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By MikeG.
#1303058
spearson92 wrote:Thanks Mike,

It's my first project and safe to say I have made a few mistakes but definitely learnt some lessons.

I shall have to go down the preventative route as Woody suggested. I'm not sure about heating it during the winter at this point in time (although I know will still be cold in the winter). If I don't heat the workshop at all I guess there will be a lot less risk of damp on the inside of the exterior layer of OSB, owing to no heat actually escaping the workshop?

Cheers,
Steve


The thing to understand, and which most people don't get, is that a heated building is wetter on the inside than the outside.
Rorschach wrote:......I'm confused, you wanted to stop moisture ingress, which he has done with the outer layer of OSB, and the membrane on the outside of that..........

Warm air holds more moisture than cold air. A lot more. Because 20 degree air can hold a lot more moisture than 0 degrees air, condensation occurs at the point where air is cooled down such that it can no longer hold all the moisture it was holding. So if you think about your wall...........on one side it will be say 20 degrees C, and on the outside it will be, say, 0 degrees C. Somewhere in the depth of the wall there is a point where condensation would occur if there was any moisture in the air within the wall. So, there are a couple of strategies for keeping the wall dry: firstly, don't allow any moisture in from the wet side (the inside), by putting in a vapour barrier (nowadays a Vapour Control Layer -VCL-, because it is impossible in ordinary construction to provide a complete barrier). Secondly, allow any vapour that gets in to the wall (and we've just seen the impossibility of keeping it out) to get out of the wall again into fresh air on the outside. Those constructions which have OSB on the outside disallow the latter, leaving you with only one strategy for fighting interstitial condensation.

Because of service penetrations and stuff fixed to walls, identical vapour-resistant layers on the inside and the outside of the wall will have differing performances. The one on the inside is almost universally less effective, so you have the situation where moisture is allowed into the structure from the wet side, and let out from the outside at a slower rate. Again, this is a recipe for interstitial condensation, which almost always manifests itself as moisture condensing on the inside of the external sheathing. This can then run down the inside face of the board and gather on plates or noggins, allowing firstly the growth of mould and then secondly the development of rot. There is a rule of thumb, therefore, that the vapour resistance of the interior of a wall should be 5 times that of the exterior of a wall (there's a whole essay on where the exterior of a wall is, BTW).

Your wall, when finished, will have roughly the same vapour barrier inside and out, albeit the interior one will be compromised as I've described. Moisture will get in, and will be trapped in. In an irregularly heated space with a poor wall design used intermittently in winter the best way of ensuring the long term health of the structure is to have good ventilation so as to remove the warm damp air. This can amount to little more than letting all the heat out when you've finished in there by, say, leaving a top light open in a window, or running a slow extraction fan, or running a dehumidifier, or opening hit and miss grills, or somesuch other. The fact is that these regimes will always rely on human behaviour, and so will never be fully followed. When you sell the house, the next guy isn't going to have read this post and is not going to have a clue how he should use the building. This is why it is better to design the problem away in the first place, and why I advocate using sheathing on the inside of the wall and a membrane on the outside.
Last edited by MikeG. on 30 Aug 2019, 09:10, edited 1 time in total.
By Woody2Shoes
#1303067
Hi Mike - point taken - I guess you're not a fan of SIPs then (accident waiting to happen?) which usually seem to be a PU sheet sandwiched between two OSBs. If the celotex is properly installed in an airtight way (and I realise that's a reasonable sized if 99% of the time) surely that helps to keep moisture migrating outwards? My motto is 'where there's a temperature difference, there's a vapour pressure'. Cheers, W2S
By spearson92
#1303074
Thanks for the posts guys!

Based on what you've said Mike, would it help at all to drill a small ventilation hole in each cavity, just above the noggins (being careful not to penetrate the membrane), so that if/when any moisture does build up behind the outer layer of OSB, it can evaporate quicker?

Cheers,
Steve
By Woody2Shoes
#1303076
spearson92 wrote:Thanks for the posts guys!

Based on what you've said Mike, would it help at all to drill a small ventilation hole in each cavity, just above the noggins (being careful not to penetrate the membrane), so that if/when any moisture does build up behind the outer layer of OSB, it can evaporate quicker?

Cheers,
Steve


I'm not Mike - I know! - but my 2d's worth is that: 1) it would be almost impossible to do without damaging the membrane; 2) halfway up (I assume that's where your noggins are) would be the least best place - I'd say top and bottom to allow a convection circulation of air; and 3) how would you arrange your celotex to allow circulation of air on the cold side of it - quite tricky. What you're currently planning to do is no worse than a lot of buildings that get built all over the place - especially in Scotland, I suspect - (Mike is pessimimistic about their long-term prospects, and time will tell) - as we've both mentioned - good ventilation and airtightness are helpful mitigations. Cheers, W2S
By spearson92
#1303089
Hi Woody,

I see your point, I'll go with the original plan then.

You're absolutely right though, all the new builds across the road have OSB exteriors and then cladding or some sort of rendering on them.

If it can go a good few years I can accept that. The next workshop will just have to be bigger and better! :D :D

Thanks for the advice Woody and Mike, much appreciated!

Cheers,

Steve