Solid masonry wall with a "cavity"

[Tim Gilbert]

In a way this question relates to module 3.

When moving windows forward in readiness for External Wall Insulation (EWI) I discovered that my solid wall isn’t exactly solid. There is a gap of a few millimetres between the sides, although bridged. The gap obviously helps reduce water ingress from the outside but also allows air to flow. If I continue with my plan for EWI the cavity will warm and warmed air may vent from the top of the wall unless effectively sealed. I cannot inspect the top of the wall without removing the roof. As the gap is less than the thickness of a mortar bed the wall measurements didn’t reveal it.
So, what do I do with the gap? I contemplated sealing it where exposed at windows or pouring floor levelling compound into the hole. The latter increases the risk of penetrating damp unless I use a waterproofing addmix and would only treat areas below windows. Neither solution seems satisfactory. Clearly the gap cannot be insulated. Unfortunately I cannot paste the photo I took into this post, otherwise I would have included it here.

Any suggestions please?

Tim

[Andy Frew]

I would think of using polystyrene bead. A cavity wall insulation contractor could blow it in if you had the tops stuffed with say mineral wool fixed in place and leakage to any neighbours was not a problem to them.

[Tim Gilbert]

Hi Andy

Thanks for the suggestion. Do you know whether polystyrene beads come in smaller sizes that would fit into and flow through a gap of just a few mm?

Tim

[Author]

Posts