Basement Finish Prep Work

Discussion in 'Remodel Forum & Blog' started by KSUDVM, Mar 12, 2017.


    KSUDVM New Member

    Mar 12, 2017
    Kansas City
    First off, thanks for such a welcoming environment for the DIY'er. I've been scouring the interwebs and was looking for some confirmation that I'm heading in the right direction and also any feedback before getting to far into my basement finish.

    • Location: Kansas City suburb
    • USDA plant hardiness zone 4a
    • New construction home, foundation poured February 2015, floor poured March 2015. Only daylight in basement is via an egress window (i.e. not a walkout basement)
    • Exterior foundation walls were damp proofed and an exterior drain tile system installed along with a sump pump well and sump pump. As a first time home buyer and first time builder, I was ignorant to how our foundation could and should have been completed at the time of construction, thus the retrofit I'm working on now. Lessons learned that won't be repeated.
    • Code called for a minimum 6 mil vapor barrier under basement slab; this was visualized in place the day before slab pour
    • Moved in June 2015
    • Record rainfall was received Summer/Fall 2015. During one gully washer, we observed approximately 1/2 cup water on foundation wall, originating from what I suspect was a leaky snap tie. The entire seam was injected with epoxy, no leakage observed since.
    • Near record rainfall was received Summer/Fall 2016. One settling crack that formed at the corner of our egress window (diagonally to the floor) leaked approximately 1/4 cup water during another gully washer. This crack was again sealed with epoxy and no leaking has been observed.
    • Efflorescence was observed at the end of last summer over a 40 sqft section of the basement slab. Basement dehumidifier was only ran sporadically as we didn't know the importance of running it continually.
    • All gutter downspouts have been buried at minimum 15 feet from foundation
    • Any settling of dirt around the foundation has been corrected to achieve minimum required slope from foundation
    • I'm working with 104 inches from top of the slab to bottom of the floor joists

    1. I've begun to seal my rim joists using the cut and cobble method. My product of choice is XPS DOW Styrofoam Utility Fit-2 inch thickness. I chose this product because of several references to the fact it does not need an ignition barrier for thicknesses up to two inches. We have since expedited how quickly we desire a finished project and the XPS will all be covered ASAP so my concern about an ignition barrier is now not applicable and am open to using a more environmentally friendly product going forward. I'm using Great Stuff Pro Floor and Wall adhesive to attach the XPS to my rim joists and sealing the edges with Great Stuff Pro Gaps and Cracks.
    2. I'm planning on attaching my 2-inch XPS directly to my foundation walls using Great Stuff Pro Floor and Wall adhesive and Hilti IDP anchors, taped seams and mastic applied. I plan to set the wall XPS directly on the basement slab.
    3. I have electrical stapled to my sill plate (see picture) thus making the installation of XPS from my sill plate over my wall XPS difficult in several locations. Would it be better to raise my wall XPS approximately 1 inch above the top of the foundation (see picture) and fill the gap between my sill plate and wall XPS entirely with Great Stuff Gaps and Cracks or to install my wall XPS flush with the top of the foundation and continue my cut and cobble approach filling the gaps with Gaps and Cracks where electrical is against the sill plate?
    4. I have two cantilevered cavities that extend approximately 24 inches beyond my foundation wall (one fireplace insert cantilever and one pantry cantilever). I'm referencing the second photo in post #17 of this thread ( for guidance on how to insulate the cantilevered sections. My understanding is that I need to remove all unfaced batts currently in place, seal all joints with Great Stuff, refill with the batts (or Roxul) and add a piece of 2 inch foam and new sheathing to the exterior under side of the existing sheathing?
    5. I plan to finish the bedroom with carpet. To make this possible I plan to lay 2 inch XPS or EPS directly on the slab butting to the edge of the XPS to be installed on the wall. I then plan on laying an appropriately thick layer of Huber subfloor which again will butt directly against the wall XPS and attaching both layers using Tapcons. Is it necessary to use another layer of 6 mil vapor barrier (even though there is a layer under the slab) and if so, where at in the layering? My thought is to run the wall XPS to the slab to prevent the Huber subfloor from contacting the concrete wall. It would NOT be recommended to lay the subfloor before the wall insulation, correct?
    6. I plan to finish the remainder of the basement (media room, bathroom and landing) with porcelain tile. To make this possible I plan to lay 2 inch XPS or EPS directly on the slab, also butting to the edge of the XPS to be installed on the wall. I then plan to lay a layer of Durock before tile installation. Is there a benefit to laying the foam/Durock first and having the wall XPS sit on top?
    7. I see no benefit to using Delta FL (or similar product) for my scenario.
    8. I see no benefit to using drylok for my scenario.
    9. Is it recommended to begin framing as close to the wall as being plumb allows or is it recommended to leave a several inch gap between framing and wall XPS?
    TL;DR or To Long; Forgot the Questions:
    1. Am I on the right track?
    2. How should I insulate between the sill plate and edge of my wall foam given that electrical is stapled to the sill plate?
    3. Is the method outlined in the linked thread the best method to insulate cantilevered cavities?
    4. Do I need an additional layer of 6 mil vapor barrier?
    5. Is it best to install the wall foam and butt the subfloor up to the wall or install the subfloor and install the wall foam on top of the subfloor?
    Thank you very much for your time.

    Attached Files:

  2. Dana

    Dana In the trades

    Jan 14, 2009
    IRC code min for DOE climate zone 4A (which is different from plant hardiness zone, but KC is in DOE zone 4A), is R10 continuous insulation, which would meet the requirements at the labeled R of 2" XPS. But XPS loses performance over a few decades as it's HFC blowing agents diffuse out, eventually settling in at R4.2 per inch. If you haven't already bought the material you might consider 1.5" foil-faced polyiso for the walls, or 2.5" EPS, both of which are blown with pentane, which is far more environmentally benign, but does not affect it's end of life R-value, since it's pretty much gone by the time the stuff leaves the factory. Polyiso has the advantage over polystyrene that in the event of a fire it's kindling temperature is higher, and it doesn't melt into a puddle of flaming polymer. It has the disadvantage of being mildly hygroscopic, so the cut edge at the bottom should not rest on the slab.

    If there's room for rigid foam to cover over the ledge at the top of the foundation, sure cut it in. If there isn't (as where you have the wiring under the joists) you may have to can-foam it.

    In climate zone 4A without a vapor barrier under the floor, the foam on the underside of the cantilever needs to be at least 17% of the total R to have reasonable assurance that it can't collect enough moisture at the foam/fiber boundary to support mold. If the batt insulation is R38 or less you should have decent margin with R10 or higher foam. Sealing the framing & framing-to-subfloor seams with polyurethane caulk might be easier and more reliable than getting it with can-foam, but either can work. Be sure to tuck the edges & corners of the batts fully into place with no voids at the corners/edge then tug it out slightly so that it's a compression fit, all to avoid convection paths around the batts, which can undercut performance considerably.

    On the floor slab it's not necessary to add another layer of polyethylene above or below the floor foam. If the floor isn't going to be heated (radiant floor style) R10 would be a bit overkill unless you were going to go better than the code-min on the walls. The vast majority of the heat loss through the foundation is through the above-grade exposed section. With R10 wall insulation, an 8" poured foundation and a finished interior wall you're looking at a U-factor of about U0.085 BTU/hr per square foot per degree-F. At KC's 99% outside design temp of +4F and an interior temp of 68F you have a difference of 64F, for a loss of U0.085 x 64F= ~5.5 BTU/hr per square foot. At the subsoil temp of ~55F you have a temperature difference of only ~13F. With an R10 slab with R1 of finish floor, even ignoring the R-value of the slab & dirt you'd have a U-factor of no more than U0.09, and a heat loss of 13F x U0.09= 1 BTU/hr per square foot. Even R4 would be sufficient for keeping the subfloor above the summertime dew point, but from a total lifecycle heating cost point of view R6-R8 might still make financial sense over 40 years, depending on interest rates and energy price inflation/deflation. Given that, 1.5-2" of EPS is probably the best choice for the slab-foam, not XPS (and not polyiso due to moisture absorption risk.)

    If the wall-foam is polyiso, the floor foam should extend out to the foundation wall, with the polyiso resting on the polystyrene foam. If the wall foam is polystyrene, it doesn't really matter which way you lap them.
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    KSUDVM New Member

    Mar 12, 2017
    Kansas City
    Good Evening Dana-

    First off, thanks for your help across the internet. Sorry it's been several months since I first posted, Spring arrived and I've been splitting time between slowly plugging away in the basement and out in the yard.

    I've poured through enough threads, I'm having a difficult time determining the most feasible yet effective method at insulating my cantilevered section in my basement. This section, which is the build out for my kitchen pantry, extends approximately 3 feet beyond my foundation wall and is four joist bays in length. The bottom of the cantilever is a single piece of LP smart siding. I've used a polyurethane caulk to seal the outside seams and began inside work this evening by removing the old insulation and sealing the seams with spray foam (easier to fit a can of foam then a caulk gun). I found out this evening that I will be unable to work in the fourth joist bay as access to it is nearly closed off (see picture).

    I believe my options are: 1) Seal the seams of accessible joist bays, stack two layers of Roxul comfortbatt to achieve R38 in each joist bay and install a piece of foam board at the sill plate for my air barrier. In the fourth joist bay I can't access I will simply install foam at the sill plate. With this option, is foam board (2 inch XPS I had purchased before creating this thread) outside of the cantilever attached to the LP smart siding a requirement? I briefly read about carpenter ants inhabiting foam board? Do I need to install another piece of LP smart siding to aid in protection of the foam?

    2) I've read your description of packing these joist bays will cellulose. I see this as a feasible option in my scenario as well and it would allow my to have a better insulation option in the inaccessible fourth joist bay then what is there now. The downside is that I would be unable to seal the seams of the fourth joist bay. With this option, is it recommended to also install foam board on the outside of the cantilever?

    I know retrofitting is never going to achieve optimal results; would you have a preference to either option listed or another alternative? Aside from taking the LP smart siding off the bottom of the cantilever I see no way of sealing the seams of the fourth joint space. Even then, I'd be able to seal the seams between the subfloor and joists and would still be missing the seams between the joists and the LP.

    Thanks again.

    Attached Files:

  5. Dana

    Dana In the trades

    Jan 14, 2009
    The bottom side of the cantilever never gets direct wetting from rain or dew, so as long as it's air-tight to the interior the moisture accumulation risk is pretty low. Blown cellulose would share that moisture burden with the wood, but renting a blower @ ~$100/day for such a tiny space seems a bit silly.

    For under $50 you can get a 12 board foot Froth-Pak kit (sometimes at the Home Depot / Lowes type places, but always online), a couple of inches of which would be more than sufficient for dew point control. The foam gun that comes with it is pretty small, and should fit pretty much any place you can get your fist into.


    That's enough foam to cover 6 square feet with 2", or 6 square feet of 1" with enough left over to do quite a bit of air sealing as well. If you can get enough line-of sight to see where it's going, covering & air sealing the subfloor with even an inch of foam would be sufficient to prevent air infiltration though seams & penetrations of the subfloor and lower the vapor permeance enough to limit moisture diffusion to tiny levels.

    For filling the cavity carefully cut up & stuff blocks of rock wool batt for a compression fit, no voids, followed by a vapor retardent interior side air barrier to block air infiltration to/from the basement. Cut-up XPS sealed at the edges with can foam or Froth-Pak is fine to use as the air barrier/vapor retarder.

    KSUDVM New Member

    Mar 12, 2017
    Kansas City
    One additional question now that I'm getting into the thick of this project. I knew my joists cantilevered into my garage, but up to now I didn't realize how insulating this section of the basement would be a bit more challenging. I essentially have two cavities, both varying in length but enclosed by my first level subfloor, my garage Sheetrock/batt insulation/rim joist, the garage floor, the foundation wall and each open space between my joists. The joists extend about 8 inches beyond the garage foundation wall before the rim joist. The garage foundation wall is approximately 8 inches high. I'm dealing with two sections, one approximately 8 feet long and a second approximately 15 feet long. Their solution was to insulate with a piece of batt insulation shoved into each cavity in an attempt to fill each cavity.

    Two things to note. First, my garage is not conditioned but is insulated completely. Secondly, my HVAC supply duct and water heater exhaust duct leaves my basement and travels to the attic through the above explained dead space. It only occupies enough space to make a 90 degree turn and then travels upward from the basement to the attic. I only mention it because if blown in cellulose is an option, I'll have to dam the cellulose with perhaps XPS to keep it from packing up against the two ducts and also falling back out into the basement(see photo). I'm open for all suggestions, including handing this one over to the professionals if what's available to me as homeowner isn't sufficient to complete the job.

    Attached Files:

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