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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorEd Davies
    • CommentTimeMay 26th 2017
     
    Posted By: djhgoodevans already explained that the test pressure isn't especially connected to anything at all. In fact the results stated at a test pressure of 50 Pa are actually calculated by regression from results measured at a range of pressures. It's just a standard value for tests.
    Of course. But if we want to get to some notion of what values we should be aiming at then it makes sense to try to work out how it relates to real-life performance.
    • CommentAuthorgoodevans
    • CommentTimeMay 26th 2017
     
    For modern build with reasonable insulation I agree that stack has a significant affect, as does wind infiltration. Without good real life research it would be difficult to separate the effects of stack against that of regular insulation. (You would need 2 identically insulated and heated test buildings - one airtight, one not and several consecutive cold days with hardly any wind).

    I have just realised that the losses due to stack affect approximately increase with the square of temperature difference (increase pressure with temp and lower air temp infiltration).

    As usual we have to guess given the present research available.
    •  
      CommentAuthordjh
    • CommentTimeMay 26th 2017
     
    Posted By: fostertomis a bungalow with walk-height loft, or a v skimpy flat roof 2 storey. 2 storey with loft more like 7m, so 14C - not just deep winter. A 2-storey maisonette above shop say with GrdFl entrance would be 9m, 11C.

    And you've neglected that the infiltration is not all at the exact bottom and top of the building, but is instead probably distributed rather randomly up the elevations.
    •  
      CommentAuthordjh
    • CommentTimeMay 26th 2017
     
    Posted By: Ed Davies
    Posted By: djhgoodevans already explained that the test pressure isn't especially connected to anything at all. In fact the results stated at a test pressure of 50 Pa are actually calculated by regression from results measured at a range of pressures. It's just a standard value for tests.
    Of course. But if we want to get to some notion of what values we should be aiming at then it makes sense to try to work out how it relates to real-life performance.

    Yes, but just trying to causally associate one magnitude with another is crazy.

    I expect the whole of this issue is dealt with at length in refereed papers, some good and some poor, so there's very little point in trying to reinvent the wheel.
    • CommentAuthortony
    • CommentTimeMay 27th 2017
     
    I like the idea of a standard method for measuring and reporting air tightness 50Pa, the ACH/m3/m2.h both have problems and interpretation issues and bid buildings automatically get good results on the latter.
    • CommentAuthordimengineer
    • CommentTimeMay 27th 2017 edited
     
    There are some comments on here about what pressure are good/bad. I work in the Pharmaceutical industry, where we have pressure differential (cascades) between clean rooms. 15Pa is the target differential. When you get to 30Pa, opening doors starts to get a bit of a struggle. 60Pa - which is the differential beteen a clean, clean room (Class B) and the outside makes opening doors a real problem. You have to start being careful about ceiling and wall strengths as 60Pa is 60N/m2 - which sounds low, but for a suspended ceiling can pull it down, especially if the pressure control fails!

    Just something to think about
    •  
      CommentAuthorfostertom
    • CommentTimeMay 28th 2017 edited
     
    Posted By: djhyou've neglected that the infiltration is not all at the exact bottom and top of the building, but is instead probably distributed rather randomly up the elevations
    But still overall height strongly indicates the order-of magnitude of stack effect; and that what may be significant in mid winter for a bungalow is significant in milder weather for a 3-storey.

    Though goodevans calc'd one tenth of the blower test (medium breeze) effect (informative - thanks), the point is it keeps going 24/7 rather than intermittent.
    • CommentAuthorgoodevans
    • CommentTimeMay 28th 2017
     
    It's also not easy to detect - unlike wind infiltration on a windy day the draft can be felt - so it is easy to appreciate, But feeling the affect of stack is not easy - I'm fairly observant about unexpected physical effects about me and other than a roaring open fire I don't think I've ever felt the effect in a regular house (but I don't doubt it's real).

    I have a sneaky feeling that it the stack effect that moves most of the moisture into roof spaces via cracks, holes etc (as opposed to permeating though plasterboard). And would explain why many condensation issues show up in the roof space. On still days this would be the dominant air flow - with no wind assisted ventilation to make matters worse as well.
    •  
      CommentAuthorfostertom
    • CommentTimeMay 29th 2017
     
    Good thinking
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