GBA Logo horizontal- lakesideca.info Facebook- lakesideca.info LinkedIn- lakesideca.info Email- lakesideca.info Pinterest- lakesideca.info Twitter- lakesideca.info Instagram- lakesideca.info YouTube Icon- lakesideca.info Navigation Search Icon- lakesideca.info Main Search Icon- lakesideca.info Video Play Icon- lakesideca.info Audio Play Icon- lakesideca.info Headphones Icon- lakesideca.info Plus Icon- lakesideca.info Minus Icon- lakesideca.info Check Icon- lakesideca.info Print Icon- lakesideca.info Picture icon- lakesideca.info Single Arrow Icon- lakesideca.info Double Arrow Icon- lakesideca.info Hamburger Icon- lakesideca.info TV Icon- lakesideca.info Close Icon- lakesideca.info Sorted- lakesideca.info Hamburger/Search Icon- lakesideca.info
Musings of an Energy Nerd

Insulating Basement Walls With Embedded Joists

If the ends of your floor joists are embedded in the foundation walls, assess the situation carefully before you insulate

No visible rim joist. In a house with embedded joists, the top of the foundation wall is usually at the same height as the top of the joists. If you have this kind of house, you'll see concrete or masonry between the ends of the joists.
Image Credit: Image #1: GBA / Joe Sweeney

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.

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 the joist pockets colder, and therefore…

GBA Prime

This article is only available to GBA Prime Members

Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.

5 Comments

  1. Antonio Oliver | | #1

    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."

  2. Kohta Ueno | | #2

    Hi Martin. One of the better resources on this subject for practitioners is BA-1508: Analysis of 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.

  3. User avater GBA Editor
    Martin Holladay | | #3

    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.

  4. Malcolm Lewis | | #4

    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

  5. User avater GBA Editor
    Martin Holladay | | #5

    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.

Log in or become a member to post a comment.

Related

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |