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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.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorjfb
    • CommentTimeJan 16th 2021
     
    Posted By: fostertomIntello etc may have a role in the very different climate of central Europe but is best omitted (and its high cost) altogether (as well as 'dumb' vapor barriers) in maritime UK.


    Ignoring any questions about moisture control membranes like that provide the airtight layer as well you so either you need something like it or something else - sheets of OSB aren't going to be much cheaper.
  1.  
    The sizes of the oxygen, nitrogen and water molecules are all about the same, 0.3nm.

    Their speeds are also similar - ideal gas law gives that their kinetic energies are the same at any given temperature, so their average speed depends on √(3/mass), which is not wildly different between those molecules.

    Their different permeability in different substrates, is not actually anything to do with speed or size, though that's the mental model that everyone gets taught at school! It's more of a chemical thing, certain gases 'stick' to certain substrates so don't pass through as readily as others. The 'stickiness' can be tweaked by temperature or concentration, like in an adsorption dehumidifier. The mental model is like how a crowd of school kids in a supermarket permeate through the confectionery and laundry aisles at different speeds.

    Air tightness is also nothing much to do with permeability or diffusion, the driving force (partial pressure of nitrogen) inside and outside the house is pretty much the same as was mentioned, unlike water vapour where partial pressure is often double inside than outside.

    Air tightness is about avoidance of macroscopic holes. As was mentioned, water and oxygen diffuse through certain plastics, but not aluminium. But a plastic sheet is as airtight as an aluminium foil if there are no tears or plumber-holes.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 16th 2021 edited
     
    Posted By: jfbsheets of OSB aren't going to be much cheaper
    Sheets of 11mm OSB3 sheathing are cheap as chips and cut like butter; butt jointed and gapfilling glued and screwed or power-nailed at every joint are easily made 100% airtight at the joints without meticulous care, by ordinary chippy skills, and are then robust indefinitely against fatigue/weathering - all quite unlike expensive membranes and tapes of notorious difficulty and rapid degradation. OSB is adequately vapour permeable and airtight, and if backed up by blown-in cellulose fibre (Warmcel) between the studs/rafters which itself gives pretty good airtightness, maintains some kind of airtightness even if later cut or damaged. No contest.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 16th 2021
     
    Posted By: WillInAberdeenThe mental model is like how a crowd of school kids in a supermarket permeate through the confectionery and laundry aisles at different speeds
    Brilliant!
    • CommentAuthorLF
    • CommentTimeJan 16th 2021 edited
     
    Will. No real arguement from me. A hole will lead to bulk air movement.
    I have used and designed membranes for gas separation in industrial processes for 25 years. If one tears they do not work very well as you get bypassing.
  2.  
    :-)
    In an early life I used a membrane stack to collect nitrogen from air, the waste gas contained most of the oxygen, it was a great hangover cure!
    •  
      CommentAuthorfostertom
    • CommentTimeJan 16th 2021
     
    Posted By: LFIf one tears they do not work very well as you get bypassing
    Yes, that's important to understand. If a membrane is 99.9% perfect, the 0.1% will have a totally disproportionate effect, to the point where if say 95% perfect it might as well not be there at all (in principle, those numbers being just illustrative).
    •  
      CommentAuthordjh
    • CommentTimeJan 16th 2021 edited
     
    Posted By: fostertomSheets of 11mm OSB3 sheathing are cheap as chips and cut like butter; butt jointed and gapfilling glued and screwed or power-nailed at every joint are easily made 100% airtight at the joints without meticulous care, by ordinary chippy skills, and are then robust indefinitely against fatigue/weathering - all quite unlike expensive membranes and tapes of notorious difficulty and rapid degradation. OSB is adequately vapour permeable and airtight

    It depends what your purpose is. If you think back a few years there was a brouhaha about the airtightness of different brands of OSB, and also questions about the water vapour permeability. There's a summary at https://www.scrantonpassivehouse.com/musings-of-an-energy-nerd/

    They led to the production of products with declared air and water vapour permeability, such as Smartply.

    edit: Here's a sample quote from the link: "Only one of the tested brands of [18 mm] OSB met the Canadian standard for an air-barrier material; the other seven tested brands were too leaky to pass the standard"
    • CommentAuthorLF
    • CommentTimeJan 16th 2021
     
    Will, I ran a fleet of 200 large nitrogen membranes and was technical manager for them as well. We could get 40%0oxygen on waste streams. Cryogenic nitrogen made some 70%oxygen which was an even better hair of the dog but not advisable for smokers.
    Industrial gas separation membranes is all to do with coatings on the polymer frame. Permeating gases sort of go into solution as they pass through coating. It also blocks up holes in polymer.
    djh has good points about different grades and links sound interesting.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 16th 2021
     
    Posted By: djhIf you think back a few years there was a brouhaha about the airtightness of different brands of OSB, and also questions about the water vapour permeability. There's a summary at https://www.scrantonpassivehouse.com/musings-of-an-energy-nerd/

    They led to the production of products with declared air and water vapour permeability, such as Smartply.
    That's true, but I rely on having being told by Peter Warm, one of the two qualified UK PH Certifiers (at that time) that "I've never had a airtightness fail due to use of OSB as the airtight layer".

    When I did my WUFI course in Dublin, a boffin from Coillte, the producer of Smartply OSB, was there on the course. I discussed with him the need for an OSB product of guaranteed air permeability (uncontrolled by the industry, alegedly variable from batch to batch) as well as guaranteed water vapour permeability (stated in product literature), and he said he would invite me on a panel to help Coillte decide what to do about the issue - but I never heard any more, and emails unanswered.

    So what did they do? Proudly came out with plastic coated Propassiv OSB of guaranteed zero water vapour permeability, as well as airtight! Which misses the point and is exactly useless for 'breathing' construction.
    •  
      CommentAuthordjh
    • CommentTimeJan 16th 2021
     
    Posted By: fostertomThat's true, but I rely on having being told by Peter Warm, one of the two qualified UK PH Certifiers (at that time) that "I've never had a airtightness fail due to use of OSB as the airtight layer".

    That specific is mentioned in the article, if you care to read it. Peter has been lucky.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 16th 2021
     
    Thanks Dave, i have now read it, like a good boy. i thiought you meant by 'that specific' that it was examining Peter Warm's statement.

    I'm not sure how to calc what the volume of such a balloon (sliver of a sphere) would be. But if 1m2 was strongly taped firmly flat without slack, I wonder how many mm it would balloon out at the centre, presumably by stretching. Say 50mm. At a guess that might be 1.6 litres. At the 0.02 liters/sec-m² of the apparently 'tough' Canadian standard, that would take 95secs; at 0.037 it's 43secs. I wonder how many secs their "immediately" was.

    My guess is that the OSB causing this 'trouble' wasn't all that far off the standard. It took more than halving its permeance, to much better than 0.02, by adding a pukka barrier's resistance, to get the house's loss down to PH. Seems to me there must have still been quite large losses elsewhere, despite their chasing round, but the OSB got more than its fair share of blame.

    I always (so far) back the OSB up with blown-in cellulose fibre (Warmcel) between the studs/rafters, which itself gives pretty good airtightness, tho (like the OSB) the manufs quote no figures.
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