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Musings of an Energy Nerd

Three Ways to Insulate a Basement Wall

For interior insulation, you'll need to use either rigid foam boards or spray polyurethane foam

If you're insulating a basement wall on the interior, one of the best approaches is to use a continuous layer of rigid foam. The illustration show the use of R-16 polyisocyanurate. In Climate Zone 3 and most of Zone 4, building codes will accept lower R-values.

Basement wall insulation can be placed on the exterior side of the wall, on the interior side of the wall, or on both sides of the wall. In this article, I’ll discuss the three most common ways to insulate a basement wall from the interior. (For a comprehensive discussion of basement wall insulation, including exterior basement insulation, see my 2012 article, “How to Insulate a Basement Wall.”)

General principles

Make sure that your basement is dry. Before installing any interior wall insulation, verify that your basement doesn’t have a water-entry problem. For more information on this topic, see “Fixing a Wet Basement.”

R-value minimums. In Climate Zones 3 and higher, basement insulation is required by the 2012, 2015, and 2018 International Residential Codes as follows: at least R-5 in Climate Zone 3, R-10 in Zone 4 (except Marine Zone 4), and R-15 in Marine Zone 4 and Zones 5, 6, 7, and 8.

That said, local codes may differ from these general guidelines, so it’s worth asking your local building department about minimum R-value requirements in your community.

Note that the IRC lists two different R-value requirements for basement walls: a lower number (for example, R-15 in Zone 5) for continuous foam, and a higher number (for example, R-19 in Zone 5) for “cavity insulation”—usually interpreted as fluffy insulation like fiberglass installed between studs. Since it is inadvisable to insulate a basement wall with fluffy insulation like fiberglass unless the wall has first be insulated with a layer of continuous rigid foam or spray foam, it’s generally best to focus on an approach that uses continuous insulation, and to ignore the “cavity insulation” approach.

Insulation choices. On the interior side of a basement wall, all three common types of rigid foam insulation—polyisocyanurate, expanded polystyrene (EPS), or extruded polystyrene (XPS)—perform well. That said, green builders usually avoid the use of XPS,…

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  1. Yeldog | | #1

    What is the sill sealer doing in the illustration with the foam ?

    1. Charlie Sullivan | | #2

      It's keeping any moisture in the slab from wicking up into the wood or the polyiso, both of which can absorb moisture. It's a nice simple solution to that problem.

      1. Yeldog | | #5

        Normally I glue (few fasteners) .. I'm always doing heated slabs with insulation and VB. So, I hope my moisture is controlled. With the sill foam strip you have to power nail.

        1. Tyler Keniston | | #17

          You glue the bottom plate instead of nail? Is that just to avoid the concrete fasteners? Specific glue preferred?

  2. Will R | | #3

    Does sill sealer need to be placed under poly iso if there is a 1" gap off the ground filled with one part spray foam?

    1. GBA Editor
      Martin Holladay | | #4

      Q. "Does sill sealer need to be placed under polyiso if there is a 1 inch gap off the ground filled with one-part spray foam?"

      A. No. Although some others may disagree, I wouldn't worry about wicking with your detail.

  3. CarsonB | | #6

    “Basement wall systems should never include polyethylene sheeting—neither between the concrete and the foam insulation”
    There are diagrams on fhb/gba that recommend installing the vapor barrier from the ground, up the wall just below the sill plate in crawlspaces. This seems like a contradiction unless there is something inherently different with the basement. We just had such as installation done, is this now not recommended? I thought polyiso wasn’t supposed to be in direct contact with ground/concrete?


    1. GBA Editor
      Martin Holladay | | #12

      You're right; the recommendations for crawl spaces and basements are contradictory. I wrote my crawl space article a long time ago, and included advice on polyethylene that was commonly expressed by sources I respected. In retrospect, I now believe that including polyethylene on crawl space walls, between the foundation wall and the rigid foam or spray foam, serves no useful purpose.

      That said, polyethylene in that location in a crawl space doesn't really cause any harm. So either way, don't sweat your decision.

      Crawl spaces tend to be more damp than basements, for a variety of reasons. (One reason: basements are more likely to have mechanical equipment like furnaces and water heaters than basements.) For basements, I would stick to my recommendation -- no polyethylene on the wall.

      The worst place for polyethylene on a basement wall would be between the drywall and the fiberglass-insulated studs. This is a very bad idea, and plenty of remodelers in Minnesota know what lurks behind poly in that position: mold and soggy fiberglass. Such assemblies have filled many dumpsters.

      1. exinreno | | #49

        Our local building inspections require 6 mil poly on any proposed basement (and above ground) wall assembly unless it is 100% foam insulation to specified thickness/R value/vapor. New Brunswick Canada. We just did one as described in your Assembly 2, but with 6mil between studs and drywall. What reference (code) should I use to have "the talk" with our inspector/permit application process next time?

        1. GBA Editor
          Martin Holladay | | #50

          Ex in Reno,
          First of all, you might want to read this Q&A thread:
          "Poly and Canadian Building Codes."

          Second, here's my advice: If a stubborn code official wants 6 mil polyethylene in the wrong place, do your best to convince the code official to accept a variable-permeance material like MemBrain instead of poly. In this location, the MemBrain is unnecessary, but at least it won't cause as much damage as polyethylene.

  4. Yeldog | | #7

    My last two projects were in the city and when dealing with 16' wide spaces -- doing the 2" gap and 3.5 stud wall is a lot of space to give up when you need the lower level for living space.

    It's the same with my current project .... while not in the city it's in an historic village and I was restricted in building size. The lower level is living space and the architect wanted to minimize the look of the foundation. I'm in zone 4. I did the R10 (-2x) .... there is not much of the foundation out of the ground. This is only a one story so we could use a strengthened 8" wall. Inches matter sometimes.

    1. GBA Editor
      Martin Holladay | | #13

      I agree that inches matter sometimes. (That said, I have no idea what you mean by "I did the R10 -2x." Sounds like an algebraic equation.)

      If inches matter, the best solution is polyiso topped by 1x4 strapping.

    2. Expert Member
      Kohta Ueno | | #39

      If you're really space constrained, a few options to minimize the thickness of your build-in:

      - THERMAX polyiso with taped seams (see Marc Rosenbaum's blog,
      - PVC Z-furring attached to the wall, with spray foam between--unlike metal studs, doesn't cause thermal bridging problems (
      - Prefabricated panel system like InSoFast (
      - Rigid foam with 1-5/8" steel studs for services; leave stud cavities empty. Unlike wood studs, you can get them to stay straight even at this depth. Could also use this for a spray foam retrofit.

  5. CarsonB | | #8

    Contractor already put stud walls against basement wall, no sill sealer underneath or vapor barrier behind. Is the best solution cut and cobble with eps? I suspect I can’t put foam over the studs, as there would be no drying potential other than back through the wet concrete.

    1. GBA Editor
      Martin Holladay | | #14

      Q. "Is the best solution cut and cobble with EPS?"

      A. No. The best solution is to dismantle the stud wall. Use a Sawzall to cut some of the nails -- that should allow you to reuse the studs.

      1. CarsonB | | #19

        Oi, not sure how that’s going to go over with the contractor. This is just so I can put foam behind them instead of in them, and rebuild the stud wall? This is for a tiny engineer spec’d utilty room in the crawl. The walls are all well below grade.

        1. GBA Editor
          Martin Holladay | | #20

          Of course, the decision on how to proceed is up to you. I'm a former builder, and my instinct is to side with builders in most disputes. But I think you are a little too worried about the contractor's feelings in this case. He or she should know better.

          1. CarsonB | | #21

            thanks Martin, going to try to remove it and go with your strapping over foam approach instead of the studs.

    2. Expert Member
      Kohta Ueno | | #40

      Question--are the studs tight against the concrete wall, or is there a gap? If there's a gap, closed-cell spray foam might be an option to consider. Also, if you can simply remove fasteners to the slab and the framing above, could you simply "shift" the wall inward a few inches for this type of retrofit?

      1. CarsonB | | #42

        unfortunately, they are tight against the wall. Cutting out part of the walls to move them inward sounds like a good suggestion and I may try that, though that may end up being more work than removing them entirely and using furring strips against the foam. It sounds like I would also need to raise them a bit and fit sill sealer to the bottoms. I'm not sure what the primary issue is - I assume moisture/mold potential of the wood against the concrete. The walls are far enough below grade I don't think thermal losses would be high enough alone to warrant the effort.

        1. GBA Editor
          Martin Holladay | | #43

          Your original question asked about "the best solution," and my answer was based on your desire for the best solution. If you are willing to accept the second-best solution, cut-and-cobble will work.

          1. CarsonB | | #44

            Fair enough, I suppose I should have worded it "what would you suggest I do"? Though it sounds like it may be the same answer. For cut and cobble, would the concern primarily be mold/rot of the studs in contact with the concrete?
            the air is exchanged with the house, so mold issues are big ones.

          2. GBA Editor
            Martin Holladay | | #45

            Without continuous insulation, several risks increase. There will be more thermal bridging, as you note, and there is a risk of rot (due to the fact that some of the studs are in contact with the damp concrete). Moreover, the cut-and-cobble approach raises the risk of air leakage due to the high number of seams that need to be sealed -- and this raises the risk that interior air will contact the cold concrete. I'm not saying the risks are significant, but there are several factors that make this the second-best way to proceed as opposed to the best way to proceed.

  6. Robert Opaluch | | #9

    The diagrams show 1/2" plywood fire block. My understanding is that a minimum of 3/4" plywood or 2x lumber are required as fire block.

    1. Expert Member
      Michael Maines | | #10

      There are a few other option, including 1/2" drywall, but you're correct that the IRC does not allow 1/2" plywood as fire blocking:

    2. GBA Editor
      Martin Holladay | | #15

      Good catch! You're right. I regret the error. The labels on the illustrations have been corrected. Thanks for your help.

      1. Robert Opaluch | | #18

        Thanks Martin for all your work over the decades educating us about a wide range of sustainable building issues. Thanks also to progressive builders like Maines and others who share their knowledge. Great community here.

  7. CarsonB | | #11

    I'm struggling to understand the need for below grade insulation. If the basement ground is not typically insulated, and in crawlspaces it's said to not be worth insulating if the walls are, what is the difference between a below grade wall and the floor? Is it purely to reduce moisture? If so, why isn't the slab the same condensing surface with the same temperature as the below grade portion of the wall?

    1. GBA Editor
      Martin Holladay | | #16

      Q. "What is the difference between a below grade wall and the floor?"

      A. During the winter, there is a temperature gradient in the first few feet of soil. The actual temperatures depend on your geographical location, the winter weather, and the depth of the snow cover (if any), but I'll describe what's possible in Vermont on a January day when there is 4 inches of snow on the ground. Let's assume that the outdoor air temperature is 0°F. Directly under the snow, the top few inches of soil might be 15°F. Two feet lower, the soil temperature might be 32°F. At 5 feet below grade, the soil temperature might be 40°F. Under the basement slab, the soil temperature might be 50°F or 55°F.

      Now let's look at the foundation wall. The top 12 inches of the foundation wall is exposed to the outside air -- so that section of the wall is at 0°F. The concrete is even colder than the top few inches of soil. Since the top of the concrete is so cold, you'll see a layer of frost on the interior of the basement wall, in a band near the top of the wall (assuming, of course, that the basement wall is uninsulated).

      If you go down to the bottom of the wall, the concrete might be at 50°F.

      The temperature gradient I just described is the reason why basement walls -- especially the tops of basement walls -- are so much colder than basement slabs during the winter.

  8. Marta Snow | | #22

    Would phenloic foam rigid insulation for the foundation walls be even more ideal since it has a higher R-value per inch and avoids the flame retardants that are in some (most?) other foam plastic insulation panel products?

    Also, how much does the vapor permeance of the insulation matter? I have always thought that with ccSPF the low vapor permeance was fine / good since it aggressively adheres to the foundation wall filling every nook and cranny and there is much less chance for water vapor to make it through the foundation wall at all.

    When applying any rigid insulation panel, in recent years, I have come to think that it is important to use a material that has more vapor permeance and allow drying to the interior since those insulation panels are not continuously adhered to the foundation wall, I believe water vapor could more easily get through the foundation wall and into the tiny air interface between the back side of the insulation panel and the face of the foundation wall. Thus I ponder whether foil faced polyiso insulation in particular should be avoided since foil has basically zero vapor permeance?

    Attached are photos from a basement that was remodeled about 20 years ago in Minnesota. The basement was build in the 1970's and has moisture resistant parging on the exterior but not a robust waterproof membrane that we normally used on new homes today. Roof overhangs are about 3 feet deep, soil is very sandy and slopes away from the house well. Despite all of this and 20 years of good performance, this past summer they had record rain of about 20" in the span of about 2 weeks and the lower portion of the paper faced drywall got moist and moldy. After removing the drywall and some of the 1.5" thick foil faced polyiso insulation, it appeared to me that the foil on the polyiso insulation has some corrosion and evidence of having seen moisture more times over the past 20 years. The electrical box appears that way also. Frankly I was a little surprised to not see mold on the back side of the foil faced insulation and the face of the CMU block foundation, but I am also curious if the alkalinity of the concrete deters mold growth on its surface as well, even if it does get moist?

    We were thinking of replacing the foil faced polyiso on the lower portion of the wall with EPS, non-foil faced polyiso or phenolic foam rigid insulation and then glass matt faced drywall this time, with the logic that more vapor permeable materials would allow the assembly to dry to the interior more quickly to better prevent any future mold risk.

    Also considering the removal of the bottom 2x2 plate that is in contact with the slab and re-install some 2x2 blocking 1.5" higher. Fill the cavity of the 2x2 removed with rigid insulation, hold the bottom of the new drywall up off the floor .5" minimum and then install base board trim again.

    The house does have a high velocity A/C system in it and in-floor heat system, but the owner does not always run the A/C, since he likes to save energy and is willing to suffer through the heat and humidity many times in the summer. Thus we are planning to have a portable dehumidifier in this basement room from now on with its humidistat set to about 30% RH as well to ensure the moisture levels are more actively managed in the future.

    Thanks in advance for any thoughts / advice you may have about all of these questions and our proposed approach to fix this basement.

  9. GBA Editor
    Martin Holladay | | #23

    Q. "Would phenloic foam rigid insulation for the foundation walls be even more ideal since it has a higher R-value per inch and avoids the flame retardants that are in some (most?) other foam plastic insulation panel products?"

    A. I have no direct experience with, or opinion on, the characteristics of phenolic foam insulation. GBA readers who have tried to buy some have discovered that the product is mostly unavailable. If any GBA readers other than Peter Yost have managed to buy some phenolic foam insulation, I'd be interested to hear of your experience.

    Q. "How much does the vapor permeance of the insulation matter?"

    A. Not very much, since useful thicknesses of closed-cell spray foam or rigid foam insulation are all vapor retarders or vapor barriers.

    Q. "I have come to think that it is important to use a material that has more vapor permeance and allow drying to the interior."

    A. I disagree strongly with your conclusion. There is absolutely no benefit to inviting moisture to flow from the damp soil outside your home into your basement's interior. For more information on this issue, see "Joe Lstiburek Discusses Basement Insulation and Vapor Retarders."

    Concerning the 20-year-old remodeling job in Minnesota: Everything looks pretty good to me. Of course, water entry into a basement is undesirable, and water entry problems should be addressed before attempting to insulate the interior of a basement wall. A little mold on the foil facing of your 20-year-old polyiso is neither surprising or alarming. After all, there is lots of mold in your soil. As long as the mold is on the exterior side of the polyiso, and you pay attention to airtightness, the mold on the aluminum foil is no more alarming than the mold in the soil adjacent to your foundation.

    1. Marta Snow | | #29

      Thanks for the replies. All very helpful, as usual!

      1. Expert Member
        Kohta Ueno | | #41

        One last point--definitely agree with installing a dehumidifier. However, setting it for 30% RH will make it run like crazy--it will run nonstop and never get there (except maybe in winter, if you have a lot of air leakage). In general, a rough target RH for basements (setpoint for dehumidifier) might be in the 50-60% RH range.

  10. Ikibbe | | #24

    I could read about basement insulation all day. Thanks Martin for all the work you’ve done to allow me to do so.

    After many enjoyable hours reading about basement insulation in old brick buildings, I’m doing a bit of a combo of these methods. Cut and cobble doesn’t seem like quite the right descriptor, but it’s close. Maybe someone else has a name for what I’m doing (continuous cobble?).

    It’s an old, 1930’s brick building in Chicago with a poured concrete foundation. Due to some bumpy walls, old house quirks, an overhead sewer along the walls, and the building schedule, I’m doing most of the wall in a continuous layer of 2” of EPS but leaving the top, bottom, rim joists and various gaps here and there to be spray foamed in with a froth pak later after the framing goes up.

    It’s helped us save money while also being be flexible in terms of creating a continuous layer while not having to stress about the areas that are difficult to attach rigid to (like the very bumpy rim joists, or the places where we did foundation crack repair). I’m being careful closing up the seams b/w the sheets of EPS, using foam and tape. 3M All-weather flashing tape has been great with the R Tech EPS boards we’re using (shiny foil facing inward).

    The one thing it seems I’ve neglected is the sill seam. Given that the walls should be well sealed and the base plates are pressure treated, is this something I should be concerned about? Basement has been very dry to date.

    And, for those interested in greater detail or how I’m dealing with my embedded joists there are more details/questions/answers in this post:

    1. GBA Editor
      Martin Holladay | | #26

      Q. "The one thing it seems I’ve neglected is the sill seam. Given that the walls should be well sealed and the base plates are pressure treated, is this something I should be concerned about?"

      A. The sill sealer shown in the illustration (between the bottom plate and the concrete slab, and between the rigid insulation and the concrete slab) is a nice detail, but it's not strictly necessary. If you omitted it, I wouldn't lose any sleep if I were you.

      For more information on dealing with embedded joists, see "Insulating Basement Walls With Embedded Joists."

      1. Ikibbe | | #28

        Thanks Martin. That’s exactly what I hoped to hear.

        For my joists I’m doing 1” of spray foam, and borate rods in the joist ends on the northern wall. Cheers!

  11. AMorley | | #25


    Great article, and incredibly timely for me - I'm working on a basement insulation plan for our home right now... Coastal Rhode Island, climate zone 5, reasonably heavy silt loam soils with high water table.

    1. Would any of your best practices change if the basement had an interior perimeter drain system with sealed sump pump, etc? That is what I have in our basement - it has done wonders to eliminate our unwanted "indoor swimming pool"!

    2. A specific question on that front - our basement is basically two basements - original rubble foundation from 1830's, newer addition has a poured concrete wall foundation from 1995 or so. The rubble wall leaks water consistently when soil water levels rise. Typically October - May here in Rhode Island. When the perimeter drain was installed a few years ago, poly sheeting was installed from the drain and up the rubble wall to guide the weeping water down the irregular wall into the drain below. Much like this assembly detailed at the end of this article by Steve Baczek back in 2015:

    Our plan was to use closed cell spray foam over the poly sheeting and up into the rim joist cavities. Our thought was that by leaving the poly sheet up against the wall, when the ccSPF is applied, the foam won't plug the perimeter drain, keeping a path for the water to run to the sump. Would you advise against this now with your anti-polyethylene sheeting recommendations?

    3. And do you have any thoughts on the use of chicken wire in Steve's detail from that 2015 article? First time I had seen that and it seemed like a neat solution! That is, if it works and doesn't compromise the spray foam structure somehow.

    Thanks so much!

    1. GBA Editor
      Martin Holladay | | #27

      A. Morley,
      Your description of the interior French drain sounds fine; it's similar to the detail that GBA has provided for years (see attachment).

      I think that Steve Baczek's trick -- installing chicken wire before spraying closed-cell foam against the polyethylene -- makes sense.

      1. AMorley | | #33

        Many thanks!

  12. drafthunter | | #30

    Are there any options that allow you to insulate, but not require sheetrock?

    1. GBA Editor
      Martin Holladay | | #31

      Draft Hunter,
      Q. "Are there any options that allow you to insulate, but not require sheetrock?"

      A. Yes. Mineral wool insulation can be used as a thermal barrier. (Of course, you should check with your local building official to make sure that he or she agrees with this opinion.) The option is mentioned in my article, "Thermal Barriers and Ignition Barriers for Spray Foam."

      More information here: "Use of Rockwool stone wool as a Thermal and Ignition Barrier."

      If you go this route, you would need to build a 2x4 stud wall on the interior side of your rigid foam or spray foam insulation, and fill the stud bays with mineral wool, in compliance with the Rockwool recommendations found in the linked document.

      1. Expert Member
        Michael Maines | | #32

        Martin, I'm curious why you are not mentioning Dow Thermax as a code-compliant solution for basements. Unless something has changed, I believe it still meets the IRC criteria for thermal and ignition barriers:

        1. GBA Editor
          Martin Holladay | | #37

          Thanks for mentioning another good solution. You're right, of course.

          I have edited my article to include a mention of Thermax, as well as a mention of the use of mineral wool insulation as a thermal barrier.

          1. Expert Member
            Michael Maines | | #38

            Martin, phew--I was worried that there had been a code change or building science development I wasn't aware of, which is not unusual.

      2. drafthunter | | #34

        What about skipping the rigid foam altogether and just using comforboard? I’m guessing that would require a vapor barrier first? Thank you.

        1. CarsonB | | #35

          I think the concern there is that warm air could move through the mineral wool and condense on the interior side of the vapor barrier, so a layer of foam of sufficient thickness would be required to be against the wall first.

        2. GBA Editor
          Martin Holladay | | #36

          Draft Hunter,
          Carson is correct. The problem has nothing to do with vapor barriers; the amount of vapor diffusion isn't really concerning. The problem arises when indoor air contacts the cold concrete, leading to condensation and moisture accumulation. Air-permeable insulation materials like fiberglass or mineral wool don't stop air flow.

          Attempts to create a perfect air barrier on the interior side of fluffy insulation usually fail. The only robust solution is to use rigid foam or closed-cell spray foam against the concrete. That way, the first condensing surface--the interior surface of the foam--stays above the dew point. (In other words, it's warm.)

          For more information on this issue, see "How to Insulate a Basement Wall." (Scroll down until you see the section that begins with the heading, "Can I insulate on the interior with fiberglass batts, mineral wool batts, or cellulose?")

  13. Joe Roman | | #46

    Great article by GBA and @MartinHalladay as always. I have learned a lot from GBA as a builder the last 2 years and happy to say, I've totally reworked my approach to basements based largely on research on GBA and some sustainable building conferences.

    I get asked a lot about basements myself and one question I get here in New England is what to do about old junk slabs or dirt floors, particularly if someone is preparing to finish a basement. Part of my prescribed answer is now to include a layer of EPS foam below the slab and thermally break the slab at the walls.

    This article appears to be primarily geared towards retrofits, however probably worth mentioning the ability to address slab insulation properties if someone is doing a full basement makeover. By my longevity calculations, it's getting harder and harder to justify building over even a decent junk slab knowing how thin it is and that it likely lacks proper drainage and for sure insulation underneath.

    Keep up the good work here guys! And @CarsonB, not even a questions on a smaller utility wall space, yank those studs and do it right! Good luck.

    1. GBA Editor
      Martin Holladay | | #47

      You wrote, "One question I get here in New England is what to do about old junk slabs or dirt floors, particularly if someone is preparing to finish a basement. Part of my prescribed answer is now to include a layer of EPS foam below the slab and thermally break the slab at the walls."

      That's good advice. At a minimum, a dirt floor needs a layer of polyethylene. And if a homeowner is considering the installation of a concrete slab -- either over existing dirt or to replace a cracked older slab -- then a continuous layer of rigid foam under the new slab makes sense.

      For more information, see these two articles:
      "All About Basements"
      "Fixing a Wet Basement"

      1. Joe Roman | | #48


        Not sure how you have time for all of of this, but it's amazing and appreciated. Yes, forgot to mention the layer of poly. I would also try to integrate a radon mitigation system anytime under a new slab in New England.


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