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Green Building Bible, Fourth Edition
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    I have been going through the multifoil threads and my eyes are starting to glaze over.

    My as-built walll detail is:

    12.5mm Plasterboard (not fitted yet)
    25mm service batten
    Superfoil SFTV VCL
    140mm Timber Frame panel(120MM foil backed Xtratherm,20mm low emissivity Cavity)
    9mm OSB
    TF200 Thermo reflective breather membrane
    50mm Clear cavity
    100mm Blockwork
    14 mm Render

    My question is that when the wind blows, I hear the foil VCL rusling like a plastic bag all over the house (windows aren't fitted yet), and I am worried that wind entering small gaps in the timber frame will cause this to happen even when the windows are fitted.

    Looking at the threads around multifoil, I am considering using rockwool to fill the air gap between the VCL and the plasterboard, meaning it pushes gently back on the VCL and would stop any rustling in high winds.

    I understand that eliminating the air gap between the VCL and the plasterboard with rockwool could render the multi-foil ineffective, but would the added rockwool mitigate this a little?

    (Understand I would need to protect any wiring in the service cavity or up-rate the cables.)


    • CommentAuthorDarylP
    • CommentTimeSep 25th 2017
    ....the multi-foil products rely on still air, low-emissivity layers adjacent to them to justify the claimed performance. They should IIRC be stapled/secured tightly, not to allow any movement (bellows effect?).
    So, is it possible to staple/tape/secure the Superfoil?
    Or else fill the void with mineral wool....:confused::cool:

    There is nothing under them to staple to (only the Xtratherm)

    Where the studs are, the foil is sandwiched between them and the timber battens for the plasterboard, as the distance between the battens is relatively large, I think this is what is generating the rusling/pulsing.


    • CommentAuthorcjard
    • CommentTimeSep 25th 2017
    Your foil is only rustling now because of internal air movements. The TF200 breather will be(should be) windtight enough to stop drafts within the walls (otherwise you're not going to have a well-performing wall insulation)

    When the windows and doors are in, internal air movements will slow considerably, and when the plasterboard is on you wont hear any minor sounds
    One thing to watch, and it's a curious effect that gets a few places in my house, warm rooms that are linked via some in wall air path to cool rooms will experience some air movement if the warm air is lower than the cool. My utility Is reasonably warm because of the energy use within it, the ensuite bathroom above is cooler. Even with the MVHR turned off I feel a slight draft coming through the plasterboard hole where the sink waste enters the wall (haven't got to stuffing it with wool yet) because of the free air path from the warm room to the cool (no ceiling in the utility yet, but it wont be sealed even when it's there, due to the number of services entering
    • CommentTimeSep 25th 2017
    Put the windows and doors in to make it weathertight then see if the problem goes away? Also a good time to do an initial airtightness test whilst it's still possible to find and fix leaks.

    Note that pressure changes as the wind gusts will still affect the walls even after the building is airtight, unless the breather membrane and/or OSB is also airtight. So there might still be some rustling, in which case mineral wool in the service cavity would seem the best option unless you haven't built the blockwork yet.

    Yes, the blockwork is built and I suspect the Timber Frame joints are fairly leaky. Relying on the internal foil VCL for the air-tightness.

    I did think about almost dot and dabing some mineral wool into the service batten zone so as to provide some light tension to the VCL without affecting too much of the total area of the air gap for the multi-foil.

    Good idea to air test before the plasterboard though, will go for that.


    I may have misunderstood, but as I read it, this does not sound very good. You could have a very airtight house with no insulation at all, or with the insulation almost totally compromised by wind-wash. Is what you are saying that the wind is blowing via the cavity and through the Xtratherm to the warm side? If so, summat's very wrong.
    Yes, I suspect that some wind is getting through at the panel joints rather than past the Xtratherm which is factory foamed into place.

    I know I could see a few slivers of daylight in places, especially around a large vertical window which has 3 studs on each side, running the whole height of the house.

    If I absolutely had to, I could remove the battens, untape the all VCL and try to get to all the panel joints with some foam, but that would be a large job.


    I think you may have just described one of my pet hates with some TF houses. I take it from your description of the lay-up that there is no additional layer of insulation to mitigate the thermal bridges through the studs. (?) Doubled- or tripled-up studs seem common for structural reasons (I get that), but with no attention paid to the not insignificant thermal bridge which arises.

    Re the leakage between studs, I would be thoroughly sealing the gaps. I'd use air-tightness tape, not foam.
    Bl---y hell! IMG_20170926_192141. That's a big 'ole!
    • CommentAuthorDarylP
    • CommentTimeSep 27th 2017
    Are those gaps/holes left as indicated, not sealed/filled at all?
    If they are left open, no wonder the shiny-stuff rustles... :shamed:
    Can you access the frame still to fill the holes, and around the PIR too? the shiny-stuff does look less taught than I have seen before....
    Good luck:smile:

    There is nothing in those gaps other than a foil breather membrane on the outside.

    The big obvious holes I will fill with foam, smaller ones I will tape over.

    Not looking forward to slicing open the VCL and checking around each panel joint/non-panel stud. Will use up tons of tape to re-seal the cuts in the VCL again. Do we think this is absolutely necessary around the joints or would it just be a margial gain?

    I was considering adding rockwool beneath the plasterboard to limit any movement, which would be expensive for the whole house. I suppose if I fill in all the holes now, I can hopefully avoid it. Just worried that I may never be able to get to all of them.

    The PIR is neatly foamed into place inside each panel, so no issues there.

    • CommentAuthorDarylP
    • CommentTimeSep 27th 2017
    Looking at :
    there seems to be a gap between the PIR and TF?

    But anyway.... why not leave the shiny-stuff intact, and 'squeeze' in lightweight mineral wool, lightly compressed, to take up the slack, in front of the shiny-stuff?

    Yes, it will reduce the claimed performance of the shiny-stuff, but the mineral-wool has a significant thermal resistance to compensate...... go for 25mm (batten depth?) cheap/cheerful stuff from the insulation merchants?
    Good luck:bigsmile:
    Thanks, that gap in the photo is all my handy-work and will be foamed up. It's filling in the web of a large steel goal-post for the kitchen sliders.

    Again, I was worried about that steel as the ends will transmit heat very effectively from in to out. So I retro-filled the cavity between it and the blockwork with rockwool slabs to try to help a little.
    • CommentTimeSep 27th 2017
    Posted By: DarylPBut anyway.... why not leave the shiny-stuff intact, and 'squeeze' in lightweight mineral wool, lightly compressed, to take up the slack, in front of the shiny-stuff?

    You'd still have a cold breeze pushing in behind the VCL with every gust of wind.
    • CommentTimeSep 27th 2017
    OracsRevenge, if you edit your post with all the links to have type 'Text', then the links will come alive!

    I was trying to be too clever with the links and using HTML tags initially, didn't seem to work right so I stripped out the tags and didn't change it back to text.
    Off topic, but the forum links do work fine for me in 'HTML' mode posts, using chrome browser on android phone, just select the text and google turns it into a working link. I know they don't work in Windows7 unless you format the post in 'text' mode, don't know about other browsers.
    • CommentTimeSep 27th 2017
    Posted By: WillInAberdeenOff topic, but the forum links do work fine for me in 'HTML' mode posts, using chrome browser on android phone, just select the text and google turns it into a working link.

    That's not the forum working fine, that's your browser performing 'magic', and I'm sure I could find some counterexample where the magic was inappropriate. As has often been said, the real fix is to upgrade the forum software.
    • CommentTimeSep 27th 2017
    In the old Wild West - 'Wanted dead or alive- The Paper Bag Kid'
    "What's he wanted for?"
    Lol, lets not start talking about Cowboys :)

    What I think I will do is wait till all the windows and doors are fitted next week, then invite my Timber Frame Company to return to look at the issue and see what they are going to do about it.

    If I get no joy with them, then i'll have to strip back the VCL at the panel joints and tape/foam as necessary, then re-seal the VCL all over the house. Following that,I will add in 25mm of rockwool between each batten and plasterboard over the top. (after getting a blower test done)

    Could do with out all that, but it serves me right for not searching the forum properly in the beginning !

    When they said they were swapping the internal 30mm rigid insulation for their new-fangled foil which does the same job, I should have done more research and not just accepted it.


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