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Housing - Renovation: cold bridging on window cill carrier; interstitial condensation

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    • CommentAuthorcc64
    • CommentTimeDec 7th 2020
     
    A query re: cill carriers on high spec triple glazed alu-clad windows

    I specified some high spec windows for a full on refurb of my terrace; the glazing has an R of ~2; the windows are installed using the Illbruck i3 '3 layer' system (external TP600 tape, middle FM330 PU foam, internal airtight but vapour permeable tape back to structural opening). Slow progress means the internal window ledges/boards and window reveals have not yet been fitted.

    I'm now surprised that the cill carrier (a thermally broken undertray to the window which extends through to the inside edge of the window) is forming condensation beads. Room humidity is 61%. Outside temperature is 2.6C.

    This alarms me; unless the installation of the window ledges/boards and liners becomes the air and vapour control layer I will have a pocket within the structure where interstitial condensation is likely. (This pocket has horizontal bounds [lower] of blockwork [upper] of window board, [outer] of window frame/cill carrier and [inner] plasterboard.) This would be unfortunate.

    Are there any experienced window fitters here that can advise?

    (The airtight but vapour permeable layer on this refurb is intended to be on the inner face of the block wall)

    Thanks
    M
    •  
      CommentAuthordjh
    • CommentTimeDec 7th 2020
     
    Posted By: cc64internal airtight but vapour permeable tape back to structural opening

    I hope that is a typo. Do you mean impermeable, or variable permeability as in ME508?

    I'm now surprised that the cill carrier (a thermally broken undertray to the window which extends through to the inside edge of the window) is forming condensation beads. Room humidity is 61%. Outside temperature is 2.6C.

    Do you have a link to the actual product you used, please? What is the temperature of the internal part of the cill carrier and of the window frame adjacent to it? (use an infrared thermometer) and also the nearest bit of internal reveal? Also what is the wall construction and how is the window fitted? Drawings/photos will probably help.

    Yes, I don't think you should be getting condensation there. The next step is determining how and why it is forming and then we can move on to remove it.
    • CommentAuthorcc64
    • CommentTimeDec 7th 2020
     
    weeeeell, it's one of Ilbruck's smart humidity-sensitive permeability - a feature of all their 500 series window membranes I think? https://www.illbruck.com/en_GB/homepage/download/24582/download.html

    Actual product is ME501; this is adhered to the side of the window before installation for the jams and header; at the base the cill-carrier is first attached to the window, then the ME501 to that (I'm surmising this last detail - didn't see the process but can see the 'finished' outcome).

    The household does not run to an IR thermometer; but the back-of-the-index finger test says cillcarrier is not as cold as a bottle of beer from the fridge; putting the 'out' sensor for an in/out electronic thermometer under the beer bottle reports 7C which is plausible; against the cill carrier it stabilizes at 11C; the out temperature it reports now is 3.2C

    Wall construction is block, brick, with 65mm cavity (currently void); and Kingspan Cool therm cavity closers in jambs. Cold bridge lintel above. On cill 25mm plywood used to raise height of inner blockwork to that of outer brickwork structural opening. windows 123mm deep, set 35mm back from brick face.

    Windows fitted as described in OP; ilbruck i3; did I omit an essential detail?








    A
    • CommentAuthortony
    • CommentTimeDec 7th 2020
     
    I removed my transit pieces before fitting my windows, are you sure that they stay on?

    You can if you wish add insulation on the inside of them, but that will make them colder still!

    Anywhere below 12 in my book is prone to collect condensation inside a house.

    Will you be fitting insulated linings? I did to all four sides 🙂 well insulation behind the reveals and under the window board
    • CommentAuthorcc64
    • CommentTimeDec 7th 2020
     
    Am I sure the transit pieces were removed? I would certainly hope so - I didn't pay some trained professionals to forget such a detail (but good point) - perhaps this is something else I should be attentive to. Is it nt possible to find staff you can simply rely upon to do a job properly?

    Linings to reveals and the window boards will be lined with offcuts of Proctor's Wraptherm (which is airtight but vapour permeable fully adH membrane bonded to 10mm aerogel)

    As for for more insulation I am simply contemplating fully purging with foam the void volume under the window board; if it's full of air and vapour impermeable foam there are no condensation possibilities. Might require a fair few cans though (34litres per FM330 according to Illbruck spec sheet IIRC)
    •  
      CommentAuthordjh
    • CommentTimeDec 7th 2020
     
    Posted By: cc64weeeeell, it's one of Ilbruck's smart humidity-sensitive permeability

    That's fine as it's variable permeability, not 'vapour permeable'! :bigsmile:

    "Due to the intelligent, variable vapour
    permeability (sd value), when used in
    an internal UK (warm) environment, it
    will be vapour-tight, and when used in
    an external UK (cold) environment it
    will be vapour open."

    Ditto for the "airtight but vapour permeable layer on this refurb is intended to be on the inner face of the block wall". I very much hope that you mean IMpermeable and/or that there is an intelligent membrane involved.

    at the base the cill-carrier is first attached to the window, then the ME501 to that (I'm surmising this last detail - didn't see the process but can see the 'finished' outcome)

    I don't understand this. What seals to the window frame? And what is sealed to the house structure? And again, what was the exact 'cill carrier' product you used, please?

    The household does not run to an IR thermometer

    I recommend buying one - they're very cheap. Oh, and what is the room temperature?

    Is there a cavity closer underneath the window as well as in the jambs?

    Is it nt possible to find staff you can simply rely upon to do a job properly?

    No, unless you also pay for somebody to watch them and take that specific responsibility. Ditto for airtightness.
    • CommentAuthorcc64
    • CommentTimeDec 9th 2020
     
    Thanks djh, attention focussing questions you ask. Useful.

    Easy ones first.
    Yes cavity closer at cill too. Since the heights of structural opening in inner block and outer brick are not the same this is topped with 25mm ply to raise inner height to match outer brick.attention was paid to ensuring seal to structure.

    Room temperature 18

    Window frame sealing. The i3 system from illbruck used (ME501 membrane) adhers with a 20mm wide Addy strip to the edge of window frame before frame is offered up into the structural opening. Once the frame is aligned and securely fixed a backer foil is removed from the membrane allowing it to be sealed to the house structure all round the window.

    Branding of cill carrier specified by Rationel is not known right now; I'll change that.

    Air tight but vapour permeable on inner face of blockwork is correct. I would be uncomfortable about adding vapour impermeable to this control layer. We accept that absolute control of moisture is illusory. So we must provide opportunity for it to escape. Hence vapour permeability to be maintained. Proctor Wraptite (the smart membrane element of wraptherm)

    M
    •  
      CommentAuthordjh
    • CommentTimeDec 9th 2020
     
    Posted By: cc64Room temperature 18

    OK, so this explains the condensation. With room temperature 18°C and RH 61% the dewpoint is 10°C. So that's close enough to your estimated 11°C that I think we can safely ascribe the difference to measurement error and say that we'd be surprised if there were NOT condensation at that point.

    So the question then becomes WHY is the temperature of the cill-carrier as low as 11°C? Answering that depends on knowing about thew exact construction of the cill carrier, which you haven't given us the information to determine yet, and the construction around it, about which I'm still confused.

    Window frame sealing. The i3 system from illbruck used (ME501 membrane) adhers with a 20mm wide Addy strip to the edge of window frame before frame is offered up into the structural opening. Once the frame is aligned and securely fixed a backer foil is removed from the membrane allowing it to be sealed to the house structure all round the window.

    Thanks, I'm familiar with the general principle of these systems. I'm interested in the specific details of how it was done on your house. Exactly what was the tape sealed to and where? Pictures and drawings may help, or detailed text. Especially how is the airtight connection made under the window?

    Air tight but vapour permeable on inner face of blockwork is correct. I would be uncomfortable about adding vapour impermeable to this control layer. We accept that absolute control of moisture is illusory. So we must provide opportunity for it to escape. Hence vapour permeability to be maintained.

    Well I understand what you're saying and the house I built is largely vapour open both on the inside and the outside but if I had your house I think I would have reached a different conclusion and would have followed something like the Illbruck system design advice to make the inside more vapour tight than the outside.
    • CommentAuthorcc64
    • CommentTimeDec 10th 2020
     
    Yes that tighter in than out is snappy logic and hard to fault for a new build; for the present project getting a joined up airtightness control layer waay inside was not only challenging but also likely subject itself to regular challenge (indeed perforation) by all manner of relativity everyday actions involving fixings through plasterboard.

    Repositioning that airtightness control layer to the inner face of the blockwork protects it from those challenges and straightforwardly allows the continued control layer around the windows using illbruck system. (Some current issues there obviously enough but we'll sort those out)

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