Workshop insulation worry

Help Support


Established Member
16 Mar 2009
Reaction score
Land o' Burns.
A couple of years ago when I built my workshop I insulated it with 75mm polystyrene sheets. I am now wondering if what I have done is correct.

It's constructed from 100mm x 50mm frame with 18mm T&G siding. The roof (pitched)is sheathed with 19mm OSB then felted, so it's a wooden workshop. I was told to insert the insulating sheet between the studs and leave a 10-15mm gap between the insulation sheet and the inside of the sheathing (OS wall) I have done this with the roof space as well. I have then lined the walls with 19mm ply and the ceiling with 3mm hardboard.

It's a hobby workshop so there is not a lot of heat coming from the wshop if that makes any difference?

I have been reading bits and pieces regading insulation, including venting etc. However I am now a little concerned that because the walls and roof are now "sealed" have I increased the risk of condensation and ultimately rot?? will I need to vent in some way or is what I have done the correct way to do it??

Any advice greatly appreciated.

Thanks, David


Established Member
26 Nov 2004
Reaction score
Its will probably be OK but what you should have done is fit a vapoir barrier over the inside of the studs below your ply internal boarding.

The reason is that warm moist air from inside the workshop will want to travel towards the cold outside and there is a risk that as the air cools the moisture will drop out of it and be left in your insulation, the point at which this happens is known as the dew point and the problem called interstitual condensation.

Its also worth fitting a breathable membrain on the outside of the studs that will let any possible trapped moisture out but stop water that may find its way under the external cladding finding its way in.

Having said all that my shed where I do my engineering is built like yours and its not fallen down after 15yrs

Latest posts