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Community and Q&A

EPS & caulk in Rim Joists: product recommendations

jgreen33 | Posted in Energy Efficiency and Durability on


My wife and I have learned a lot by reading through previous GBA articles and forum topics about the pros/cons of various insulation options.  Thank you to all who have contributed for sharing your knowledge and opinions. 

We need to insulate our basement rim joists (19th century house in southeastern PA) and can’t quite get comfortable with using closed cell spray foam due to the potential health concerns.  So, we’ve decided to use EPS insulation and caulk for air sealing (and perhaps some spray foam sparingly if absolutely necessary around pipes/wires). 

Our contractor is willing to go with whatever approach we like, but has asked us to select the products.  While we understand the basics, we were hoping some here could recommend specific products for the rigid foam, caulk, and (if necessary) spray foam (great stuff?).  Obviously, we’re hoping that the caulk we use won’t have safety/health concerns of its own.

Very much appreciate any input!  Thank you!

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  1. Expert Member
    BILL WICHERS | | #1

    You’ll want to use great stuff and not caulk for this. Caulk really needs a pretty close fit between parts to work well, and you are unlikely to be able to get that tight fit in the rimjoist area, especially around any penetrations. Cut the rigid insulation to allow about a 3/8”-1/2” gap all the way around the perimeter, then fill that gap with great stuff. The great stuff will expand and make a good seal regardless of the exact shape of the gap.

    I’d recommend using the great stuff “pro” cans and the gun for this. You’ll be using enough to make it worth the money, and it’s a lot easier to use than the little one-time cans.

    BTW, most of the issues with spray foam have to do with getting the mix right. Experienced contractors shouldn’t have a problem doing that. I think most of the problems people have seen are new spray foam crews that aren’t very familiar with the equipment yet. There are likely thousands of good applications for every bad one but you never hear from the people who haven’t had problems.


  2. jgreen33 | | #2

    Thanks very much for the reply, Bill. Good to know about the great stuff pro. That said, I think we are committed to caulking the perimeter to limit the amount of spray foam as much as possible. Would you or anyone else have specific brands / products to recommend for the EPS and caulk? Thanks again.

  3. Expert Member
    Akos | | #3

    Get a bucket of acrylic duct mastic. You can brush it on and trowel it to cover gaps, much easier than dealing with a caulking gun. Installed over the fiberglass mesh tape, it does a great job of air sealing and staying put even with some building movement.

    I can understand the reluctance with spray foam but it is the best way to do this air sealing. One part spray foam in a can is a different material than the spray applied stuff, it is pretty benign. The problem with most 2 part systems is that if the mix is not right, the foam won't cure. The one part doesn't have this issue.

  4. jgreen33 | | #4

    Thanks Akos, we only discussed the caulk approach with our contractor but I will mention the acrylic duct mastic and see what he says. Also, point taken on the spray foam, but we have decided to err on the side of caution and limit its use as much as possible.

    Appreciate any comments on our product selection if we can do better or there's something missing/incompatible. Perhaps this list will be helpful to someone else in the future:

    EPS rigid foam:

    Caulk for perimeter (thank you Charlie Sullivan):

    Canned spray foam for penetrations:

    1. Expert Member
      BILL WICHERS | | #5

      I think you are confusing spray foam with the great stuf material, which isn’t exactly the same thing.

      Spray foam as is usually discussed when there are problems is either closed or open cell sprayfoam. This is the stuff you often see abbreviated as “ccSPF” or “ocSPF”. Sometimes even just “SPF”. Either are two-part materials that are mixed onsite along with a gaseous blowing agent. The cure primarily happens by the two chemical compounds reacting with each other. If the mix is off, or the temperature of the applicator is incorrect, you can get an impartial cure. This is where the messy goo, smell, and offgassing complaints come from. This is why anyone using spray foam should use an experienced contractor to make sure the spray foam is applied correctly.

      There are also two-part spray foam kits for small projects. These kits are very expensive, and have the same potential issues with the chemical mix mentioned above.

      Great stuff is “one part” foam. It’s not the same as spray foam. Great stuff cures by exposure to moisture in the air. There is no chemical mixing to worry about. I am not aware of any complaints about offgassing with great stuff, and there is no concern about a bad chemical mix since it is only one material — there is no mixing.

      If you’re concerned about using spray foam that’s ok, but I wouldn’t lump the one-part foam like great stuff in with the two-part foams that you sometimes read about people having problems with. Great stuff is going to be far more effective for you in your application than caulk or mastic.


  5. jgreen33 | | #6

    Thanks Bill for continuing to drive home the point Akos raised. I've looked around a bit and haven't come across any off-gassing complaints with GS. Using GS will certainly simplify things. With all the concern surrounding ccSPF, I wonder why no one is selling a one part foam in quantities large enough to do the entire job? My understanding is that Great Stuff is polystyrene and thus would provide the same R-value as that not right?

    1. Expert Member
      BILL WICHERS | | #7

      I think great stuff is polyurethane based but I’d have to check to be sure. It is lower density than the two part closed cell spray foam and much less tenacious in terms of stickiness (that may be hard to believe if you’ve worked with it, but it’s true). Great stuff doesn’t expand as much either. They are very different products intended for different applications.

      Note that you’ll want the regular stuff, not the more squishy “door and window” version of the foam. I also recommend wearing nitrile gloves while using it. Great stuff isn’t really hazardous, but it sticks like glue and is hard to clean off (acetone works while it’s still gooey though).

      If you use the one-time cans and not the “pro” gun, check your glove periodically and make sure the push-down part of the can hasn’t worn though your glove to let great stuff get on your skin. It’s a pain to clean off. Ask me how I know...


  6. jgreen33 | | #8

    Thanks again for all the help Bill. I was going to get the pro version of the GS "Gaps and Cracks" foam. One last question if you don't mind...our basement is unfinished but houses the furnace, water heater, it recommended to use a faced product because of flammability concerns or is the unfaced product I linked considered ok because it's not a finished living area? (we have no intention of putting drywall up)

    1. Expert Member
      BILL WICHERS | | #9

      The facing is usually for vapor control or manufacturability and not fire resistance. There are some products out there that DO have a facer rated for fire resistance, the one that comes to mind is Dow Thermax.

      If you’re just putting little pieces in around your rim joist area those can usually be left exposed. The code allows for exposed insulation like this in that particular area. If you were insulating your basement walls, then you’d either want to use a fire rated product or put up a fire rated barrier material like drywall. Most rigid insulation is not rated to be left exposed in accessible areas.


  7. jgreen33 | | #10

    Gotcha, thanks Bill!

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