There are foam compatible adhesives available.
I would make a little rig out of scrap lumber and a cinder block or sand bag to push against the board while the adhesive cures.
Thanks again, Dana.
On the wall with no exterior insulation, I can't go thicker than 2" because I can't lose any more thickness due to the non-ideal staircase placement. So, I will go with 2" XPS everywhere. I am not clear on your suggestion for a 2x3" studwall. I am planning a 2"x4" studwall with R11, or whatever I can get for unfaced batts. Will this all be ok, or do I really need to make some effort to balance the Rs? If so, I could scale back to 1.5" XPS on the walls with insulation on the outside of foundation, giving me a total of 15 (XPS) + 11 (FG) = 26 on some walls, and 12.5 (XPS) + 11 (FG) = 23.5 on other walls.
I was thinking kraft paper batts previously because of you comment "A batt left open to the room or a cavity is an air-filter, not insulation." But perhaps you were referring to a batt that did not have XPS between it and the foundation. I am also anticipating that the inspector may want to see a vapor barrier there, but will keep my fingers crossed. Would they generally allow a pass on this since the XPS will be there, and taped? In any event, your suggestion makes sense, and I will do it if the inspector allows it.
Do you generally use an adhesive to fasten the XPS to the block foundation, or tapcons, both, or other?
There are foam compatible adhesives available.
I would make a little rig out of scrap lumber and a cinder block or sand bag to push against the board while the adhesive cures.
Yep- foam-board adhesive is enough if putting up a studwall that would trap it in place. If going with a foam-only solution with furring to hold the wallboard, the furring would have to be through-screwed to the foundation, not merely glued to the foam in order to meet fire code. (The thermal barrier against ignition is the gypsum, and it would collapse unless mechanically secured to something other than the foam.)
And yes, I was talking about an air-barrier between the fiber and your previoiusly proposed cavity. If it's snug against the foam AND the greenboard, there is no convection path out the side cutting into performance.
If you print out those pages from the Building Science website (or just show them the simple-math model on where the dewpoint is likely to occur) you can usually get the inspectors to go along if they're otherwise reluctant. In Canada national building code still requires interior poly on insulated foundations, and while that presents less of a problem in 10,000+ heating-degree-day (Fahrenheit) climates than in warmer regions, it's still arguably not the best practice in every stackup.
Vapor diffusion is a slow process for getting water into wall assemblies even through painted gypsum- orders of magnitude less water than typical air-leakage, and achieving/maintaining a perfectly air-tight poly is a losing proposition. But while water gets into the assembly by any number of means, it gets out primarily via vapor diffusion, and that's a function of surface area: Even a ripped up sheet of poly with multiple holes is still a pretty good vapor barrier, even if it's a lousy air-barrier, often letting more moisture in via air leaks than it can purge via vapor diffusion. In a basement situation you have the added problem of ground moisture wicking in and ZERO capacity for drying toward the exterior- it MUST be allowed to dry toward the interior to avoid potentially damaging moisture levels at the foundation sill & above-grade exterior portion of the foundation.
Thanks again for all your help on this. This is great info.
Well, the inspector has said that he will either require a vapor barrier on the warm side of the fiberglass, or he would allow me to leave the studwall cavities empty. I told him about the building science reference, and it didn't help my case. I am leaning toward leaving them empty. What would you do? Would kraft faced batts be a better approach?
Kraft facers would all but guarantee that a problem gets started. Kraft facers run about 0.4 to 0.5 perms, which is already about as low as you can get away with for allowing the foundation to dry, which means you wouldn't be able to use foam against the concrete, creating wintertime rot conditions at the studs. Unfortunately your inspector is unwilling/unable to do the math on it, but adding a vapor barrier on the interior would add sigficant mold & rot risk from ground moisture to both the studwall and the foundation sill. What makes sense above grade just plain doesn't work for a foundation wall.
Best/cheapest all-foam solution would be to go with 4" of unfaced bead-board EPS (not XPS) for about R16. The fattest you could go with XPS is 2", for R10, and it would cost about the same or even slighly more. Use multiple layers taping the seams as you go, and overlap the seams by 12" or so for best possible air-tightness. If you go any thicker than 4" on the EPS it too will be come to vapor retardent as well.
To save on depth, use 1x furring (or even cut down half-inch ply if you're fighting for minimum depth) through-screwed into the concrete with 5" screws, hang the gypsum on the furring. The point furring & screws rather than relying on glue to hold both the foam & wallboard in place is a fire-code issue- the long screws continue to work at temps that would cause adhesives to fail. But gluing the foam in place as you go makes it easier to assemble.
With 4" foam + 1x (3/4" true thickness) furring, and 1/2" wall board the finish surface is 5-1/4" out from the concrete.
about the same as 2" XPS + 2x3 studwall solution:
With 2" XPS + 2x3 studwall + half-inch furring the finish surface is 5" out from the concrete.
With 2" XPS + 2x4 studwall + half-inch wall board the finish surface is 6" out from the concrete.
Thanks Dana. I wish I could hire you to debate this with my inspector.
What are the potential ramifications of having 2" of XPS, with a 2x4 stud wall, and no cavity insulation in the stud wall? This is the path that I am currently heading down. I have about half of the XPS glued in place now.
With no cavity insulation, the studs stay warm and dry- not a problem other than at R10 it's 2x the heat loss of an R20 wall. R10 is more than enough from a comfort point of view though- the walls won't feel too cold even when is below zero out. Tape or mastic-seal the seams in the XPS, and sprayt-foam seal the top to the foundation sill & band-joist insulation. (I assume you going to do a cut'n'cobble with XPS to insulate the band joist & sill?)
If at some later date fuel prices skyrocket you can probably rationalize retrofitting a cementitious or highly permeable foams in the cavities such as Tripolymer or Airkrete to boost the total R without a dramatic reduction in drying capacity for the foundation. Both of these products (and there are others) are non-expanding foams, with vapor permeability greater than 10perms at 3.5" thickness, so blowout risk during installation is very low. Half-pound polyurethane pours (slow rise, open cell expanding foam) would run about 2-3 perms, which might be acceptable, but 2lb polyurethane slow rise permeability is too low.
Thanks Dana. My inspector did not like XPS on the rim joist, as he wanted to see a vapor barrier. He suggested Kraft faced batts for that. I removed unfaced fiberglass that had been up there most likely for 25 years, and it appeared to be dirty from filtering air, but not damp moldy. Granted, it wasn't behind finished walls.
So, Kraft faced batts, bad idea here?
Perhaps he doesn't understand that the XPS is the vapor barrier (really a vapor retarder similar to Kraft paper). Functionally, you could run 2" XPS on the rim and seal the edges with Great Stuff or a similar foam. Seal joints with Tyvek/Tuck tape. Make sure to seal between the sill pate and the foundation. I found quite a bit of air leakage there (dirty fiberglass similar to what you have seen). Once you have your XPS on the rim, you could then add some unfaced batts to fill in the area.
If he won't go for that, I don't really see anything worng with the faced batts for the rim. The rim should be above grade, so it is more like insulating an above ground floor than a basement.
For the walls, I guess you'll have to look at some other options:
- faced fiberglass alone = bad. I pulled out a bunch that was damp/moldy from seepage/vapor from the cinder block wall. I previously had 2x2 furring strips with faced fiberglass in between. Not much insulation and the setup just doesn't work
- XPS alone. Will work, but you probably won't be able to meet energy code requirements for your area. In my area (quite a bit warmer than yours), they call for R-10 in the basement. Since you can really only get R-10 before running into permeability problems with XPS, this can be an issue. Using EPS, you could go thicker and a higher R value, but still might not be enough for your location
- spray foam - Will work and should have no trouble getting the R value. Expensive, though.
- XPS + unfaced batts. This option would work the best, but the inspector doesn't seem to like it. You also have the issue that MN and similar cold areas typically want two barriers. This is usually asking for trouble.
-XPS + faced batts. Makes inspector happy, but could cause issues with drying. You could slash the barrier after he leaves, though.
With the all foam options that Dana mentioned, remember that using furring strips can make electrical, etc. a pain. If you go this route, plan on using 4" metal boxes for electrical (1.5" depth) and add a mudring to mount your outlets to. I also don't like furring strips as you are putting holes in the wall plus the foundation walls are also not usually real plumb/square/straight. In my situation, the nails used to attach the furring to the block were pretty rusty as the penetrations made a weak spot for water to seep through. This is a block wall, so you would be better protected from this if you have a poured concrete foundation, though.
I'm going with 3/4" XPS and filling the 2x4 wall built in front with rockwool batts (unfaced). I don't think I will have an issue as I have it this way on the plans and the inspector saw the XPS during the rough inspection, but I have not had the insulation inspection yet, though.
Nukeman- I'm assuming that if the inspector even knew the vapor retardency of the XPS he would insist that it's on the "wrong" side of the fiberglass if it's between the studwall & foundation.
With spray foam you still have to watch the total vapor retardency- you can't go more than ~R15 with closed cell polyurethane (2.5-3" max). If adding it to the 2" of XPS you really can't go more than about R6 (1") without pushing the total permeability limits. But you can blast away with open-cell polyurethane. A full cavity-fill of the 2x4s with o.c. foam would yield about R12-13, with essentially zero air-transport of moisture to the cold edge of the stud, which would be at least as good as unfaced R11 fiber batts from a total moisture loading of the stud POV. A full cavity fill of c.c. foam would cause spalling on the exterior of the foundation at best, and would very likely cause rot on the foundation sill in places. Only VERY highly permeable foams like cementious foam/triypolymer foam or open cell polyurethane foam can be used as cavity fill here. (That would be true in VA's climate as well, but you might get away with high-R levels of cc foam in desert area of NV/AZ.)
Slashing the kraft facers does almost NOTHING to improve drying via vapor diffusion, which since vapor diffusion rates are a function of surface area- it requires the square footage to dry. You'd have to actually strip them to get a drying benefit. Slashing them only ruins it's air-barrier capabilites. It's a major issue with interior vapor barriers/retarders (and why it's better to design wall stackups to be resiliant without them.) They're hard to make perfectly air-tight and are easily damaged, so in practice they will often let more moisure IN by air-transport mechanisms than can get back out via vapor diffusion, since the moisture takes a long time to get out through a vapor barrier/retarder. In all but the leakiest of structures vapor-diffusion will be the primary drying factor, but wetting comes from multiple sources, of which vapor diffusion is the LEAST factor. A square inch of air leak is worth several times more than a whole wall's worth of vapor diffusion through latex painted wallboard, and bulk waterincursions (water leaks) is at least an order of magnitude higher than air-transport.
In basement walls in cold climates you have to both let the ground moisture out of the wall assembly and into the room to protect the studs & foundation sill, yet keep room moisture from accumulating when it's wicked cold out for weeks/months on end. Interior vapor barriers help the latter, but render the former hopeless. It HAS to be designed for resilency without the use of vapor barriers in order to work. The lower the vapor retardency the better, since ground moisture flow into the foundation can be seasonally quite high during the Minnesota mud-season on into the spring, if fairly low through the winter.
By contrast, designing above grade stackups for resiliency is dead easy, but often gotten wrong, with upquitous mis-application of that evil 6mil poly instead of focusing on building for interior airtightness, and making the exteriors too vapor retardent. With back-ventilated siding (aka "rainscreen gap"), and inch or so of exterior XPS, and reasonable (not hermetically sealed) interior air-tightness you don't need anything more vapor retardent than standard grades of latex on the interior to keep moisture levels out of the mold enhancement range at the sheathing. But skip the foam and rainscreen and let the interior drywall leak into the stud bays, things can go downhill fast. A sheet of 6 mil poly with a bunch of holes in it only cuts the drying capacity in half, and limiting the moisture loading from the interior by only a small fraction.
Right. Should have been more specific on the open/closed cell. For the faced batts, I do agree. The problem is that his inspector is looking at the "traditional" way of doing things, even if it is not correct. Unfaced would be the way to go, but he does have to stick with what the inspector will go along with.
It looks like for what I saw on the energy code that R-10 is still okay for you. Maybe the easiest solution in this situation is to use the 2" XPS (since you have it) and leave the cavity insulation out. However, if he still wants a barrier over that, then that could be a problem.
Funny thing. I saw a show on DIY sometime back. They were doing a basement (in Canada, I believe). They took out the old fiberglass batts (wet/moldy). I thought sure that they understood the problem and were going to use XPS/batts or perhaps spray foam. Nope..they put up new fiberglass batts and put 6 mil poly over it "to keep out the mold smell". It would be nice to stop the mold in the 1st place, but I guess they figured it was better to cover it up...
The Canadians sometimes get it dead-right (as in requiring 10mm rainscreen gap under all siding, all Canadian climate zones) in their national building codes, but they've missed the boat on the more comprehensive understanding surrounding vapor barriers. In Canada 6 mil poly vapor is REQUIRED for foundation insulation, independent of insulation type or stackup, despite ample real-world evidence that it causes at least as many problems as it solves.
From that MN document you linked to:
BASEMENT FOUNDATIONS AND CRAWL SPACES – GENERAL REQUIREMENTS
• Must be installed to an R‐10. Adding additional insulation to increase R‐value or
adding adding additional vapor retarder to foundation wall assemblies is
prohibited, except for the installation of R‐13 when using fiberglass batt insulation
on the interior.
• Must be insulated from the top of the foundation wall down to the top of the
footing or from the top edge of the interior wall to the top of the slab if insulation
is on the interior.
My interpretation of the part I emphasized in boldface is that you're required to insulate to R10 (OK, you're there with 2" of XPS) but prohibited from adding interior vapor retarders or insulation beyond R13 (in which case unfaced R11 or R13 would comply). This is DEFINITELY worth taking to the inspector!
And it's consistent with the simple-math model of where the dew point occurs within the stackup that I ran through on a prior post. R13 is taking it to closer to the edge, but if one makes the realisitc assumption that at high delta-T R13 batts actually perform only to ~R11, you're probably safe in Minneapolis (but could still have some risk in International Falls.) A WUFI model of the stackup would be able to verify just how much margin you have, or really need. The inspector might interpret that to mean that if fiberglass is added it MUST be R13, which will still be OK in most instances. But the prohibition against additional vapor retarders appears absolute (and that would include kraft facers.) It also seems to preclude the use of high-perm foams though.
pmayer- print out that document, and get your salesman shoes on. This is well worth pursuing before closing in the studwall. Getting it to R20 now is a lot cheaper & easier than doing it later. What's clearly cost-effective with cheap low-density batts isn't necessarily the case with high-perm foam pours at current prices.
I still can't fathom that the inspector would nix XPS on the band joist in favor of kraft faced batting (which is nearly impossible to air-seal there.) One 600board-foot closed cell SPF foam kit (TigerFoam, FomoFoam, et al) is usually enough to do the whole house at 2" (R12), completely air-tight, with the same vapor retardency as kraft facers. It's more expensive than cut'n'cobbled R10 XPS, but usually worth it if your labor means anything to you. Insulating and sealing wiht spray foam right up to & over the top of the XPS on the wall guarantees a good air & vapor seal. If you want to cheap out, 1" of SPF is enough to seal it and protect the wood, at which point stuffing in kraft-faced batting (even air-leaky) would be fine, and do at least some good (may be R8s worth for a leaky R13 batt.)
One the 2nd page, there is some stuff that might help as well. They do talk about perms, unfaced batts, rigid foam, etc. However, when they talk about the R-13, it isn't real clear if they mean:
1. You are limited to R-10 (pretty much no matter what) unless you use an R-13 batt instead
2. You can have an R-13 batt in addition to the R-10
Now, I am not sure why they would limit the insulation levels to R-10 unless:
1. They know issues with the perm rating being too low and many types of insulation will have too low of a perm rating above an R-10 thickness
2. Perhaps the way they seal things up, above an R-10 causes too high of a dT and there are problems with condensation/mold
You'll just have to talk to the inspector and see if you can get him to see it your way. I agree with Dana in that going with a good setup with >R-10 would be the way to do it. Easy and cheap to do it now and the additional insulation should pay off in your climate without a problem. If he doesn't see it your way, I guess you'll just have to live with just the 2" XPS.
I was reading that as number 2 "You are limited to R-10 (pretty much no matter what) unless you use an R-13 batt in addition (not instead of)". In other parts of the document they prohibit you from putting the studwall up against the foundation, which kinda steers you away from the "instead" interpretation.
R10 of foil- faced iso or foil/poly faced EPS would be a disaster against the foundation, and you're getting close to the danger zone even with low density XPS, but still "safe enough".
The section on page 2 for using batt insulation probits the use of facers or wall coverings with a permeance of less than 1.0. (Kraft facers are ~0.4-0.5, and would violate that prescription.) They seem to prohibit total R greater than 13 (and R13 batts would only perform to about R8-R9 when it's -10F on one side, 70F on the other.) It's not the condensation/mold issue their addressing with that- an unfaced batt only solution sans foam would have a fairly high mold hazard in cold climates like MN. The only rationale I can speculatively come up with is that going higher-R might run into ground frost pressure cracking the foundation or frost-heaving the footings, since heat leakage from the house keeps the soil thawed and malleable next the foundation, raising the frost line considerably from where it would be in an open field(???). If that's the case, a ring of buried horizontal 2" XPS extending 2-3' from the foundation outward around the perimeter would allow much higher Rs without frost damage. (It would also be like adding another ~R5 to the below-grade portion of the foundation.)
I'm not sure how they deal with insulated concrete form foundations in MN, since it's hard to find any with R-values less than R16, which would clearly violate the outlines in that document.