Hi all,
I have been working over the last month or so on replacing a suspended timber floor, and insulating it and the internal walls (200 year old house in conservation area).

I have had dampness on the internal wall in the bottom left corner since moving in about 6 years ago. There have been other issues, including ground high external ground levels and the suspended floor/subfloor ventilation which I have hopefully resolved. However, at this particular spot, directly above where the metal vent pipe comes in, there has been a vertical line of dampness that doesn't seem to fit with the other issues. As I had the skirting off to do the floor I gouged out what seemed like soggy clay or maybe lime mortar and found that there was a void at this point, directly above where the pipe comes in and a draught coming in the hole I just created.

The metal vent pipe looks very old and is actually two pipes at an angle to each other in order to create the bend. My theory is that the draught I am getting through the internal wall is coming from this join in the vent pipe getting into this void, which in turn is creating a cold bridge in the thin mortar fillet where condensation has built up over time. There was previously an electrical socket cut into a skirting board at this point, so an easy route for warm damp air to get to this point. At first I thought this void might have been intentional, so as not to bridge dampness from the higher solid floor in the next room, but I put an endoscope in there and it doesn't seem to run the whole length of the wall, just about 300mm or so.

My question is, does this theory sound credible? And should I attempt to fill the void, and if so with what? I thought about maybe attempting to pour some EPS beads in but if there is a gap the beads might end up in my subfloor void which I can't access now that it is closed up.

Any thoughts welcome.

Thanks. I thought about expanding foam, would expanding foam work ok in a situation like this where there could be water getting seeping into it from the higher level floor on the other side?

Posted By: Kenny_MThanks. I thought about expanding foam, would expanding foam work ok in a situation like this where there could be water getting seeping into it from the higher level floor on the other side?

Expanding foam can be either closed cell or open cell. Make sure you use a closed cell foam.

Posted By: Kenny_MIts the vent for the suspended floor. It looks like it has been fitted this way in an attempt to get through ventilation which otherwise wouldn't be possible with a solid floor in the adjacent room, so no it can't really be removed. I am looking to see if I can somehow line it with plastic though as I think air might be condensing on the metal.

Hmm. Condensation is certainly a possibility, however I wonder if it could be water (from outside / from within the higher floor) tracking along the outside of the pipe. Especially as you seem to have been having damp problems and describe the soil / mortar as soggy; that sounds like more than condensation. Even more so if the vent slopes down into the house?

I'd be tempted to make a larger hole to get a better idea of what's going on, rather than filling it with foam right away.

I have used squirty foam under a floor, but it was soon chewed out again by mice. They find it irresistible.

No substitute for mortar and rubble, for filling gaps like this.

In this cold weather can you use an IR thermometer to trace the void (or a thermal camera if you have access to one?)

Posted By: Kenny_MThere are some other damp problems in the outside wall at the corner near the pipe that I am also working that could be connected too, but it seems like this is an isolated issue. Its always difficult to diagnose when there are multiple things going on.

Yes, that does complicate things.
Are you able to leave it until the spring, when condensation should no longer be a problem, to see what happens then?