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?

In Canada they darent have any pin pricks in the vb as it freezes and icebergs form which can force off the cladding!

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 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.

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?

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

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.

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.

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?

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.

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.

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.

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.

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.

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.