Basement Insulation Options

Discussion in 'Remodel Forum & Blog' started by Yankee1423, Jul 30, 2010.

  1. Yankee1423

    Yankee1423 New Member

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    Iowa
    I am in the process of finishing a basement (location is Iowa with cold winters and humid summers). The builder had put up all of the exterior walls already and put in unfaced fiberglass between the studs. After reading several articles and posts, I think putting in XPS foam board would be a better choice due to potential mold issues. The problem is that the walls are up and the plumbing in. The walls are off of the exterior concrete by maybe a ¼†at best. Additionally, the tub/shower is already in place with the fiberglass/vapor barrier behind it.

    What I was thinking about doing is slide some 6 mil plastic between the stud and concrete wall leaving a couple inches poking out on either side. Then glue 2†XPS foam board in each stud cavity. Once that is in place, I can take some tyvek tape and seal the edges by taping the exposed 6 mil plastic on top of the foamboard on each side of the stud. After that is done, I could fill the rest of the cavity with the unfaced fiberglass. Will this approach work or be my best option? I don’t think I want to move the shower unit now although I could still pull out the fiberglass if I need to. Maybe a blow-in option exists for that area?
  2. Dana

    Dana In the trades

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    Cut'n'cobble on a bunch of XPS can be very time consuming, but the approach is sound. Rather than using poly film & tape to seal, if you cut the XPS sheeting with ample space between it and the stud you can use closed-cell foam sprayed with the gun tip at the gap between the studs & concrete, which will then expand into the gaps between the stud and the XPS for a perfect seal. Use at least an inch of XPS, but not more than 2". If you use 2" and they're 2x4 walls that leaves you only 1.5" of space to fill, which is hard to do with split up batts (even R8 econo-batts are 2.5" thick) but wet-spray cellose or ultra-fine wet-spray fiberglass like JM Spider fills all voids and would work well giving you another R5-R6 (~ R15 total, center-cavity.)

    It's well worthwhile to treat the concrete with a spray on acrylic or silane based masonry sealer, which will still be breathable enough to keep the foundation from saturating and rotting out the foundation sill.

    Going with the spray-foam sealer approach you can be really crude cutting in around already-installed pumbing & electrical, since you can fill arbitrarily large gaps with the stuff. If it's more than 2000 square feet of wall area it's probably worth springing for one of the 700 board-foot 2-part foam kits (Fomo-Foam, TigerFoam, FrothPak, etc.) If the foundation sill & band joists aren't already foam insulated, an 1-2" of foam to seal that from interior humidity & condensation during the winter, with a batt or spray-fiber insulation beyond that to bring it up to a similar level as the rest of the foundation wall R is the right way to go there. Just be sure the foam R is at least 40% of the total R where the floor joists are, or you could have localized mold conditions on the joists where they pass through your band-joist sealer. If you give the outer 3" of the floor joists an inch of foam that'll be "good enough", even if you get occasional condensation or frost conditions at the foam/fiber boundary on the band joists during extreme cold snaps. In your climate, with 2" of foam on the band joist + R12-13 of fiber on the interior that wouldn't be often enough to cause mold or rot issues. But for every inch of thickness a square foot of coverage is going to cost you ~$1.25 in closed cell foam. (If you used 2lb Icycnene- a contractor-only solution, that would typically run 75-90cents per board-foot, but only delivers R5/inch rather than the R6/inch you get from standard polyurethane foams.)

    If you're looking at over 1000 board-feet (square feet x 1" depth) it's often cheaper to hire a contractor than to use the kits, so look at it carefully. Figure on about 1 board-foot per stud for sealing the 2" XPS and stud-edge, then look at the running length around the band joist x 1" or 2" to figure out just how much spray foam you're talking.

    Spray foam and rigid XPS both need a thermal barrier against fire- half-inch sheet rock meets the required rating. But so does 3" of spray cellulose or Spider even without sheet-rocking over it.

    Neither spray foam nor Spider is a DIY project, but if you went with 1.5" of XPS and don't have lots of blockages cutting across cavities, dry-blowing cellulose isn't rocket science, and a decent job is possible with a box-store rental blower. Be sure to seek out "borate only" or "sufate free" material though, since sulfated cellulose will corrode copper pretty quickly if it ever got wet. All wet-spray cellulose is sulfate-free, and dry-blowing the wet-spray stuff works just fine. (You won't find wet-spray goods at the orange or blue box stores, but building goods distributors that handle cellulose insulation would have it.)

    If there's a liklihood that the place will see occaisional flooding near the floor, it may be better to use spray fiberglass, as cellulose will wick more flood moisture up into the cavity and require somewhat more extensive repairs.

    Hopefully the slab has a poly sheet in it &/or the studwall builder put at least a sill gasket under the studwall to limit capillary draw of ground moisture into the wood. (Or at the very least, used pressure treaded lumber against the floor.) It may never have problem, but if no measures were taken there's a liklihood of at least localized issues a decade or three down the road.

    Behind the shower you're kinda stuck, but pulling out the fiberglass will run the studs much warmer, with a much reduced chance of condensation/moisture issues on those studs. Slow rise foams, etc are almost certain to destroy the shower unit (and would make ripping it out to start over even more difficult.) If you have line-of sight from above into the stud bays and can get the spray gun in there it may be possible to get at least a 1" shot of spray foam in there on the foundation down to at least the frost line, which will be WAY better than nothing. Don't go for a full cavity fill of foam there, but if you can get it up to 2" you'll have R12, which isn't bad at all. But even a 1" shot will be ~R6. (Over 2" of foam on the concrete would be too vapor impermeable and the foundation sill might rot in that area over time.)
  3. Yankee1423

    Yankee1423 New Member

    Messages:
    15
    Location:
    Iowa
    Thanks for the reply. In order to keep costs down and within my budget, I'm trying to limit the amount of spray foam that I use and time is less of a factor so the cut'n'cobble approach will have to be my first choice. How does this sound for a plan:

    1. 1.5" XPS in all stud cavities leaving a little room on each side to fill in with spray foam of some sort.
    2. Fill in the remaining area with a blow in product (is this do it yourself? and do I need any kind of vapor barrier between this and the drywall that is going on top of it?
    3. Try to do spray foam in the band joists around the perimeter and then fill in with the fiberglass R12-13. Just wondering if I could use more XPS here instead so that I limit the amount of spray foam to the four edges.
    4. Pull out the fiberglass behind the shower unit (I believe I can get to it). Is my only real option for behind this area the spray foam? Plus, I'm assuming I need to pull out as much of the vapor barrier back there that I can.

    The basement is about 50' by 30' square with about a 30' section on oneside being unfinished.
  4. Dana

    Dana In the trades

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    If you're in heavy cut'n'cobble mode insulating the band joist & sill with XPS works as long as seal the edges air-tight with spray foam. If you use batts on the band joist over the XPS, kraft-faced, with the facer toward the interior taking care to make it reasonlbly air-tight works. Unfaced goods with no air-barrier to the interior will lose R value when it's cold outside because room air can convect pretty freely into the insulation layer. At times that will add up to increased moisure on the floor joists as well. Kraft paper is a vapor retarder & air barrier, but not a true water vapor barrier (vapor retardency of ~ 0.5 perms.)

    Dry blowing cellulose into a cavity wall isn't rocket science, but read up on it first. (There's lots of online tips on cellulose installaion available from a guy named Karg google it.) At anything over 2lbs density it won't settle much over a coupla decades, but if you're ambitious you can also "dense pack" it to 3lbs+ for slightly higher performance and zero settling.

    As long as the vapor retarders behind the shower are ripped in every cavity they'll pass sufficient moisture- you don't need to go nuts on it. But getting the insulation out is key to keeping the studs warm enough to not condense moisture on them all winter.
  5. Yankee1423

    Yankee1423 New Member

    Messages:
    15
    Location:
    Iowa
    Great. Thanks for the help. Hopefully I can get started on this project this weekend. Will need to call someone about spraying behind the shower surround. Not sure if someone will come out for that small of a job. Any other options for behind that area other then spray foam? That wall is below grade and the garage sits above the backside. Also, if I use "Great Stuff" or something between the cut XPS and stud, will that be an air tight seal? Just wondering if I should spray that and still tape the 6 mil over the "seam".
  6. Dana

    Dana In the trades

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    Dow Great Stuff will work, but it's expensive. If you're going to do a lot of it, buying a sealant gun that takes the 22-24oz. cans of 1-part works better, costs less and wastes less. (The valve is in the gun tip rather than at the can, so you don't have to burn through the whole can in one go to avoid clogging and having to throw away the rest.)

    Cutting in 6-mil poly to slip in behind the studs does almost no good. It'll limit some amount of ground moisture from wicking into the wood, but not substantially better than 1/8" of closed cell foam. The vapor permeability of the wood itself is roughly the same as the XPS. If you tape over the seam with plastic or foil tape, you've limited the ability of the wood next to the concrete to dry. Spray foam at less than 1" thickness is fairly permeable, and will still let that water through, if slowly.

    The bigger mold issue in the studwall in an otherwise dry basement is room air humidity reaching the colder part of the studs, as long as the whole assmbly is vapor permeable enough that the concrete doesn't load up with ground moisture. If you insulate the rest of the cavity with cellulose, the cellulose will have a drying effect on the studs overall. Cellulose is also treated with mold inhibitors & fire retardents- it's definitely a better way to go. Putting the XPS against the concrete A: slows but doesn't stop the transfer of moisture from the concrete into the room, and B: Ensures that the coldest edge of the air-permeable fiber insulation stays well above the dew-point, so moisture won't accumulate creating mold conditions.

    Any insulation or materials in the assmbly from the cut foam/spray-sealed part in toward the room need to be water-vapor permeable. Even kraft facers would be too much- if batts, unfaced only, if celluose, no poly or foil between it an the wallboard. Also, use only standard latex paints for the interior finish. NEVER use foil or vinyl wallpapers, or you'll be ripping it all out to get rid of the mold in under a decade.

    The unfinished 30' section of wall it's still worth insulating. Without the studwall to interfere it's actually pretty easy to take 4x8' sheets of XPS or EPS (no foil or poly facers) and glue them to the wall with walnut-sized blobs of foam-board construction adhesive (special purpose- has solvents that won't eat into the foam). 3 courses of lateral furring over the top held in place with screws through-drilled into the concrete can then hold the (fire-code required) 1/2" gypsum in place. In $/R EPS (fused bead-board, like coffee cups or packing material) is usually the better deal, but it takes 2.5" to get to R10, where the pink/blue XPS only takes 2". But wiith EPS you can go to R16 before it's vapor retardency becomes too high, whereas with XPS you have to stop at 2", and further insulation has to be done with something that is vapor permeable like cellulose or unfaced batts, blown fiberglass.
  7. Yankee1423

    Yankee1423 New Member

    Messages:
    15
    Location:
    Iowa
    Dana,

    A few last questions. In order to save a little cash, do you see an issue with cut and cobble the 2" XPS between studs with foam in stud gap. Then take the unfaced R-13 that was in there already and compress that between the XPS and Gypsum? I see that compression would reduce the batt to r-7 or so but that gives me r-17 total.

    There is a large section (top half of the wall between all the egress windows) that is above grade with OSB behind the current insulation. Does this need to be treated any differently?

    Finally, I was looking at the shower area and I may be able to slide some XPS back behind there. If I can't do that, I could also move a stud temporarily and slide almost a full sheet of 1" foam behind the entire assembly (between the surround and studs with nothing in the cavity. Not sure if that would do much good.
  8. Dana

    Dana In the trades

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    Compressing the 3.5" thick batts to fit in to a 1.5" space will reduce it's R down to ~ R5.6, not R7 (R13 x 1.5/3.5= R5.57) , but yes, that would be fine.

    The above-grade portion need not be treated differently. A somewhat lower vapor permeance on the interior there would acceptable, but not necessarily desirable. With more than half the R-value in exterior-side foam, the temperature at the interor surface of the foam won't be cold enough to condense until it's -30F outside (and surely it won't stay that cold for weeks on end, eh? ;-) )

    An inch of XPS would do a world of good slipped in behind the shower- go for it! (It'll be ~ R5, compared to the ~ R1 represented by the concrete foundation, for an ~83% reduction in heat loss for that area.) The wall temp of the surround will be noticably warmer in winter.
  9. Dana

    Dana In the trades

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    It just occured to me, but there is an injectable foam product out there that could be retrofitted at full stud-depth that wouldn't be too vapor-impermeable to use behind the shower surround: Tripolymer. I have no idea what to expect for installed cost relative to other foams- never used it, but it's drying capacity is 5x higher than other foams- higher than 1" of XPS. Using 1" XPS against the foundation would be a good enough to use in combination with Tripolymer as cavity fill.
  10. Yankee1423

    Yankee1423 New Member

    Messages:
    15
    Location:
    Iowa
    Ok, thanks for all the guidance. The city inspector said we are required to have a vapor barrier between the gypsum and any type of insulation so I'll either need to try and present enough evidence to get a pass or get something up there to get me through inspection that won't create problems down the line.
  11. Dana

    Dana In the trades

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    (apologies for the delay...)

    Your city inspector is obviously unclear on the principles, but hopefully he/she's educable...

    Start here:

    http://www.buildingscience.com/documents/reports/resolveuid/4ba77ccd34c76e58d45214a1b5b961e7

    The move on to:

    http://www.buildingscience.com/documents/reports/rr-0202-basement-insulation-systems

    and

    http://www.buildingscience.com/docu...ion/files/bscinfo_511_basement_insulation.pdf

    (All printable with pretty pictures & discussion.)

    A poly vapor retarder on the interior would be a guaranteed mold trap. Kraft facers would be too, in a large fraction of cases. True vapor barriers are to be avoided when possible. A foundation wall has no capacity to dry toward the exterior, and thus MUST be allowed to dry toward the interior. Putting moisture-tolerant foam insulation at the depths where the winter dew points occur keeps the studs & wallboard from becoming wet.
  12. Yankee1423

    Yankee1423 New Member

    Messages:
    15
    Location:
    Iowa
    Basement Insulation - Updated

    Well after getting the inspector's approval before tackling the project, I decided that it might be just as easy to cut loose all the exterior framed walls and do it right. After tempering fears from the wife that it would not add weeks to the project, I went to work with a saw-zaw and started to release everything. I was able to get 1.5" foam on every exterior except in the bathroom (due to the piping already being installed and one short section where duct work and lots of wires made cutting impossible. I then put the walls back up against the foam and will re-use the fiberglass batts to get a total r-value of around 20. Plus I got it all done in two days with minimal interruptions and the electrician will be much happier. For the bathroom and that one wall, I'll use the original approach.

    For the shower, I will remove the batts behind it and try to slip 1.5" in there. If that proves too difficult I'm pretty sure I can get 1" to bend enough. Do you think 2 layers of 1" would be helpful since the problem is really getting the thicker stuff to bend enough? The tripolymer is not a local option and we are talking maybe 3 or 4 stud bays worth of work.

    The last thing I'll do is start putting 1.5" foam board on all the rim joists and the few 4' sections that are above grade. One area I may have trouble with is underneath the first floor gas fireplace. That rim joist is extremely deep (cantilevered out) and the builder stuffed batts all the way to the back. I can get the batts out but getting a foam board back there and sealed might be tough. If I can get it pretty tight, I might just try to push it back and not deal with sealing it. I guess I could also just lay some foam board on the floor of that section and then stuff the remaining cavity with the unfaced batts.
  13. Dana

    Dana In the trades

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    Sometimes it IS simply easier to just start over. I'm glad you're able to get the XPS in there, and that the inspector is OK with unfaced batts w/no vapor barrier in basements! For the moisture to not build up, it has to be let out, and even vapor barriers with holes block drying, while letting air-transported moisture IN.

    Pulling the batts out of the cantilevered section is the right thing to do- they let the moist air in to condense against the cold wood. If you can seal & insulate in there with a couple inches spray foam you'll be way ahead of the game, after which you can stuff it full of batts again. Any wood that's adjacent to the exterior really needs the foam needs to be protected. At the very least, put an air-barrier like Tyvek or 15lb felt over the side of the batt stuffing facing the room to limit the amount of air-transported moisture getting in there.

    To get the most out of your studwall batts split them rather than stuff them where they go over wiring/plumbing etc, and trim carefully around & behind electrical boxes, etc. If you're sloppy you can cut their performance by 30-50% pretty quickly. "Typical" detailing on batt installations by pros will run ~15% below the rated-R (and they're PROS!) Truly perfectionist installers can get close to full rating, as long as the air-sealing of the cavity is up to snuff. Foaming/gluing the interface between the timbers & foam board in every cavity bay prior to installing the batts, and glueing the wallboard in place with construction adhesive as you put it up, then foam-sealing around all electrical & plumbing penetrations are all necessary to get the full R out of it.
  14. Yankee1423

    Yankee1423 New Member

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    Location:
    Iowa
    Last question I promise. For behind the shower unit, could I stack 2 1" pieces? The one inch board can bend enough to get behind the unit and I figure that if I put one piece in and then PL300 the heck out of a second piece and shove it in there, I have essentially 2" of foam board even though I can't spray foam the edges.
  15. Dana

    Dana In the trades

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    Sure- stacking them works. By not making them air-tight you run some risk of condensing conditions in winter on the foundation sill & rim joist in that area though (if you can at least seal that area you'll be golden). It won't be dramatically more than if you'd left it un-insulated, but some. The studwall will be a lot happier for it though.
  16. Yankee1423

    Yankee1423 New Member

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    15
    Location:
    Iowa
    I was putting up drywall in the basement this weekend and took a quick look behind the insulation on the exterior wall before putting the gypsum up. In the above grade area of the basement wall, I put in 1.5" XPS between stud bays and then spray foamed the edges. I then put in unfaced batts to fill in the remaining stud space. Removing the studs here was not an option. The temps have dropped to the single digits and when I pulled the unfaced batts back, I noticed some condensation on the interior side of the XPS. In one spot it was frosting on the XPS. I used a ton of the Great Stuff to seal all the edges completely and then some. Is this going to be an issue once I put the drywall on?
    Last edited: Dec 6, 2010
  17. Dana

    Dana In the trades

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    Air leakage transports 500-1000x more moisture than vapor-permeability through unpainted wallboard. With unfaced batts and no interior-side air barrier a LOT of room air will convect through the fiberglass and condense on the cool XPS. Without an air-barrier copious condensation on the XPS would be expected during single-digit outdoor coolth.

    Once you have the drywall up the amount of convecting room air reaching the XPS will go down by 90% or more. If you seal & caulk every seam, edge, and plumbing/electrical penetration it'll be something like a 99%+ reduction. The better the air-seal, the less condensation potential there is- and it becomes only a matter of the water-vapor permeability of the wallboard & paint.

    Keeping the wallboard at least semi vapor-permeable will allow any condensation or ground moisture to dry toward the interior when temperature & humidity are favorable. Standard latex paint will reduce the vapor permeabilty of the wallboard from about 50 perms to about 2-5 perms- which is plenty of drying capacity, but allows less than 0.01% of the amount moisture to reach the XPS that your unfaced-no-air-barrier fiberglass lets in.

    Make it air tight on the wallboard, and it'll always be dry.
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