Plasterboarding over bowed insulation ?

[Jimmy78]

Ok the topic is not completely accurate, let me explain.

I have erected a large "shed" at the bottom of my garden, (5M x 5M) from 4 x 2 framed 12mm WBP ply. The outside coated with exterior paint then featheredged completely, I am very happy with the outside.

However I am now looking to insulate the void before plasterboarding over the top. I am using 90mm Celotext/Kingspan as the 4 x 2 used are regularised so actually come up around 92mm instead of 100mm. So you would think the 90mm insulation would slot in nicely.

Problem is that virtually every panel of ply seems to have a significant bow running down the middle of it, bowing inwards so when the Celotex goes on/in it wont run flush with the inner edge of the timbers so plasterboarding will have problems.

Does anyone have any idea of how to overcome this, short of sanding the ply or cuting some kind of recess into the insulation ?

Thanks chaps

 

[Richard C]

Use 80mm Celotex instead or battens on the inside to increase the depth to provide clearance.

Plaster Board (even Moisture Resistant) is a very poor choice to internally line a shed; the way it’s constructed & the ambient environment will be damp especially in winter & plasterboard will quickly warp & eventually disintegrate when exposed to damp.

 

[Jimmy78]

Thanks for your input.

The shed is raised from the concrete slab by 40mm, then constructed from treated timbers underneath a Damp proof membrane, so I'm hoping damp won't be a problem.

Should've check the boards before purchasing 90mm celotex obviously, but a bit late for that now. Was hoping for a miraculous solution !

 

[Richard C]

I think your hopes will be dashed; raising the shed off the slab, treated timbers & a DPM will still not prevent condensation & high internal moisture levels unless you permanently heat/ventilate it. Even a modern cavity wall constructed building will suffer internal condensation & damp if left unheated/unventilated for any length of time, especially over winter.

Gypsum plaster board is not designed to be used in damp environments & even MR or Duplex board will warp & suffer eventually; damp turns it to mush.

 

[Jimmy78]

The "shed" is a little more than a storage space and will be in virtual constant use as I intend to have my office in there, which will mean ventilation/heat should be sufficient. At least I hope so.

Looks like I may have to exchange the 90mm for 80mm which is a shame as the 90mm sits perfectly flush with inside of frames.

 

[Richard C]

I know & work with plaster & plaster board all the time, don’t say you weren’t warned.

 

[foxhole]

Should have built with ply on the inside and feather edge on the outside of the frame.A vapour barrier under the feather edge to protect insulation.Richards warning should be heeded or you are looking at bowing moldy plasterboard in a shed.