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    • CommentAuthorsquowse
    • CommentTimeSep 24th 2021
     
    Hi. Long time since have been active on here - hopefully will be able to join in more again.

    I'm constructing a timber cabin - exterior wall consists of (from inside to outside)

    1) 9mm OSB
    2) 100mm Sheepwool Insulation between studs
    3) 9mm OSB
    4) Glidevale TF200 Protect Breather membrane
    5) Vertical and HorizontalBattens
    6) Vertical Timber Cladding

    Just concerned about breathability. I'm under the impression that an internal vapour control layer is required, and that OSB is about the right permeability if the joints are sealed. So proposing to do this on layer 1)

    As regards the outside membrane, this should be more permeable than the internal VCL. I was thinking of gluing the OSB 3) for strength. This would seal it quite well and I'm thinking I need some holes drilled in it to give it some breathability.

    I'm probably overthinking this but want to get a nice breathable air-tight structure.

    Thanks
    ROger
    • CommentAuthorSimonD
    • CommentTimeSep 25th 2021
     
    Hello Roger,

    Scrub the outside layer of osb, this isn't needed. I'd suggest going for 11mm osb on the inside - tape the joints and corners, your wool, breather membrane, battens and cladding. I think the Glidevale TF200 is designed to sit on an osb substrate. Instead you can use any generic breather membrane. If you're concerned about compatibility Thermafleece's membrane is suitable for roof and wall applications, For extra tightness you can also tape the membrane overlaps. No need for glue, just use 50mm nails at 150mm spacing around the edges of each osb board and 300mm elsewhere.

    HTH.
    • CommentAuthorsquowse
    • CommentTimeSep 25th 2021
     
    I did consider that but the osb is required for panel strength. If I used the inside osb as the structure then I would be insulating from the outside. In the current weather conditions /season that's not an option but I may try it in future if I can prefab the panels under cover.
    • CommentAuthorSimonD
    • CommentTimeSep 26th 2021
     
    Posted By: squowseIf I used the inside osb as the structure then I would be insulating from the outside.


    Not at all:

    1. build your stick wall sections and raise them into place.
    2. square each section, fixing cls diagonal bracing to maintain squareness and temporary racking resistance.
    3. build your roof frame and roof covering (temporaray or not)
    4. wrap the outside of your walls with membrane and if you choose you can also batten for the cladding (you can even use tarpaulin for temporary wrap if you prefer)
    5. go inside and install your insulation and once done, install the internal osb sheathing boards, removing the diagonal cls bracing as you go.
    6. tape up your osb joints and corners and you're done.


    Posted By: squowseI did consider that but the osb is required for panel strength.


    You only need one layer of osb for racking strength and if you install osb to both the inside and outside you will need to reduce vapour permeability at the internal face of the wall. You would need an additional vapour barrier to the inside of the inside layer of osb, so you're adding unnecessary complication to your wall buildup. You're also adding risk of condensation and moisture buildup within the frame structure with your proposed double osb buildup, even with the moisture buffering capability of the sheepswool.
    • CommentAuthorphiledge
    • CommentTimeSep 26th 2021
     
    Going back 20 years when we built in timber frame there was a rule of thumb that the the outer skin had to have 4 times the vapour permeabilty of the inner skin, on the warm side of the insulation( i think it was 4 times but may have been more). As already suggested, OSB inside and out could cause problems??
  1.  
    There'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.
    •  
      CommentAuthordjh
    • CommentTimeSep 26th 2021
     
    I used to worry about the inside having greater vapour resistance than the outside, but then I built with lime on both the inside and outside of my straw bales (some people even use clay on the inside) and I decided not to worry. This year (six years on) we repaired some cracks on the outside and the straw appeared dry immediately inside the lime, even at the cracks. So now I don't worry at all. (Well, not about that!)

    But that's just one data point.

    SimonD's suggested build method sounds reasonable though. For myself I just used 18 mm OSB wherever I needed OSB, to simplify ordering and stock.
    • CommentAuthorSimonD
    • CommentTimeSep 26th 2021
     
    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.


    There are actually quite a number of houses of a certain modern timber frame generation built in Sweden suffering decay within the timber frames for this reason. The typical problem has been found to be due to breaks in the vcl and thus moisture buildup within the frame. I suspect we simply don't build timber frame houses to such a scale in the uk to really know about this problem, apart from some of the early tf buildings from a few decades ago that decayed like 1980s Italian cars! :wink:

    Vcl and airtightness layers are typically one and the same in modern timber frame buildings e.g. Intello Pro Clima

    SIPs panels are often foam/eps core so are a completely different buildup compared to the op but the issue you've mentioned above illustrates the risk of moisture ingress.
  2.  
    The 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. Generations of Irish and Scottish TF houses were built with External OSB and they're still standing. SIPs is a form of Timber Frame construction with OSB on both sides and no issues with breath-ability.
    • CommentAuthorSimonD
    • CommentTimeSep 28th 2021 edited
     
    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.


    What I'm proposing is to do with using the osb as your airtightness/vapour control layer and not 'what side of the frame' the osb is installed on. This depends on the whole buildup of the wall system, not just one component and where it lies. As I'm sure you well know the typical buildup of a timber frame with osb on the outside is to use a vapour barrier/retarder on the inside and has not been traditionally designed as a breathable wall system.

    It's an interesting differentiation you're making given that they're different processes from a building physics perspective. The differentiation of these processes isn't without its problems as the all work together for a healthy building. 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 contro and/or breathability - in Swedish it's referred to as ångspärr or ångbroms.

    What 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.

    But 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 fine :wink:
  3.  
    I 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.
    • CommentAuthorSimonD
    • CommentTimeSep 29th 2021
     
    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.


    I agree. If I was building a passivhaus, I'd probably opt for the now special Air/Vapour barrier osb on the market unless I found that this negatively affected the hygroscopic nature of the boards in some way. In practice, if you have hygroscopic materials as the buildup such as the sheepswool above, I think the vapour retardance properties of normal osb suffice, but additional insurance could be thicker boards. Another option could be clt.

    If I were to use a material without capillary function like mineral wool, or cellotex, then I believe a vapour dedicated VCL/airtight layer is a necessity.

    As always, I think it depends on the wall/roof/floor system being employed and the system's overall hygrothermal performance.
  4.  
    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 fine
    I 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.
    • CommentAuthorSimonD
    • CommentTimeOct 4th 2021
     
    Do you have links to the research you refer to? This paper (amongst others), does, for example, call much of that into question. Whilst it focuses upon mould growth in timber frame built to ph standard, it very much demonstrates the importance of wall buildup. https://core.ac.uk/reader/82393645

    If you don't want to read the whole paper, here are some salient points from the discussion:

    "The hygrothermal performance of the timber-frame external wall was found to be dependent most of all on the thermal resistance of the wind barrier and vapor permeability of the wind barrier and vapor barrier layer. The importance of air&vapor barrier layer can be seen, if to compare points AP1 (wall with PE-foil air & vapor barrier layer) with AP3 (without air&vapor barrier layer) –the absence of a vapor resistant layer contributes to higher moisture load from the indoor side, because of vapor pressure differences during the majority of a year, and therefore to higher mould growth risk in the structure. In the case of constructions that have been insulated with non-capillary mineral wool, a vapor barrier has to be included to keep moisture load below critical.

    In the comparison of insulation materials – mineral wool and cellulose insulation –the cellulose insulation showed lower mould growth risk than mineral wool in conditions of cold climate. This was mainly due to the differences of moisture capacity and capillary moisture transfer properties of insulation materials. Cellulose insulation has higher moisture storage capacity than mineral wool and therefore moisture increase is slower. Therefore, the wall remains in the dry condition longer and RH stays lower. If the originally designed by engineer wind barrier (for example, wall type 3 of the studied house) would be combined with mineral wool insulation, it would lead to serious mould growth (mouldindex M|4). Therefore, it is needed always to perform the hygrothermal analysis before changing any materials . Andersen et al. [21]showed thetendency that the moisture content behind wind barrier, measured in the north-facing facade element with mineral wool, was higher than in the facade element insulated with cellulose insulation.Pihelo and Kalamees [20]compared mineral wool and cellulose wool behavior in timber-frame structures and pointed out that mould formation risks are considerably lower with cellulose insulation and when a proper air&vapor barrier layer is installed .Therefore, capillary transport properties and moisture capacity of wood and paper based materials should be exactly known and considered."

    So WUFI doesn't pay heed to moisture build up over time in wall and roof assemblies? I'm very surprised as I thought that was exactly what it is designed to model, especially given how we're nowadays typically building homes with less permeable envelopes. But maybe it just doesn't cover capillary moisture transfer very well?
    • CommentAuthorphiledge
    • CommentTimeOct 4th 2021
     
    Not knowing what WUFI is, I googled it and the WUFI home page says-

    "WUFI® software uses the latest findings regarding vapor diffusion and moisture transport in building materials"

    Whether they consider vapour diffusion or ignore it I guess depends on what the latest findings were!
  5.  
    No WUFI pays little heed to internal water vapour, Moisture build up inside a wall is most likely coming from rain, the second most likely cause is from lack of air-tightness with internal water vapour falling into the highly unlikely category. We can agree that cellulose wicks away moisture better than rockwool but it didn't mention where the moisture came from.
    Here's the Spokane and Tsongas field studies where the walls of 200 houses were opened and checked for signs of decay;
    http://www.viking-house.ie/downloads/Tsong79.pdf
    http://www.viking-house.ie/downloads/Spokane.pdf
    The walls were built with the 5:1 ratio going in the wrong direction with the outside layer 5 times less breathable than the inside layer, no sign of decay was found other than from a few leaks.
    • CommentAuthorSimonD
    • CommentTimeOct 6th 2021
     
    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;

    Thanks for those. I shall read and digest. Another couple of bits to add to the picture of conflicting findings. On skimming, one thing I did notice which is positive is that they are careful to highlight regional/local climate playing a role. Which seems sensible

    Intrestingly, a similar picture is coming together re Etics where in Norway they now provide a chart for windriven rain recommending in certain areas that thin coat render is not to be used, but instead a rain screen. In Finland, after numerous failures of the render, they've derived a national test in addition to the ETAs that tests specifically for the ability of the render to cope with rain/frost cycles which are common in Finland but the ETAs for these products doesn't test for this.
    • CommentAuthorsquowse
    • CommentTime7 days ago
     
    Thanks for all your comments. Building without OSB on the outside ia certainly an interesting idea and could be done with temporary bracing as described. I'd probably use metal band.

    I have built the cabins with the idea of the being liftable so the additional strength of osb top and bottom on the roof is welcome.


    For this build I've put 20mm holes - about 6 per 1/4 sheet "compartment" formed by studs/rafters and noggins. Hopefully any water vapour will find its way out.
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