Basement insulation

Discussion in 'Remodel Forum & Blog' started by Something Sexy, Jan 21, 2014.

  1. Something Sexy

    Something Sexy New Member

    Messages:
    4
    Location:
    Wisconsin
    I have been a long time lurker but finally posting. I have gotten a lot of ideas from here but I am still not sure what my best solution is yet.

    We purchased a home 3 years ago in southeastern Wisconsin. It is a ranch and the basement was finished when we purchased it, with wood planks/paneling (not terrible on the eyes). There is a utility room that is unfinished, which is fine with us. The basement stays very dry, even in the heaviest of rains and we run a dehumidifier in the summer time. We want to tear down the paneling and hang drywall. The finished part is heated and in the coldest of tempatures we can usually maintain around 60-65 degrees, however I still think we are losing a lot of heat around the sill. At first I wasn't too concerned with this job but now that I am inspecting things closer, I do not have confidence in the job the previous owner did. We have two closets in the unfinished part, against the foundation where the back side wasn't completely closed off so I an take look at how they did it. It appears they put up a vapor barrier against the cement wall and then put the studs directly against the wall. Inside both of the closets they used rigid foam core as insulation but it is for the most part tossed in there, nothing is sealed. I haven't taken anything down yet to see if that is what they did with the rest of the walls against the foundation but I wouldn't be surprised it that was the case. They at least used PT wood for the bottom of the wall against the foundation floor.

    I am wondering what my best approach would be (assuming no mold problems either) knowing that the studs are directly against the wall. Is closed cell spray foam still possible even though the studs are directly against the wall? There might be a gap but it doesn't seem to be more than a 1/4". Or can I get XPS or something and place it between the studs and seal it? Or should I really move the wall to be at least an 1" away the foundation and then get spray foam done? I haven't got any estimates yet on spray foam (maybe it will be out of budget for me) but I feel like that is what I want to do if I can.

    Any help from the experts around here would be most appreciated!
  2. Dana

    Dana In the trades

    Messages:
    2,783
    Location:
    01609
    If you haven't had moisture problems at the foundation sill from the vapor barrier, you probably won't, so you can leave it there if that's easier..

    Studs against the cold foundation is prone to becoming a mold-farm if you insulate the wall cavities. If you don't want to lose interior floor area, pull the studs (carefully- you can re-use them) and put 2" of rigid EPS (the stuff of cheap coolers and coffee cups) up against the foundation, tacking it in place with foam-board construction adhesive, then trap it in place by re-installing the studs, turned sideways. Seal the seams of the foam with duct mastic, liberally applied. Caulk or can-foam the studs to the wall foam, and insulate between the studs with unfaced split batts, compressing them in place. The compressed 1.5" of fiberglass or rock wool is worth about R6, and the EPS foam is about R7.8-R8.4m depending on density (the 1.5 lbs density "Type-II" EPS is the higher-R, and is less crumbly, and by far the better choice beyond mere R-value.)

    EPS is more enviro friendly than XPS and has many similar characteristics, such as increased R value at lower temperatures. The HFC blowing agents used for XPS have a global warming potential of about 1400x CO2, compared to 7x for EPS. Worse yet, as the HFCs leak out over a few decades the R-value of the XPS falls to that of EPS- at 50 years they're essentially the same.

    The band joist can be insulated & air sealed with a flash-foam of 1-2" of sprayed closed cell polyurethane followed by at least 3" of rock wool (Roxul is now sold at box stores) on the interior. Make the sprayed foam continuous with the rigid EPS and the top plate of the studwall for optimal air-tightness. The spray foam should wait until May or June to give the wood sufficient seasonal drying time, and to have a sufficiently warm surface that the spray foam will stick and seal well enough. If it's too small a job for the local pros, a 600 board-foot TigerFoam or Fomo-Foam kit (about $700-$800 if you include accessories) is enough to do most houses, and if you pay close attention to the instructions, (including the tank temperature ranges) it's a fairly straightforward DIY. (Band joists & foundation sills together typically comprise the single largest air leak in a house, even if it was built with sill-gaskets, adding up to more air leakage than all windows & doors combined. It's just one big long skinny hole that will often add up to a square foot or more.)

    To do the whole wall in 2lb spray foam you're looking at $2 per square foot for 2" (R12-R13) and an environmental footprint from blowing agents nearly as big as XPS ("only" about 1000x CO2 :) ) In your climate (US zone 6), as long as the ratio of exterior foam-R / interior fiber-R is greater than 1/3 you will not accumulate moisture in the stud cavities from wintertime moisture drives. So if you can give up the bit of floor area, putting 2" of EPS against the foundation trapped in place with a 2x4 studwall 24" oc. with R13-R15 batts would give you "whole wall" R (after the thermal bridging of the studs is factored in) of about R19-R20, and that's probably going to be your best bang/buck. UNFACED batts are by far the least risky, since it allows ample drying rates toward the interior, and the exterior foam is sufficient to avoid wintertime condensation/frost issues inside the wall cavity, as long as the wallboard is painted with a standard latex paint and reasonably air-tight.

    Do NOT add any sort of interior side vapor barrier or the assembly will be a moisture trap. (Avoid wallpapers, especially foil or vinyl wall papers or you'll trap moisture in the studwall.) If you pull the bottom plates of the studwalls, add an inch of EPS under them when you re-install as a thermal and capillary break to keep it from wicking moisture from the slab. (They're not structural- they only need to hold up the insulation and finish wall materials, not the house.)

    With an R15-R20-ish whole wall the basement will likely stay above 60F even WITHOUT actively heating it, but the heat load will be miniscule compared to an 8" poured concrete wall with an empty studwall (a whole-wall R of about R1.5, R2 best-case.)
  3. Something Sexy

    Something Sexy New Member

    Messages:
    4
    Location:
    Wisconsin
    Thanks for the quick reply!

    OK, so if I do move the studs out, do you recommend I just take down the vapor barrier or can I tack the EPS to the vapor barrier?

    What would you recommend to do if I did not want to move the studs out?
  4. Dana

    Dana In the trades

    Messages:
    2,783
    Location:
    01609
    If you pull the studwall out if it's any easier if you then also pull the poly sheeting and use blobs of foam-board construction adhesive (it's available at box stores, comes in caulking tubes, but uses solvents that won't degrade foam board) to tack the EPS in place before you seal the seams & edges and put the studs back up. It'll stick to the vapor barrier just fine, but it won't be as rigid, and may make it a bit more awkward to install at corners, etc. Either way it's your call. My inclination is to yank it out if you're going to put up foam.

    If you don't move the studs out, you can't insulate with out risk of moisture accumulating in the cold edge of the stud, and the performance of any insulation you DID install would be undercut by the thermal bridging of the studs. Stuffing R13 batts in a 2x4 wall 16" o.c. only yields about R9.5-10, and only if you add R1 for the wallboard + concrete. But continuous R8 foam that has no studs or screws cutting through it performs at R8.
  5. snokel

    snokel New Member

    Messages:
    41
    Location:
    Wi
    are you sure the studs are tight against the concrete? When you frame a basement if you want a straight wall they have to be at least 1/2 inch from the concrete to handle bows etc in the concrete
    wall, also how old is the house? Newer houses have 1 inch of the XPS foam on the outside of the concrete already and I am not convinced you need to put foam on the inside of the wall if the wall surface temp is not colder
    than the dew point.
    I have a ranch in SE wisc and it's two years old with 1 inch XPS on exterior and I don't heat it and it never got colder than 59 degrees even when it was -17 outside.

    If your humidity level in the basement is say 36 percent, the surface temp of the concrete would have to be lower than that in order for water to condense.
    its the same deal as why when it gets really cold out you get condensation on the edges of your windows.
    Last edited: Mar 12, 2014
  6. snokel

    snokel New Member

    Messages:
    41
    Location:
    Wi
    sorry browser posted twice.
  7. Something Sexy

    Something Sexy New Member

    Messages:
    4
    Location:
    Wisconsin
    This house was built in the 50's. Part of the stud wall is exposed inside the closets and they are right up against the vapor barrier/wall.

    I run a dehumidifier in the summer but it doesn't get too bad.
  8. snokel

    snokel New Member

    Messages:
    41
    Location:
    Wi
    ah, so it would be a block wall and probably a lot straighter than a poured wall. The poured wall in my basement is not straight at all and no way you could put the studs right against it.
    Did you pull any of the wall covering yet to see if there is any mold/dampness issues?
  9. Something Sexy

    Something Sexy New Member

    Messages:
    4
    Location:
    Wisconsin
    Not yet, I won't be starting this project until early next year unfortunately. So far the basement seems to be staying dry, that doesn't mean there won't be problems when we eventually pull it back. There is a section of the basement that is unfinished and that hasn't had any problems yet.
Similar Threads: Basement insulation
Forum Title Date
Remodel Forum & Blog Basement insulation question Mar 12, 2014
Remodel Forum & Blog Uneven walkout basement external wall's insulation Feb 2, 2013
Remodel Forum & Blog Basement Wall Insulation and Buildout Jan 1, 2013
Remodel Forum & Blog basement wall insulation Sep 25, 2012
Remodel Forum & Blog XPS basement insulation and foundation sill plate Jul 30, 2012

Share This Page