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Community and Q&A

Exterior vs. Interior Basement Insulation

JennL | Posted in Expert Exchange Q&A on

Building in CT zone 5. I’ve been advised by our builder not to insulate the exterior foundation. He has done this in the past and there has been significant termite damage despite use of termite flashing and treatment. Another consideration is that I am very sensitive to mold and would like to observe the foundation for 2 yrs for cracks resulting in water intrusion so they can be accessed for repair. I’m aware of the possibility of dew point condensation (especially on the above grade foundation interior) as a result of not insulating. We will run a Sante Fe (Thermastor) 70pt dehumidifier but I’m not sure if this will fully control dew point condensation. The basement will be heated.

I’d greatly appreciate feedback regarding these possible solutions.

1. Insulate the exterior foundation but leave a 2-3 inch gap in the above grade insulation to check for termites. This would complicate the issue of above grade protection of the foam (not sure how this would be done) and the uninsulated section would still be vulnerable to condensation on the interior.

2. Insulate the interior foundation at the above grade section only where it is most vulnerable to condensation. This would allow for observation and intervention if cracks occur. Any water intrusion occurring under the foam could easily be identified as it would drip down the wall. (The space would be dehumidified.)

3. A hybrid solution in which the exterior foundation would be insulated below grade and the interior foundation above grade. I’ve never seen any discussion about this approach and I am always hesitant to be first out of the gate on anything due to unintended consequences. Removing exterior foundation insulation would be a nightmare.

4. Leave it uninsulated for 2 years and use heat and dehumidification to control any condensation that might occur. Insulate the interior with EPS w/fire barrier or Thermax would remain a possibility at a later date.

Thoughts? Thank you

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  1. Expert Member


    There is no reason to believe your concrete walls will be appreciably drier in two years than they will be by the point of construction when you would normally expect to be insulating them. It simply isn't a factor in whether the walls will grow mold or not.

    Properly constructed concrete walls do not have to be left open to look for possible structural failure, or bulk water intrusion.

    The problem with a hybrid insulation approach is it leaves the unprotected concrete as a large thermal bridge allowing heat to bypass both insulated surfaces.

    I understand your reticence to use the standard, approach to insulating the inside of the walls due to your fear of mold growth, but done properly it's exactly what you need to do to prevent it.

    1. JennL | | #3

      I suspected that was the case with a hybrid approach... I guess the thermal bridge concept rules out option 2 as well?

      I've been told that the concrete foundation can take up to 2 yrs to dry to the interior. Sounds like you disagree?

      My builder told me that cracks in the foundation are inevitable as the foundation settles. I'd imagine locating a leak under the foam would be difficult as it should be tightly adhered and the water would take a convoluted path to where it might exit at the floor. Can you suggest a strategy for locating a leak should it occur? I doubt if a moisture meter could work over Thermax.

      We had a crack that leaked (and was easily identified and fixed) in the uninsulated part of the basement in our previous home so experience tells me this happens.

      And can dehumidification control the possibility of dew point condensation at the interior of the above grade foundation? I never observed this to occur in my previous home even in winter. The dehumidifier was set to 45% and only turned on June to Sept.

      Thanks for your insight.

      1. Expert Member
        MALCOLM TAYLOR | | #4


        If you are going to only insulate one portion of the wall the above grade section makes more sense, as the lower areas are partly buffered by the surrounding soil - but yes you will get significant thermal bridging. The first place to start when designing options is what your building code will require. Do these partially insulated walls meet your code?

        Concrete does cure and dry for a long time, but just like its strength, most of the drying occurs immediately after the pour and then tapers off, so that the drying that occurs as time goes on becomes increasingly insignificant.

        Hairline cracks are common, but with properly executed exterior waterproofing should never lead to bulk water intrusion. There are a wide variety of treatments to choose from. Membranes, dimple mats, crystalline coatings like Xypex, or drain-rock up the whole height of the walls.

        If you include your basement as part of the conditioned area of the house the risk of mold growth should be the same as anywhere else in the structure. It shouldn't need stand alone humidification - alth0ugh the whole house may require it for a period immediately after construction

  2. plumb_bob | | #2

    My first thought- do you need a basement? What about slab on grade? Then basement moisture issues are a non-starter, and if mould sensitivity is a real concern you could eliminate the most likely location for problems.
    However, if you follow good building practices you should not have to monitor the foundation for cracks, and therefore not worry about mould. Well installed foundation drainage and damp proofing should trake care of any water that makes it to the foundation. And then using systems and materials for insulating the interior of your basement that are proven. Building Science Corp has covered this extensively and has free articles on their website.
    I do not live in a termite zone, but can appreciate how the fear of termite intrusion will have implications on how you build.

  3. GBA Editor
    Kiley Jacques | | #5

    We have some excellent recent articles on this topic that include key details (with illustrations) and how to get them right:

    Three Ways to Insulate a Basement

    Insulating Concrete Foundation Walls

  4. AlexPoi | | #6
  5. PAUL KUENN | | #7

    Think of it this way... if you insulate on the interior, you lose all that thermal mass. When done on the exterior, the foundation mass is warmed all summer and then does not get cold until February without any heating. Of course I use 6" of exterior insulation to keep the R value high.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      I'm not sure I agree with that description. If you can arrest the concrete losing its heat through months of cold weather, surely the same strategy also stops it from taking on that heat for the same period of warm weather.

    2. AlexPoi | | #10

      Thermal mass is a double edged sword. If you have too much of it, it can takes forever to warm or cool your basement temperature because of the thermal lag. Thermal mass and insulation are the equivalent of an electric RC filter (low pass) for which you don't control the C value. It's nice if you live somewhere where the temperature is pretty consistent and you are at home most of the time. Otherwise it can make things worse.

    3. Expert Member
      NICK KEENAN | | #12

      Please do this:
      1. Estimate the mass of a foundation.
      2. Google the specific heat of concrete.
      3. Estimate how much heat it takes to warm a foundation by 10F.
      4. Estimate how much heat it takes to warm your house for one year.
      5. Compare the magnitude of the numbers you got in #3 and #4.

      Typically #3 comes in at around 10-15 hours worth of heating. "Thermal mass" -- which isn't a real scientific or engineering term -- just doesn't work the way people think it does.

      1. Expert Member
        NICK KEENAN | | #17

        Extra credit: calculate the dollar value of the heat in #3 above. It's typically in the $10-20 range.

    4. JennL | | #14

      Our builder has done exterior foundation insulation and reported termite problems despite flashing and treatment. I see lots of info about insulating the exterior of the foundation, but without a viable solution for termites it really isn't an option.

  6. PAUL KUENN | | #9

    Heat always goes to cold. So the summer's warmth warms up the interior side of the concrete and is not lossed to the exterior grade dirt. Then most of the winter the warmed concrete heats the basement until equalized. We don't heat our basement other than the dump load from the solar hot water system when tanks get hotter than 170F.

    1. Expert Member
      Akos | | #11

      Lets do some maths!

      Say 1500sqft 30x50 basement with 8' foundation 8" thick. In total you have ~850 ft^3 of concrete so ~120000lb. That works out to about 24000BTU/degF heat capacity.

      People complain if the temperature swing is much more than 5F, so at most you are storing 120kbtu. Lets say the basement is very well insulated and has a heat load of 1200BTU/h. That 120k will heat that basement for 100h or 4 days.

      Because is is heavy, it feels like concrete can store a lot of heat, but it is actually not all that much. At best it can store enough heat for a day or two. Definitely not months.

      The good news is that a basement with 6" of exterior rigid will be very comfortable no matter what the foundation is made out of.

      1. Expert Member
        NICK KEENAN | | #13

        We must have been typing at the same time when I did my post #12 above. You've given all the answers!

      2. Deleted | | #15


      3. JennL | | #16

        Our builder has done exterior foundation insulation and reported termite problems despite flashing and treatment. I see lots of info about insulating the exterior of the foundation, but without a viable solution for termites it really isn't an option.

  7. onslow | | #18


    Although my climate and termite risk are very different from yours, I became concerned when a neighbor at a lower altitude discovered a termite infestation while renovating his kitchen. My build was just starting, so I elected to encapsulate the 3" of (reclaimed)XPS I was attaching to the exterior of my foundation. Partly to prevent water intrusion, partly to create a barrier against possible termite invasion. I use Grace Bituthene which ran from sill to footing sides.

    I also made very certain that all building waste was removed from the trench around the house on a daily basis to deny potential food sources. Termites need food and moisture, so basically I am attempting to deny them a foot hold any where near the house. My friend's kitchen reno revealed a hidden leak that had saturated framing which extended to the exterior sill drawing them in.

    I do not have any vegetation around the house (due to fire concerns) which would need watering. Damp soil and food sources next to unprotected foam would likely make a pretty good homestead for many forms of critters and insects. Certainly my efforts to deny building rights to Mother Nature are not perfect. Mice are very determined to explore all possible options. My flashing details have seemingly stymied them, but they are relentless.

    I don't think the first three options given are sensible. The fourth might give you peace of mind if the contractor is that adamant about cracks. If the footings are doing their job and the rebar is sufficient, I would not expect settlement to occur. There are chemical treatments against termites and in the southern regions they also set food traps out beyond the foundation much like ant traps. I am not sure that approach is good for northern infestation levels, but maybe the climate shifting of viable termite zones is worse than I know.

  8. JennL | | #19


    Thank you for your insight.

    It was my builder's practice to encapsulate the foam, use termite flashing, etc. but there were problems despite this.

    I've read about leaving 2-3 inches of foundation visible to check for termites but I have never seen any drawings or detail showing this. My guess is that it would create a thermal break as described by Akos.

    1. karlb_zone6a | | #20

      Hi JennL,

      Joe Lstiburek of BSC published an article addressing your question/concerns this past fall:

      Figures 4 and 5 should help out. I'm not in the industry (just a concerned homeowner), but I'm convinced that the northward march of termites is going to be fast and devastating. You're wise to put some thought into getting the details right now.

      1. JennL | | #24


        Thank you for the link to the article. It was informative.

        First time I've seen the detail of a removable protection strip. I wonder how labor intensive (and costly) it is to implement? And durability?

    2. Expert Member
      Akos | | #21

      I'm generally not a fan of rigid mineral wool, but I think in this case it will fit the bill. Insulating on the exterior with mineral wool warms up the whole foundation so there is no issues with condensation. It also won't become food for critters.

      A bit harder to finish above grade than foam. Simple options are metal coil stock cap or cement parging.

      The termite inspection gap is a thermal bridge but not one I would loose too much sleep over. Keeping critters at bay is more important.

  9. PAUL KUENN | | #22

    Science is based as much on observation as math. I've done this on seven homes in 12 years ranging from 1880s stone foundation to 1968 concrete basements. None have sub slab rock for drainage and all sitting on our glacial clay. As they all hire me back for other issues, they are pleased it all works. No need for dehumidifier in summer as it stays between 68 & 70F all summer and rarely goes below 62F in winter. I always use 4-6" of EPS foam on the exterior down at least 5' so we're a bit below frost level. Given global warning, in the last 20 years I've never seen it freeze below 2 foot sub surface levels. To avoid insects, I have a heavy galvanized drip pan made with a return edge and angled slightly down. Below that is a heavy self adhesive barrier from the wall over the EPS and down into at least 8" of soil but stuck over the below grade fiberglass (roll)protection layer (found at most big box stores). Nobody has seen any degredation in these 12 years. Also keep in mind additionalheat is used in summer as the heat dump load from solar thermal panels on many of these homes is run through baseboard heaters to keep the tanks cooler so that does warm the walls a bit warmer but not on all the homes I've done.

    1. Expert Member
      NICK KEENAN | | #23

      I don't question your observation, but I'm questioning your analysis. What you're seeing isn't due to the "thermal mass" (whatever that is) of the walls. It's geology. If you dig down about 10' the ground will be at the same temperature year-round. Dirt typically has an r-value of about R4 per foot, so you can get the same effect if you have R-40 of insulation above, and insulation to the sides. If you put insulation on the walls of a basement and a house above it, the earth below that basement is going to be well-insulated and stay at a pretty constant temperature. If you live in a moderate climate that temperature is going to be pretty comfortable.

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