Image Credit: Image #1: GBA / Joe Sweeney
More Musings of an Energy Nerd
If you visit the basement of an older home, you may notice that the floor joists above your head are embedded in the foundation walls. This is rare in a modern home, where floor joists usually rest on a 2×6 pressure-treated mudsill that is bolted to the top of the foundation wall.
In a house with embedded joists, the top of the foundation wall is often level with the tops of the joists. This construction method required builders to frame the floor (supporting the joists with temporary beams in some cases) before the wall was complete, and then to finish the wall by placing concrete around the ends of the joists. The same method was also used for some homes with stone-and-mortar foundations or brick foundations.
Builders used to embed joists in the foundation walls to prevent air leakage into the joist bays. While it’s true that poured concrete is a decent air barrier, embedded joists have a downside: the joist ends are susceptible to moisture accumulation and rot.
Interior insulation makes the situation worse
The ends of most embedded joists don’t rot, of course. While these joist ends face a daunting number of moisture sources — capillary moisture rising up the foundation wall, condensation of moisture from the interior air, and foundation moisture from wind-driven rain — the lumber is also able to dry out when conditions are favorable. Joist ends on the south side of a building usually have a lower moisture content than joist ends on the north side, since sunlight warms a foundation and helps keep embedded joists dry.
If homeowners get tired of the energy losses associated with uninsulated basement walls, and decide to install interior insulation, there’s a potential for greater problems. In a basement with embedded joists, the new interior insulation will now make…
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19 Comments
Oddly, my house (built in 1954) has joist hangers with a foundation similar to the picture above whereas a neighboring house (built in 1951) has joists resting on a masonry wythe somewhat similar to the photo above. However, the joists are not embedded. Perhaps embedding the joists would have kept some of the joists from twisting as they have over the nearly 70 year period. The twisting joist occurred mostly under the bathroom. Those joists supported a few inches of mortar until the owner removed it recently while remodeling the bathroom. Sometimes it good that "they don't build them like they used to."
Hi Martin. One of the better resources on this subject for practitioners is BA-1508: Analysis of Joist Masonry Moisture Content Monitoring (https://buildingscience.com/documents/building-america-reports/ba-1508-analysis-joist-masonry-moisture-content-monitoring). An annoying 80+ page report, but some key takeaways buried in all the research-y geekiness:
- The European researchers did a literature search, and referred to a field survey where they looked at joist ends in insulated mass masonry buildings. The takeaway is that if there are visible, macroscopic cracks near the embedded joists that let in bulk water/rainwater, that's what causes problems. But if the masonry is in good condition, they didn't observe durability problems (see 2.4 Morelli and Svendsen (2012))
- Putting the embedded joist below grade is *the* way to destroy them with moisture damage. This sometimes occurs when grade is pushed upward over a building's lifespan. In any event--a terrible idea, with sometimes-terrible consequences (see pages 10-11, under 2.7 Field Observations of Embedded Wood Members).
- We pulled a "stub" of a joist end that was cut off during the renovation out of the wall. This joist end had *no* insulation (ever), and ran both through the life of building--both heated and unheated conditions. It smelled like mold, and was a little bit soft... but no punkiness or structural damage. A lot of observations that corroborate our field measurements: orientation matters (west stub was fine, east stud was wet, highest MCs at bottom of joist), the wood survived high MCs without a problem (wood from that day was sterner stuff than what we build with nowadays), and the joist ends accumulate moisture even without insulation... but seem to survive. (see 6 Joist End Samples).
- I won't try to summarize the entire conclusions section here--it's just six pages, take a look (7 Conclusions).
- Our guidance to practitioners is in 8 Guidance for Retrofit Projects. Again, worth reading in full, I hope. One useful technique--if everything is looking pretty safe, but you're still worried and want to be a bit more conservative, consider using borate rods, drilled into the embedded joist ends--they are used for preservation of outdoor wood bridges and similar structures. The beauty is that if the joist end gets wet, the borate will start to dissolve and spread out (see 8.6 Joist End Preservation (Borate Rods)).
- Options for cutting off the joist ends and supporting the floor otherwise can be found in 8.7 Elimination of Embedded Beam Condition.
Kohta,
Thanks for taking the time to post your detailed and helpful comments. In my article, I have added a link to the excellent resource, “Analysis of Joist Masonry Moisture Content Monitoring.”
I have also added information in my article on the use of borate rods.
Martin,
I also read Joe Lstiburek's BSI #105 on "Avoiding mass failures." This was a small departure it seems from what he wrote about earlier in BSI #80.
In the earlier article he recommended just being able to use high density SPF against the brick. In the new article he recommended a fluid applied water control layer before applying SPF. I was a bit confused about the need for the extra water control layer, doesn't high density SPF do that as well?
Thanks
https://buildingscience.com/documents/insights/bsi080-tailor-made
https://buildingscience.com/documents/building-science-insights/bsi-105-avoiding-mass-failures
Malcolm,
I can't speak for Joe Lstiburek. While closed-cell spray polyurethane foam is generally an excellent air barrier if it is applied at a thickness of 3/4 inch or more, some spray foam installers make mistakes and leave voids. It's possible that the fluid-applied water-resistive barrier is applied (a) as insurance against imperfections in the spray foam, and (b) to ensure that there is no possibility that wind-driven rain will find its way indoors.
This type of insulation retrofit work is expensive and hard to repair if done wrong, so it makes sense to get everything right the first time. A belt-and-suspenders approach isn't a bad way to proceed.
Hi Martin,
I have a similar assembly as shown in the first picture of this article but with a slight twist. Joists rest directly on a 6” wide concrete foundation, however I have more of a concrete sandwich between the joist: 2x8 bridging flush with concrete wall—behind that about 4” concrete—then the 2x8 exterior rim board (see picture).
I could insulate, as is, but that seems to leave 6” of wood and concrete to dry to the outside, not to mention the possibility of cold floors above near the wall.
Should I remove the interior 2x8 and chip out the concrete between the joists to get better insulation? Or, in other words, would it be worth my time to do that?
I live in North Dakota.
Thanks for your thoughts!
Steve
Steve,
If this were my house, I wouldn't chip out the concrete. I'd leave it where it is, and I would insulate on the interior side of the rim joist assembly. As with any rim joist insulation job, it's important to pay attention to airtightness. Good luck.
Thanks, Martin. As I believe you have mentioned before, remodels are a series of compromises!
Martin,
Is there any issue with removing the cement between the joists and then insulate the space? It does seem that at least one was removed at some point to run a pipe to the outside spigot. I have a home that was built in 1914 and in a cold zone 6 climate. The concrete seems like it was poured on top of my concrete block foundation. Other than it taking an extremely long time to remove, could there be any issues with this? I can't find a lot of information on the subject.
Thank you.
Emily,
It would take a site visit to answer your questions. If the concrete was installed between the joists after the structure of the house was complete, it may be possible (although quite difficult) to remove the concrete. However, in some buildings this concrete will be structural, and may support the wall above. If the concrete is structural, you wouldn't want to remove it.
Hello,
Basically I’m reading that there are 2 approaches to this situation, insulate on the exterior face (best) or on the interior and cut the joists free of the foundation - supporting them on the new 2x6 wall. Is there a third direction where you could cut the floor free of the foundation and lift the house supporting it on a new 2x6 pony wall that sat on the foundation? Any special considerations be for that method?
Thank you from inside a ‘54 bungalow in Calgary AB.
BDGartner,
The best place to start is with a section through whatever option you propose. How will the joists if cut free still bear on the existing foundation walls? Can pony walls extend down to the existing footings or do they necessitate pouring new ones?
Of all the options raising the house involves the most complications and expense. Extending the plumbing, electrical, and interior walls or posts, providing access for the equipment necessary to raise the house, and building new entrances to compensate for the increased height from grade. That said, it's the one option that also may expand the usable living area.
Houses in Calgary benefit from the dry climate which makes things like embedded joists much less of a problem than elsewhere. You may find they are fine now, and will continue to be if you insulate while leaving them where they are.
Hi Malcolm,
I’ve attached a couple sketches: at left is generally the existing condition at right is the structural change - I haven’t sketched the new exterior cladding or the insulation above and below grade. I’m just trying to weigh the full cost/sequence of each of the 3 or 4 options. The fourth being a frost wall on the existing without cutting the joists free as you say this may be fine in Calgary. It’s a small 900 sf bungalow so any increase in living space is welcome and likely cheaper than an addition so the lift might be nice/worth while especially if it results in a more resilient wall assembly.
Any thoughts or considerations for the lift in the sketch? Likely looking at around 2” of exterior semi rigid insulation that would transition to an interior insulated frost wall. I could easily get the exterior semi rigid to extend a foot down from the cut line without demolishing sidewalks etc.
Thanks again for any thoughts.
BDGartner,
It's all doable - and unfortunately only a contractor in Calgary can give you an idea of what it will cost right now. If the aim is to0 increase space it might well be the best option, but if it is to make the house more energy efficient, I'd work on every other part of the house before deciding to lift it or even worry about the rim-joists beyond air-sealing them.
If you do decide to lift the house I'd go high enough to provide space in the framed portion of the basement walls for good sized windows.
Great info; thanks. Any thoughts on this situation? In Lawrence, KS (zone 4A) with stone foundation and stone and mortar between joist ends. House has brick skin on first floor/limestone block foundation where it is visible. Some of the stone/mortar fill between joists has been removed over the years where pipes or wires were run, leaving the inside of the limestone blocks exposed.
As I need to frame and blow insulation to insulate the walls, does it make sense to limit to 1" at the joists? Is it wishful thinking to imagine that the joists will "breath" to the outside? (Note the limestone blocks have been repointed with the the correct mortar, not concrete.)
Thanks again, Dale
Dale,
I'm not sure what type of insulation you intend to "blow" against the stone-and-mortar between your joists. If this is a foundation wall, you don't want to install blown fiberglass or blown cellulose. Instead, you'll probably want to consider using closed-cell spray foam. (For more information on insulation choices for this location, see these two articles: "How to Insulate a Basement Wall" and "Insulating Rim Joists.")
Concerning the moisture risk to the embedded joists, my advice in in the article on this page. Risks vary, depending on the factors listed in my article. You'll have to perform your own site inspection and assessment before proceeding.
Thank you Martin,
Yes, I am guilty of not being correct with my use of terminology. As you note, I plan to use closed-cell spray foam. I appreciate you responding.
--Dale
I am not trying to insulate my basement walls, but I am trying to seal the air channels that go from basement to attic (low slope roof) between the uninsulated interior of the brick walls and the lathe. Those channels are behind stones nestled between the joists (see photograph).
If I airseal the foundation rim (which is ~4' above grade) between the joists, it seems to me that I could cause a similar problem that this article about insulating the basement walls. Or, perhaps not, because there is still plenty of drainage/moisture movement down the uninsulated stone wall below the rim?
I did read https://www.greenbuildingadvisor.com/article/insulating-old-brick-buildings, which I got to from Holladay's response #2 to https://www.greenbuildingadvisor.com/article/reviving-old-masonry-buildings, but, as the title suggests, it is about how to properly insulate this type of construction, as opposed to airsealing the channel.
I would appreciate a recommendation on how to seal those channels. Thanks!
Samuel,
If these joists are 4 feet above grade, and you don't have vegetation close to your foundation, I'd be inclined to insulate and air seal the stone between the joists with closed-cell spray foam. It might also be worth removing the baseboards on the first floor, and removing a strip of plaster near the floor, to airseal the crack near the floor behind the laths. But that's a lot of work (and repair work).
If you follow this advice, be sure to keep an eye on the embedded joists to see if there are any signs that they are accumulating moisture.
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