Dealing with floor joist thermal bridges

In most terraced houses, the floor joists run from front to back. The joists sit in sockets in the external walls, which means that they act as a significant thermal bridge. The impact of this thermal bridge is enhanced if the external walls are insulated above and below the joists, which is the traditional way of applying internal insulation.

As well as the thermal bridge issue, there is an additional concern - that the joist ends will be exposed to dew point temperatures. Wet joist ends are worrying because the wood can rot and, ultimately, break.

Addressing this issue is one of the major engineering challenges of our project, one that we have spent a great deal of time considering and one to which we are applying an extremely innovative solution. In brief, it involves cutting the floor joists away from the external wall, hanging a steel beam off the party walls, insulating outside the steel beam and then hanging the floor joists off the steel beam; i.e. now entirely within the thermal and airtight envelope. As with pretty much everything on this build, there is an enormous amount of fine detail that I haven’t covered here, but hopefully you get the general idea.

As far as we’re aware, it is not a solution that has been implemented before, so we’d certainly be interested in hearing from anyone who has tried it!