Question on foam board at bottom of cantilever floor insulation

Discussion in 'Remodel Forum & Blog' started by LesterBK9, Aug 24, 2018.

  1. LesterBK9

    LesterBK9 New Member

    Aug 24, 2018

    Just found this forum and decided to post question.

    I am working on insulating cantilevered floor ( 2' deep, 16 wide ). Home is in Connecticut ( zone 5 ? ), built in the late 70s.. There is direct access to the bottom of soffit. I had frozen pipe problem last winter when temperature was at -5 degrees for over a week, and looking into getting the pipe moved if feasible. Regardless, the floor gets cold, and insulation improvement is clearly needed. I opened up the soffit, and found 2" EPS loosely attached to framed blocking with Batt insulation above. But it did not appear it was sealed well. Also, there were no blocking between joists at edge of bearing wall sill. I also see signs of old infestation of some creatures ( bee, mice ??? ) Clearly, if bugs can get in, so could air. This is what I am doing and with questions.

    I am throwing away the old insulation, by the way.

    1. Blocking joists with 1" wood piece, and seal all corners with foam. Or, do I need to use EPS board for blocking ?

    2. Attaching 1" EPS board to subfloor and seal corners. I attempted with 2" EPS, but floor has a lot of nails and staples sticking out under subfloor. 2" is too bulky, and leaving space. I figure that it is better to use 1" board so that I can attach board tighter to the floor. I assume glueing is fine, correct ?

    3. Joists are 2x8 @ 16" OC> So that leaves appx 6.5 " . I plan to use R30 Rockwool to stuff cavity. ( 7.25" insulation stuffed in 6.5" cavity. Is that ok ??? )

    4. Big question. Shall I still use 1 or 2" EPS board at bottom of Joists ? Energy Star ( ) states you would need to put board under floor joists and seal completely. It makes sense for insulation, but, that also sounds like it is considered a vapor barrier ( foam without lining ) ? With EPS under subfloor and bottom of joists, would there be a concern of moisture trapped in the sealed cavity causing framing to rot ? What is the recommendation here ? However, given the issue I had, I am also reluctant to reduce the insulation.

    5. I will most likely use painted 3/8" ply to seal the bottom. Once again, I assume I shall seal tight at edges and joints between... Comments ?

    Thank you for your advice in advance.

    Thank you
  2. Dana

    Dana In the trades

    Jan 14, 2009
    Is any of the plumbing (heating or potable) in cantilevered part of the floor, or is it all inside the boundary of the foundation footprint? If any of it is outside the foundation, how far is it from the subfloor?

    1> If by "blocking" you are referring to the air barrier where the joists cross the foundation sill, no that does not have to be EPS. Plywood or OSB or 1x milled lumber would be fine, but it does have to be air tight. Sealing the seams with polyurethane caulk (not the cheap stuff) is usually a better long term air seal than can-foam, but either can work. Can foam works better if the cut is so loose that the gaps are a half inch or more. Put the air barrier on the interior (basement) side of the foundation, so that the foundation sill is covered with the insulation.

    If the foundation has no insulation, put the air barrier ~4" toward the interior from the foundation, and install a bottom side air barrier on the interior under the joists. That way if/when you insulate the foundation in the future (highly recommended) it will be easier to have a continuous insulation layer. If you go that route, the bottom side air barrier needs to be caulked/foamed too. The best material for the bottom side air barrier would be 1.5" EPS, since it's the same depth as a 2x foundation sill, and will slip snugly in there even if there is a bit of foundation ledge between the inside of the foundation and the inside of the foundation sill. Caulk & tape that bottom side air barrier to the foundation sill before installing your joist-bay blocking air barrier.

    2> If the subflooring is sheet plywood or OSB (rather than plank), there is no point to installing foam board to the underside of the subfloor. The subfloor itself is a Class-II vapor retarder- it just needs to be made air tight. If there are seams in the subfloor tape them with housewrap tape and trowel a bit of duct sealing mastic over the edges to ensure the tape sticks. (If the subfloor is super-clean the tape alone would be fine.) Caulking the subfloor to the joists and band joist with polyurethane caulk (or construction adhesive) air seals and glues the subfloor to the framing, minimizing any flexing that might break seal at taped seams.

    Sometimes it's worth installing 1" foam on the band joist at the far end of the cantilever, but if it's air tight to the subfloor and basement, that's overkill. If you have the leftover stuff old stuff in sizes big enough to cut'n'cobble in there, it doesn't hurt.

    3> Once the cavities are all sealed, R30 rock wool (at full loft) would meet code-minimum on it's own. You could just stop there, but the thermal bridging of the 2x8s will have detectable "cold striping" on the floor during the coldest weather.

    4> If you install foam on the bottom (highly recommended, due the cold striping issue), there needs to be enough foam-R to prevent wintertime moisture build up on the cold side of the fiber insulation layer. If the foam is too thin any minor air leaks from the interior risks mold/rot levels of moisture accumulation at the bottom edge of the exposed joists, since the moisture can't dry at a reasonable rate through the foam and bottom sheathing. In a zone 5 climate (that would be you), a cantilevered floor the foam needs to be at least ~25% of the total R or more to achieve dew point control at the foam/rock-wool boundary. ( For walls it would take 28%, but walls get more wind-washing and more radiant cooling in winter than floors. ) So 1" EPS (~R4) is not going to be nearly enough.

    With 2" of EPS (R8-ish) it would be enough if you backed off to R25 fiberglass (which really performs at R24 when compressed to 7.25" in a 2x8 cavity). With R30 rock wool better bet would be to install 2" polyisocyanurate (~R13), which would give you R13/R43= 30% of the R as foam, and it would more than double the R-value of the framing, minimizing the temperature striping effects. (That's what I'd do.)

    To install 2" polyiso on the underside of the cantilever use a thin bead of foam board construction adhesive on the band joist and where the edges of the panels meet the floor joists to better air-seal it. You may need to leave a ~1/4" gap where it comes into the foundation to be able to seal that seam well with can-foam. Nail the foam in place with 12d or 16d cap nails sufficient to hold it up before the bottom sheathing goes on.

    Tape the seams of foil-faced polyiso with a high quality temperature rated foil tape, such as Nashua 324A (sold in most box stores.) If fiber faced, uses housewrap tape.

    >5 Sheathing the bottom with 1/2" OSB is is cheaper and more rigid than 3/8" plywood. Half inch CDX is probably cheaper than 3/8" plywood too. But any of the above can work. With 2" foam and 1/2" sheathing you can use 3.5-4.5" pancake-head timber screws (not pan-head, pan-CAKE head, eg. FastenMaster HeadLok ) through-screwed to the joists 16-24" o.c. to securely hold the thing together over the long term. They're pretty pricey, but it takes fewer fasteners. If you want to nail it, hot-dipped ring-shank nails 12" o.c. should do it.

    If this is going to add up to a lot of floor area, to take the bite out of the foam cost, using reclaimed 2" or 2.5" fiber faced roofing polyiso would work. At 2" most 2lb roofing foam is good for at LEAST R11, which would leave you at R11/R41= 27%, which is still adequate for dew point control at the foam/fiber boundary.

    Here's a detail drawing from that's a good approximation of what I've laid out here. (The draing is of a retrofit that leaves the prior bottom sheathing in place, but you don't need two layers of bottom sheathing.)

    Last edited: Aug 24, 2018
    LesterBK9 likes this.
  3. Sponsor

    Sponsor Paid Advertisement

    LesterBK9 likes this.
  4. LesterBK9

    LesterBK9 New Member

    Aug 24, 2018
    Thank you very much for your thorough response.

    There are four occasions.
    a. Heating pipe in one bay. This is fary close to subfloor. This did not give us any problem, so I plan to keep as is and insulate.
    b. HW, CW pipe feeding bathroom upstairs ( froze last year, did not burst ) - hang low, feels too close to bottom, and the edge. Trying to see if I could relocate.
    c. HW, CW pipe feeding kitchen right above. - Have not visually checked yet.
    e. HW, CW pipe feeding dishwasher. - This one seems to be behind caulked silver lined insulation board. Board is fixed inset to joists, not below. I have not opened yet, but will as it is most likely open into the house side.

    Understood. I put the barrier on outside edge of sill plate, which was shown in that energy star web site, but it makes more sense to extend to the other edge of sill plate. I will redo these.

    Basement is finished in this portion behind drywall. I don't have visual until I open up the wall one day. Thank you very much for the remark. Basement gets rather cold, so it is conceivable the insulation was not done abundantly. One step at a time :)

    Thank you for this explanation. Subflooring is plywood with wood flooring nailed to it. I misunderstood the role of the EPS board below the subfloor. Floor above does get quite cold exactly at the portion of cantilever, so I assumed the foam would be to keep the floor warm. Now, would it be an overkill to install anyway ? Or , even a negative impact to have too many layers of Vapor retarder ?

    R30 Rockwool is about 4x the cost of R30 fiber ( may be too thick ). Rockwool should be worth it, correct ? It is not really the cost at this point, but availability. I need to order them. By the way, the one I can order from big store says 'rock wool with acoustic barrier', although it is to be with no facing. Would that also become a problem for moisture ?

    Great information. Will follow this. A bit of strange thing they did was to attach deck joists directly on the rim joists of cantilever without pillars. Granted weight that the joists carries is only the deck, so that is not much, and the house has been doing fine for almost 30 years, I do not want to get into second guessing what they did too much. However, what that makes it tricky for me is that I do not have access to the face of the joists. I think I will install the 2" polyiso a little shy of the edge so that I can seal the edge of polyiso that is attaching to the joist above.

    Thank you very much. The digram is extremely helpful !
  5. Dana

    Dana In the trades

    Jan 14, 2009
    The closer the plumbing is to the warm side of the assembly, the better. If its' too far away and can't be moved. make an narrow air channel from the pipe to the subfloor with rigid foam board, and insulate the space from the pipe to the exterior sheathing.

    High density R30 fiberglass (designed for 2x8s) is different from low density R30s designed for 2x10s. Low density R30s only perform at R28 when installed in a 2x10 framing bay(!), and that drops to R25 if compressed to 7.25" in a 2x8 framing bay.

    A high density fiberglass R30C "cathedral ceiling" batt performs at R27 when installed in 7.25" deep 2x8 framing.

    A rock wool R30 designed for 2x8 framing are manufactured and tested at 7.25" of loft, and actually perform at R30.

    A standard fiberglass batt compression chart looks like this:


    A fundamental difference between high density fiberglass or rock wool compared to low-density batts is their superior air retardency, which will slow down the flow volume of any air leakage paths, and blocks convective air motion in the batt layer itself. A major advantage of rock wool vs. HD fiberglass (aside from the marginally higher R value) is that it is completely fireproof, whereas fiberglass melts at temperatures relevant in a house fire.

    The vapor retardency of the cavity insulation (faced, or unfaced) hardly matters, but since EPS runs between R4-4.2/inch, and R30 rock wool is also R4.15/inch, is usually cheaper and easier to install in cavities, rock wool (or HD fiberglass) is the right solution. Save the rigid foam budget for where it's actually buying you something that fiber insulation can't. A continuous layer thermally breaking the bottom side of the joists, detailed as an air barrier to limit infiltration is something that rigid foam board can do (and fiber insulation can't).

    If you end up biting the bullet on insulation the basement walls check back here, or here. There a many ways to insulate a basement wall, but several of them are at high risk of turning it into a mold farm. For now concentrate on making our air-barrier blocking and the exterior foam board & sheathing as air-tight as possible. An square inch of air leak near the bottom or top of the house can move a lot of heat (the stack effect) and is more important than other leaks at mid height in the house. In addition to heat those leaks can also move a lot a moisture. But in winter the direction of the air flow in winter is predominantly from outside to in at the bottom of the house, so during the months that matter from a wood-rot point of view it would tend do remove rather than accumulate moisture, but also makes the floor (and pipes) colder. So air tightness and air retardency matter here more than it does in your walls.
Similar Threads: Question foam
Forum Title Date
Remodel Forum & Blog Another foam over concrete slab floor question May 2, 2019
Remodel Forum & Blog Basement Foam Board Question Jan 3, 2013
Remodel Forum & Blog Replacement of Drop-In Stainless Kitchen Sink Questions... Feb 15, 2020
Remodel Forum & Blog Shower Rough In Questions Feb 3, 2020
Remodel Forum & Blog Basement wall insulation question Jan 16, 2020

Share This Page