How do I stop a cold RSJ rotting my joists?

[Fowlet]

Hi, we're doing up a stone house, and have lots of vapour-related potential problems. Today's problem: we have an RSJ across a room, both ends in external stone walls, supporting the first floor joists. We're installing internal insulation (stud etc.) on all external walls, so the RSJ is a cold bridge through said insulation. I was thinking I would wrap the RSJ in insulation, a VCL, and fire-proof plasterboard - but how can I seal the VCL where the joists rest on the RSJ?

Would be very grateful for any vapour-wizard advice!

 

[^woody^]

You are trying to solve a problem that does not exist.

 

[Fowlet]

That would be very welcome if true! Why will I not get condensation on an RSJ that's the same temperature as the outside?

 

[Notch7]

That would be very welcome if true! Why will I not get condensation on an RSJ that's the same temperature as the outside?

The only place it could do that is the top of the rsj where the joists rest, I’m not sure the condensation risk there would be enough to cause any rot. I suppose what needs to done is to use a vcl to stop vapour getting into the ceiling void - given that heat rises, the most important place is the ground floor ceiling.

 

[^woody^]

The factors that create condensation are actually quite complex, and so that means that just because a surface is colder than others around it, does not mean there will be condensation. Likewise the environmental factors for mould and wood rot to occur are quite specific.

So whilst a steel beam may benefit from insulation to deal with a thermal bridge and potential impact on the internal surface (mainly appearance) there is no need for vapour barriers, or complex sealing.