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By spearson92
#1302894
Hi guys,

Just me again. I've pretty much finished my workshop now, and starting to look to the inside construction, but first I have a couple of questions:

1) The wall construction is (outer to inner): featheredge cladding, battens, breathable membrane, OSB, stud framework (no insulation or internal cladding/boarding yet. Question is: have I correctly added OSB to the outer side of the studwork?

From videos and pictures I have seen of shed construction, all other projects seem to do the same however, looking at posts on this site, it suggests that by doing this, and then adding insulation in between studs then boarding, that moisture could get trapped on the inside part of the outer layer of OSB, and run down creating damp and rot. I guess that the answer to that question partly depends on my next question which is...

2) what is the best way to insulate the shed to give best value for money? I don't want to go overboard on the cost with insulation, but don't want to go too cheap so as it doesn't sufficiently insulate the place. With the wall construction as above, is there any other way I should be insulating between studs so as to prevent rot?

Sorry for bringing up the same old questions!

Cheers,

Steve
By Woody2Shoes
#1302896
Hi - you need a vapour-impermeable (a 'vapour-check') barrier on the 'warm' (that is, inside) side of any insulation. OSB is considered vapour-impermeable, but since you've got it on the "wrong" (of course there's nothing wrong with it!) side of any insulation you put between the studs - you'll need to add another vapour-barrier. The whole idea is to stop warmer (moisture-bearing) air from inside migrating into the structure and cooling on its way towards the outside and dropping excess moisture on the way - https://en.wikipedia.org/wiki/Interstitial_condensation

As a vapour barrier you could use, for example: polythene sheet sealed with tape, foil-backed plasterboard, or more OSB (say 9mm or 12mm).

As far as insulation is concerned, the best insulation value for a given thickness (U or R value) is probably foil coated polyisocyanurate board (Kingspan/Celotex) e.g. https://www.insulationsuperstore.co.uk/ ... lotex.html

It's not the cheapest, but you get what you pay for, but - for me - money spent on the best insulation is not wasted.

It's worth paying attention to both airtightness and ventilation if you're going to do the best job you can.

Cheers, W2S

PS given that you're probably in a relatively exposed location (in Inverness) your first outer layer of OSB is probably a really good idea - making the whole structure very robust.
By spearson92
#1302899
Hi W2S,

Thanks for the quick reply!

Ok that's good to know, I suddenly had a panic thinking that the outer layer of OSB would cause me problems in the future by trapping moisture, but panic over :D

My plan as you say would be to use OSB internally, with insulation of some description in between the studs. I know with wool type insulation you have to ensure an air gap, is that the same with foam insulation as well? Or can you stuff the whole cavity with foam board (Kingspan etc) and then OSB?

Cheers,

Steve
By Woody2Shoes
#1302920
Hi - I think you could fill the gap between the studs completely if you wanted to, but two thoughts cross my mind:

1) the studs will probably be CLS sizes or similar so, for example, a nominal 100mm will be say 95mm - the celotex on the other hand would be exactly 100mm with an annoying discrepancy to get rid of with firring pieces/battens or somesuch.
2) You might find it handy to have a small gap between the insulation and the OSB for wiring etc.

So, maybe an option would be to go with say 80mm Celotex between the studs - with gaps sealed with PU gun foam (helps with airtightness which makes a huge difference overall) - and then say 12mm OSB3 on top directly onto the studs (you could add a polythene vapour check membrane under the OSB as belt and braces but it would probably be overkill e.g https://www.insulationsuperstore.co.uk/ ... -roll.html ).

That's what I'd do anyway! Cheers, W2S
User avatar
By MikeG.
#1302962
spearson92 wrote:.......Question is: have I correctly added OSB to the outer side of the studwork?.......


No, it is very much the wrong way if you are proposing to heat the workshop and work in it during the winter. You have effectively put the vapour barrier outside the insulation (ie on the cold side of the insulation), when it should be on the warm side (the inside). People you watch on Youtube might well live in different climates, but more likely, are just ignorant of the issue.

W2S has indicated the correct redemptive procedure......a less permeable vapour barrier on the inside of the studs. However, this is very much a second-best solution, particularly in a workshop, because there will be all sorts of penetration s of this for electrics etc, and from everything that gets screwed to the wall.
Last edited by MikeG. on 29 Aug 2019, 19:16, edited 1 time in total.
User avatar
By MikeG.
#1302979
No, the breather membrane (or building paper). This is critical for forming a ventilated void behind the cladding, and for keeping the frame dry in all circumstances.
User avatar
By Benchwayze
#1302994
My garage shop has only one outside wall. West facing, cavity wall, single-brick and thermalite inner courses. It's about four feet from the next door property and is clearly sheltered, except when it's very windy, when it becomes a wind-tunnel. Would insulation help at all. At the moment I don't get any condensation in my shop, due mainly I suspect to the metal, south-facing up and over door .
which gets blister hot in the summer and still collects heat in the winter.

John (hammer)
User avatar
By Benchwayze
#1303009
Thanks Mike.

I am leaving well alone. I can work in there at any time of year in relative comfort. If it's extremely cold weather I have a heater, and in summer I have to have the door open or I'd fry. Yet still I get no condensation despite the heat exchanges. My bench is well toward the rear of the shop of course and not too close to the open door. Planing soon gets a sweat on!

John (hammer)
By Rorschach
#1303020
MikeG. wrote:No, the breather membrane (or building paper). This is critical for forming a ventilated void behind the cladding, and for keeping the frame dry in all circumstances.


So what about houses that use sheathing over the framework? Every house in the US is built that way.
User avatar
By MikeG.
#1303022
Yep, and it's a massive store of potential problems they're creating for themselves. They're building standards are so rubbish that no-one should be trying to copy them.
By spearson92
#1303036
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
By Rorschach
#1303045
MikeG. wrote:Yep, and it's a massive store of potential problems they're creating for themselves. They're building standards are so rubbish that no-one should be trying to copy them.


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 will help stop the OSB getting wet and he has battens providing a vented void behind the cladding. As long as he makes sure he has a vapour barrier on the inside too he is fine. The insulation is protected from moisture in both directions then.

According to this page his wall construction is correct.

https://www.designingbuildings.co.uk/wi ... _buildings