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Community and Q&A

Dow Styrospan and Roxul Batts

Ken Wigboldus | Posted in Energy Efficiency and Durability on

I have followed a long list of questions regarding closed cell foam, flash and batts etc. None of which quite answered my question. That said, here goes my very first internet posted question!

I have a poured rubble foundation(85 yr old home) in southern ontario…zone 6 or 7? I would like to insulate the walls to R-24. The walls are quite sound save the odd area where I have reparged. I have priced 4″ closed cell foam but I am still very reluctant to go ahead with it for two reasons. Number 1 is the cost and a very close second is the fact that I live in a subterranean termite zone within my city and I do not want to create a protected tunnelling haven for them.

My real question is this. My next best idea is to lay a tar paper membrane against the foundation(or a recommended alternative), followed by R21 roxul then finished with 1-1 1/2″ of foam board sealed individually between the studs. I would not install a vapour barrier but would drywall lastly. This may be more time consuming but it is all removable if need be for inspection. Am I looking at any condensation and frost problems down the road?


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    I don't live in a termite zone, so I'll defer to those who do if they care to contradict me. But I think most people solve the termite inspection problem by leaving a narrow inspection strip of uninsulated basement wall right at the top of the wall, so that any termite activity can be spotted.

    Of course that's no good from a thermal performance perspective, but sometimes termite inspection protocols trump superinsulation strategies.

    I've heard of people using mineral wool batts to insulate basement walls, but its use makes me nervous. I know that fiberglass batts perform very poorly in this location. I would stick with EPS, polyiso, or closed-cell spray polyurethane foam.

  2. Jonathan Hambides | | #2

    Ken, I am also in southern Ontario and have done something similar to what you are considering. We removed all the walls down to the inside of the outside walls and reinsulated with 3 inches of rigid foam insulation (rated at R-7 or R-5 an inch depending on the manufacturer), and then filled all the gaps between the studs and the rigid insulation with spay foam (Tiger Foam or FOMO brand). We then spray-foamed the remainder of the cavity to seal any airflow and to bring the level to around 3.5 inches of foam. We have had no condensation issues over the last two years but, if you are thinking of possibly removing the rigid foam for inspection – especially if you seal between the studs and rigid foam with spray foam – it is almost impossible without tearing apart and ruining the rigid foam insulation. We have gone through the process when re-wiring the house (I won’t go into the very good reasons we had for insulating before re-wiring). So if the idea of being able to remove the rigid insulation to inspect for termites is guiding your insulating decision, I suggest it shouldn’t be a strong factor. If you don’t seal around the rigid insulation I think you will have moisture problems. I believe the Ontario Building Code requires a minimum of 50 mm of rigid foam to act as a vapor barrier; engineering reports I have read support a minimum of 50 to 75 mm. We haven’t had any termite experience but I have seen demonstrations on This Old House where a pest control company sprayed a barrier solution around houses. That might be something to look into. Best of luck.

  3. Ken Wigboldus | | #3

    My plan, should I go ahead with the spray foam, is to leave the headers unsprayed and to put two batts or more of r14 roxul and finish with 1 1/2" of styrospan. I guess I would need to make that 2" if it is to provide a proper vapour barrier? My thought was to seal the styrospan with silicone(stripable perhaps) or whatever caulking material would adhere yet still be able to cut or remove for inspection purposes. Is this a sound idea.

    To further this, come spring my neighbour plans to dig up his foundation to put a water barrier and proper drainage around his cinderblock foundation. If I were join his efforts and tackle my foundation as well, do I risk creating any vapour lock in my foundation(below grade) having the closed cell foam on the inside and a barrier on the outside??

    Thankyou for sharing your experience and responding so quickly. This is fantastic!! I'm sold!


  4. Ken Wigboldus | | #4

    Sorry to beat this to death but there is another nagging question I have which I forgot on my last posting. Does what ever insulating material you use on the basement wall have to be applied tight to the foundation? If I were to put a rigid foam board against my rubble foundation(as Jonothan and others I have seen do) it would leave many cavities between the foam and wall because of the inconsistencies in the wall. Does that allow more humidity to condense and collect and/or freeze on the wall?

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Q. "If I were join his efforts and tackle my foundation as well, do I risk creating any vapor lock in my foundation (below grade) having the closed cell foam on the inside and a barrier on the outside?"

    A. I'm not sure what you mean by "vapor lock" -- it's a phrase that's always mystified me (I always thought it had something to do with balky internal combustion engines).

    Some experts come up with a logical explanation for why it's useful for basement walls to be able to dry to the interior, but I don't buy it. A basement wall will always be damp, unless you install a submarine-quality waterproofing system on the exterior. Old-fashioned uninsulated basement walls DO dry to the inside, but that's not good. Since the source of moisture is basically infinite, the only result is that you add lots of moisture to your home. It's not as if the basement wall ever dries out.

    So install exterior drainage and waterproofing if you want; both are excellent things to have. And go ahead and install low-perm insulation on the interior; no harm done. Masonry and concrete walls aren't hurt by a little moisture.

    Q. "Does whatever insulating material you use on the basement wall have to be applied tight to the foundation?"

    Well, you certainly need to stop air movement behind the insulation. If you want to use rigid foam, you need to come up with an excellent method of air sealing the perimeter of the foam -- presumably using spray foam as an adhesive and air sealing material. No gaps in the perimeter seal are allowed.

    It's tricky. If you have an irregular stone foundation, it's much easier to just insulate with closed-cell spray foam rather than rigid foam.

  6. 5C8rvfuWev | | #6


    locally, where "heavy infestation" is the area's middle name, code and inspectors actually require both a 6" area at the top of the foundation wall for inspection; in addition, they require batt (or similar ... whatever that means) insulation material in the rim for removal during inspection.

    That double whammy is what moved me to change my preference to a slab which can be insulated from beneath, and on the exterior (perhaps! they say) by the newer RFB treated with "Preventol" or -- another option (again perhaps) -- the exterior foam/form reviewed by Michael Chandler in FHB.

    For the OP -- you might be satisfied if you can do a search for insulation sold in your area which is treated with "preventol," not a deterrent but a killer.


  7. Jonathan Hambides | | #7

    I don’t think you have to be concerned about insulating tightly to the wall as long as you have an acceptable vapor barrier. Air on the ‘house side is considered ‘conditioned air’ while air on the outside of the insulation is considered not conditioned – same as in an attic.
    Ken, the Ontario Building Code says: Termite and Decay Protection
    (1) In localities where termites are known to occur,
    (a) clearance between structural wood elements and the finished ground level directly below them shall be not less than 450 mm and, except as provided in Sentence (2), all sides of the supporting elements shall be visible to permit inspection, or
    (b) structural wood elements, supported by elements in contact with the ground or exposed over bare soil, shall be pressure-treated with a chemical that is toxic to termites.
    (2) In localities where termites are known to occur and foundations are insulated or otherwise finished in a manner that could conceal a termite infestation,
    (a) a metal or plastic barrier shall be installed through the insulation and any other separation or finish materials above finished ground level to control the passage of termites behind or through the insulation, separation or finish materials, and
    (b) all sides of the finish supporting assembly shall be visible to permit inspection.

    There are other sections of the code that defines requirements for metal barriers as well as termite protection, but nothing that I can find which defines ‘finish supporting assembly’ or ‘visible to permit inspection’ as stated in 2(b). I suggest you call your local municipality (or go visit them as it sometimes takes a long time to get a return call) and ask them how the requirements are interpreted. In almost all cases I have found inspectors to be very helpful.

  8. User avatar GBA Editor
    Martin Holladay | | #8

    I disagree with your statement, "I don’t think you have to be concerned about insulating tightly to the wall as long as you have an acceptable vapor barrier."

    Vapor barriers have nothing to do with it; this is an air barrier question, not a vapor barrier question.

    The extreme example: what do you get if you string a clothes line down the middle of your basement, and you hang a piece of rigid foam from the clothesline with two giant clothes pins? Is your wall insulated? Of course not -- because air is circulating around all sides of your insulation.

    For the rigid insulation to be effective, it needs a perimeter air seal all the way around its perimeter, as well as air seals at all joints. There must be no way for interior air to connect with the air behind the insulation. It's tough to do that well when the wall is very irregular.

  9. Jonathan Hambides | | #9

    Martin, yes, I agree with you of course. I thought we had established in earlier posts that there had to be a full perimeter seal around the rigid insulation, at the depth which Ken had stated, and at minimum thicknesses to meet the vapor barrier requirements of the Ontario Building Code, so I didn’t refer to that again. Thanks for reinforcing that though.

  10. Ken Wigboldus | | #10

    I was put in touch with a termite specialist who actually works for the city. He has done experiments with our particular termite, on different building materials to see what they are prone to tunnel through or eat. As it turns out, the high density foam board, both blue and pink, are very attractive to the termites. There is no food for them but they have no trouble tunnelling. He has also used a sample of the sprayed closed cell foam which he found they did not touch, however he does not know the length of time required for the foam to off gas and thinks that could be a reason they left it alone.

    I have decided against the spray foam and will go ahead with the tar paper against the foundation followed by roxul insulation and 6 mil vapour barrier. This will allow me to inspect when needed. If there is any additional steps I should consider I would love to hear them. Thankyou!

  11. User avatar GBA Editor
    Martin Holladay | | #11

    The usual result of your approach is a moldy mess. The mineral wool batts will not prevent the migration of warm, humid interior air toward the cold tar paper and cold foundation wall, so condensation will accumulate at the back side of your batts, leading to mold.

  12. Jonathan Hambides | | #12

    I’m not sure what else I can offer Ken. In support of what Martin has said, my experience, when opening a wall insulated with batts and a plastic vapor barrier is that there is almost always mold on the plastic and studs at least. It’s a difficult decision for you to make. I’m not sure of the cost involved but what about using a rolled galvanized metal barrier, as the building code suggests, and then using rigid insulation as previously discussed? Best of luck in whatever you decide.

  13. Ken Wigboldus | | #13

    Darn! To be honest I don't quite follow you on the metal barrier. Where exactly is the metal barrier being applied, the entire foundation? I can't picture what your posting of the code 2 a) states : metal barrier through the insulation and finished material.
    Forgive my ignorance but I feel like I'm back to ground zero and can't help but ask the most basic question again... why a vapour barrier at all then? What is actually wrong with insulation, foundation, framing, drying in either direction? If the house can breath and there is no 'barrier' to prevent the direction of moist air escaping, would that not prevent mold and the like from starting. What I know of straw bale homes is that they have the ability to highly insulate, yet breath, eliminating the need for air exchange/supply devices that are needed for today's super air tight homes. It seems the more I delve into this the more complicated and difficult this decision is becoming. Perhaps if dollars and insects were not an issue it would be a no brainer.

  14. Jonathan Hambides | | #14

    Hi Ken. Well, good question about the need for a vapor barrier as the idea of when and how to use vapor barriers and types of barriers is coming under some scrutiny. However, there are minimum building codes to follow. With respect to the use of termite barriers, Google ‘metal termite barriers’ and you will find lots of information about them. Here are two sites: and and one more: Good luck.

  15. User avatar GBA Editor
    Martin Holladay | | #15

    A polyethylene vapor barrier should never be installed in a basement. The only appropriate insulation against a below-grade concrete or masonry wall is a foam product -- either rigid foam or closed-cell spray polyurethane foam. These foam insulations stop warm, humid indoor air from contacting the wall, where moisture can condense.

    Don't think that a poly vapor barrier will keep warm interior air from passing through the mineral wool batt. It won't -- we've all learned this from experience. You need an insulation that is not air-permeable.

  16. Bob Coleman | | #16

    they do make foam boards with the borate termite preventive, and you can buy the borate yourself and probably soak EPS fairly well; doubtfull XPS or polyiso would absorb much

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