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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorMike George
    • CommentTimeApr 19th 2007 edited
    Hi all, I am working on some figures for a publication and I want to check that my understanding of conversion rates is correct.

    Here goes:
    Part L1 states guidance figures of 10m3/m2/hr @50Pa for new housing
    [whole building volume] [m3] per m2 of external fabric [in contact with outside air] per hour @ 50Pa

    The software I am using works on values inputted in ach [air changes per hour] @ 0Pa

    Converting the requirement to air changes per hour is buiding specific
    As a simple example 1ach for a building 200m3 with a surface area of 100m3 would be 200/100 = 2m3/m2/hr @ 0Pa
    To convert to values at 50Pa multipy x 20 = 40m3/m2/hr

    Therefore in order to comply with Part L1 this example would require a maximum air infiltration rate of 0.25ach

    Is this correct? In particular the rule of thumb figure of 20 to convert from 0Pa to 50Pa?
    • CommentAuthorGuest
    • CommentTimeApr 19th 2007
    From Paul T,

    Hi, I am performing Air tightness tests and SAP work now.

    you are correct on the calcs.

    Unfortunately PArt L1, SAP and ATTMA (the air tightness guys) have got in a mess!

    We should all be using the internationaly accepted copnvention - air changes per hour. - [Whole building air change rate][m3] per [Volume][m3] per hour @ 50Pa
    (Permeability is just an average performance indicator of the building envelope)

    The rule of thumb to convert to normal operation (4 Pa average assumed) is 1/20. If however you want tto get the real value use:
    1/10 for a sheltered site
    1/20 for 'average'
    1/30 for exposed

    Imagine this for a block of flats. Regulations use 1/20. The basement flat will be half this and the top floor + 50%.

    How significant are these errors?
    For a 60m2 square flat, 2.4 m high the difference between the surface area and Volume will be 40% This means a building with n50 =14 ach could pass the regulations (Permeability, q50=10). For a high rise flat then use the 1/10 rule. the heat loss due to infiltration will then be 180% greater than assumed in SAP! (even for the closest that SAP gets, a basement, it is still out by 33%). Typicaly ventialtion heat loss is around 30-40% for a new build
    As government predictions are for 60% of new homes to be flats it negates all of the Part L improvements in 2006!
    To scale up from background ach/hr to the air ach/hr @ 50pa you multiply the background ach/hr by 20, to the best of my knowledge this scaling does not work for m3/m2/hr@50pa.
    Thanks for the help Paul, I was hoping you might reply. The figure of 20 is okay for the site in question.
    • CommentAuthorMike George
    • CommentTimeApr 19th 2007 edited
    Also thanks Mark, Can you explain a little more? Do you mean the way I have calculated the conversion is incorrect?
    • CommentAuthorGuest
    • CommentTimeApr 20th 2007
    I went astray when I read your posting. Sorry if I've caused confusion.
    I can now see the intuative leap that you made 40m3 = 4 times more than B.Regs therefore divide by 4. The thing is that this intuition in unlikely to work when the air permiability target is say 3.5m3/m2/hr@50pa or when the volumes are not quite so nice and round. Having worked your calcs from first principles: -

    If you building had an air leakage of 1ach per hr at 'zero' pa (as your post) then to scale up to 50pa = 1 x 20 = 20ach per hr
    20 x 200m3 = 4000m3 per hr @ 50pa
    4000 / 100sqm = 40m3 per m2 per hr @ 50pa (as you say, however this fails the B.Regs some what)

    However working backwards:-
    100sqm x 10m3 per m2 per hr @50pa = 1000m3/hr
    1000m3 per hr / 200m3 = 5ach per hr @ 50pa
    5ach per hr / 20 = 0.25ach at background pressure

    This agrees with your conclusion.

    • CommentAuthorMike George
    • CommentTimeApr 22nd 2007 edited
    Thanks Mark, I see where you are coming from now.
    • CommentAuthorGuest
    • CommentTimeApr 27th 2007
    Paul T,

    I am interested in air permeability so I was looking at these posts. Are the sheltered and exposed figures not given the wrong way round? In that sheltered 1/10 gives a greater ach?


    Steve H
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