Basement wall insulation question

Discussion in 'Remodel Forum & Blog' started by Jotun, Jan 16, 2020.

  1. Jotun

    Jotun New Member

    Joined:
    Jan 8, 2020
    Location:
    Hampstead, MD
    I've been reading a lot of threads on basement insulation and would like some guidance/clarification. I'm getting a bit overwhelmed with information and different methods I've seen used.

    I'm in the process of remodeling my basement (in Maryland), and none of the basement walls are insulated. In some areas the studs are a little over 1" away from the wall, and I think I can get 1" EPS foam behind them. But most of the studs in the basement are only about 1/2" from the concrete block. I'd really rather not remove all of the stud walls around the perimeter of the entire basement if possible...

    About 3/4 of the house is below grade, with part of one wall exposed for a walk out exit.

    My plan was to put 1" EPS behind the studs where I can, and cut and cobble 2" EPS in the wall cavity between the studs. Then using (closed cell) spray foam (like the “great stuff pro”) behind the studs and around the EPS.

    1) Where I can't fit any rigid foam behind the studs, I thought about cut/cobble 2" EPS between the studs, with the 1" EPS over top the entire stud wall before drywall goes on. Is this a good idea, or would I bet better off just going thicker, like to 3" foam between the studs here (and not doing an inch of continuous over the studs)?

    2) My understanding is that I don't want any kind of vapor barrier in the wall, to permit drying. Is it best to spray foam behind the studs (where there is currently a 1/2" gap)?

    3) Do I need to put fiberglass batts in the wall cavities, or is the EPS a sufficient insulator depending on what amount I end up using?

    4) Am I better off using a spray foam kit? It seems like it would be more expensive and a pain to do my entire basement over time, but if it's a better alternative I would consider it.

    I'd like to order my foam soon but I have to have it shipped, so I'd like to make sure I've got everything I need before I order. Thanks everyone.

    Edit: I called some basement insulation companies this morning to get some quotes and info, and one of them recommended R13 fiberglass batts and not doing EPS or spray foam. I have no idea what to think at this point!
     
    Last edited: Jan 17, 2020
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    I'm not sure what question #1 is asking, but in general it's better to put the low vapor permeance foam layer between the foundation and studwall, and NOT between the studwall & wallboard. At 1" most Type-II EPS has a vapor permeance comparable to standard interior latex on wallboard, cutting the drying rate of the wall assembly in half. EPS with facers will be an order of magnitude lower, essentially blocking drying completely.

    #2: Yes, you want to install lower vapor permeance material between the potentially damp foundtion and the moisture susceptible studs. Can-f0am would serve that purpose on the studs that are too close to slip in foam board.

    3# The IRC 2018 thermal insulation requirements for US climate zone 4 where you live calls out continuous R10, or a 2x4 R13 wall. (A 2x4/R13 wall with no foam is a bit moisture risky though.) It takes ~2.5" of EPS, uninterupted by studs for an all EPS solution, so you're not going to make it unless there is a stackup adding up to at least R13 in each stud bay. As a practical matter, contractor roll R13s are pretty cheap, and easy to install, and are a generally all around better product than R11s, though R11 would probably get you there.

    4# The 2 part foam kits really only make sense when you have a lot of volume to fill, so it's not in the cards for filling in behind studs. Using 2 part foam kits in lieu of EPS is a crime against the planet, since they all use HFC245fa (an extremely powerful greenhouse gas) as the blowing agent. HFC blown closed cell foam is better than XPS (blown primarily with HFC134a, an even MORE powerful greenhouse gas), but it's not exactly great compared to (low impact hydrocarbon blown) EPS- it's not even close:

    [​IMG]
     
  3. Sponsor

    Sponsor Paid Advertisement

     
  4. Jotun

    Jotun New Member

    Joined:
    Jan 8, 2020
    Location:
    Hampstead, MD
    Dana, thanks again for your reply. Since posting, I have decided that it might be worth removing the stud walls after all. The time I would take cutting down foam board to fit between the stud bays just isn't worth it when I could spend that time moving the walls. We plan on being in this home very long term, so I'd rather take my time and do things right.

    1) There is one wall where I already sprayed foam behind the studs to put some XPS board that I already had in the studs. This is one studded wall that I would rather not try to move back. What would you recommend for that wall? It's a standard 2x4 studded wall with about 1/2" of spray foam behind the studs and XPS in the stud cavity. I had planned on putting 1" EPS over the studs, since that isn't optimal is there a better option? The existing foam is 2" so there is only 1.5" of space, not enough for R13.

    2) I can do 2.5" continuous EPS on most of the walls; I will do one layer of 1" and another 1.5" overlapping the seams unless you have any other suggestions. I can do a new 2x4 wall in front in most areas, and the one wall where I can't I'll just have to use 1x3 strips. Do you think it's worth adding R13 batts into the studded walls in addition to the 2.5" EPS?

    3) Where I'm moving walls, I read that it's best to put 1" EPS on the floor underneath them. Is it a problem if I have existing interior walls where the pressure treated floor plates are directly on the slab? What about any new interior walls I put up?

    4) What sealant should I use where pieces of the rigid foam meet each other, like the top of the block (where full sheets on the face meet the small strip going back)? I've seen the spray foam used to fill gaps, but wasn't sure about these other seams.
     

    Attached Files:

  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    The 2" of XPS is labeled R10, an will perform to at least R8.4 even 50 years from now. So at most you only need ~R5 out of that 1.5". (Going by labeled-R you'd only need R3.) Compressing an R11 into that space would deliver about R 7-R8. Splitting an R13 into a pair of 1.75" thickness batts would deliver about R6.


    Use 1x4s rather than 1x3s- the latter are far more likely to split, and tend to have more dog legs, bows and twists.

    In very high energy cost locations or if there's enough left over from a cheap contractor-roll that would otherwise be scrapped it might be worth adding the R13s. But from a comfort point of view it isn't going to move the needle muchn and from a raw energy use point of view it might not be fully rational on a net-present-value-of-future-energy cost-savings within 40 years.

    Fully interior partition walls will run warmer than those at the foundation walls, and pressure treated lumber isn't going to get moldy. Leaving it alone is fine.

    If the edge of the foam is super smooth housewrap tape can sometimes seal it. A thin bead of foam board construction adhesive or polyurethane caulk works if there aren't big gaps, otherwise a more flexible "windows & doors" can foam works better than big-gap filler can foams.
     
  6. Jotun

    Jotun New Member

    Joined:
    Jan 8, 2020
    Location:
    Hampstead, MD
    Thanks so much. I can’t tell you how much I appreciate your time and insight. I hope some others can get some good info from my situation as well!
     
Similar Threads: Basement wall
Forum Title Date
Remodel Forum & Blog Mounting Many Big Mirrors to Basement Walls on ICF Foundation? Jan 21, 2020
Remodel Forum & Blog Basement finishing: existing insulation half covering foundation walls? Apr 7, 2019
Remodel Forum & Blog Basement 1/2 Wall Framing Help Mar 2, 2019
Remodel Forum & Blog Basement insulation for walls and floors Mar 26, 2018
Remodel Forum & Blog Furring out basement wall Jan 21, 2018

Share This Page