Green Building Bible, Fourth Edition |
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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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Posted By: squowseIf I used the inside osb as the structure then I would be insulating from the outside.
Posted By: squowseI did consider that but the osb is required for panel strength.
Posted By: Viking HouseThere's never been a proven case of structural decay in a building caused by water vapor diffusion so I wouldn't get too hung up on breath-ability, concentrate on air-tightness. SIPs Panels have 12mm OSB on both sides and the only decay found, on an internet search, was where 2 SIPs Roof Panels were joined without air-tightness tape.
If it makes it easier to build there's loads of houses built with OSB externally and Plasterboard internally. There's been no large claims against timber frame companies regarding decay from lack of breath-ability.
Posted By: Viking HouseThe Swedish houses had air-tightness problems, not breath-ability problems, it wasn't caused by fitting the OSB on the wrong side of the stud.
Posted By: Peter_in_HungaryI don't think that OSB can be relied upon to provide a VCL by itself. I recall previous discussions here that OSB vapour permeability is inconstant and varies between batches.
Posted By: SimonD But essentially the moisture got into the frames causing excess moisture buildup within the frame. The moisture didn't have anywhere to go, thus the damage to the frame. You call it what you like, but in the reference texts, it's not referred to as airtightness problems, but rather one of vapour control and/or breathability - in Swedish it's referred to as ångspärr or ångbroms.Was moisture ingress by lack of air-tightness ruled out by an air-tightness test and was it proven that the moisture build-up in the frames was caused by water vapour diffusion? If that's the case then it's the first time I've seen decay in a wall caused by water vapour diffusion, WUFI for example pays little heed to it.
Posted By: SimonDWhat we do know is that if you do build an airtight building that is not breathable (e.g. using a plastic vapour barrier) you will get moisture problems within the building unless you also provide sufficient ventilation. If you build a building that is breathable, you also need ventilation, but necessity of ventilation for moisture reduction is significantly reduced.Passivhaus says if you build a house with an airtightness level of 1ACH or less, the wall build-up is irrelevant. The Fraunhofer study shows the moisture buffering ability of hygroscopic materials is reduced by 80% behind unpainted plaster-board, so any moisture buffering by sheepswool on the far side of an airtight OSB is minimal. The Spokane/Tsonga study on retrofitting existing un-insulated Timber Frame houses that had no VCL with blown-in insulation showed no signs of moisture or decay in the walls or the outer OSB. The 1:5 wall breath-ability (not 5:1) made little difference.
Posted By: SimonDBut regardless of this and to clarify my suggestion re the op further, I am doing so from a practical building and cost perspective, because why use two layers of what is now an expensive material when you don't have to and why add to the complication of the wall buildup when you don't have to? The benefit of this is also that it reduces risks of moisture buildup within the frame and it takes advantage of the chosen insulation material and its properties. The choice is also to build a more traditional modern timber frame like I've illustrated above, but by all means, if you prefer to spend more time and money unnecessarily, that's fineI agree with keeping it simple but disagree with every discussion on wall build-up centering on breath-ability when air-tightness is 100 times more important.
Posted By: Viking House
Here's the Spokane and Tsongas field studies where the walls of 200 houses were opened and checked for signs of decay;
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