Timber frame - membranes

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Hi. This is a first post, and I'm in a bit of a quandary about membranes on timber frame builds.

This is not a 'conventional' timber frame build - at least not initially. I do not have planning permission for a house, so I am following an unusual precedent set by a neighbour.

A bit of background ....
I currently live on a large (several acres) plot, in a large residential mobile home. I have full planning permission for this. The legal term for this is 'caravan', and the legal definition of a 'caravan' in a nutshell amounts to a transportable modular type structure (not necessarily on wheels), made up of no more than two modules/parts, up to about 65 feet long and 23 feet wide. There's obviously a lot more to it than that, but that's the basics.

My neighbour (with the same permission) has got around this by effectively building the inside of a 1,000sq.ft timber framed bungalow, on discrete columns of concrete blocks, with a full slate tiled roof, and a suspended timber floor, but without the masonry skin, theoretically qualifying as a 'caravan'. He then applied for planning permission to turn it into a permanent dwelling by adding a blockwork skin, on the basis that there was 'no material change' and there would be no additional vehicle movements. [We also have an unrelated bargaining chip which helps]. The permission was approved.

The council have advised me that although I cannot build a house outright, I can follow his precedent as a route to getting a permanent dwelling.

I am doing the same. Last year I discreetly buried a full set of house foundations under a portion of my land and covered them over. On this I am building columns of concrete blocks which will support a timber frame on a suspended timber floor with a slate tiled roof. Initially I will only build the timber frame, then I'll apply for the same planning permission, wave my bargaining chip and hopefully get the masonry skin/permanent dwelling approval, on the same grounds as my neighbour PLUS the precedent set by him.

Problem.
For a year or so, it's going to have to survive as just a timber frame with no masonry skin, and look half decent. I know this is possible, as my neighbour managed it. My initial plan was to build the frame as (starting from the inside) Plaster, Plasterboard, Vapour check membrane, 140mm timber frame with 140mm frametherm insulation, followed by 11mm OSB skin to which I was going to apply a temporary coating of primer followed by some kind of textured masonry paint. The paint protects the OSB for the year it has to spend un-clad. At the end of the year, if I get permission to turn it into a dwelling, I add a breather membrane and the masonry skin. If for some reason I don't get permission and it has to stay a 'caravan', I then add a membrane, battens and cedar cladding.

BUT. Is the temporary coating of primer+textured masonry paint going to form effectively an external vapour check membrane and ruin the outward path of condensation?

AND. For the year it spends un-clad, do I need to consider a breathable membrane BEHIND the OSB?

Thanks. I'm due to commence the timber frame just after Easter, the timber goes on order tomorrow!
 
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Interesting loophole :idea:

If you are diligent with your vapourcheck internally, and use full fill inso, there will be nothing coming through from the inside cept maybe any moisture that may exist within the timber (newly delivered timber is often still wet from the pressure treatment process, or may have been stored outside before delivery)

I would put a breather membrane and 25mm batten before the osb. Leaving a gap top and bottom for airflow with insect proof mesh covering.

I would also prefer ply to osb, then painted with a breathable coating. oil/woodstain/shed treatment/other.

or: clad the building in featheredge board with some easily removable nails (big heads)

Then you could remove this and reuse it for fencing/give away/freecycle/eBay/whatever.... and use new dry osb when the time comes.
 
Thanks for the reply.

I do like the idea of the frame+membrane+batten+OSB, but I've already installed my foundations (and actually the outer skin of masonry to just below ground level) for a 50mm cavity - the batten would use up 25mm of it. I'm reluctant to make my frame any thinner to incorporate it the other way as I believe 140mm is the current standard.

I guess a breather membrane without a breathable space behind it is not the greatest idea ... however it works the other way around ... ie when you pin the breather membrane directly to the outside of the OSB on a conventional timber build. What I'm really trying to avoid is condensation buildup on the inside face of the OSB while it's in the 'caravan' state, but this will only happen if there's water present - which there shouldn't be if it's externally coated and internally vapour lined.

What about doing it with masonry paint as planned, but making up some kind of surface-distressing tool (like a roller with lots of nails sticking out) with which to 'puncture' the masonry paint in many places before putting the final membrane on?

Maybe the idea of a shed type treatment is a good'un - I'll look into this - thanks. I had really decided on the masonry paint route as this would most closely resemble the proposed final block/render finish, making the 'planning decision' easier.

Ply rather than OSB ... can I ask why? I did originally plan to do it in ply, but then read up on the main differences between the two and decided on OSB largely for it's (apparent) greater resistance to swelling at seams, and neater cutting when sawing. Also OSB seems to be the 'norm' for (proper) timber frame houses. Structurally and price-wise there isn't a lot between the two. I'm due to be placing the order for the bulk of the timber tomorrow (Tues) so I could really do with making my mind up!
 
As for pricking holes in masonry paint, well you're in experimental territory there. Breathable masonry paints are available. and masonry paint on wood?

Ply has superior weather resistance than OSB, it will take a soaking and won't fall to bits but will defo swell up over the course of a year, whereas wbp ply will hold together much better if treated.

You could do away with the battens and membrane altogether, use ply, treat it properly, cross your fingers etc

Regarding the founds, assuming the wall will be tied to the timber structure with frame ties, you will probably be able to oversail the masonry by 25mm.
 
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Breathable masonry paint ... that sounds interesting and is a route I shall definitely investigate. Yes - on wood - hence why I thought a primer coating might be in order first. It only needs to do its' job for one year max.

Ply - OK - you're making a very good case for it. Can I assume we're talking about an exterior-use shuttering-type ply here, but not necessarily pretty? If I step up to a marine-type ply I don't think the purse will be very happy but I'll check prices all the same.

I'm half tempted to wing it in ply alone. Breathable membrane without a circulating air gap is likely to be fairly ineffective and I'm out of space for extra air (I'm interested in your oversailing idea - is this common practice?). Although the outer walls may get wet during construction, they will dry out well from the inside while I'm roofing and 1st fixing (which should be during summer months too) especially once they're coated from the outside, so no additional moisture should get in.

I have a trailer body outside made from shuttering ply, irregularly treated, and it's sat outside solidly for the last 8 years or so with little ill-effect. Maybe this is indeed a panic over nothing .... !
 
I think you`re right - and I happen to know a Park Home owner where they have units re furbed . they look good and it does indeed last well :idea: Never had cause to ask about it - but I`ve put your lik in favourites now . Good Luck ;)
 
Good find! Whats the damage for 5 litres?

Will you need to even bother with your masonry outer wall if using this?
 
I don't know what the price is. I've emailed my requirements and am pending a reply. Will post an answer when known ....

I will do the masonry wall, even if only to make it a proper/standard dwelling and mortgageable. I might want to sell it one day and it helps if folk can get a loan on it. Having got the foundations in already, it's only going to be the price of a few hundred blocks, some wall ties and a membrane.

Cheers for help and pointers, much appreciated.
 
Got a price and product guide for the coatings, for those who asked/are interested ...

Primer currently 3.10/litre, est coverage 4-6m2/litre
Textured finish currently 2.60/litre, est coverage 1-1.5m2/litre

plus VAT, plus delivery.

My project (approx 100ish sq.m of external wall) comes in just under £500 (inc VAT & delivery) to prime both sides of each external board (recommended), to scrim tape the joints and give it a layer of the textured stuff.

Having seen a few park homes, if this is the same stuff the manufacturers use (which it is claimed to be) it's quite beefy.

If I remember I'll post how I get on with it .. but that won't be until towards the end of the summer. Thanks to all who helped.
 
Price not too bad for what it claims.
Don't forget to take pics of your handywork! Good luck with the project.
 
Hi

Why do you need to bother with these fancy texture finishes, if you want to save some money get hold of some 'weathershield' exterior masonary paint and simply paint the boarding and forget about it for a year! Rather than worrying about filling the gaps between the boards use a small section of softwood timber say 12mm x 50mm (or maybe thinner section) to act as a cover strip across the joins in the boards, again paint with weathershield - job done for a year.

Regards
 
It's a good point - Dulux Weathershield masonry paint does also appear to be breathable. Whether it is OK for going straight on wood I don't know.

The textured finish discussed above buys me a bit of time - manufacturers claim 10 years - which may be useful. The planning permission should last for five years.

Right next to this structure I'm building, is an existing outbuilding which I'm hoping to extend the completed house into in due course. It needs a chunk of work doing on it, including the addition of a pitched roof, windows etc. If I do the major structural work to this building while my project is still a 'caravan', it won't need building regs approval, as it will qualify (I think) as a 'Schedule E small detached outbuilding'. However if I do it while my project is a 'dwellinghouse', ie post conversion, it is close enough to the 'dwellinghouse' that I think I will need building regs approval. A bit more time with the structure as a 'caravan' could be really useful.

It's not that I'm trying to avoid doing the work to a certain standard, it's just one less bit of red tape to cut ...
 
Almost there. It was PT coatings I used, bought their primer and stipple coat, to cover the whole thing (primer both sides of the boards) including delivery (with a small overage) came to just shy of £600 ... not the cheapest stuff in the world.

We've now just finished building the bare structure and roofed it, the primer is all on, I'm aiming to do the topcoat in the next week or so.

PICT1490.jpg


Will post an update when it goes on.
 

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