I just found this very interesting article:

http://www.housebuildersupdate.co.uk/2007/07/little-known-history-of-vapour-barrier.html

I'd be interested in forum members thoughts - especially as there still seems to be a lot of debate on this forum about breathing wall timber frame construction as described in the Whole House Book and advocated by Warmcel. (e.g. see here: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=532&page=2 )

I'm about to embark on a self funded new build in Ireland. Building control and codes are a non-issue for me, so I'm contemplating something radical. A timber frame house with electric fan assisted passive stack ventilation to remove moisture laden air from all rooms. Wall construction (inside to out): sasmox boards on 6x2 studs with sheeps wool insulation, then 100mm tektalan e21 magnesite wood wool insulation boards and then a render of waterproof rein fibre cement on a mesh (rein fibres and mesh will prevent cracking).

Conventional wisdom says this is madness.

My logic suggests that it will be super insulated, the studs will be warm - hence no due point on them, and internal most moisture will be moved by the ventilation. Obviously I will need to carefully keep an eye on the rein fibre render, but unless water is pouring in from the outside, the timber is 100mm away and should be safe. The advantages of taking this route are simplicity, cost effectiveness and super insulation. The danger according to conventional wisdom is that the timber will rot. I don't think it will.

More info on tektalan insulation boards is here:

http://www.skanda-uk.com/heraklithProducts.htm

More info on Rein fibre cement which I have used in a 7mm coat to create waterproof bird baths that haven't cracked in years:

http://www.koicymru.co.uk/const9c.htm

http://koicymru.co.uk/q&a7.htm

I'm almost tempted to try the construction as an experiment. The thoughts of the forum are very welcome!

(This is my first post - I've been a long time lurker - many thanks for all the insights on the forums.)

Just to add to tonys post, I do not like rendering over external insulation of any kind. I worry that large areas of thin dense material cannot expand and contract with seasonal temperature fluctuations without cracking, leading to moisture ingress, exacerbating the [already high?] risk of interstitial condensation.

Posted By: tomsuswebI just found this very interesting article:

http://www.housebuildersupdate.co.uk/2007/07/little-known-history-of-vapour-barrier.html" rel="nofollow" >http://www.housebuildersupdate.co.uk/2007/07/little-known-history-of-vapour-barrier.html

Got to love this part of the article:

"Now, in the 1930s, placing insulation in walls was a relatively new idea and the businesses making insulation were keen to see that this new business venture wasn’t killed off by complaints from painters before it had got going. So they set out to investigate."

Insulation a NEW idea in the 1930??!! My parents houes was built (timber frame) 1921, it has a vapour barrier (in the walls and roof) consisting of paper soaked in tar and is insulated with sawdust (roof) and wood shavings in the walls. Never been any issues with damp there...

Fit a vapour barrier. If you don't want use polythene use tarred paper...

Tom, how are you planning on insulating the floor and the roof?

The Sasmox boards are very nice indeed, but very expensive...Fermacell is a little bit cheaper, but equally good.

Mike

This is one of the products I have used. There is no need for expansion joints but when used with heraklith tektelan a glassfibre reinforcement mesh is required with an additional layer at the corners of an opening. Thickness of up to 15mm.

http://www.lime-green.co.uk/images/Product/MONOSTASTIER.pdf

Posted By: tomsuswebThanks for the comments everyone. So I take it that Nigel and Johan are suggesting the system I described would be OK as long as a vapour barrier was fitted and also I used the correct rendering process?

I like the idea of Warmcel - would you recommend it sprayed in wet or pressure injected behind boards - our local guy doesn't like wet spraying.

Floor is a suspended timber floor (house built on peers) so probably warmcel between the joists. Roof would be warmcel also.

Thanks for the input - still not sure what way to do it - a more conventional approach with an air gap does sound safer but hard to get such good insualation. Any more thoughts are welcome!

I agree with Nigel. Warmcel or some other eco fiber is good for walls and roofs, not sure how it would work for a floor though. Pricewise it is also competative with glassfibre wool, which Thermafleece isn't...

If you have a vapour barrier on the inside of the wall you can dry blow it before you fit the plaster boards.

Personally I would recommend a service void between the vapour barrier and the internal plaster board, 1x2. Just makes life easier. You can always fill the service void with another 25mm of insulation if you want to,rule of thumb is you can have the vapour barrier up to 1/3 into the insulation. A dew point calculation is always recommended though to make sure...

You can blow the warmcell into the void between the sheathing and the external insulation.
You would need a 100mm hole to be made in the sheathing for each void which is then replaced and taped up.

The Vapour contrrol layer is then fiited after the insulation is installed.

Can you please tell us what the U values will be?

How does plaster reduce the need for air changes? There's more to air changes than just removing humidity. Of course, if you don't build airtight in the first place, there's no need for mechanical ventilation.

Paul in Montreal

Posted By: Paul in MontrealHow does plaster reduce the need for air changes? There's more to air changes than just removing humidity.
Paul in Montreal

Some info here:

http://www.housebuildersupdate.co.uk/2007/07/how-should-we-handle-humidity.html

And here's a quote from the page:

There are alternative water vapour management strategies out there. The best established one hails from Germany and it consists of fitting highly permeable materials to be used as a reservoir to store moisture, with a view to letting it be evaporated back inside when conditions allow. In Germany, it is seen as part of the Building Biology movement and they regard the use of humidity-buffering materials as one of their key principles. Needless to say, they regard the use of mechanical ventilation as an anathema.

Also the Neil May paper linked above seems to concur with this.

Posted By: nigelMy rough calc says about 0.21 for the walls and the roof is less good at 0.23.

It would be better to make the roof better at the expense of the walls.

I know you dont need to satisfy them but the roof would not satisfy building regs in the uk whereas the walls are well below building regs requirements.

Thanks for the calculation Nigel. What would you recommend as a good way for use to improve the roof? (Hopefully compatible with the kind of approach described.)

Hi Thomas,

If the answer in this link is correct 11mm osb interally on your studs will serve a double purpose, Vapour barrier and racking strength.

http://www.osbguide.com/faqs/faq11.html

We currently use 9.5mm OSB but are looking into using 11mm instead, we still use a plastic barrier where f-floor panels tie in as this is a difficult area to seal.

Thanks again for the comments. Strawbale was considered but a friend of ours has a strawbale house out here and is not happy that the bales are dry enough due to the very damp climate.

The OSB and pavatex ideas sound good. I'll see if I can get an alternative softboard to pavatex that might be cheaper.

PS

has anyone come across 'thermowall' wood fibre board used without an air gap over a timber frame as a render sub base..?