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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorGreenfish
    • CommentTimeSep 13th 2014 edited
     
    In another thread about air test experiences this came up

    Posted By: PeterStarckApparently high levels of airtightness affect the level of moisture in the wall more than heat loss. I asked this question on the AECB Forum.

    http://www.aecb.net/forum/index.php/topic,3928.0.html" >http://www.aecb.net/forum/index.php/topic,3928.0.html

    Realy interesting Peter (and Tom), however I am not sure I understand it. I have masonry walls with insulated cavity, wet plastered inside and rendered outsde or cedar cladding in places. How does more or less air permeability affect these walls? The wall is air tight, the edges (floor, ceiling, window reveals etc.) and any penetrations could leak, or I could just open a window. Humidity is driven by what it is outside despite MVHR window open or not. Position of insulation means not going to suffer from intersital condensation.

    How much heat I lose from air leaks depends on how windy it is, but the cost difference between a sub 0.6 air permeability and one twice as much (but still pretty good) is not noticeable.

    So sub 1 (cubic metres per hour per square metre of envelope area) air permeability is desirable because.... it seems perceived comfort is the answer. Can we really perceive the comfort difference between a 0.6 and say a 1.2 house?
    • CommentAuthortony
    • CommentTimeSep 13th 2014
     
    yes you will in your pocket, probably not if you have a thermostat unless it is very windy. In Canada or Scandinavia you may hit problems with ice build up in you walls, much greater air tightness is common there.

    do you mean by perceive feel warm with no draughts?
    • CommentAuthorGreenfish
    • CommentTimeSep 13th 2014
     
    Posted By: tonyyes you will in your pocket, probably not if you have a thermostat unless it is very windy. In Canada or Scandinavia you may hit problems with ice build up in you walls, much greater air tightness is common there.

    do you mean by perceive feel warm with no draughts?

    But that's the point Tony, I doubt if I would notice a difference in energy consumption, thermostat or not, between 0.6 and 1.2, best I can model it anyway. OK, if was not in balmy Cornwall then intersitial condensation and icey walls could be an issue. But I thought that in a masonry build more air flow was good to keep things dry? Confused!!

    When I talk about the perceived comfort factor I think it is not just about obvious draughts. Old house does not feel draughty, I don't notice any air flow on a windy day, but it was not designed to be air tight so I am sure it is average for a building of 1929. But new house feels more comfortable at lower room temps. My question is how far do we need to go with air tightness for comfort. I can get wrapped up in the best number quest with the rest, but just pausing to examine why PH values are desirable?
    • CommentAuthorEd Davies
    • CommentTimeSep 13th 2014
     
    You could have a house with zero air leakage as shown on the blower door test at the end of construction but which has dreadful airtightness problems with leaks everywhere bypassing almost all the insulation, only stopped by what tony calls a plasterboard tent.
    • CommentAuthorGreenfish
    • CommentTimeSep 14th 2014
     
    Posted By: Ed DaviesYou could have a house with zero air leakage as shown on the blower door test at the end of construction but which has dreadful airtightness problems with leaks everywhere bypassing almost all the insulation, only stopped by what tony calls a plasterboard tent.
    That is true with plasterboard, and a reason to go wet plaster like we have. But good point, a good blower test does not tell the whole story. How could buildings be tested for this problem?
    • CommentAuthorEd Davies
    • CommentTimeSep 14th 2014
     
    Even with wet plaster the insulation behind could be being bypassed. The outer layers don't need airtightness as such but they do need good protection against windwashing. How to verify that is a puzzle.
    • CommentAuthortony
    • CommentTimeSep 14th 2014 edited
     
    Thermal imaging picks up plenty of problems. This one is of a lounge in a fairly airtight house, clearly a lot of thermal bypass going on.
    • CommentAuthorEd Davies
    • CommentTimeSep 14th 2014
     
    Good point, but it's a bit late then. You really want to pick up the problems before it's all sealed up and the house is heated.
    •  
      CommentAuthordjh
    • CommentTimeSep 19th 2014
     
    Posted By: Ed DaviesGood point, but it's a bit late then. You really want to pick up the problems before it's all sealed up and the house is heated.

    I don't know of any better technique than thermal imaging to detect heat loss, and for that to work you need a temperature difference. So I think the answer to your quandary is to heat the house before it is all sealed up, but clearly you do need it pretty airtight. The slightly less sure and more glib answer is of course 'good design' and 'good build'.
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