Hello,

I have a question about how best to install a vapour control layer in the floor or my timber framed garden room.

The VCL will sit above the joists (with insulation between) and under 18mm OSB, which will form the subbase of the garden room. My question is, should the VCL be cut to the same size at the floor area, or should there be extra membrane left that can be folded back over the 18mm OSB floor?

If I installed the VCL with additional membrane, the wall bottom plate could sit on it and the extra membrane could be wrapped up the internal wall and fixed to the VCL I install in the wall. Would this make sense? The VCL that sits under the walls bottom plate would be punctured by the nails that fix the bottom plate in the subframe. Butyl tape could be used between the bottom plate and subframe to seal the nail punctures. My concern with using butyl tape is the thickness of it and I feel it would be better to have a solid contact between the bottom plates of the walls and the subframe.

I might be over thinking it all and it’s best to simply to install the VCL cut to the same size at the floor area.

I have attached two drawings to illustrate the build-up of the project.

Posted By: djhIs the garden room heated?

yes it will be heated.

Posted By: djhHow will the floor be secured to the joists?

I will screw the plywood boards to the joists. So i don't need to butyl tape every joist.

Do you think its best to allow for additional to fold back over the floor, under the bottom plate, and up the internal wall and join it to the interior wall vcl?

or is there an alternative i should consider?

Hi Tom, you can use the OSB as a VCL on the floor and the wall, just tape the joints or put silicone on the joists before you put down the OSB. Externally insulating the stud brings the Dew Point out into the External Insulation (usually within 10mm of the outside) so the OSB can be the vapour control layer. You could use the stud as the service cavity.