the head of the window. It is not affected by the presence of the plywood box.

    The plywood box fits around the window & so needs to be accommodated by the inner leaf. So, for an 18mm plywood box, you just set the inner leaf sill 18mm lower & pack the inner leaf lintel 18mm higher. You can just cut the block around each sill/lintel. You don't need to carry these levels around the rest of the house.

    Could you post a drawing of your proposed installation?

    David
  3.  
    Are you assuming the plywood box completely closes the cavity, i.e. spans from inner leaf to outer leaf? It's not normally done like that as it would form an unnecessary thermal bridge & you'd need to make sure it didn't get damp where it contacts the outer leaf.

    Instead its normally cantilevered from the inner leaf reveal & either stops flush with the inner face of the frame or laps 50mm onto the frame to provide support. If it stops flush with the inner face of the frame then support is provided by window fixing straps.

    David
    • CommentAuthorjfb
    • CommentTimeApr 11th 2014
     
    And what is normally used to finish off the external reveals that minimizes thermal bridging? Didn't denby dale use alluminium reveals, but if that is not an option?
  5.  
    I wouldn't bother with external reveals. If the outer leaf is masonry then you can fit the window tight to the inner face of the outer leaf. Heat loss is slightly increased compared to putting the window in the middle of the insulation, but the install is a lot simpler & more weather proof.

    David
    • CommentAuthorwoodgnome
    • CommentTimeApr 11th 2014 edited
     
    Thanks David.
    The different lintel heights didn't occur to me until the other day when setting out for the door openings. I think Tony's method is cheaper and simpler, but i think the fixing method is better with a ply box as opposed to drilling into the outer wall as Tony did.
    I'm considering a hybrid method of ply sides, fixed to inner leaf and Tonys method top and bottom. This way i can keep the lintels the same height and get a good fix into the ply at the reveals. I can then fix some PUR insulation to the back of the ply to fill in the gap between ply and outer leaf.Ply sides could be fixed prior to building the opening or fixed afterwards, depending on how good the blocklayer is with his reveals!
    Obviously the outer and inner leaf need to be set out to accommodate the 18mm ply sides and also allowing the window to sit behind the outer leaf as well.
    I was considering Velfac 200 energy series windows as other half prefers outward openers, but looking at their standard interface drawings, the window sits between the outer leaf, not behind due to the profile of the window. I'm not sure if this is the best way for weathering, relying on some compriband.
    Here is the section details for Velfac as you can see there is no way to sit the window behind the outer reveal
    http://www.velfac.co.uk/Projects/Search_details

    Installation interface details
    http://www.velfac.co.uk/velfac-uk/download.nsf/web_all/7BD4EE60DD491368C1257426003C07EB/$file/200_ren.pdf
    Ill try to draw up a proposed detail and post it up.
    •  
      CommentAuthordjh
    • CommentTimeMay 24th 2014
     
    I know this is a slightly old thread but I just came across a new [to me anyway] method of installing windows in cavity walls and wanted to post a link to it. It's by Tremco Illbruck

    http://passivlux.com/window-support-bracket/
    • CommentAuthorNovice1
    • CommentTimeMay 28th 2014
     
    I'm also using velfac windows and glazed sliding doors and a ply box to put window / doors into 140mm of EWI.
    Challenges
    1. As with woodgnome I'm struggling find an effective way to protect the edges of the external ply and edges of the window from the elements. What is best practice?
    2. To complicate matters we are planning to re-use the roof slate to clad the walls. Any suggestions on how to detail the reveals to protect the windows?
    3. Can ply boxes be used for glazed doors as well? Worried about the weight of the door plus human traffic
    4. One solution was to accept the cold bridge in the name of a strong build and put the windows/doors midway in the box (partly on outer leaf and partly within EWI. The outer projecting part of the ply box would then be clad in breather membrane and aluminum.
    Any suggestions
    •  
      CommentAuthordjh
    • CommentTimeMay 28th 2014
     
    Posted By: Novice1I'm struggling find an effective way to protect the edges of the external ply and edges of the window from the elements. What is best practice?

    The edge of the window shouldn't be exposed to the elements. It's inside the box and should have a weatherproof tape and/or foam seal at the outside edge.

    We're planning to use OSB for the box instead of ply, which means I'm not concerned about delamination to the same degree. But the edge of the box and part of the face of the window will be covered in insulation and render, so again it's not exposed to the full force of the elements.
    • CommentAuthorNovice1
    • CommentTimeMay 30th 2014
     
    Thanks dgh. Makes sense
    Doors
    Are you using the OSB boxes for doors as well? Worried about the weight of the door plus human traffic?

    Windows
    As we are not rendering we wanted to use the outward projecting edge of the ply to support the battens/counter battens that we are using to fix the slate onto?
    •  
      CommentAuthordjh
    • CommentTimeMay 31st 2014
     
    Posted By: Novice1Are you using the OSB boxes for doors as well? Worried about the weight of the door plus human traffic?

    I think that's the plan. The doors are actually quite a bit lighter than the biggest windows, even with somebody standing in them. They'll be supported underneath and at the head as well as the sides. I believe the main concern is wind loading, not deadweight.

    Posted By: Novice1As we are not rendering we wanted to use the outward projecting edge of the ply to support the battens/counter battens that we are using to fix the slate onto?

    I'm no expert but that sounds a bit dodgy. Nailing/screwing into the end-grain doesn't sound strong or good for weatherproofing. But are the windows set back? If so then there's no need for the box to go forward of the window so it isn't exposed to the weather. How is the tile hanging detailed into the reveals, or are you doing something different there?
    • CommentAuthorDarylP
    • CommentTimeJun 2nd 2014
     
    We have used the Paslode ProScrews, G3 finish.
    These do not shear, we have used them to lift oak beams on blocks/hydraulic jacks, they do bend, but never give up!

    Cheers :smile:

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