basement wall insulation - help me avoid a mistake

iron Posted By iron, Jul 28, 2016 at 4:42 PM

  1. iron

    iron
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    Sep 23, 2015
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    in the upcoming months, i will be remodeling our basement. there are two primary goals:
    1. seismic retrofit (basically involves anchors bolts, tie down straps, and adding plywood to the interior face of cripple walls)
    2. improve insulation

    my insulation scheme is as follows:
    - 1" XPS on the floor with 3/4" tongue and groove plywood over it (for carpet)
    - concrete foundation wall gets 2" of XPS or spray foam (closed cell)
    - cripple walls get R13 faced batt insulation
    - insulate joist cavities at the rim board as well as general air sealing

    please take a look at this image for my plan:
    Wall Insulation-SH-2.jpg

    i have several question areas which are indicated by the dark dashed boxes. let's start from the top and work down.

    1. at the interface of the joists and the wall, how should i best insulate this? i know you can spray foam or XPS foam the cavity, but i'm curious about transitions to other wall/ceiling surfaces. it seems to me i need to fully cover any potential cold surfaces to prevent cold air from contacting my future finished interior wall.

    2. at the transition between cripple wall and concrete foundation wall, what should i do here? i've seen some suggest just putting a vapor barrier over the cripple wall (like 6 mil plastic) and then taping it to the XPS foam (assuming i use XPS and not spray foam). but, that method seems inadequate to me since it feels like that's a good spot for cold air in the winter to come off the concrete, then reach the backside of the interior (heated) wall.

    3. where the floor insulation meets the concrete wall insulation. should i leave a gap or make it tight? some say that a gap is good in the event it gets moist. as far as i can tell, we have a pretty dry basement, though there is a musty smell (likely in the floor which i'll be ripping out). no matter what, i plan to hold the subfloor a few inches away from the wall insulation.

    bonus question:
    on the surface of the interior plywood used for the seismic retrofitting, should i just spray foam or XPS foam over this to create a vapor barrier? this is my biggest concern of the project. i do not want to inadvertently create reverse vapor barriers or not insulate correctly.

    i can see moisture/cool air getting in through the cripple wall foundation plate and into the cripple wall (which will have batt insulation). once there, it seems easy enough for it to permeate through gaps and the plywood and then reach the back of the interior (heated) wall. i feel like foaming over the entire wall surface could avoid this potential issue, but then i am worried about doing something stupid like trapping too much moisture in the batt insulation or reversing a vapor barrier. please see the pic below. this kind of "insulation" is underneath my siding. i call it structural cardboard --- it's about 1/8" thick and has a silver facing.

    IMG_6263.JPG
     
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  2. vinny11950

    vinny11950
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    You have a lot going on here, Iron.

    Basements are big deals because you are constantly fighting moisture and water damage, while keeping access to all the plumbing and electrical you need access to.

    Honestly, in your region, I would not worry about losing too much heat from the floor. I would do away with the carpet and T&G plywood and floor foam insulation because one leaky pipe at the wrong time will have you ripping all that out. Leave the concrete bare. Worry more about air sealing the basement and controlling the air that comes in and goes out. In my basement, I try to open the windows only when the humidity is low. Still, they used all the wrong materials and now it smells moldy in the summer. Sheetrock has to be ripped out.

    Anyway, for your cripple wall, I would continue with the 2 inch XPS, sealed seams with tape all the way to the bottom of the floor joists where they meet the insulation in the joist bays.. And before putting it up, seal the cripple wall as much as possible with foam - gaps and cracks and holes.

    If you do decide to put the foam on the floor, I don't think having the foam go to the walls or leaving a gap will make much difference. The foam can handle the contact and a small gap at the same time will not do much to dry the floor beneath should it ever get wet.

    Anyway, I recommend that after you demo the basement, leave it bare for a few months (a year to see all the seasons would be best), just to learn the what's happens at the concrete walls - do they weep, leak or have efflorescent spots? Any of those issues should be addressed first before rebuilding it.

    I know they are expensive, but those basement wall panels from Dow sure look nice and functional for a basement.
     
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  3. begreen

    begreen
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    I'm not a builder but for question #1 would Roxul batt insulation sections work there.

    Note that this electrical box needs to move so that the wire can be stapled within 6" of the box.
    IMG_6263.JPG
     
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  4. iron

    iron
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    we're doing carpet for sure. the basement will be a living room/play area for soon to be future kids (<6 weeks) and we will have a guest bedroom and bathroom down there. it needs to be comfortable.

    there currently is carpet in a small section. i believe it's just laid over 6mil plastic. i will pull it out and give it the smell test.

    BG: my buddy the electrician either didn't care about that wire not being stapled, or fixed it before we finished up the wall. in either case, there's tile and such already installed, so it is where it is :)
     
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  5. iron

    iron
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    also, could anyone speak about heat recovery ventilators? our house is 1979 construction, but with recent air sealing and insulation efforts on my part, i suspect it's going to be much tighter. our windows are all new (2011) and exterior doors as well. i'm starting to think we may benefit from a HRV.

    heating is provided by an electric furnace with an old heat pump. of course, our goal is to heat 100% with wood this upcoming year, but we'll see how that goes. i do see using the recirc fans from the furnace to move air from the warm upstairs to the basement.

    thanks all!
     
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  6. LowbanksArcher

    LowbanksArcher
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    I'm planning a similar installation for the flooring, except I plan to lay a basement foundation wrap membrane down first under the insulation. The stuff with the dimples that lets the concrete to breath and will channel potential moisture away. This will mimic the design of the DRIcore/Amdry subfloor panels but save some $.
     
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  7. vinny11950

    vinny11950
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  8. sportbikerider78

    sportbikerider78
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    I'm doing this as well. I will be following this video to a "T". But I will be using double faced foil polyiso and taping seams and sealing it to the wall with an adhesive. I know that polyiso is not 100% a vapor barrier but it is pretty darn good and way better than bare concrete. My objective is not to finish it, but to decrease moisture and cold from coming through the wall.

    http://www.thisoldhouse.com/toh/video/0,,20503345,00.html
     
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  9. iron

    iron
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  10. moey

    moey
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    I used 2" XPS on my walls. I glued/attached them to the wall with "great stuff" and 1*4s with 3.5" concrete screws. Then put drywall up attached to the 1*4s. Basement is much warmer and much less humid now. I didnt tape the joints I would just fill them with great stuff when I did the adjoining section.

    If your finishing your basement ( I was not ) I would run some serious numbers on how much it would cost to put the 2*4 frame up to the wall with a one inch gap and have 4 1/2 spray foam done. XPS is pricey and its a lot of labor drilling holes. Adding the extra R-13 in the walls is not saving you much money at the end of day.

    A 4*8 panel drywall XPS boards and screws ended costing me about $35 and thats was using 10% off coupons and any way I could get a discount at lowes.
     
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  11. gzecc

    gzecc
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    For fire code the ceiling drywall need to extend past the wall top plate to the foundation sill. Easy to do prior to building the wall. Not so much after.
     
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  12. iron

    iron
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    it looks like i'll be doing either 2" or 3" of CCF sprayed by a contractor. quote came in very reasonable, especially considering the time tradeoff.

    one question: i know i have some water lines and such that run pretty close to an exterior wall. if those are too close to the future interior plywood face (plywood will be on the exterior wall studs, on the inside of the room), should i have them spray right over it?

    followup related question: if my water lines are in the stud cavities (to be insulated with R11 or R13 and then covered with plywood), and then i have foam sprayed over the plywood, are those lines at risk of freezing since they'll be very isolated from a heated space (interior sheetrock wall, 2-3" CCF, plywood, and R11)?
     
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  13. Buzz Saw

    Buzz Saw
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    Can you reroute the water lines to the inside? With some PEX it wouldn't be much work ( Easy to say from my house ).

    Why batt insulation then spary foam? Why not spray foam in the stud cavity?

    Maybe I missed something....

    Sent from my SM-G900V using Tapatalk
     
  14. iron

    iron
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    i'm performing a seismic retrofit. this involves adding plywood sheathing to the inside face of the studs. it's going to take me a few weeks to complete all of this. it's recommended to not leave your walls without drywall since that is a bit of a stabilizer. therefore, i will be doing sequential demo (demo, clean, retrofit, and repeat) until everything is retrofitted. once it's retrofitted, i will hire out the spray foam insulation for a single day's work --- mobilization is the highest cost.

    also, because of the junction between plywood and concrete foundation walls, i believe it will make more sense to spray foam on the plywood down to the floor rather than in the cavities then down to the floor since i will need access to install the plywood to the sill plate (very critical)
     
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  15. Buzz Saw

    Buzz Saw
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    I'm going to have to trust you on this one. I know nothing about seismic structure building. We don't have that stuff here in Ohio....

    Sent from my SM-G900V using Tapatalk
     
  16. iron

    iron
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    one more question:

    i was all set to pull the trigger on getting the closed cell foam installed, but then read this:
    http://www.greenspirationhome.com/sprayed-foam-insulation-why-homeowners-must-proceed-with-caution/

    my wife is nearly 9 months pregnant, so the idea of having the basement sprayed in the next week or two and then having horrible chemical smells (possibly dangerous) is deeply concerning to me. i understand that most of these issues are contractor-related installation methods, but still.

    price for 464SF of spray foam @ 3" thick is $1800, so $3.90SF. i can buy 3" XPS for about $1.26/SF. add in some great stuff and manual labor and not worry about possible chemical smell issues....

    hmm.

    i would love to hear your thoughts on this (soon!).
     
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  17. drz1050

    drz1050
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    I've also heard those complaints about the smell/ outgassing.. it seems they're all in cases where the foam was sprayed too thick. From what I've read, you can only spray 2" of closed cell foam at a time. If you go thicker than this, you're risking the foam not curing completely, and that's what gives you the smell.

    Regarding the water lines- how deep is your basement? Is the entire basement under grade, or is there an exposed wall? If the basement is deeper than the frost line in your climate, those pipes won't freeze.
     
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  18. semipro

    semipro
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    There are a lot of stories out there about nuisance out-gassing from sprayed foam. I think even the most reputable installers have had issues with bad batches of chemical components, bad mixing valves, etc. Its definitely something to take into consideration.
    That's one reason I went with foam XPS board in our basement.
    I'm a big fan of Roxul also but had one batch that had ammonia-like odor problems. I've had no problems with recent purchases.
     
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  19. DickRussell

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    [QUOTE="iron, post: 2091368, member: 46385"..... it's recommended to not leave your walls without drywall since that is a bit of a stabilizer.also, because of the junction between p
    ..... since i will need access to install the plywood to the sill plate (very critical)[/QUOTE]

    I'm inclined to suggest skipping the spray foam and do it all with rigid board foam. Applying it to the concrete foundation is straightforward, and even easier onto the studs of the cripple wall. I wouldn't use sheet polyethylene anywhere in the wall assembly, as that would preclude any drying to the interior. Even 2" XPS has some low permeability, somewhere around 0.6 perm, whereas poly is a tenth that, essentially a vapor barrier, rather than a class II retarder. I might also be inclined to rip out that foil-covered 1/8" cardboard stuff. If that is a foil surface, it would would be a vapor barrier by itself, and on the wrong side of things. The shiny surface perhaps is intended as a radiant barrier, but if you put insulation right up against it then you lose the radiant barrier property; it needs an air gap to be effective. I agree with recommendations by others to air seal carefully at the sill area on top of the concrete foundation, using caulk or can foam after the plywood goes up over the studs. Seal also where the plywood meets the ceiling.
     
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  20. vinny11950

    vinny11950
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    Yeah, I have read about a few horror stories with spray foam insulation. Either the installer screws up, the chemicals are bad, or some time in the near future we find out the bad health effects of these products.

    I would go the foam board route instead. Make sure you buy a reputable brand, maybe even Google it and see what hits you get. Can't be too careful. These boards are supposed to be inert so no off gassing (unless they burn). But then you have to cover them to be code compliant so check your building code department and see what they suggest.
     
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  21. iron

    iron
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    semi-related question:

    i know a lot of people that install rigid foam on the floor followed by a wood subfloor will use tapcons to screw things down. after installing a few tapcons in my concrete for other purposes, i've learned that they do not work well for me unless the hole is perfectly clean. even then, it seems like it's hit or miss. therefore, i'm thinking that instead of using tapcons, i would just do the following:
    1 use some great stuff spray foam, spray the backside of the rigid foam
    2. place on ground and put on temporary counterweight 'til it sets
    3. spray foam on backside of plywood
    4. place on foam and put on temporary counterweight 'til it sets

    think this is a problematic approach? the biggest drawback i see is that with the tongue and groove plywood, i wouldn't be able to pound it into place as easily.
     
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  22. sportbikerider78

    sportbikerider78
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    From what I have read, on covered XPS and EPS aren't code compliant. But foil covered poly iso is compliant. It makes sense. Foil is spark resistant.
    If anyone has any other input, I'd like to hear it. In the next month or so, I'm buying foil iso and doing the walls...and plan to leave uncovered.
     
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  23. iron

    iron
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    IIRC, you can have issues with the foil surfaces on concrete.

    also, with the polyiso, i believe it performs horribly at cold temperatures. at normal temps, it does fine and is slightly better than XPS. at lower temps, it's R-value drops from 6-something down to 2-something.

    i'm planning to build a wall in front of my foam, so XPS is the way to go for sure (if i don't do spray foam)
     
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  24. sportbikerider78

    sportbikerider78
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    Could you elaborate? Everything about poly iso up against concrete is fine, as long as it is foil faced and air sealed along the perimeter.

    My basement is 40-65F, so never really cold or really hot.
     
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  25. vinny11950

    vinny11950
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    Putting great stuff on the backside of the rigid foam and the plywood sounds like a big old mess. The few times I have worked with it, it has been a mess. Not to mention the foam expanding.... Will it cause bumps in different areas of the board? Also consider once you spray that stuff on the concrete floor it is not coming off easily, so you can forget about exposing the concrete floor ever again.
     
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