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Community and Q&A

Minnesota basement remodel: Insulation questions

wood0619 | Posted in Energy Efficiency and Durability on

I am doing a basement remodel and am looking to upgrade the basement energy efficiency on this 1976 rambler as much as possible. Any suggestions are appreciated.

My plans:

Added oversized gutters with raindrop gutter guards to move bulk water far away from the house.
Inspect existing sump pump system to verify continuous drain to the sump.
There is a sign of water in the past in one corner of the foundation, but I have not seen water there since I moved in in 2012.

-Add 2″ of XPS foam T&G glued to directly to concrete
-Add 2″ of XPS foam T&G glued to the floor
-3″ of medium density spray foam (R18) for rim joists
-Add 1/2″ T&G plywood glued to foam, screwed if necessary
-1-5/8″ steel studs for exterior walls with rock wool insulation between studs

I would like to start on adding the wall foam board in the very near future. Is there any downside to doing the walls first, then the floor?

Thank you,

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  1. Expert Member
    Peter Yost | | #1

    Hi Mike -

    The most challenging time for basement slabs in cold climates is when an intense and susatined freese is followed by a big rain event. There is then a thin zone of thawed soil right next to the foudnation wall and it is the only time that you can get an actual head of water (hydrostatic pressure) the height of the below-grade foundation. Problem is this might only happen once every 15 years or so (ask me how I know my own home and basement, this happened about 15 years after we bought our 100+ year old home while I was traveling and my wife had to deal with liquid water in the basement...).

    The test for how wet your basement slab is you can find here: I would NOT glue down the XPS and I would not put it down before completely understanding the moisture performance of your slab, per above.

    And why not just let your floor system "float?" (sure hope use of the term "float" is not prophetic....)


    1. wood0619 | | #6


      When you had basement water issues, did you have foundation drainage and a working sump?

      I plan to have my drainage tube video inspected. I am also adding radon mitigation in the sump which I have read has the benefit of reducing slab and foundation vapor pressure.

      Also, I am OK with not gluing the foam to the floor if there is a benefit in that for water control? My thought is it would control bounce of the floor. It is my understanding that if you end up with bulk water under pressure it's coming in unless you have foundation drainage.

  2. Expert Member
    Dana Dorsett | | #2

    Minnesota code diverges significantly from the IRC when it comes to foundation insulation, even restricting the total R when all of the insulation is on the interior side. Though most inspectors won't force you to follow the code strictly on a retrofit, be sure to get sign off on the plan from your local building department first.

    XPS is one of the least-green commonly used insulation materials due to the high polymer weight per R and more importantly the HFC blowing agents used (soon to be banned under the Kigali amendment to the Montreal Protocol.) As it loses the HFCs over several decades performance eventually drops to that of EPS of similar density.

    Both polyisocyanurate and EPS are blown with comparatively very benign hydrocarbons (usually a pentane variant). Polyiso has a higher R/inch and greener polymer, and would be fine for use on the walls. But since it can potentially wick moisture it's best to use EPS rather than polyiso on the floor.

    Steel is extremely thermally conductive- that 1-5/8" of rock wool isn't going to buy you more than R2 after thermal bridging. Going with 2.5"-3" of polyiso on the walls would be higher performance and simpler, skipping the fiber insulation altogether.

    Like XPS, most closed cell polyurethane is blown with HFCs, but not all. The HFC blown goods can only be applied in lifts of 2" with a cooling/curing period between lifts, which makes 3" an awkward number to work with. Many or even most installers would just go ahead and do it in one pass, but that is both a quality issue and a fire-hazard while curing. Though it's more expensive, HFO blown closed cell foam runs about R7/inch, and can be installed safely with high quality in deeper lifts. (Some even claim 7" per pass is fine.)

  3. wood0619 | | #3

    Hi Peter and Dana,

    Thank you for your insights and feedback.


    I based my 2" of XPS off of what my building inspector would allow and the article below from Building Science Co.
    "Up to two inches of unfaced extruded polystyrene (R-10), four inches of unfaced expanded polystyrene (R-15), three inches of closed cell medium density spray polyurethane foam (R-18) and ten inches of open cell low-density spray foam (R-35) meet these permeability requirements."

    My inspector has advised me to use 1.5" of XPS, but he would approve up to 2". He said he would also approve other methods if he saw a technical and validated insulation assembly for the basement interior that proved that assembly (it was a good sign he was open to new options).

    What are the other downsides of EPS besides the lower R-value?

    Does it make sense to do EPS on the walls, but XPS on the floor?

    I had not considered too much the HFCs associated with XPS, thanks for pointing that out. If I go with 4" of EPS to achieve a greater R-value and have the permeability necessary (I want the basement to feel dry in the summer), would 4" of EPS produce less HFCs than 2" of XPS? I have considered all spray foam, but it may be cost prohibitive for me, but it is still on the table until I have some quotes. One other advantage of 4" of EPS at R15 is it's not necessary to insulate the 1-5/8" stud cavities (and you are right it does not add much anyways).


    When you had basement water issues, did you have foundation drainage and a working sump like I do?

    I should also add that I plan to have my drainage tube video inspected. I am also adding radon mitigation in the sump which I have read has the benefit of reducing slab and foundation vapor pressure.

    Also, I am OK with not gluing the foam to the floor if there is a benefit in that for water control? My thought is it would control bounce of the floor. It is my understanding that if you end up with bulk water under pressure it's coming in unless you have foundation drainage.

    1. charlie_sullivan | | #5

      The climate impact of XPS is about 300X worse than EPS. So 4" of EPS would have much less impact than 2" of XPS--you'd need 50 feet of EPS before you accumulated the same impact as 2" of XPS! XPS is really an outlier in how bad it is.

      The lower R-value per inch of EPS is offset by having a higher R-value per dollar spent. The only other disadvantage is that if you buy the lowest density EPS, it's kind of crumbly. So you might want to specify higher density EPS for the floor.

      If you are constrained on thickness, you can specify graphite infused EPS, often branded "neopor" which gets up near the XPS R-value per inch.

      I actually don't think the permeability specified for drying to the inside, as specified by the BSI article, is necessary. For the basement to feel dry inside in the summer, you need the insulation to keep the warm interior air away from the cold concrete. And you need good enough air sealing that you can dehumidify in the basement without that being undermined by new humid outside air coming in.

  4. wood0619 | | #4

    Also, I have explored this excavation-less exterior foam insulation method. It is very intriguing and revolutionary. However, I could not find a contractor willing and able to do this or even provide an estimate.

  5. Expert Member
    Dana Dorsett | | #7

    At 4" EPS would have a signficantly higher R-value than the rated R10 of 2" XPS. No down side there!

    EPS produces zero HFCs (none), but at the rated R-value about 15% more polymer per R. The polymer isn't as important as the HFCs, but 4" of EPS has about twice the polymer as 2" of XPS, but it's a true R16-R17 forever, compared to a "warranteed" R9 that you would get out of XPS.

    At 4" Type-II EPS (1.5lbs per cubic foot nominal density) is a minimal Class-II vapor retarder, still slightly more vapor open than XPS at 2", and not a big deal either way. Type VIII (1.25lb) EPS would be right on the Class-II/Class-III boundary, still not a big deal either way. Used Type VIII EPS is often available at a huge discount from reclaimers, the most common density of EPS used in roofing applications. Run this search on your local craigslist (and other nearby locations)- there are multiple foam reclaimers operating in the upper midwest, some of which may be near enough for a pick-up or cheap shipping:

    EPS is fine on the floor- it used under slabs, airport runways and roads for frost heave control all the time, and won't be "crushed" under a residential subfloor when fully supported by a slab.

    There is no "bounce" to a slab/EPS/subfloor sandwich type floor- it's stiffer/harder than a floor supported on joists by quite a bit.

    1. wood0619 | | #10

      I am looking for options for foam board on the walls and floors. For the walls, I am thinking of aluminum faced polyiso over EPS. I worry about polyiso's moister resistance, but because the foil is against the foundation it should limit absorption, and the R-value is great. Is this a bad idea?

      For the floor, I am thinking this:

      or this:

      Any other recommendations? I am hoping to find EPS and polyiso in tongue and groove format.

      I am thinking of doing a floating vinyl plank floor. Can I eliminate the plywood layer and place the planks directly on the foam board? I may use a small amount of glue on the foam EPS to concrete prevent bridging depending on the flatness of the slab. Overall my slab is in great shape, very flat. 8.5' from slab to joists which is pretty rare for 1976.

      I have been completely dry even in this very wet Minnesota spring. No water in the sump either. I believe my soil is mostly sand.

      1. Expert Member
        Dana Dorsett | | #11

        Polyiso on the walls (foil faced or fiber faced) is fine.

        They don't specify what density they mean by "High Density", or what the R-value of that Insulfoam EPS is, but any density of 1.5lbs/cubic foot or higher would deliver R10 @ 2-3/8".

        With EPS or polyiso you're probably going to have an easier time finding ship-lap versions than tongue & groove.

        Without at least a half-inch OSB or plywood subfoor to distribute the weight, vinyl planks are likely to permanently deform the EPS. It's fine to glue the EPS to the slab, and the subfoor to the EPS using foam board construction adhesive, as long as there is a vapor barrier under the slab.

        If there isn't a vapor barrier under the slab it's prudent to install one between the subfloor & EPS to keep the subfloor dry. This would mean screwing the subfloor to the slab with masonry screws. Screws are not vapor permeable, and the annular ring hole around the screw is miniscule in cross sectional area- no significant moisture will be coming through. But if you're really concerned you could always put a blob of polyurethane caulk or foam board construction adhesive on the tips of the screws as a sealant.

        1. wood0619 | | #12


          Thank you for your help; it is much appreciated.

          There does not appear to be a vapor barrier under the slab. I had to trench a few areas for plumbing and did not see one.

          Would it be a good idea to use double-faced polyiso on the floor to create the vapor barrier?

          Because plyiso will absorb moisture, am I making a mistake using it in the basement walls or on top of the slab? My basement has been dry so far, but I want a robust system that can dry if something happens.

          Thank you,

          1. Expert Member
            Dana Dorsett | | #13

            Foil faced polyiso works great on walls as long as it stops above the high tide mark of any historical flooding.

            On the floor it polyiso could take on water in a minor flood that could take decades to dry even if the vinyl planks were removed to speed the drying of the subflooring. Unfaced EPS would dry orders of magnitude quicker, even through a wood subfloor.

  6. wood0619 | | #8


    Thank you for the great information.

    How do you recommend to fasten down the subfloor? Can a float the EPS and plywood on top of the subfloor?

    I would prefer not to use fasteners into the concrete to prevent a vapor or water path.


    1. GBA Editor
      Martin Holladay | | #9

      Q. "How do you recommend to fasten down the subfloor? Can I float the EPS and plywood on top of the subfloor?"

      A. Here is a link to an article that will answer your questions: "Installing Rigid Foam Above a Concrete Slab."

  7. wood0619 | | #14

    There really needs to be an XPS material that uses different blowing agents which does not have a huge environmental impact.

    Is it recommended to use poly between the subfloor and insulation for any type of insulation, EPS, polyiso, or XPS?

  8. wood0619 | | #15

    I am getting ready to insulate my basement concrete slab and am having some second thoughts. I plan to do EPS directly to the concrete then OSB Tapcon to the concrete where necessary. I have read a few articles that suggest a dimple mat should be used in
    contact with the concrete, followed by insulation. Is the dimple mat needed or nice to have for any reason?

    One source that talks about the dimple mat:

    1. Expert Member
      Dana Dorsett | | #16

      A dimple mat makes a lot of sense in basements with a flooding history or a history of bulk water leakage through the foundation walls. For basements that are pretty dry, never flood, with no signs of bulk water leakage, and no significant efflorescence evident except perhaps a very short section of wall above the slab the dimple mat is probably overkill.

      If you're the kind of person who would harbor nagging doubts about it it for years/decades, by all means, install the dimple mat. But know that many homes are doing just fine without it.

      Have you drilled through the slab to determine definitively the presence or absence of a vapor barrier? (You indicated that there wasn't any where you installed the plumbing.) A dimple mat is a vapor barrier, which is fine on it's own if installed,, but if there isn't a vapor barrier under the slab and you skip the dimple mat it's worth installing 6 mil polyethylene between the EPS & slab.

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