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Green Building magazine: Major article about Thermal bypass

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    • CommentAuthortony
    • CommentTimeJun 24th 2009 edited
     
    Great article Mark! Green Building Summer 2009.

    I have long suspected that small gaps and cracks in insulation have a disproportionate effect on heat losses.

    Your reporting the 158% reduction in performance was found when there was a 3 mm crack must put an end to partial fill with sheet insulation

    I recon it puts an end to sheet insulation unless inspected and controlled on an hourly basis.

    lots of good points made

    hopefully it will stop the nonsensical way we build party walls (you didn't mention the thermal bridging into the attic through the structure).

    I am unconvinced that any ammount of training will make any difference to grass roots practice, at least not for at least 25 years.

    tony
    • CommentAuthorchuckey
    • CommentTimeJun 25th 2009
     
    If sheet insulation material was smaller ( say 2' X 4') in size and had proper moulded "click?" or tongue and groove edges then the sheets could be assembled properly as the the construction got under way, rather then just being leant against or stood on their sheet edges with a bit of desultery tape stuck over the joint (optional in most cases). Of course they would have to handled correctly then - another problem!
    Frank
  3.  
    Posted By: chuckeyIf sheet insulation material was smaller ( say 2' X 4') in size and had proper moulded "click?" or tongue and groove edges
    That's exactly what we have over here - except it's a shiplap edge rather than tongue and groove and the sheets are 2'x8' - makes it easier to carry them home in the car too! The sheets also have wooden battens moulded in to allow drywall or furring strips to be screwed to the sheets. Typically, this kind of insulation is used on the inside of the concrete walls of basements. Easy to install and easy to install a finished wall afterward.

    Paul in Montreal
    • CommentAuthorSaint
    • CommentTimeJun 25th 2009
     
    Recognising this problem years ago XPS boards have been available with shiplapped edges for use in partial fill cavities since the 80s and the XPS roofing boards with that edge profile even further back
    • CommentAuthortony
    • CommentTimeJun 25th 2009
     
    ever seen them fitted? 3mm is a very small tolerance almost universally unknown among bricklayers
    • CommentAuthorSaint
    • CommentTimeJun 25th 2009 edited
     
    Yes I have and done them myself. There's a good 15mm overlap to play with although I did once inspect a job where they had laid the boards such that the rebated edges abutted one another so it looked like the boards had been specially routed out for conduits. The manufacturer had to put arrows on the side that faced out and upward after that
    • CommentAuthortony
    • CommentTimeJun 25th 2009
     
    the 3 mm we are talking about is between the blocks and the insulation :smile:
    • CommentAuthorSaint
    • CommentTimeJun 25th 2009 edited
     
    Then I'd challenge that. There's little room for free air movement in 3mm and in thermographic scans when tested by Lloyds Register 3mm doesn't show up even in vertical joints in insulation in coldstore panels where the store has been taken down to working temperature, -28degC or so and where the thermal gradient is obviously far higher than in residential situations
  9.  
    ...my understadning was that the thermographic test did not show the gap itself - the thermography was just to assess the overal heat loss of different sample panels some with crack some with out.

    J
    • CommentAuthorSaint
    • CommentTimeJun 26th 2009
     
    James, this was the simple scan done with a camera to check for hot spots (or cold spots in this case) It was found, and this is empirical data, that gaps less than 3mm did not show up as coloured lines on the image
  11.  
    Tony,
    Thanks for your positive comments.

    Saint,
    Perhaps the condition that you mention was such that air movement behind the insulation was not possible. If this is the case then I agree that a 3mm gap would pose little threat (as the air is still), however, if there is air gap behind the insulation (warm side), then thermal bypass (in this case convective air movement behind the insulation and then through the gap) can occur resulting in a significant degradation in thermal performance.

    Mark

    P.S. You can find the article in GBM: http://www.greenbuildingmagazine.co.uk/summer09/
    • CommentAuthorbrig001
    • CommentTimeJun 26th 2009
     
    Just seen some partially built houses with partial cavity fill using rigid foam insulation, Some of the boards were bowed such that I reckon I could get my hand between the insulation and the inner leaf. No evidence of taping either. I nearly asked if I could take some as it would do no good like that. Seems like 3mm is a pipe dream with some builders.
    • CommentAuthortony
    • CommentTimeJun 27th 2009 edited
     
    Could you have taken us a photo?

    there again any of could do that on any site in the country using boards

    Here is a challenge "can anyone post a photo of partial fill with boards with less than a 3mm gap between the boards and the blocks?" :boogie:
    • CommentAuthortony
    • CommentTimeJul 3rd 2009
     
    no takers then?
    • CommentAuthorJohan
    • CommentTimeJul 3rd 2009 edited
     
    Posted By: SaintRecognising this problem years ago XPS boards have been available with shiplapped edges for use in partial fill cavities since the 80s and the XPS roofing boards with that edge profile even further back


    You can get PUR boards with tongue and groove in UK, Recticel do them. Called Eurosarking SL, http://www.recticelinsulation.com/UK/EN/Product/Eurosarking+SL/
    • CommentAuthortony
    • CommentTimeJul 3rd 2009
     
    Isn't the gap we are talking about between the blocks and the sheets? t & g etc do not effect this gap.
  17.  
    Posted By: tonyno takers then?


    Done plenty, but no photos. Next extension I will photograph as I go.
  18.  
    Posted By: tonyIsn't the gap we are talking about between the blocks and the sheets? t & g etc do not effect this gap



    That was my understanding, but also coupled with an inevitable gap elsewhere, which allows the cold air into the gap behind the boards, a la dot and dab...

    ...but to be honest I should probably re-read the article to do it justice.

    J
  19.  
    To differentiate between the various types of cavities I have been using the terms 'crack,' a 'gap' and a cavity. A crack is such that it is continuous through the depth of the insulation but for the worst effects of convection to present themselves there is also a gap behind the insulation (this allows warm air to rise behind the insulation, within the gap, which then travels out through the crack). A cavity can be, and often is, present on the cold side of the insulation. If the insulation is not adequately protected a cavity can then allow windwashing. This also has a negative impact upon performance. Convective bypass is far more onerous than a simple crack i.e. a crack on its own, that is less than 3mm, has little impact upon performance (and can be determined in a standard U-value calculation) but when gaps and cavities occur the risks are far more significant.

    I hope this clarifies things.

    Mark
    • CommentAuthorJohan
    • CommentTimeJul 7th 2009 edited
     
    Posted By: tonyIsn't the gap we are talking about between the blocks and the sheets? t & g etc do not effect this gap.
    Sorry, I read Pauls post and took it as meaning the 'crack' between the edges of the sheets. From Mark S. reply I take this is what we're talking about.

    I would've though any air leakage between sheets degrades the thermal performance more then the 'gap' between the sheets and the inner leaf?
    • CommentAuthorGBP-Keith
    • CommentTimeJul 7th 2009 edited
     
    I would just like to do the advert bit (site owner prerogative) so other forum users can see where the article was published.
    http://www.greenbuildingmagazine.co.uk/summer09/
  22.  
    Is there anyway to pay for an electronic copy of the article?


    I'm interested in this as it impacts a little on my thoughts for wall construction.

    I was/am thinking of
    solid blockwork inner leaf
    100-150mm PU boards
    Cavity
    Rainscreen (possibly brickwork)

    effectively an externally insulated wall.

    This problem of gaps between the insulation slabs and between the insulation slabs and the blockwork seems a bit of a problem.

    Here's a thought.

    If, rather than using a single 100mm thickness of slab, I used 2x50mm slabs that could be laid in a broken jointed fashion so the edges don't line up through the thickness would I reduce the gaps through the insulation?

    Secondly, would it be possible to use something like bathroom tiling grout to spread (using one of those combs) on the outside of the blockwork to stick (just like giant bathroom tiles) the first layer of PU insulation on. Once "stuck" up the edges could be taped. The grout behind the PU would act to help fill any gaps between the plaster and the blockwork and the taping would then prevent any air infiltration through the insulation. The second layer of PU slabs could then be laid, possibly using an adhesive to stick it to the first.

    Finally cavity ties designed for thin block work would be "nailed" through the insualtion into the blockwork. Insulation clips would then fit over to ties to hold the PU slabs in place permanently (the tile grout only being a tempory bond and a cavity filler).

    Then the outer leaf could be built up as per normal.

    I would end up with a partial fill cavity BUT as the outer leaf was built after the insulation has been installed I can pay extra attention to getting a good seal and fit.

    The tile grout would act as a tempory fix to hold the insulation in place and as a filler/flow disruptor to reduce the convection between the blocks and the PU.
    • CommentAuthortony
    • CommentTimeJul 7th 2009
     
    Nail in ties sound like a dangerous idea? Though the idea of fixing ties through the insulation sheets is a good one. Once you can see the joins then why not tape them?

    I recon we are heading back towards retro filling the cavities.
  24.  
    @tony: The "nailed" ties I was referring to are designed for thin jointed blockwork, where you can't use the traditional built in ties.

    see: http://www.ancon.co.uk/main.asp?pageID=159

    I admit i'm not too sure about the idea as it seems a bit odd, but they are an approved product. They seem to have some type of high pitch thread on them (like an impact driver) so maybe as you bang them in they cut their own thread.

    After reading a paper on the effects of gaps in the insulation it would seem pretty vital to either seal the insulation layer totally or fill the gaps behind the insulation layer.

    As sealing the insulation layer is likely to be impossible that leaves filling the gap behind.

    I could move to a fully filled cavity of rockwool but I need 200mm of rock wool to get the same effect as 120mm of PU.

    Maybe 100mm of PU backed by 50mm of rockwool, still built up in the same way

    If the tile grout used for "tacking" the first layer of PU slabs onto the blockwork can act to effectively seal the back cavity I could be onto a winner!

    here's the paper on the effects of gaps behind insulation etc

    http://www.viking-house.ie/downloads/Lecompte%20paper_partial%20fill%20cavity%20walling.pdf
    • CommentAuthortony
    • CommentTimeJul 7th 2009
     
    Nice link thanks --- But ....you say :-

    I could move to a fully filled cavity of rockwool but I need 200mm of rock wool to get the same effect as 120mm of PU.

    this is not true as 1 the 120 of PU will have little gaps reducing its performance possibly to below that of a similar thickness of batts!
  26.  
    Posted By: beelbeebubI could move to a fully filled cavity of rockwool but I need 200mm of rock wool to get the same effect as 120mm of PU.


    Don't forget that PU needs a 50mm air gap, so 120mm of PU needs a 170mm cavity. Rockwool can fully fill the cavity, so 200mm of rockwool only needs a 200mm cavity. Shrinks the wall thickness difference to just 30mm. (Not to mention any argument about gaps around the PU etc.)
    • CommentAuthortony
    • CommentTimeJul 8th 2009
     
    :swingin: glad I chose full fill batts now
  28.  
    <blockquote><cite>Posted By: MarkBennett</cite><blockquote><cite>Posted By: beelbeebub</cite>I could move to a fully filled cavity of rockwool but I need 200mm of rock wool to get the same effect as 120mm of PU.</blockquote>

    Don't forget that PU needs a 50mm air gap, so 120mm of PU needs a 170mm cavity. Rockwool can fully fill the cavity, so 200mm of rockwool only needs a 200mm cavity. Shrinks the wall thickness difference to just 30mm. (Not to mention any argument about gaps around the PU etc.)</blockquote>

    Why does PU need an airgap and rockwool doesnt?
  29.  
    Posted By: MarkBennett
    Posted By: beelbeebubI could move to a fully filled cavity of rockwool but I need 200mm of rock wool to get the same effect as 120mm of PU.


    Don't forget that PU needs a 50mm air gap, so 120mm of PU needs a 170mm cavity. Rockwool can fully fill the cavity, so 200mm of rockwool only needs a 200mm cavity. Shrinks the wall thickness difference to just 30mm. (Not to mention any argument about gaps around the PU etc.)


    Sorry, that should be 25mm air gap, not 50mm. Hence 120mm of PU needs a 145mm cavity, compared to a 200mm cavity for Rockwool.
  30.  
    Posted By: bot de paille
    Posted By: MarkBennett
    Posted By: beelbeebubI could move to a fully filled cavity of rockwool but I need 200mm of rock wool to get the same effect as 120mm of PU.


    Don't forget that PU needs a 50mm air gap, so 120mm of PU needs a 170mm cavity. Rockwool can fully fill the cavity, so 200mm of rockwool only needs a 200mm cavity. Shrinks the wall thickness difference to just 30mm. (Not to mention any argument about gaps around the PU etc.)


    Why does PU need an airgap and rockwool doesnt?


    I believe that for PU the gap is needed to stop water that has penetrated the outer leaf from coming into contact with the insulation. The gap ensures it runs down the inside skin of the outer leaf.

    Not sure why rockwool doesn't need this and can therefore be full filled. I can only guess that it is more tolerant to water ingress. Tony?

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