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Help with air sealing our 1920s sill plate – and plans to finish the basement

homeowner007 | Posted in General Questions on

We’re finalizing the plan to finish the basement of our 1920s, 1,500 sqf, 2 stories, daylight basement in zone 4a. The plan is to lower the basement slab (requiring underpinning). We’ll also waterproof the exterior walls and add exterior insulation. We haven’t decided on the details yet – party because we haven’t found a reference assembly to guide us.

There isn’t a band joist per se in the assembly – instead, there is a 4” x 6” sill plate and the joists are notched to sit on the sill, next to the balloon frame studs. The sheathing is 1” t&g, with tar paper and lap siding. Eventually, we’ll update the siding assembly with a WRB (Henry Blueskin VP100) over the t&g sheathing, new siding, and dense pack in the walls – we’re holding off on defining the wall assembly further until we decide how to handle insulating and air sealing the basement as there are some dependencies between them.

The particular construction is illustrated in the following rendering and photos that I created for this post:

We’re looking for help on two major questions, but open to any thoughts you have.

1)    The CMU below grade is a smooth finished cinder block. ~6’ below grade assembly will probably get damp proofing, dimple mat, and 2.5″ of EPS. But we would like to preserve the ~3’ of split-faced cinderblock above grade – albeit with industrial epoxy paint to seal it.

Questions: Anything we should be concerned about if we don’t insulate the top ~3 feet of the basement wall? Any ideas on sealing the top of the edge of this assembly at grade?

2)    I’d like to reduce the stack effect and air leakage in the basement. I was thinking of using 2” THERMAX against the sill and firestop block with beads of canned foam. But even though I’ve seen 100’s of assembly illustrations I can’t confirm that this is going to work in our particular situation.

Questions: Is there any harm in sealing the inside of our sill with THERMAX before the new siding and WRB over the next year? Can we air seal around the sill and firestop from the inside, and fill the CMU cell voids? Or maybe wait to fill the CMU voids until the damp proofing is done? I’m concerned that we’re eliminating the ability for the wall to dry out – which could create issues we don’t have at this point.

Notes on the foundation and framing:

– 10 ½” cinderblock CMU foundation wall, cells are empty
– Finished depth of the foundation wall (after underpinning) will be ~6’
– New basement slab will have a VB and be insulated to R10
– Adding interior and replacing exterior drainage. Adding a sump pump

thanks in advance for your thoughts!

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  1. b_coplin | | #1

    I'll make a few comments to give your question a bump.

    Try to get a capillary break between the block foundation and the sill plate, no matter what you end up doing. If the sill plate is 3 feet above grade, your risk is probably fairly low, but it would be better to have a capillary break.

    It seems simpler to work from either the exterior or the interior, rather than trying to transition between the two. Exterior work will likely be more difficult and expensive, but probably work better. Interior work would be easier and preserve the above grade block look. Take a look at this for working from the interior:

    You have cinder block, which would allow you to use less expensive/carbon intense sheet goods on the interior, rather than the closed cell spray foam described in the article.

    Another member shared this interesting case study with me earlier this year, should you decide to work from the exterior:

  2. homeowner007 | | #2

    Wow! thank you for the thoughtful response, Bryan. Until you mentioned it, I hadn't recognized that providing a capillary break is the first step in sealing up the basement. As it is today, the sill shows zero signs of rot - but I get your point that if we sealed everything up from the interior then some of the moisture entering through the CMU would naturally have less drying potential and get drawn into the sill. Quoting Dr. Joe from the link you sent: “Energy and airflow is no longer going to be available to assist in drying. Wetting from underneath needs to be eliminated and wetting from the outside needs to be controlled..." but once the new slab is installed and the underpinning is finished (filling the bottom cells of the CMU with concrete) and the exterior damp proofing it done, the CMU should not see bulk water or capillary wicking. If that's accurate do you think it is plausible to insulate the exterior below-grade and not insulated above grade other than the sill plate/CMU connection and the ceiling - leaving a 3' section of the wall (and the daylight basement windows) uninsulated? Also, the case study on the method for exterior insulation was fantastic! thanks for that too!

  3. b_coplin | | #3

    Omitting a capillary break is probably a very small, but non zero, risk. I would be curious to see if others have views on this. My point is that if you are going to the trouble of excavating and insulating on the exterior, I think it would be wise to install a capillary break.

    The 3 feet of exposed foundation represent the lion's share of the heat loss through the assembly. If you want to insulate this portion on the inside, fine, but it should be insulated.

    Referring to your drawing, the strips of Thermax will be about 7" tall to insulate/air seal the rim joist. Cut a 7" strip off of the long edge of each sheet, then use the remaining ~41" x 96" sheets to insulate the top ~41 inches of the foundation wall. Build a stud wall on top of the polyiso.

    If you cover the studwall with 1/2" drywall, (including the sill beam area), you could forgo the Thermax (which is pretty spendy) and use regular polyiso. You might be able to find reclaimed foam sheets on Craigslist. Also, consider going with 1" rather than 2" foam, and filling the stud bays with batts. (Likely to be cheaper still, while offering better performance).

    My suggestion would be to cut blocking to support the bottom of the batt; you would use fiber insulation in front of the foam-insulated portion of the wall only. The bottom portion of the wall would be empty to run wires, water lines, plumbing, etc.

  4. homeowner007 | | #4

    thanks Bryan - a lot of great ideas. I was thinking about the capillary break a little more. Maybe another way to do this would be to install the break under the existing CMU as part of the underpinning. Going this route would alleviate needing to jack up the house or create a large enough gap to a slip the break in at the sill. It would be interesting to model the difference of placing the capillary break at the top, or near the bottom, of the foundation walls. I don’t have those skills but there may be some folks that are reading this might take a stab at this. thanks again.

  5. homeowner007 | | #5

    this video is one of the best I've come across

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