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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorrsk1
    • CommentTimeMar 5th 2021
     
    I am designing a timber frame building: internal osb , stud frame, wood fibre between and on outside of studs, membrane, battens, timber cladding. The internal osb with taped joints is the vapour barrier, as per pavatex recommendations. My question is: why does a breathable wall need a vapour barrier?

    I've often wondered the same thing about cold roof insulation on rafters: if there is good ventilation around rafters, why is it important to have a VB on the warm side?
    • CommentAuthortony
    • CommentTimeMar 5th 2021
     
    If you allow moisture to travel from the air in the house (which will have higher partial vapour pressure of water) into the wall there is a significant risk that as the temperature in the wall falls towards the colder outside moisture levels in the insulation could rise with disastrous results
    • CommentAuthortony
    • CommentTimeMar 5th 2021
     
    In Canada they darent have any pin pricks in the vb as it freezes and icebergs form which can force off the cladding!
    • CommentAuthorSimonD
    • CommentTimeMar 5th 2021
     
    To be ever so slightly pedantic, it isn't a vapour barrier but a vapour control layer. Vapour barriers are typically as good as vapour impermeable, whereas vapour control layers aren't. In a breathable, hygroscopic wall system the materials are able to buffer moisture within and through the wall fabric. However, too much moisture through a lack of vapour control would be bad for these materials, perhaps even leading to newly formed icebergs in some climates..
    •  
      CommentAuthordjh
    • CommentTimeMar 5th 2021
     
    Posted By: rsk1My question is: why does a breathable wall need a vapour barrier?

    As Tony and Simon have said, the problem is not so much with blocking all moisture as with ensuring that whatever moisture does get to wherever in a wall doesn't reach concentrations where it causes a problem. That means some combination of reducing the amount of moisture that gets there, ensuring there's no damage at the place whilst there is moisture there, and ensuring that moisture can evaporate/remove itself/dry from wherever it is.

    In a breathable construction in a cooling climate it's often said that the inside should be more vapour resistant than the outside. Since I live in a house with lime render both inside and outside and straw bales between, I hope that isn't really required :bigsmile: A lot depends on the local climate.

    Regarding your roof question, it's a matter of doing the sums and making sure water doesn't build up anywhere in the construction. With traditional engineering techniques, the easiest way to guarantee that is to limit the quantity of water reaching the timber in the roof.
  1.  
    Hi rsk1, concentrate on airtightness and don't get too hung up on breath-ability, I've built numerous houses with the build'up you're considering and learned the hard way. Air-tightness is 99 times more important than Breathability! Good Air-tightness (1ACH) drastically reduces the movement of moist air towards the outside of a wall so there's never moisture present at the Dew-Point to freeze. I've stuck moisture meters 10mm into numerous externally insulated houses where the Dew-Point is supposedly and never encountered moisture.
    Plasterboard on a Wood Fiber Insulated wall reduces it's moisture buffering effect by 80% and then you paint it!

    Regarding your roof query you're correct, if there's enough ventilation you don't need a VB but it reduces the risk of condensation on the OSB, I've encountered spongy OSB roofs that didn't have sufficient ventilation, I've also encountered a roof with a bulging membrane that was full of water from a roof leak.
    So I probably prefer warm roofs without OSB directly beneath the Fiberglass or Parlon.
    •  
      CommentAuthordjh
    • CommentTimeMar 6th 2021
     
    Posted By: Viking HousePlasterboard on a Wood Fiber Insulated wall reduces it's moisture buffering effect by 80%

    Not sure what you mean by this?

    Posted By: Viking HouseRegarding your roof query you're correct, if there's enough ventilation you don't need a VB but it reduces the risk of condensation on the OSB

    What OSB?
  2.  
    Possibly the taped OSB in the wall, which was described in the OP as a vapour barrier, is actually the airtight layer? So moisture can diffuse/permeate freely in and out of the osb and woodfibre and be buffered, and escape outward through the vapour permeable membrane, but nice warm air cannot leak out carrying moisture with it.

    Condensation doesn't necessarily happen at the dewpoint in a wall - if the vapour is free to carry on diffusing/permeating/convecting outward to an even colder or dryer place, it prefers to do that, rather than condense. It will only condense it it meets a vapour-resistant layer somewhere outboard of the dewpoint.

    RSK, no need for a vapour barrier in a cold roof if as you say there is plenty of ventilation round the rafters. IE ceiling-insulation-airspace-rafters-waterproofing, with a good breeze through the airspace, like in a loft. However the term 'cold roof' often refers to a flat roof, with the ceiling fastened to the underside of the rafters and the deck on top of them, so minimal ventilation, so vapour really has to be kept out. They're banned now in Scotland, don't know about anywhere else.

    DJH, the deck of a cold flat roof, to carry the waterproof layer above the insulation, was usually OSB.
    • CommentAuthorSimonD
    • CommentTimeMar 6th 2021 edited
     
    Posted By: Viking HouseHi rsk1, concentrate on airtightness and don't get too hung up on breath-ability, I've built numerous houses with the build'up you're considering and learned the hard way. Air-tightness is 99 times more important than Breathability! Good Air-tightness (1ACH) drastically reduces the movement of moist air towards the outside of a wall so there's never moisture present at the Dew-Point to freeze. I've stuck moisture meters 10mm into numerous externally insulated houses where the Dew-Point is supposedly and never encountered moisture.
    Plasterboard on a Wood Fiber Insulated wall reduces it's moisture buffering effect by 80% and then you paint it!

    Regarding your roof query you're correct, if there's enough ventilation you don't need a VB but it reduces the risk of condensation on the OSB, I've encountered spongy OSB roofs that didn't have sufficient ventilation, I've also encountered a roof with a bulging membrane that was full of water from a roof leak.
    So I probably prefer warm roofs without OSB directly beneath the Fiberglass or Parlon.


    Here's a link to an interesting study re internal wall and moisture buffering

    https://orbit.dtu.dk/en/publications/full-scale-tests-of-moisture-buffer-capacity-of-wall-materials

    I think the moisture buffering nature of the walls and roofs has two perspectives, which are often confused.

    The first is how those moisture buffering materials behave within the wall structure itself to prevent excess moisture buildup within the wall fabric. One of the drivers for this has, for instance, been a growing problem in countries like Sweden where they've experience high levels of timber frame failure and degradation from houses built using the 'standard' plastic vapour barrier, stud fill glass wool, osb, breather membrane, cladding buildups. Through failure of the vapour barrier, moisture is transported into the fabric, condenses and can't get out efficiently enough. Hygroscopic materials are design to combat this and some papers I read from Swedish building research studied just this aspect of, for example, wood fibre insulation, measuring only moisture in the fabric over long periods of time. This I view as the fabric perspective. (we obviously know of similar situations here in the UK with period properties).

    Interestingly, if you look at the information provided by many natural insulation manufacturers and suppliers, this is the implied benefit of the material.

    The second is internal moisture buffering and related indoor air quality. The paper I've linked to above indicates that this is an area lacking in research and understanding. I also think that drawing conclusions from fabric hygroscopicity to use for internal moisture buffering is a mistake and much more needs to be understood.

    Somewhere I have some research from Denmark and Canada which makes some interesting statements about this. I'm going from memory here so may not be totally exact. The Canadian study found that buildings with hygroscopic fabrics generally showed a reduction in indoor RH of between 10-25% but it didn't look at internal wall linings. The Danish paper suggested that it is more important to consider hygroscopicity of building fabric than vapour permeability as the effects of vapour permeability are negligible.

    Even with hygroscopic materials, airtightness is paramount.

    For me this all renders the 'breathability' as pretty meaningless and a term to be banished :wink:

    As for roof buildup, I think it depends on the materials. I've just installed a metal roof on my house and the guidance much prefers a cold roof saying that a warm roof is extremely risky. If building a warm roof, a self-sealing vapour barrier properly detailed is absolutely essential.
  3.  
    Was just looking up the section in Scottish building stds tech handbook about cold roofs

    "cold, level-deck roofs, should be avoided because interstitial condensation is likely and its effect on the structure and insulation can be severe and many instances of failure in such systems have been recorded
    ....Both the warm deck and warm deck inverted roof constructions, where the insulation is placed above the roof deck, are considered preferable. However fully supported metal roof finishes including aluminium, copper, lead, stainless steel and zinc are regularly used in conversion work, and they should have a ventilated air space on the cold side of the insulation in addition to a high performance vapour control layer near the inner surface."
    • CommentAuthorSimonD
    • CommentTimeMar 6th 2021
     
    Posted By: WillInAberdeenHowever fully supported metal roof finishes including aluminium, copper, lead, stainless steel and zinc are regularly used in conversion work, and they should have a ventilated air space on the cold side of the insulation in addition to a high performance vapour control layer near the inner surface."


    Interesting. Federation of Traditional Metal Roofing Contractors UK Guide to Good Practice provides details for both a cold roof with minimum 50mm ventilation and a warm roof buildup (i.e. insulation above rafters) that also requires a 50mm ventilation gap on the cold side of insulation. These are the preferred options. I've actually got a bit of a hybrid where I have insulation both between rafters and above.
  4.  
    Excellent link you posted SimonD, will read with interest and coffee.

    "the experiments... demonstrated how cellulose insulation, as a very hygroscopic material, is a good buffer compared to the almost non-hygroscopic mineral wool. ...tests demonstrate it is not possible to take advantage of the moisture buffer capacity... if covered with plasterboard, painted or not."

    Which is also what GP was saying.
    • CommentAuthorrsk1
    • CommentTimeMar 6th 2021 edited
     
    Posted By: WillInAberdeen

    "the experiments... demonstrated how cellulose insulation, as a very hygroscopic material, is a good buffer compared to the almost non-hygroscopic mineral wool. ...tests demonstrate it is not possible to take advantage of the moisture buffer capacity... if covered with plasterboard, painted or not."


    This is an interesting statement, could someone explain it a bit? I would have thought plasterboard is fairly vapour permeable (please correct me..), so how does it impact the hygroscopic behaviour of wood fibre? And getting back to the original question, wouldn't an internal taped osb vcl have a far greater impact?

    Also , some of the early replies assume that the wall in the OP is in a house, where the vapour pressure is inside to outside. It's actually a building that might only get used intermittently so it's not so simple. In which case is there not an argument for having a wall that dries IN as well as OUT?
    •  
      CommentAuthordjh
    • CommentTimeMar 6th 2021
     
    Posted By: rsk1Also , some of the early replies assume that the wall in the OP is in a house, where the vapour pressure is inside to outside. It's actually a building that might only get used intermittently so it's not so simple. In which case is there not an argument for having a wall that dries IN as well as OUT?

    Hmm, if you wanted us to consider some other type of building then IMHO it would have been wise to state that!

    Even if it is used only intermittently the vapour pressure inside is likely to be the same as or greater than that outside so inward movement won't be very important. It only really becomes important in cooling climates where air-conditioning is used a lot.

    I'm puzzled by the plasterboard bit as well, but I haven't read the paper yet.
    •  
      CommentAuthordjh
    • CommentTimeMar 6th 2021
     
    We have a metal roof on our house. The build-up from inside is:
    - clay paint on plasterboard
    - Intello Plus membrane
    - Panelvent boards
    - Warmcel insulation between wooden roof structures (complicated multi-layer arch structure)
    - 18 mm ply (two 9 mm sheets overlapped)
    - Solitex UM connect membrane
    - standing seam aluminium

    EBS https://www.ecologicalbuildingsystems.com/ did a WUFI analysis and provided a guarantee for the roof on the basis of that.
  5.  
    OK the Danish research report is about how materials buffer humidity over a daily cycle, 12 hours of getting damper and 12 hours getting dryer.

    Similar to how heat cycles only get so far down into a surface - eg the surface of the ground gets hot in daytime and cold at nighttime, but a few inches down stays the same temperature all day. But the winter-summer temperature cycle has an effect several feet down into the ground.

    The Danish experimental team found that day-night moisture cycles only penetrate a few mm into the surface of plasterboard because plasterboard is reasonably good at humidity buffering*. So it doesn't matter what is behind the plasterboard. You could use woodfibre or cellulose, but equally you could use something cheaper or more insulating, such as polystyrene or PIR.

    Presumably winter-summer humidity cycles would penetrate further into the wall, so then it does matter what else the wall is made of. The experiments were not looking at that. Is relevant for traditional stone walls.

    Presumably hour-to-hour humidity cycles have even shallower effect: showering, cooking, etc.

    The experiments were looking at absorbing humidity from the room and releasing it back into the room. Not at transporting moisture from the room to the outside or vv. So they aimed to get !more stable! humidity, not !lower! humidity.

    As Simon said, the sellers of 'breathable' materials tend to be a bit vague about what cycle they work over.

    *The exposed plasterboard buffered humidity in the experiments, but not as well as exposed cellulose insulation. Both were much better than exposed mineral wool.
    •  
      CommentAuthordjh
    • CommentTimeMar 6th 2021
     
    Posted By: WillInAberdeenPresumably hour-to-hour humidity cycles have even shallower effect: showering, cooking, etc.

    Thanks for the summary of the paper. I suspect that having clay paint on my lime walls and plasterboard ceiling is quite important when it comes to showering etc. Another reason I'm glad I don't have any tiles. :wink:

    The humidity behaviour of our house has been a bit different starting this summer and continuing into the winter. I haven't quite got my head around what has happened yet but one hypothesis I'm considering is that after five years the bale walls have finally reached some sort of equilibrium. Another possibility is that it's simply due to the quantity of rain last year and things will gradually revert to how they were. I'll let you know more when I understand what the pattern is now.
  6.  
    This study came to mind when posting "Plasterboard on a Wood Fiber Insulated wall reduces it's moisture buffering effect by 80% and then you paint it!" https://www.ibp.fraunhofer.de/content/dam/ibp/en/documents/oeVB_eng_3_tcm1021-30995.pdf
    Humidity spikes are taken up by plasterboard with little or no humidity reaching wood-fiber board, cellulose or sheeps-wool insulation, a VCL probably eliminates it totally. SimonD's Danish link shows similar results, moisture buffering occurs in the first internal 10mm of wall so anything beyond that is irrelevant.
    There's a differential vapor pressure between the walls and the external-internal air for only 1-2 months in Ireland and the UK because windows are usually open in summer so Wall-Partial Vapor Pressure is in equilibrium or in drying mode for 10-11 months of the year. A humidity activated fan installed in wet areas eliminates any benefit of moisture buffering materials in the colder months.
    •  
      CommentAuthordjh
    • CommentTimeMar 7th 2021
     
    Posted By: Viking HouseThis study came to mind when posting "Plasterboard on a Wood Fiber Insulated wall reduces it's moisture buffering effect by 80% and then you paint it!"https://www.ibp.fraunhofer.de/content/dam/ibp/en/documents/oeVB_eng_3_tcm1021-30995.pdf" rel="nofollow" >https://www.ibp.fraunhofer.de/content/dam/ibp/en/documents/oeVB_eng_3_tcm1021-30995.pdf

    I read the first sentence "A uniform indoor climate with minor variations in temperature and relative humidity contributes to establish a healthy and comfortable environment for the occupants" and was immediately struck by how that would offend the likes of Sue Roaf and others who believe in the adaptability of humans to changing environments. Whilst I dislike her views overall, I have a lot of sympathy for that particular viewpoint. And we, like most people I suspect, run our house noticeably warmer outside the winter period than we do during it.

    I have a lot more sympathy and expectation of the paper as a whole so I'll reserve judgment, but that initial sentence is a singularly poor start. :cry:
  7.  
    GPs link is another good one, a study looking at a cycle of a couple of hours of humidifying followed by a few hours drying, repeated twice a day. Like showering in the morning and cooking in the evening.

    So as you'd expect, the results were like the Danish study, only more so. Only the very surface layer mattered, over that short time frame.

    It would be nice if someone did that study over a couple of years timeframe, if anyone would fund that, then it wouldn't be just the surface effects that showed up.

    If I grappled with the maths sufficiently, which are ideally the same* for diffusion of humidity as for diffusion of heat, the depth that the effects of the cycle are felt into the wall, is proportional to the square roots of the cycle time and the thermal diffusivity.

    So if the Danish 24 h humidity cycle were able to sink say 10mm into the surface of the plasterboard, then the German 4h cycle would affect only
    10xSqrt(4/24). = 4mm of the plaster surface

    No need to provide breathable materials any deeper than that.

    But a 365day cycle would sink
    10xsqrt(365/1) = 200mm into plaster, IE right through the plasterboard and deep into the materials behind the plasterboard. Then it would really matter how breathable those materials are, as the depth that interacts with the moisture would depend on the moisture transporting properties of the material, analogous to its thermal diffusivity.

    *But gets worse if the material has long pores!
    • CommentAuthorSimonD
    • CommentTimeMar 8th 2021
     
    I've been digging through my archives and I'm going to chuck in some further links to papers that arrive at some interesting findings. These tend to look at the building from a wider, more functional perspective.

    Potential for Hygroscopic Building Materials to Improve Indoor Comfort and Air Quality in the Canadian Climate - https://web.ornl.gov/sci/buildings/conf-archive/2004%20B9%20papers/002_Simonson.pdf

    Improving Indoor Climate and Comfort with Wooden Structures - http://www.vtt.fi/inf/pdf/publications/2001/P431.pdf.

    There may be more to come if I can find them on my old computer.
    • CommentAuthordickster
    • CommentTimeMar 10th 2021
     
    Just rebuilding wall (and floor) of mobile home (rotten through bad plastic window fitting, rain into caravan, chipboard floor soup.).
    Build. Wallpaper, oiled hardboard, 30mm wide stud with 30mm glass fibre insulation fill. Aluminium skin.
    Evidence of condensation on ally, not a sign anywhere!.

    50+ years old and lived in for 40 continuously. Surprising, to say the least.
    •  
      CommentAuthordjh
    • CommentTimeMar 10th 2021
     
    Posted By: dicksterWallpaper, oiled hardboard

    Oiled hardboard is a vapour barrier, I think. Wallpaper might be as well; depends on the type and whether painted etc.

    Evidence of condensation on ally, not a sign anywhere!

    What evidence would you expect condensation on ali to leave? Genuine question.
    • CommentAuthordickster
    • CommentTimeMar 11th 2021
     
    Genuine answer: Streaks of cleanliness or a dotty pattern when seen under my very good battery light, but I know what you're saying. No staining on studs.

    Yeah I reckon hardboard + wall paper acts as a semi breathable skin. It's all pretty damn clever and superbly built.
    New floor down soon then wall, which is built on the cantilevered floor edge. Such fun.
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