Single skin timber construction - is airflow necessary?

[Single-Skin9000]

Hello

I am currently building a single-skin timber extension (will house a utility room and downstairs loo.)

Original plan was to fix expanding metal mesh (basically chicken-wire looking stuff) to the outside OSB and then apply sand and cement to that to finish. I was recommended that actually instead I should fix some 25mm battens to the external OSB, then cement/render board and render on top of that.

This allows for airflow between the timber and the cement board and therefore less risk of damp and mould, however this method will cost ~2k more.

I've spoke to a few other builders who have also said my ORIGINAL plan should be fine, they've done lots of single-skin timber work with NO air gap on the outside. However I've done much reading and it seems most everyone has a ventilation layer (and it certainly makes sense to me). I'm getting conflicted messages and really need some clarification!

Does anyone have any experience here please? How long can I expect the extension to last with no airflow between the timber and render/cladding? When I come to sell, will a surveyor mark me down because there is insufficient airflow?

Thanks

 

[jeds]

Nearly all methods of construction and detailing of timber framing etc. are well known and come from standard manuals. You won't find your proposed detail in any standard detailing manual. You'll get people (builders) saying I've done it this way and I've done it that way, and it was fine, and it might well have been fine, but that doesn't mean that you doing it that way will be fine. Different environmental conditions, different internal conditions, different patterns of use, may not be suitable in your case. So, the standard details found in the manuals are designed and tested in different environments and are proved to be safe, and that is the best option. Everything else, including your proposed detail, is a risk, it's just a question of how much of a risk?

To try and answer your last paragraph; I have seen poorly detailed timber roofs rotted in less than 10 years. Usually, the issue that shows first is condensation causing dampness and spoiling of internal finishes - and this can happen much quicker. A couple of years or even less. I can't say if a surveyor would pick it up?

 

[freddiemercurystwin]

Rendalath is the belt and braces way to go. Or a rendered cement board on battens.

 

[martygturner]

I have good results from wrapping in breather membrane and then using fiberglass render mesh on 30's semi referbs . The metal lath can be a pain if it rusts, fiberglass won't and will allow flex of the wall.. Your choice really is in the render, k rend or sand and cement with some lime....
Its just a case of controlling the moisture in the osb..most of it will come from the inside so a good vapour control layer and insulation will sort that out. The outside will be pretty waterproof, sand, cement,lime and a good paint or waterproof k rend...
Plenty of 1930's semi houses out there with the curved front bedroom / lounge that are still going strong with a bit of ply and some render as the the bedroom wall
eg