Hi All. Our house is over 100 years old, and it's in a damp, alternately cold and hot climate (southern Ontario). I've read a couple of threads on the forum about insulating old double-brick homes, but I've got a unique question.

I am considering filling the 3/4" gap between wythes with expanding polyurethane. I know this won't add much insulation (Maybe R3 or 4), but anything would be an improvement. I'm more interested in what filling the cavity will due to the moisture flow in the wall. There is evidence that our wall is already performing poorly - certain exterior bricks are spalling, and certain interior bricks in the basement (above the foundation!) show efflorescence. Both symptoms make me think the gap between the brick wythes is a very wet place

So will adding a 3/4" thick layer of foam help protect the brick, or make it worse?

Alternatively, should I be drilling vents in the top and bottom of the exterior wythe to drive airflow in the cavity, or will this just provide a new vector for water to get in? I have an ulterior motive. I'm hoping to remove the interior plaster on one of the walls to expose the brick, but I don't want to do that until I stop moisture (and bug) flow through the brick.

Venting the cavity top & bottom would reduce both the spalling and the efflorescence. Using a sacrificial parge of lime mortar over the areas with efflorescence will also save the mortar longer-term. The efflorescence is more likely to be a "rising damp" issue of groundwater wicking up the foundation than an exterior moisture drive problem, but if the cavity isn't vented it's forcing most of the drying to be toward the interior. Venting the cavity provides a better path for groundwater moisture to dry toward the exterior as well, as well as helping the exterior wythe shed moisture from exterior drives.

Vented cavites between wythes behave as something of a vapor-barrier, providing a neutral zone at roughly the outdoor dew point for both the inner & outer wythes to dry into. If unvented it still provides some protection, but not nearly as much as when open at the top, with weep holes at the bottom to behave as convective air inlets as well as condensation & bulk-water outlets. It's exactly the same thing as a rainscreen cavity, just wider than the 10mm Canadian code-min for other types of siding.

Insulating cavity walls in climates as cold as Ontario are better done on the interior, between the interior wythe and the finish wall. In the (much warmer) UK filling cavity walls with polystyrene beads or blown mineral wool works in some cases, but even there it puts the masonry at higher risk.

Hi Brendan, I am in Toronto and I have been involved in, unfortunately, removing ureaformaldahyde from older double brick walls (back in the 80s). when taking off the old plaster and lath we could see that there was not much insulation at all between the lath and the brick, and usually none between the inner and outer layers of brick, because usually the plaster or morter would make bridging that the poured in foam could not go around.

There are people promoting pour in urethane for this but I think it is bogus. If you really want to insulate and make use of the thermal mass of the brick, some form of exterior insulation is needed. I am in the process of doing this on my double brick house in Toronto as we speak. We have already dug down to the weepers and installed Styrofoam SM (XPS) and for termite control (termites love SM) we made a heavily sealed aluminum cap at the bottom to hold the sheets and coated the insulation all the way above grade.

We are having 4" of a mix of ISOboard and a German urethane product (because I have the German stuff for something else and need to use it somewhere) which will have, first horizontal strapping then vertical strapping then a wood clapboard (not sure if we should put on tyvek or not). Dana could answer that one.

We did most of the exterior basement insulation a few years ago and it dried up the basement beautifully and the spalling we did have, stopped.

Exterior insulation over brick works great, and removes the issues of rot potential where structural timbers penetrate (colder) brick when taking an interior insulation approach. Higher R values are achievable that way too. With an interior approach ~2" of closed cell foam, thinning to ~1" around timber penetrations is pretty typical for an whole-wall R of R11-12. With exterior insulation the thickness is somewhat arbitrary- you can take it as high as you like (or whatever cost-effectiveness & comfort dictate.) Putting the thermal mass of the masonry completely within the thermal & pressure boundary of the building lowers energy use and increases comfort as well.

Where and how to apply the drain-plane material (be it Tyvek or other) in MikeSolar's stackup depends on how the windows are placed & flashed. If the windows are set-back behind the foam and roughly co-planer with the brick, the drain plane goes between the brick & foam. If the windows are co-planar with the outermost foam or siding, it goes between the foam and rainscreen/strapping. Getting the window flashing and drain-plane right is critical to the moisture performance of the building. The brick can take a lot of bulk moisture without damage, but the interior finish materials usually can't.