View Full Version : Insulation where floor joists overhang foundation

01-01-2013, 05:09 PM
I am continuing my basement remodel and following the great advice that I have received here in the past regarding insulation. In fact I should mention that the previous basement room that I finished is by far the most comfortable room in the house thanks to you guys. In the area that I am currently finishing there are a couple places where the floor joists extend a couple feet past the foundation wall (one is for a bay window and the other is for a patio door bump-out thing. Currently there are just unfaced fiberglass bats in these cavities, installed with the bats laid flat apparently to allow some air flow and to make a nice comfy home for mice. :) Now that I am replacing all of the fiberglass that is up against the rim joist with Thermax (inspector won't allow unfaced XPS), but I won't be able to reach in far enough to install this on the protruding rim joist in these two areas. So, my question is, how should I insulate in these areas? Do I need to use spray foam? Or replace the unfaced bats with paper back ones? What about the air flow that appears to be currently in place; is this important?

Also, on the exterior side of 2 of my foundation walls there is a layer of 6 mil plastic directly against the foundation, and 1" of XPS on the outside of that. Am I OK to install another 1" XPS against the inside of the foundation wall? The other wall that I am finishing had no XPS or plastic on the outside and I am adding 2" of XPS on the inside foundation wall as I did on my previous remodel.



01-02-2013, 12:44 PM
If you're blocking the cantilevered bays at the foundation with Thermax there is no rationale for a ventilation gap between the subfloor under the bay-window bump-out and the fiber insulation (if there ever WAS one.) A nominal 1" skim-coat shot of closed cell foam (you can use the 12 board-foot FrothPak (http://www.lowes.com/pd_144279-236-157802_0__) kits from box stores, if it's a small enough area) on the subfloor would be sufficient vapor retarder to allow you to just fill up the space with fiber insulation, so long as the bottom sheathing of the exterior overhang doesn't have any strong vapor retarders like poly or foil.

On the fiber insulation, both density, and snugness (preferabably a compression-fit) count. I personally prefer to use rock-wool (http://www.lowes.com/Search=roxul?storeId=10151&langId=-1&catalogId=10051&N=0&newSearch=true&Ntt=roxul), which is about the same cost & R value/inch as "cathedral ceiling" batts. It's probably not a huge area, so there's no real savings by to going with low-density batts, which are much better mouse-nests. Are they 2x10s, 2x12s or... ???. If it's more than arm's length it can be awkward shoe-horning in batts (especially if there are nail-heads interfering and catching). It's easier to get a tight fit with blown fiber, but it's probably not worth the hassle & expense of renting a blower.

Then, just blocking it in with fire-rated Thermax and sealing the edges (with 1-part can-foam or FrothPak) works.

Where there is poly on the exterior of the foundation, unless you have a metal-flashing or membrane type capillary break between the concrete and foundation sill it's better to use something more vapor-open than XPS on the interior. Using 1.5-2" of unfaced EPS (not the goods with poly or foil facers sold at some box stores, which are <1 perm) would give it 3-5x the drying capacity of 1" XPS for about the same or better R-value. If XPS is all you have available, hold the line at 1" (which runs ~0.8-1.2 perms, typ.).

01-02-2013, 01:31 PM
Thanks, Dana. On the cantilevered bays, I believe that the joists are 2x10, and yes, it is a bit of a reach of erhaps 2 feet in each area (probably 8 or so total cavities to deal with). The reach is for sure too far to do anything nimble out at the rim joist. If I am interpreting correctly you are suggesting to spray the foam directly onto the rim joist, then fill the cavity in behind the foam with rock wool or blown fiber. But then are you suggesting putting Thermax between the joists right at the foundation, directly behind the rock wool or blown fiber? I'm trying to understand the benefits of foaming the rim joist and again at the foundation, sealing in the cantilevered area. Did I misinterpret this?

01-02-2013, 02:25 PM
The rim joist on the cantilever is way the heck out there, and the bulk of the moisture drive and heat loss is through the sub-floor of the bump-out, not from the basement out to the band joist. Spraying the foam on the sub-floor (not the cantilevered band joist) to air seal it and give it a reasonable vapor retardency keeps wintertime moisture from the conditioned space above the cantilever from accumulating in the cold under-side sheathing or band joist of the cantilever.

Blocking air & moisture movement from the basement out to the cantilever at the foundation sill with the Thermax keeps wintertime moisture from the basement from accumulating there.

|...bay window cantilever area...
|^^^the foam only goes here^^^
|<< band joist
|{fill space between foam & bottom sheathing with fiber}
|______bottom sheathing_____

...outdoor air...


With a 2x10 bay and a nominal-inch of closed cell on the subfloor you have about 8.25" of space between the foamed subfloor and bottom side sheathing. Compressing a standard density 9.25" R30 batt in there would be the cheapest and may be the "right" option, and would perform at about R28. (Better R/inch at the compressed density, but fewer inches.) Unfaced R30s in 300 inch rolls (http://www.lowes.com/pd_177781-1722-B390_0__?productId=3141353&Ntt=r38+fiberglass&pl=1&currentURL=%3FNtt%3Dr38%2Bfiberglass&facetInfo=) are usually sub-$15 at box stores.

R15HD or rock wool batts are about 3.5" and R23HD/rock wool are 5.5", which would give you a compression fit if stacked. It'll be a pain, but it's doable. You'll be into it for $75-80 and you may have a lot left over- I'm not sure if it's worth the extra expense if there is going to be much scrappage after it's all cut & fit.

R30HD/ rock wool batts or high density fiberglass batts are nominally 7.25" inches. If the budget allows it might be better to go with 2" of foam and a snug fitting high density R30.

The key to avoiding rot/mold is the spray foam air seal & vapor retarder. Since the bottom side sheathing never gets rained on, it can dry toward the exterior even in the winter. As long as it can dry to the outside air faster than the interior-side is delivering moisture via air-leaks or diffusion, it'll stay in great shape. If the previous configuration had the air space above the fiber and no vapor retarder, that was a much worse situation, since in winter the sheathing was constantly being supplied with air-transported moisture from the warm interior.

01-02-2013, 03:29 PM

This is so helpful. I really appreciate your thorough response once again.

01-05-2013, 03:32 AM
Here is how I ended up doing this after some input from a contractor friend. I cut XPS panels to fit the bottom of the subfloor, extending from the rim joist to the foundation block. Then I heavily foamed the perimeter of the XPS and pressed it into position so that the foam oozed and filled the gaps around the perimeter. Then I wedged scrap material against the XPS to hold it in position until the foam cured. Then I cut pieces of Thermax to seal the cavity to the basement and foamed that in as you suggested. Worked great! Thanks again for the advice, Dana.

01-05-2013, 07:45 AM
A "vaapor barrier" goes between a heated and unheated area, so you would need a vapor barrier on top of the insulation, assuming there is a heated area above the insulation.

01-05-2013, 07:54 AM
wouldn't the XPS sealed with spray foam constitute an adequate vapor barrier? It sits between the heated room above and the exterior.

01-05-2013, 12:42 PM
Don't mind HJ his knowledge in this department is 60 years out dated.

01-05-2013, 04:28 PM
I was thinking that perhaps I had not communicated my approach clearly enough.

01-07-2013, 08:43 AM
wouldn't the XPS sealed with spray foam constitute an adequate vapor barrier? It sits between the heated room above and the exterior.

An inch of XPS runs between 0.8-1.2 perms.

At 2" XPS runs 0.4-0.6 perms (which is about what you get out of an asphalted kraft facer on a batt.)

The legal definitions of vapor retarders come in three classes of vapor permeance (US- units) based on ASTM E96 lab testing:

Class-I: 0.1 perm or less (poly or vinyl sheeting, foil facers etc.)

Class-II: 0.1-1.0 perms (kraft facers on batts, closed cell spray polyurethane foam @ 1"+ thickness, XPS @ >1", or EPS @>4")

Class-III: 1.0-10 perms (most grades of roofing felt, open cell foam @ 3"+, some types of housewrap)

Only class-I vapor retarders are true "vapor barriers", but the vapor retardency required to be protective depends on the type of assembly & stackup and the local climate. With wall assemblies designed to dry toward the exterior in a US climate zone 6 climate (eg Minneapolis) a class-II vapor retarder or tighter is necessary when wooden exterior sheathing is used (OSB/ply, or planking), but that is relaxed to class-III if fiberboard or gypsum structural sheathing is used, as long as the exterior siding is back-ventilated (with an air space between the siding and sheathing.)

This is built into prescriptive wall stackups in the IRC section N1102.5. at this point- no calculations necessary. (Note, Minnesota has amended many parts of the IRC prescriptive issues, including putting a ridiculously low maximum R-value for foundation insulation based on an unrealistic and strictly theoretical risk of frost heaving- an R value so low as to prohibit the use of insulating concrete form type construction if followed to the letter of the law.) MN has chucked IRC section N1102.5. in it's entirety replacing it with:

Subp. 12.
IRC Section N1102.5.

IRC Section N1102.5 is deleted in its entirety and replaced with the following:

N1102.5 Vapor diffusion management.

N1102.5.1 Exterior wall vapor retarder. Above grade frame walls, rim joists, floors, and ceilings shall be provided with an approved vapor retarder as defined in IRC Section R202. The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation. Subfloor materials that meet the requirements of a vapor retarder are allowed. The vapor retarder does not need to be continuously sealed unless it also serves as an air barrier.


1. In construction where moisture or its freezing will not damage the materials.

2. Where other approved means to avoid condensation are provided, such as when rim joists, crawl space walls, or basement walls are insulated on the exterior or are integral to the building assembly and meeting the vapor retarder requirements.

N1102.5.2 Under-slab vapor retarders. Under-slab vapor retarders shall meet the provisions of parts 1322.2100 to 1322.2103.

N1102.5.3 Crawl space floor vapor retarder. The floors of insulated crawl spaces shall be covered with a vapor retarder meeting the provisions of parts 1322.2100 to 1322.2103.

Unlike a wall assembly a cantilever overhang there is ZERO exterior wetting from rain/snow penetration that needs to dry, and SUPERIOR drying capacity to the exterior, since it usually has no felt or housewrap slowing it down, nor is there a siding layer. (It's typically CDX plywood or T111 siding with a 3-5 perm latex paint. So even if the 1" XPS is a shade over the class-II vapor retarder, it's "good 'nuff", plenty vapor retardent for the application. Without going so far as to run a WUFI simulation on the exact stackup, I'm confident that the assembly as-described would avoid damage, even if only 1" of XPS (rather than 1.5") meets the letter of MN code, should it prove to be 1.2 perms rather than 0.999 perms. The single most important aspect is air-tightness at the interior, with sub 1-perm vapor tightness being a distant second.

As built, the most likely "issue" if any that might crop up is a premature failure/peeling of the paint on the underside of the cantilever if the paint is a more vapor-retardent type, but even that risk is remote. It's neither going to rot off or develop a mold problem. Only air-leakage (or bulk water leakage from a plumbing drip or something) could move enough moisture out to the cold underside sheathing to create those types of problems, and you've foam-sealed the edges. (Stack effect infiltration would also be more likely pulling drier outdoor air in than warm moist air out since the cold surface is at the bottom. But air-handler driven leakage from hot air furnaces doesn't follow gravity, so air-tight is still always right.)