Opinions on Treating Embedded Joists in Brick Basement
Hello all,
We’re turning our unfinished Chicago basement into a living space and I’ve been working hard on a plan for insulating the ~80-year-old brick & concrete walls (below grade is poured concrete and upper 2/3rds is brick). After a number of interesting/informative/frustrating/enthralling rabbit-holes on the risks and best practices of old brick basement insulation, I think I have an acceptable (and affordable) plan.
It includes a combination of 2″ EPS on the majority of the wall, 2″ spray foam along the lower third (where the electrical and heating pipe will run), 1″ spray foam around the embedded joists, and Rockwool w/ a smart vapor retarder (maybe not needed?) to help reach the required r-value. The spray foam I’ll be doing myself with help from a friend who has experience with it.
One of my remaining questions is whether I should do anything additional for the embedded joists. From the reading it seems I could…
A) Inject the joists with borate rods
B) Insert metal plates.
C) Run the pipes for the baseboard heating along the ceiling below where the joists enter the walls, which in theory would provide some heat in those areas. I can’t find much about how to go about heating joists but wonder if this would be a reasonable option. This will slow me down and cost a bit more, but not a ton.
D) Of course, my preferred option would be to just insulate with foam and not worry about it. Ha.
**The practical Yankee solution of reinforcing and cutting scares me a bit. Would rather not disturb the joists at all, and it’s beyond my skillset. **
Thanks to all who have weighed in the community on all these issues in the past! It’s been hugely helpful. If anything else in this plan seems suspect, I’m welcome to all feedback.
***Note: The basement is pretty dry and exterior water issues have been addressed as best as possible – ground pitched away, downspout drain directs far from the house, masonry tuckpointed a year ago. Joists all are dry and in good shape.***
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Replies
I’m giving your question a bump. You are right to be giving thought to those embedded joists. Joseph Lstiburek and Kohta Ueno from Building Science Corporation conducted a field study of embedded joists at a brick building in Lawrence, Mass (climate zone 5). The problem of joist-end rot in multi-wythe brick buildings is similar to the problem of joist-end rot in residential basements with embedded joists. “In both cases, interior insulation makes the situation worse.” While you wait for experts to offer opinions on your situation, check out the article to learn more about the study findings and their advice for homeowners. (They talk about a few of your suggested ideas): Insulating Basement Walls With Embedded Joists.
Thanks Kiley!
Yes, that's a great article. I've been told by my family if I mention basement insulation or the names John Straube, Martin Holladay, Joseph L. or Kohta Ueno I'm going to have to sleep in the yard.
It seems like there is still some debate exactly how big an issue this is, depending on climate. Chicago is somewhat like Lawrence and Winnipeg in terms of temp and humidity. Both places are mentioned in Martin's article, and though one is data-driven and the other is anecdotal, their findings suggest different approaches.
I also should have added that I plan on continuing to run a dehumidifier. Moisture content in the basement during winter without a dehumidifier seems to be consistently around 50%, and 65% in the summer. This is before we installed drain tile, put in an overhead sewer and patched some small cracks.
Anyway, thank you again. More thoughts are certainly welcome!
Hi Ikibbe--
You've definitely done a good pile of 'due diligence' with exterior grading, repointing, and directing surface water and ground water away. These are typically the most important steps. A European researcher did a survey of joist ends post-insulation, and found problems were specifically linked to macroscopic cracks and bulk water leakage, wetting the beam ends. Another disastrous failure occurred when exterior grade was built upwards, putting the joist ends below grade... and they turned to mush.
As you can tell, embedded joist proximity to grade impacts risk--due to both ground water/wicking and splashback. Some of Joe's thoughts on the relative areas in the last figure in this article.
BSI-011: Capillarity—Small Sacrifices
https://www.buildingscience.com/documents/insights/bsi-011-capillarity-small-sacrifices
As to what to do: my thought is that installing borate rods is not that bad of a retrofit to do, and buys a good amount of safety. If you haven't seen it, the conceptual detail I'm thinking of is in Figure 72 in this report:
BA-1508: Analysis of Joist Masonry Moisture Content Monitoring
https://www.buildingscience.com/documents/building-america-reports/ba-1508-analysis-joist-masonry-moisture-content-monitoring
It is also useful because it can give you an 'early warning' of rot issues deep inside the joist and the pocket. If this is too much of a pain, perhaps consider adding borate rods to the highest risk joists--the ones near grade, or on the north or east facing side of the building.
Or you could just go ahead and install the spray foam without doing anything else.
Sorry I can't give more definitive guidance--it would be great if there was a huge population of retrofitted buildings where we could suss out where the 'edge of failure' occurs... but I do not know of such a data set.
Good luck!
Hi Kohta,
I have a very similar situation. When you say, "Or you could just go ahead and install the spray foam without doing anything else.", is that specifically because the plan involves only 1" spray foam between the joists? Or would more foam probably be fine too?
What's the energy penalty for doing R-20 from floor to bottom of joists with R-6.5 between the joists versus universal R-20?
Thanks,
Drew
This is great, thank you Kohta!
(Also, very cool to have the people who are writing “the books” on these things responding to the questions).
I’d read that capillarity article early in my research and don’t think I understood it, nor retained it so it was great to re-read. Thank you.
After re-reading that and your paper on Moisture Content Monitoring, and I think I’ll do as you suggest and install rods into the most vulnerable/high risk joints. All the joists are about 4’ above grade and the bricks seem to be in good condition. There isn’t a capillary break between the concrete and brick, but hey, you can’t always get what you want in a retrofit.
Thanks again for the feedback Kohta.
Here is just some extraneous house info if anyone is interested or working in somewhat similar conditions...
The house faces East-West on a corner. The southern wall is largely protected from wind and driving rain by another house, but the northern side is open. The eastern (rear of house) is an enclosed, vinyl-sided porch and the western front of house has some larger trees that keep it shielded.
The house was actually moved to it’s current location in the late 50’s to make way for the Kennedy Expressway that runs through Chicago. It was also raised during the move, which is probably why the lower course of brick is in better shape and also “better brick” (so I’ve been told). The move did cause some settlement and foundation issues, likely due to poor backfilling, but after consulting with a structural engineer and many others, I’m relatively certain that the settlement finished a long time ago (fingers crossed). I’ve been in the house for 13 years, owned it for three and never noticed anything. The recent tuckpointing shows no signs of opening. The only flooding has been surcharge from the cities aged combined sewer system, which is why we also installed an overhead system.
Just some interesting quirks.
Anyway, thanks again!
This has been a super helpful conversation - thank you!
I am in a similar situation with partially embedded 2x7 joists in a 1940 poured concrete foundation (about 4.5" of the joist is in concrete, 2.5" above). Any recommendations on what size of borate rod to insert? The foundation wall is about 8" thick. The joists ends are about 30" above grade on the north side of the house.