Finishing basement interior brick walls
As part of an addition (including a basement addition), some of our existing basement exterior walls (double wythe brick foundation) will become interior walls. The new foundation will be detailed appropriately (including drainage) and insulated with 2+ inches of EPS on the inside walls, finished in drywall. I’d like to drywall the brick walls that will become interior walls as well. The basement is overall dry even though there will be no footing drain on the to-remain portion of the house.
Given that a brick wall (even if interior only) is somewhat of a thermal bridge to the uninsulated footing/slab, should I be concerned about installing drywall over furring to this surface, or should I look to get some foam on it to be safer?
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Patrick,
It would be useful to know the climate zone or geographical location of this house. The colder the climate, the more you might have to worry about thermal bridging issues in winter.
Apologies! I meant to add that when I originally posted. New Jersey/4A.
IRC code min foundation insulation for 4A is R10 continuous insulation. With 1.5 of foil faced polyiso and an the furring/air-gap you'd be there, but with EPS it'll take 2.5" minimum.
The subsoil temps in 4A NJ aren't really all that cold- above 50F, and brick has a real enough R-value- a foot above the footing you'd have ~R2- R2.5 between the footing and the rest of the wall. The soil has R-value too. Unless the slab is poured on damp clay soils (not likely, if it's a pretty dry basement) your slab temp and wall temp will always be well above the deep subsoil temperatures in a heated finished basement.
Where the brick meets the exterior walls would be a more relevant thermal bridge. Installing 3/4" foil faced polyiso on both sides of the brick out to a couple feet from the exterior might be "worth it".
Patrick,
Unless you are insulating the floor slab, that will be a significantly higher heat load than the walls, though an inch or two of foam on the walls wouldn't hurt. Pay special attention to air sealing the old brick walls, especially at the top where the wood framing sits on the brick. Air leaks in old foundations are the biggest contributors to heat loss.
You should also be careful of moisture management. Those old brick walls are constantly wicking moisture up from the soil below. The moisture evaporates naturally into the basement and exterior right now, but when you cover them, you're trapping some of that moisture inside the walls. I always recommend repointing the walls before covering using mortar compatible with the original. If these are more than 100 years old, the mortar probably needs to be custom mixed - the bag mixes are generally not appropriate. Once repointed, apply a vapor barrier paint. Drylock or Thoroseal are both available locally. Also always use moisture/mold resistant drywall in finished basements, especially brick ones.
And warn your contractor to be careful around the brick walls. Most old brick walls have no footings. If your new basement is going to be deeper than the old one, you will need to underpin the brick walls to stabilize them and prevent future settlement.
Thanks Peter.
> Unless you are insulating the floor slab, that will be a significantly higher heat load than the walls, though an inch or two of foam on the walls wouldn't hurt.
I do plan on putting about R-3 on the floor of the existing basement before putting a subfloor on top of it.
> You should also be careful of moisture management. Those old brick walls are constantly wicking moisture up from the soil below. The moisture evaporates naturally into the basement and exterior right now, but when you cover them, you're trapping some of that moisture inside the walls.
This has been something I've been mindful of. Comments I've read from Dana and others seem to suggest that this is relatively low risk in 4A, though considerably more risky further north. The two wythes have an air space between them, so perhaps this will facilitate some overall vapor movement and drying to the exterior, even if I have foam inside. (Obviously this doesn't work with the walls that will now be interior only.)
> I always recommend repointing the walls before covering using mortar compatible with the original. If these are more than 100 years old, the mortar probably needs to be custom mixed - the bag mixes are generally not appropriate. Once repointed, apply a vapor barrier paint. Drylock or Thoroseal are both available locally.
The brick and mortar inside appears to be in very good condition, despite being 100 years old. There is some paint peeling here and there, but zero spalling that I've been able to note. Should we need to do some repointing, I'm familiar with Lime Works in PA as source for compatible mortar.
> And warn your contractor to be careful around the brick walls. Most old brick walls have no footings. If your new basement is going to be deeper than the old one, you will need to underpin the brick walls to stabilize them and prevent future settlement.
My mason is one step ahead of me, fortunately. He warned me of the same thing and is prepared to underpin as necessary. He just finished doing it on another 100 year old home in my town.
Thanks again. I happened to notice yesterday that you're a PE and not too far south of me. If some other aspects of our significant renovation get a bit uncertain, I may be giving you a call. ;-)