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General: suspended floor insulation - an interesting link

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  1.  
    Crossover with many threads here. Not read it all yet - please discuss.

    http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces?full_view=1
    • CommentAuthortony
    • CommentTimeOct 25th 2008
     
    Is this in a country where it very cold in winter way below freezing?
    •  
      CommentAuthorfostertom
    • CommentTimeOct 26th 2008
     
    Nice one Mike
    • CommentAuthorskywalker
    • CommentTimeOct 26th 2008
     
    Good find Mike

    They've missed our wizard wheeze of using the magic gel though!
    • CommentAuthorMike George
    • CommentTimeOct 26th 2008 edited
     
    Posted By: tonyIs this in a country where it very cold in winter way below freezing?


    Washington DC, so yes I believe so.

    Is any of it relevant to the UK?
    • CommentAuthortony
    • CommentTimeOct 26th 2008
     
    I dont think so. Clearly there was no vapour check on the floor -- I advocate one for UK floors they are a must have in cold places.
    • CommentAuthorJackyR
    • CommentTimeOct 27th 2008 edited
     
    No, it claims moisture from ventilation air (warmer than the cold side of the insulation) is condensing on the insulation and timber, and that the level of moisture in the timber is being exacerbated by the presence of a vapour check at floor level.

    The reason given is that the cold side of the insulation will be at the same temp as the ground under the house, which is not sun-warmed. (If I've skimmed it correctly and not misread again...)
    • CommentAuthorEd Davies
    • CommentTimeOct 27th 2008 edited
     
    Posted By: tonyClearly there was no vapour check on the floor


    But wasn't that exactly the point of the article? There was a vapour check on the floor which was preventing removal of vapour from the ventilation air. Moist ventilation air plus a cold surface can obviously be a problem sometimes in Britain though nowhere nearly as much of the time as in Washington D.C. This article, it seems to me, is interesting as a description of the general physics of what can happen but is not likely to be very directly applicable here.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 14th 2012
     
    Mike's first mention of this interesting paper http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces?full_view=1
    eventually led to http://uploading.com/files/ad26df12/285Ja-ufloor.pdf.
    Building Inspector is keen to support it, wanted the 'argument' submitted - this is it. Hope he approves, as an02ew of this forum and I have just started building it!
    • CommentAuthortony
    • CommentTimeMar 14th 2012
     
    As far as I can see they are saying vapour closed or vapour barrier which is what I have been saying and doing for ages.
  11.  
    Main message for me is "Warm wood is happy wood" and make it as airtight as you can to reduce air movement around the timbers.
    • CommentAuthoraa44
    • CommentTimeMar 15th 2012
     
    Would dramatically increasing the air flow make things better or worse?

    We live in Shetland where the wind gusts probably aren't a good thing for MVHR units. As a result I was planning to take the input for the MVHR from the solum, with lots of vents in the wall to allow a good airflow into it.
    • CommentAuthortony
    • CommentTimeMar 15th 2012
     
    No difference, well a tiny amount more heat loss as air will be colder and may be more

    You could suck in scotch mist which would not be good as it is "wet"
  14.  
    This paper is entertainingly written, but I dont think it applies in my neck of the woods.

    He seems to be saying that the underside of the insulated suspended floor will stay close to yearly-average temperature, so will collect condensation from the ventilation air, if the dewpoint of external air in midsummer is warmer than that. (this bit is nothing to do with permeation through the floor)

    For Washington DC the year-average temperature is lowish 12C (snows in winter) but the mid-summer dewpoint is very high 21C (US Capital was deliberately built in a hot swampy place). As 21 is much more than 12 there is a condensation risk.

    For me the year-average temperature is 9C and the midsummer dewpoint averages about 11C so theoretically there is a slight risk of condensation, but in practice I suspect the timber will always warm up by a couple of C during summer and that will take care of it. During winter the air dewpoint is about 3C so that will dry the timber out nicely.
  15.  
    Posted By: fostertomMike's first mention of this interesting paperhttp://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces?full_view=1" rel="nofollow" >http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces?full_view=1
    eventually led tohttp://uploading.com/files/ad26df12/285Ja-ufloor.pdf." rel="nofollow" >http://uploading.com/files/ad26df12/285Ja-ufloor.pdf.


    Hi Tom, I have still not managed to look at your pdf, any chance of making it available in a different way?

    Peter
    • CommentAuthorMike George
    • CommentTimeMar 15th 2012 edited
     
    Posted By: WillInAberdeenThis paper is entertainingly written, but I dont think it applies in my neck of the woods.

    He seems to be saying that the underside of the insulated suspended floor will stay close to yearly-average temperature, so will collect condensation from the ventilation air, if the dewpoint of external air in midsummer is warmer than that. (this bit is nothing to do with permeation through the floor)

    For Washington DC the year-average temperature is lowish 12C (snows in winter) but the mid-summer dewpoint is very high 21C (US Capital was deliberately built in a hot swampy place). As 21 is much more than 12 there is a condensation risk.

    For me the year-average temperature is 9C and the midsummer dewpoint averages about 11C so theoretically there is a slight risk of condensation, but in practice I suspect the timber will always warm up by a couple of C during summer and that will take care of it. During winter the air dewpoint is about 3C so that will dry the timber out nicely.


    Well said. Most seem to be overlooking the fact that the paper refers to a totally different climate to the UK.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 16th 2012
     
    Mike, don't you think that half the damp dankness in underfloors is due to moisture imported and condensed from the through-ventilation? We're used to most so called rising damp in fact being condensation at the coldest point - how wd that not also apply in the underfloor? What's the point of the ventilation anyway, apart from 'tried and tested'?
    • CommentAuthortony
    • CommentTimeMar 16th 2012
     
    Because a house is loosing heat the underfloor void will be warmer than outside nearly all the time and so condensation of outdoor air under will be extremely rare

    On the other hand warm and consequently with more moisture in it air from inside the house will tent to want to condense there.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 16th 2012 edited
     
    Posted By: tonyBecause a house is loosing heat the underfloor void will be warmer than outside
    Used to be true, but now minimal, given significant insulation - and that readily overcome by evaporative cooling esp in spring/autumn (the UK danger time), which alternates or is concurrent with condensation in the ventilated void.

    Posted By: tonywarm and consequently with more moisture in it air from inside the house will tent to want to condense there
    Not if there's a VCL.

    Whatever, the benefit of the through ventilation is somewhere between irrelevant and actually a possible problem - so IMHO it's best eliminated. Then other benefits of the underfloor and/or subsoil can be brought into play, and hopefully the need for floor VCL eliminated too.

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