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Fundamentals: Airtightness Detail Around Windows

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  1.  
    • CommentAuthorjonespr1
    • CommentTimeFeb 13th 2019 edited
     
    We’re building a house, we inherited the plans and it wasn’t originally designed with air tightness and insulation in mind, so we’ve been making improvements where we can, as we go along. We’ve put a lot of focus into air tightness in particular, but have just received our triple glazed Internorm windows and am concerned about the air tightness detailing when in fitting them.

    The wall makeup is from the outside in -> render, blocks, 50mm cavity, Protect TF200 Thermo membrane, 9mm OSB, timber frame with 100mm of PIR insulation, Protect VC Foil Ultra air and vapour membrane, service cavity, plasterboard and plaster.

    The builders have started to fit the windows in the outer blocks, over a stone cill. They’ve fitted over a DPC which spans the cavity, the internal membrane has then been folded over the DPC, the cavity isn’t closed. My concern is that any air in the cavity is driven behind the internal membrane, bypassing the insulation. We could close the cavity with cavity closers, but I’m concerned that they still won’t be airtight, particularly the headers.

    I would appreciate any advice you can offer on the window detailing, recommended tapes etc…

    Robert
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  2.  
    I assume the cavity is ventilated, for the 'health' of the timber frame. I share your concern, but perhaps the problem could be mitigated if the dpc were glued to the TF opening. Not normal practice, I know, but pragmatic.

    I might be equally concerned about the depth of the external reveal. I feel that the tendency for windows and doors to be fitted with only a slight reveal can put timber windows at risk. Are yours alu clad? That would reduce the risk.
    • CommentAuthortony
    • CommentTimeFeb 13th 2019
     
    Very simple, join your internal air tightness barrier hermetically ro the window frame.

    For me this would be a continuous sheet of polythene with welted joins, no holes or discontinuities and a wide strip sealed to the window frame before the window goes in ALL round and sealed to the vb in the reveals ALL round.
    •  
      CommentAuthordjh
    • CommentTimeFeb 13th 2019
     
    Ideally the windows shouldn't be fitted in the outer blocks, since that means both the inner and outer surfaces of the frame are cold, making a thermal bridge that will encourage condensation in the corners of the windows.

    The best arrangement is to mount the windows in the plane of the insulation. In that case it is obvious to seal the outside of the frame to the cold side of the insulation, and the inside of the frame to the warm side of the insulation.

    One surface should be the main airtightness barrier. In your case with a vapour barrier behind a service cavity, I presume (you haven't stated) that is your primary airtightness barrier. Clearly that should be connected to the inside of the frame.

    The outer surface should also be airtight but especially watertight, although ideally breathable. Are your breather membrane and/or your OSB airtight? Drainage also needs to be incorporated below the outer edge of the frame.

    If your windows project outward beyond the insulation, you will need to provide both airtightness and insulation alongside the window somehow. Insulated, airtight cavity closers, perhaps?

    If there's anything else the builders can be doing I would stop them installing the windows, work out the best solution first and get it properly drawn up for clarity.
    • CommentAuthorjonespr1
    • CommentTimeFeb 15th 2019
     
    Thanks everyone for your replies and apologies for the delayed response, I didn't get any forum notifications, so assumed no replies!

    Sounds like the general opinions are the same - any recommendations for really good tapes for doing what you have suggested FJW - I have used the Protect tapes elsewhere in the house and not been very impressed, they don't seem particularly sticky or durable!

    Also, if I take the approach of sealing inside and out, is it worth pulling the windows out to seal the cavity?

    Thanks

    Robert
    •  
      CommentAuthordjh
    • CommentTimeFeb 15th 2019
     
    Siga or Pro Clima are most often recommended. They both proved good ranges of products in my experience. I mainly used Pro Clima tapes, but used Siga Rissan where its stretchability was useful (around my ventilation ducts, for example) and Siga Sicrall where I needed the most sticky for special purposes (like sticking a polyethylene sheet to external lime render for the winter to cover an unfinished detail until it was warm enough to complete).
    • CommentAuthorbhommels
    • CommentTimeFeb 15th 2019
     
    Second djh on Siga tapes. Fentrim allows plastering over, Sicrall sticks like nothing else, Twinet double sided tape is useful when attaching airtightness membranes to other building elements.
    See if you can mechanically fix/bury the tape in position if you have the opportunity, so you don't have to rely on the tape remaining where you stuck it for its lifetime. And fold in stress relief loops if you expect movement.
    • CommentAuthortony
    • CommentTimeFeb 15th 2019
     
    I don’t like the idea of relying on tapes, better to be sure without them then there is less to go wrong or fail
    • CommentAuthorjonespr1
    • CommentTimeFeb 19th 2019
     
    Posted By: Nick ParsonsI assume the cavity is ventilated, for the 'health' of the timber frame. I share your concern, but perhaps the problem could be mitigated if the dpc were glued to the TF opening. Not normal practice, I know, but pragmatic.

    I might be equally concerned about the depth of the external reveal. I feel that the tendency for windows and doors to be fitted with only a slight reveal can put timber windows at risk. Are yours alu clad? That would reduce the risk.


    We haven't been told whether it is ventilated for a reason, my presumption was that the timber batons wrapped in DPC provide the fire protection at lower cost than cavity closers.

    Agree with sealing DP to timber frame as pragmatic option.

    With regards to the windows they are a combination of UPVC and aluminium - so no concerns about wood.
    • CommentAuthorjonespr1
    • CommentTimeFeb 19th 2019
     
    Posted By: djhIdeally the windows shouldn't be fitted in the outer blocks, since that means both the inner and outer surfaces of the frame are cold, making a thermal bridge that will encourage condensation in the corners of the windows.

    The best arrangement is to mount the windows in the plane of the insulation. In that case it is obvious to seal the outside of the frame to the cold side of the insulation, and the inside of the frame to the warm side of the insulation.

    One surface should be the main airtightness barrier. In your case with a vapour barrier behind a service cavity, I presume (you haven't stated) that is your primary airtightness barrier. Clearly that should be connected to the inside of the frame.

    The outer surface should also be airtight but especially watertight, although ideally breathable. Are your breather membrane and/or your OSB airtight? Drainage also needs to be incorporated below the outer edge of the frame.

    If your windows project outward beyond the insulation, you will need to provide both airtightness and insulation alongside the window somehow. Insulated, airtight cavity closers, perhaps?

    If there's anything else the builders can be doing I would stop them installing the windows, work out the best solution first and get it properly drawn up for clarity.


    We've made as much effort as we can to ensure the external OSB and membrane are airt tight (taped joints etc) and the internal vapour membrane is also airtight.

    Agree, the builders are busy on the MVHR whilst we sort this out :)
    • CommentAuthorjonespr1
    • CommentTimeFeb 19th 2019
     
    Posted By: FJWHi There,

    Ideally the window should be connected both wind tight externally to your tf200 breather membrane and airtight internally to your vc foil airtight layer.
    As you say, sealing the dpc around the reveal prior to sealing the air barrier to your window is a very good idea - otherwise a bypass of air from cavity to the internal is quite likely.
    You can peel back the folded vc foil and tape the dpc to the timber surround at the reveal, head and cill timbers.
    Once you have sealed this, the vc foil internally can be folded back into reveals. Cut four corner pieces approx 200-250mm and apply 100-125 to each side of corner around reveal and tape back to foil already installed. you should also consider releasing slightly the battens installed nearest the cill level to allow you to get a good air tight seal with the tape here. externally depending on access tape between the window frame and tf200 breather using an airtight tape also.
    good luck


    Thanks for the advice, we can pull the windows out if required as we have only installed a few as yet :)

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