Garden Room Gym - is stud wall sheathing really necessary?

[kpstone123]

Hi guys, I'll be building an insulated 3.6m x 4.8m garden room/gym for my brother and was just wondering why it's recommended to sheath the outside before the cladding? I understand this is for strength (to prevent twisting) but surely the cladding itself will provide this? Obviously I'd understand twisting would happy if the weatherboard cladding were to only have one nail/screw at each stud, but it wouldn't. I've built plenty sheds with T&G weatherboard that are solid as a rock and wouldn't budge (some with diagonal braces between end studs), so don't understand why it seems over-engineered, especially considering the price of wood these days?

I was thinking for the sake of costs maybe leaving out the OSB/ply sheathing on the outside and instead just use vapour barrier and battens on top for the cladding air gap....would this work or is that cutting too many corners? The inside of the studs will likely be fibreglass, to save internal space and costs.

My other option would be to forget the insulation (as it's mostly just a gym to be used less than 5 hours a week) but at this stage it seems a shame to completely prevent insulation in the future by design as the cladding would need an air gap and obviously couldn't be modified if put directly onto the stud walls.

 

[big-all]

what cladding is on the inside ??
is it just a normal wood door or a heavy byfold door wall

 

[kpstone123]

what cladding is on the inside ??
is it just a normal wood door or a heavy byfold door wall

I'll be going for plasterboard, and the door will be a standard UPVC one, or French door. I guess the joints could be an issue with the plasterboard (?) if no sheathing is used on outside, but moreso at the corners which I'd use caulking on anyway.

 

[^woody^]

The sheathing is to stiffen the frame, and the cladding does not do this. Yes it's acceptable for a shed, and if that's what this building is then it won't need sheathing. However, if it's a room, with the usage, larger size, loadings, movement inside, heat and humidity etc and intended to last, then the frame should be strengthened.

Cutting back on insulation, or using a small amount of quilt is a false economy and makes any insulation a complete waste of time. Saying it's only going to be used for a few hours a week and then reducing insulation based on that is illogical reasoning. All that means is that the room will just be cold for those few hours a week.

 

[big-all]

any heavy load hung on a wall will cause racking [sideways movement] without bracing
without insulation it will turn into a cold dumping ground or a very very expensive to heat room thats always cold

 

[kpstone123]

Thanks guys, that's what I needed to hear! I'll go with the sheathing. Anything wrong with 9mm vs the 12mm? Any difference with OSB vs ply? Personally prefer ply as it holds up better with moister.

I was thinking of using fibreglass in the walls to maybe help deaden sound a bit more (and cheaper) but happy to pay more for insulation board if it'll be a big difference to temperature and other things?

 

[kpstone123]

I've come up with a plan that kills 2 birds with one stone - ply sheathing on the INSIDE. That way I get the structural strength and don't need to mess around with plasterboard. It'll give it a more rustic look, which is a bonus for a man-cave/gym

Surprised this isn't a standard thing really? I shouldn't need a vapour barrier this way either

 

[big-all]

you still need cladding/15-25mm air gap breathable membrane insulation

 

[noseall]

I shouldn't need a vapour barrier this way either

You will still have warm side/cold side issues no matter what you fix to the walls. Ply will go just as mouldy if you don't insulate/ventilate/VCL etc and build the walls robustly.

 

[kpstone123]

Ok thanks for the warnings!...I might be missing something here - what I was leaning towards is:

Ply on top of studs > Foil backed rigid insulation between studs and foil taped at joints > Battens on studs > Cladding

I understand that the foil backed insulation doubles up as moisture barrier too (if taped at joints)? In that case the barrier would be on the warm side as recommended to prevent warm/moist air penetrating the wall and condensing on the colder outer surfaces. Would the ply which is on the inside then be protected as it's always warm and hence not able to let warm air condense on it? It's also open on the inside so will be able to dry out?

I often see on new builds that there is a barrier on the outside of the build (before the battens and cladding goes on....I take it that's purely for water proofing (in case of cladding leak) rather than vapour? IMO if there is a 15-25mm gap as mentioned, then the extremely low chances of leaks getting on it would dry out anyway? In fact, wouldn't the barrier impede it from drying out? VCL on the outside just seems redundant to me.

Sorry for all the questions! Still learning a lot

 

[stuart45]

The external one is a breather membrane, which allows moisture out of the wall, but stops any rain or condensation into the wall.

 

[kpstone123]

Ah makes sense, thanks Stuart