Insulation where floor joists overhang foundation

[Pmayer]

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.

Thanks!

Paul

 

[Dana]

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 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, 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.).

 

[Sponsor]

 

[Pmayer]

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?

 

[Dana]

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

...outdoor air...





_____ground__________



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 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.

 

[Pmayer]

Dana,

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

 

[Pmayer]

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.

 

[hj]

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.

 

[Pmayer]

wouldn't the XPS sealed with spray foam constitute an adequate vapor barrier? It sits between the heated room above and the exterior.

 

[Dlarrivee]

Don't mind HJ his knowledge in this department is 60 years out dated.

 

[Pmayer]

I was thinking that perhaps I had not communicated my approach clearly enough.

 

[Dana]

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.

Exceptions:

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.)

 

[Dave KP]

Hi, new here, I know this thread is 2 years old, but I'm hoping Dana or the OP is still around.

I'm having a very similar issue. I live in Topeka, KS, so zone 4. 85-year-old house, plaster walls, no insulation to speak of.
Our dining room has an area that bumps out - i.e., the floor joists extend about 1.5' out from the foundation; I reckon it's about 8 feet long.

I was thinking - could the Thermax be used on top of the bays, under the sub-floor, instead of spray foam? I'm looking to save money. And then fill with r30 unfaced fiberglass, and close off the bays with the same Thermax? And speaking of Thermax, is this the same product?
http://www.homedepot.com/p/Thermash...-Rigid-Foam-Insulation-Board-787264/100549260
I have to admit, I'm a little nervous spending about $50 on a spray foam kit and not doing it correctly.

 

[Dana]

What is the sheathing on the underside of your cantilever overhang?

Is the subfloor sawed planks or ship-lap or... ???

 

[Dave KP]

What is the sheathing on the underside of your cantilever overhang?

Is the subfloor sawed planks or ship-lap or... ???

Hi Dana, I'm not entirely sure. It's old. The underside sheathing looks and feels like thin planks that are nailed in, I'm guessing to the floor joists; I can't quite tell what's on the inside of the bays yet. (When we moved in, the opening of the bays were filled in with fiberglass, which of course was nasty and inside was very dusty, spidery, etc. No water damage, I don't think [the bumpout is the bottom of a major roof slope and has guttering on top of it]).
The subfloor looks to be the same as in the whole basement - wide planks under the hardwood floor above. I can post photos, if it'll help.

 

[Dana]

Pictures of the overhang from both the interior & exterior might be useful.

 

[Dave KP]

Pictures of the overhang from both the interior & exterior might be useful.

Here's a view of the basement side of the bumpout joist bays, and inside one of them, and some exterior shots.
A problem will be the two that are blocked by the I-beams, plus all the nails make complicate things if I want to use rigid foam.
This pro recommended 2" XPS on the bottom and top of the inside of the bumpout, and a piece on rim joists, or a large piece on the outside, covering the bottom sheathing:
http://www.homeconstructionimprovement.com/bay-window-floor-insulation-detail/#comment-1589217

 

[Dana]

That pro is recommending making a moisture trap- not particularly well thought out:

^^^DON'T DO THIS^^^


In your case it would be OK (but not great) to use 2" XPS, which has a vapor retardency of about 0.6 perms but foil faced polyiso has a vapor retardencey of less than 0.05 perms, and is a true vapor barrier.

When installing a true vapor barrier in an assembly it should go on only one side of the assembly, and at a sufficient R-value to provide dew point control at the seasonal average temperatures (which is pretty easy to do in eastern KS, which is US climate zone 4A.)

Their second detail would almost work:

The foam board on the band joist is not necessary in your climate as long as you can-foamed the band joist to the floor joists, subflooring, and can-foamed or caulked the exterior foam on the cantilever bottom planking. It would be fine to cut in and can-foam the foam board on the interior side of that planking, but in your climate it would be fine to use #15 felt as the bottom side air barrier (inside the cantilever) can-foam sealed to the joists and foundation sill.

With your ship-lap subflooring you need an air barrier on the underside of the subfloor too. It doesn't need to be anything fancy- sheet cardboard would do, but with the long nail-points visible in your pictures it might be better to do that with cut'n'cobbled polyiso.

Ideally you'd be putting R8-R12 foam from the slab all the way up to the subfloor on the interior side of the foundation once you've sealed & insulated the rest of the cantilever cavities, which would be roughly the range of IRC 2012 compliance (=R10 continuous insulation for zone 4.) Using 1.5-2" fire rated Thermax wouldn't even need an interior side thermal barrier against ignition in mode jurisdictions. Can foam the edges to make it air tight. Using 2-3" of EPS would also be fine (faced, unfaced, doesn't matter.)

On the exterior side it looks like the brick pointing needs to be touched up in spots, which will tighten the foundation up a bit. Use can-foam, not mortar to seal between the foundation and any wood.

Where the plumbing goes out over the subfloor you may want to insulate only between the plumbing and the exterior to limit the risk of freeze-up during the next Polar Vortex weather event.

 

[Rengel]

Hi, Dana.
I agree with much of what you said for all of the above posts. However, I have one point of contention: the complete sealoff of the cantilevered joist or the use of a vapor barrier on a subfloor. Several years ago I had my cantilevered joists completely sealed with open-cell foam (closed cell was too costly, and I didn't want the vapors). In truth, I felt little difference in comfort on the carpeted floor above (naturally, things improved in the basement). I attribute this to a continuous thermal bridge from the wall sheathing.

In retrospect, I would have preferred to foam-seal the interior of the bottom sheathing and the band joist only (closed cell foam), allowing some heat from the conditioned basement to continue to warm the subfloor above. I am now in a new house with a non-insulated cantilever issue (gotta love sloppy builders), so I am trying to figure out how I should proceed.

I don't know about Ohio codes, but I have never ever seen poly vapor retarders used anywhere on the rim joists or sub floors. In fact, I think that it could produce a diaper-effect, much maligned by the likes of building gurus such as Martin Halliday and Joe Lstiburek. Perhaps I am misunderstanding??

Very curious to know your thoughts. Go easy on me....I'm an optometrist by trade and an energy-nerd hobbyist on the weekends
Thanks!
Rob

 

[Dana]

Open cell foam is not a vapor barrier. But it IS an air barrier. Any air barrier in that location would need to be at least semi-permeable to water vapor if it's built with very low permeance or foil face foam on the cold side of the assembly.

I don't think we disagree about use of poly vapor barriers, or the very real hazards of multiple vapor barriers. Poly sheeting can be detailed as an air barrier, but it's a lousy choice in almost any assembly due to the moisture trap issues. But in a heating dominated climate it can still be used on the conditioned space side of the assembly, as long as there is sufficient drying capacity toward the exterior side (as in, no foil-faced foam, and no XPS thicker than about an inch or so.) Foil-faced foam is just as vapor-tight as (or tighter than) 6 mil poly, so care must be taken when designing it into any stackup.

Insulating the exterior band joist and the bottom of the cantilever leaving 3-4" of air space reduces cold-striping, and can make the floors there a few degrees warmer, but only if the basement is fully heated and the cantilever isn't too deep.

 

[miracj]

I'm located in the Boston, Massachusetts metropolitan area (Zone 5) in a split-level house built in 1986 with a 2 foot cantilever using 2 x 10" joists sitting on a brick wall on top of a cement wall (although I suspect the joist load is on the inside stud walls transferred to the cement foundation). The cantilevers undersides are covered by vinyl siding that have no air holes (but the attic ones do). The floor above the cantilever and below the cantilever are both heated.

At some point, I had sealed the edges of the floorboard with canned foam (as I could feel air penetration), and that helped.

Then, some years after that, a yellow-jacket nest formed in the overhang. After killing the nest (about the size of a basketball!), I noticed there was some crumpled up R13 insulation (not even close to filling the 10" joist cantilever of 2 feet) and upon inspection, the entire 42 feet front of my house's cantilever had mostly no insulation (I bought the house new, but never thought I needed to check. Have I learned a lot! Builders like that, and the town inspectors, should be removed). I then added R-30 Kraft Faced insulation batts 16 in. x 48 in into each one with the faced-side up towards the floor. This helped a lot.

However, upon reading here and other forums, it appears that I should seal the joist areas which allows air into the rest of the floor.

I should mention, there are house heating hot water pipes used for heating that pass into these cantilevered areas at 4 different joist areas. Since these pipes are always warm during the cold (freezing season), do I need to keep them in the warm side of insulation, or does it not matter. And do I have to worry about any condensation issues from them?

But now to the specific question about insulation. I was considering using DOW TUFF-R 1-7/8 inch thick R-12.2 Foam Insulation, which has one facer with a continuous sheet of aluminum foil; the other side is a Tri-Plex facer consisting of a three-ply laminate of durable kraft with aluminum foil on both sides. I would seal with the foam insulation and foam spray can the joist area to the subfloor, and the subfloor area at the top of the cantilever, and then reuse the R30 batts cut to size.

So my questions are;

• Is the DOW TUFF-R 1-7/8 inch thick R-12.2 Foam Insulation correct to use, and if not, what is recommended to maximize R value and eliminate any condensation issues?
  
• Which side of the DOW TUFF-R foam insulation facer goes to the inside of the house?
• Should I remove the facer to the R30 batts?
• Should I include the hot water heating pipes inside (warm side) or outside the foam insulation?

And are there any other recommendations?