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Q&A Spotlight

If Ants Like Rigid Foam, Should We Stop Using It?

Rigid foam insulation is routinely used in high-performance buildings, but it also seems to attract carpenter ants

Ants seem to like rigid foam, as this perforated sheet of insulation seems to show. Builders have tried a number of preventive measures to keep insects at bay, but some builders wonder whether the industry should switch to a different type of insulation and avoid the problem altogether.

Writing from the Pacific Northwest, Malcolm Taylor dives into a problem experienced by many homeowners and builders: a carpenter ant infestation in rigid foam insulation.

“I am involved with two projects right now that have carpenter ant infestations — and in both cases they are in the foam,” Taylor writes in this Q&A post at GreenBuildingAdvisor. “One is particularly difficult to fix as it is a flat roof with tar and gravel above and a wood tongue-and-groove ceiling, making it hard to get at the nests.”

For a season, the owners tried to deal with the problem themselves, but their efforts backfired. Instead of a single nest, there are now “multiple nests across the whole roof,” Taylor says. Exterminators brought in to find a solution say that it may take more than a year to get rid of the insects, and of course during that time the ants will continue to damage the building.

“When this topic comes up, typically the responses tend to fall into three categories,” Taylor says. “First, ‘don’t think about it — [foam] is used all the time so it must be all right’; second, ‘as long as the foam is dry they won’t move in’; and third, ‘treated foam is available.’ “

Taylor has found that carpenter ants will move into foam whether it’s damp or not. All they need is a source of moisture nearby. As to foam treated with an insecticide, he adds, no one seems to stock it.

“As foam is now increasingly being used as an integral part of many structures, and often in a load-bearing capacity, how wise is it to use a material that is so vulnerable to pest and insect damage?” he asks. That’s the topic for this…

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  1. bytenik | | #1

    So is this all foam?
    The article only mentions EPS but seems to imply all foam. Do we have to worry about polyiso and XPS, or are they sufficiently dense to ward off attack?

  2. Expert Member

    All the foam I've had problems with was XPS. I don't know about polyiso.

  3. Expert Member
    Dana Dorsett | | #3

    It's not a matter of density
    Until recently one of the guys in my office lived in a nice 1980s post & beam house with 2lb polyurethane core stress skin panels and was constantly battling ants. I've seen carpenter bees & wasp nests in fiber faced 2lb roofing polyiso too.

    I would expect aluminum facers on foam to be a pretty good ant-armor if you tape over the exposed top & bottom edges with an appropriate foil tape, but that won't help you for below-grade exterior applications.

  4. jackofalltrades777 | | #4

    Man vs Nature = Nature Wins
    I was told by a wise architect that when it comes to building homes and dealing with pests, it's a never ending battle and nature will always persevere. What one can do is try and REDUCE the threat with ants, termites, rats, bats, bees, mice, etc but in the end it will always be a battle of Man vs. Nature and nature will win.

    So I don't believe the problem is rigid foam because termites will eat wood, so should we stop using wood in buildings? Rats and mice like nesting in cellulose, mineral wool and fiberglass, should we stop using it? Every material out there will be used by some type of pest except for maybe concrete and steel. The latter two are great for long term building materials but make for lousy insulation.

    No matter what you use, one cannot "build it and forget it" when it comes to pest protection. One has to remain vigilant and keep up the battle with nature. I got mice in my garage this winter and if I didn't address & attack the issue then the mice would make their way into the home. It's a constant battle. Welcome to nature and mans struggle with the earth. Don't get me started on battling weeds in my vegetable garden ;)

  5. Andrew_C | | #5

    nesting in mineral wool?
    Peter L, while everyone has seen mice in fiberglass batts, I haven't heard of mice in mineral wool, especially the denser version that come in board form for exterior use. Is this common? I also wonder about dense pack cellulose as well; properly packed, it would seem like it would take an awful lot of effort for mice to tunnel into.

    I understand your general comment that nature tends to prevail in the long run and material selection is not always simple.

  6. Tim C | | #6

    Insecticide Stability
    My concern with the insecticide treated foams - how stable are the insecticide compounds? If they're going to decay in 10 years and leave you with non-insecticidal foam, they haven't gotten you very far.

  7. christopherw | | #7

    Animals do nest in mineral wool, at least the less dense stuff
    Apparently birds like to nest in the less dense CavityRock boards at least: Wrapping an Older House with Rock Wool Insulation

    And mice seem to like the batts: Mice nesting in Safe and Sound

    I don't know how resistant the significantly denser ComfortBoard might be, but apparently rock-wool isn't really pest proof. If pests like plastic and glass, perhaps they do like rock when it's been spun into wool. I'd love some more solid information on this myself.

  8. Expert Member

    Andrew and Chris
    I guess for me the distinction between battling rodents and insects is that you have a fair chance of building a mouse-proof exterior, but I can't imagine being able to build an envelope tight enough to keep out ants.

  9. Andrew_C | | #9

    Rodents vs insects
    Malcolm, that's a good distinction. I've moved a lot, and while looking at houses at some point I decided that if there were no signs of mice, it was an indicator that the house was reasonably built (at least by normal production builder standards).

    I do appreciate that this thread was chosen for a Q&A Spotlight. I look forward to any additional expert input (from the bug people), and to more articles on how to detail with rigid exterior mineral wool. I've heard that some people have techniques for getting good rain screens that allow for straight forward cladding installation...maybe using panels instead of just fiber cement siding...

  10. jackofalltrades777 | | #10

    Andrew C
    Yes, most definitely, mice, rats, birds, etc. can and do nest in mineral wool and cellulose.The dense packing means nothing since a rodent will simply just remove the cellulose and create a nesting pocket.

    In my area pack rats will actually gnaw their leg off in an attempt to escape a snap trap. I've seen snap traps with only a rodent leg in it and no rodent and a 3 legged rodent running around. They nest in thick brush packed with twigs, cactus and rocks. So mineral wool and dense packed cellulose house walls is the Hilton resort for them.

    Mineral wool is susceptible to infestation. Posts and articles here on GBA and what I have observed and read about shows that it does happen. Even birds have been observed nesting in mineral wool. I've personally seen squirrels and rats nesting in cellulose and mineral wool.

    I quote:
    "One slightly amusing problem that we did not anticipate: birds like to nest in rock wool! As soon as we installed insulation above head height, the word got out to the bird community that there was some prime real estate to squat in! The birds would quickly burrow clean circular holes to claim their own little condo.

    In the few weeks that it took to install the insulation and await delivery of the metal siding, there were a dozen or so uninvited guests in our wall assembly. ..."

    Read more:

  11. AlanB4 | | #11

    Maybe they need to add
    Maybe they need to add diatomaceous earth to the mix when making foam?
    Would it work?

  12. 00kingston00 | | #12

    A green home in all sense
    While building homes , we should ensure that they are sustainable and healthy . I f this foam is leading to accumulation of ants then we might as well avoid using it . A green home should be environmentally stable in terms of the materials being used and the energy it consumes.

  13. charlie_sullivan | | #13

    Law of conservation of toxicity
    Just as proposals to remove toxic flame retardants from below-grade foam are being seriously discussed, we have to consider adding another toxic substance to them.

    An organization promoting the first change has a webinar about it tomorrow and is asking for letters of support by April 1st.

  14. jkstew | | #14

    What a depressing article. I
    What a depressing article. I am nearly ready to build my dream home and after all my study I have decided that SIPs are the way to go in my zone 7 area, but this area definitely has carpenter ants. Back to the drawing board?

  15. sharonsec | | #15

    interior insulation
    A reason to insulate on the inside.

  16. JC72 | | #16

    We're generally talking exterior insulation are we not?
    If so I don't see how birds or rodents could physically get into a rain screen and set up shop in dense mineral wool board.

  17. GBA Editor
    Martin Holladay | | #17

    Response to Chris M
    I don't have enough experience with mineral wool to know whether rodents or birds like to nest in mineral wool.

    But it's obvious to me that rodents can get there if they want to. Rodents have teeth. The word "rodent" comes from the Latin rodere, "to gnaw." Rodents are gnawers.

    Different species of birds have different behaviors, of course. But a woodpecker can make a hole you can put your fist through.

  18. jackofalltrades777 | | #18

    Chris & Martin
    In my area I joke that woodpeckers have created an alliance with rodents. The woodpecker pokes a hole in the soffits & walls and then the rats, mice and even squirrels then make their way into the homes walls. It's a team effort to break and enter into your home ;)

    That's why I believe and stated that it's an ON-GOING due diligence effort that must be undertaken by the homeowner to watch over the house and see what pests are attempting entry. You can build a better pest resistant home but you CAN'T build a pest proof home. Unless you want a home made of concrete and steel with no insulation, one will have to deal with insulation which by nature is soft, warm and provides a safe haven for pests like termites, ants and rodents.

    Fact is NO insulation material is 100% pest proof. All have their pros and cons but NONE are 100% pest proof.

  19. STEPHEN SHEEHY | | #19

    My old farmhouse, built when the Founding Fathers were just getting started, had mice. My new, super tight (.59ach50) house has had a few. Lucky I have a cat.

  20. DavidJones | | #20

    Don't give up on SIPs
    Jay Stewart

    I have built with SIPs, spray foam, cellulose for 25 years. In my opinion, SIPs are relatively protected from insects because the foam is completely sealed in wood. The article correctly pointed out that carpenter ants can get into virtually any building material, so you don't eliminate the danger by using a different material. Ridged foam behind a rain screen wall is an insect superhighway. Ridged foam at ground level (exterior foundation insulation) is just begging for problems. I have seen only a few minor instances of ants in SIPs and I have seen plenty of ants in wood framed houses. Moisture is the enemy of all building materials. Control moisture carefully and your risk of ants is low. FYI, many SiPs are available treated with borates , (non-toxic to humans).

  21. jkstew | | #21

    Thanks David
    That's a little encouraging, but my plan was to build an ICF-style basement and put polyurethane foam blocks under the basement floor. Foams are wonderful insulation. Nothing can touch them in R-value/inch. It never occurred to me that ants would love them and destroy them.

  22. DavidJones | | #22

    ICF and ants

    I am proponent of foam, but I have always considered ICF to be scary product because they were so difficult to protect from insects, that I have never used them. I am not worried so much about the ants getting to foam below grade, but the foam provides a hidden path from the ground to the rest of the building. I often use a pre-cast concrete foundation that comes on the inside surface of the foundation from Superior Walls.

    Foam under a basement floor is not likely to have an ant problem. Ants have to travel a long way under ground to get there, there is no food source down there, and that foam doesn't provide an easy path the rest of the building. Foam makes sense there.

  23. Irishjake | | #23

    ant won't travel far......
    I was told by a scientist once that ants won't go further than 10' feet from a source of moisture. So there is truth that they burrow in dry foam (poly-iso, XPS, EPS, PU), but if ants are on a roof there is a leak somewhere high.

    Once I heard this from the scientist, it really helped me during investigating for leaks/causes. Not that I should assume, but based on that statement, once moisture was removed, the ants would retreat. I haven't ever gone back to look at any situation where corrective action was taken though to see if that was truly the case. All I can say is that I never heard complaints post leak remedy.

  24. Buzz_Burger | | #24

    Some ICF proponents claim that a Bituthane barrier will block access to termites (and I assume ants). Any experience or opinions on that?

  25. Rob Myers | | #25

    This is a rather depressing
    This is a rather depressing read through - I find it amazing that more is not known about the problem. Years ago I had ant problems in my geodesic dome which had EPS insulation in the wall. It seemed to be related to tree branches contacting the house and vents that allowed easy access to the foam. Once I took care of these, the ant problem seemed to go away. More recently I found that ants had chewed into a section of XPS foam that was left with wet OSB in contact with it (it was also exposed to sunlight so it was warm). There were no problems with any other areas on the foundation but it was disconcerting to say the least. These were very tiny ants (not carpenter ants) - I should have kept a few to identify. Strangely, I have never seen an infestation in a scrap pile of foam - perhaps they prefer finished houses!

  26. greenhouse437 | | #26

    styrofoam? dense packed cellulose with fire retardant?
    Are those two insulators susceptible too? Styrofoam would seem to be too hard for ants to penetrate--unlike the cellulose. But are they attracted to either?

  27. user-2069108 | | #27

    Under-slab Insulation
    So, what is the solution? Insecticide impregnated foam? Rock wool?
    There does not seem to be a clear remedy for bugs/rodents in foam. Perhaps it is not as much of a problem? Have there been serious problems with GeoFoam in highways?
    I have seen on this site that someone used Roxul Confortboard CIS (maybe renamed to Comfortboard110) under a slab but there seems to be no documentation that this an approved use of the product.

  28. Expert Member
    MALCOLM TAYLOR | | #28

    Martin did a blog on sub-slab mineral wool. I think Roxul has approved it for that application.

  29. ethan_TFGStudio | | #29

    AAC + Rockwool?
    This article again makes me question the soundness of the idea of relying on thin films, tapes, membranes, etc to ensure building performance over future decades, particularly as we face climate change which may bring unanticipated pests to a building which cannot migrate north. The idea of building with autoclaved aerated concrete + rock wool, as Dan Levy has done, is increasingly attractive as it seem to follow the directive of the third little pig who built his house out of masonry units so the wild forces of nature couldn't knock it over.

  30. Expert Member
    MALCOLM TAYLOR | | #30

    I think it's a good exercise to think about who does the bulk of the renovations in our neighbourhoods. Are they building science buffs who will understand and respect the meticulously assembled envelopes that we are encouraged to build?

  31. Melina_Boukis | | #31

    Interior Insulation x2
    Avoiding insulation on the outside has more advantage than just keeping the pests out. Keeping the weather barrier layer (sheathing/WB/cladding) pure from the insulation layer will simplify construction, reduce maintenance, simplify future remodels and repairs. And yes, if those pesky varmints want to get into the walls to nest, they will. However, a pure, quality weather barrier layer should drastically minimize that (with the vigilant care mentioned by others).

  32. Jon_Lawrence | | #32

    I have been flip/flopping
    I have been flip/flopping between MW and EPS and now leaning towards MW again.

  33. StollerB | | #33

    Mineral Wool Densities
    I think the distinction between the different mineral wool products needs to be reinforced. The typical batt products used in framing cavities definitely pose no difficulty at all for mice, ants, termites or birds to nest in. Same as fibreglass or blow-in cellulose. The Roxul Cavity Rock MD, as referenced by Peter L. in his post about the deep energy retrofit project ( I think also has a low enough density that nesting would be pretty straightforward, as the author discovered from the birds. That product almost seems to have a thin layer of higher density material on the outside, and then just a slightly more dense batt attached to the backside of it.

    Roxul Comfortboard IS, however, is a totally different beast altogether. Conventional batt products are soft and fluffy. I'm even tempted to nest in them some days at work! The Comfortboard product, however, is very dense in comparison, and has a far scratchier, grittier texture. When working with it, you will have a pile of dark green sand at your feet afterwards. It really is nothing like other mineral wool products at all. It really makes it obvious that mineral wool is indeed made of rock and steel slag. I would be very surprised to see any kind of rodent, bird or insect have anything to do with the Comfortboard. Not saying it's impossible, but of all the insulation products available, Comfortboard would by far by the most resistant to pests, in my opinion. It also seems much more hydrophobic than other mineral wools. When used in a vertical application, water does tend to bead and roll down it, unless it gets completely hosed down, at which point it will begin to absorb. It does dry out quite easily, however.

    Buzz Burger- If I was building an ICF foundation, and was concerned about possible connecting paths for pests between the exterior foam and other assemblies in the building, it would be a no brainer to install a continuous sheet of self-adhered waterproof membrane (bituminous peel-and-stick) from the top of the concrete stem wall, over the top of the outside layer of EPS, and all the way down connecting it to the concrete footing. I have used Bakor Blueskin and their paint-on Blueskin on top of foam before, and it works very well. Astoundingly, it does not melt EPS or XPS foams, and provides a tenacious grip for the peel-and-stick. Of course it should be used anywhere you are sticking the peel-and-stick to concrete as well.

    To further add to Malcolm's comment (and Martin's article) that Roxul Comfortboard is supported in under slab applications, here is a link to Roxul's technical document regarding it:

    I haven't yet done a cost comparison between Roxul and EPS for under slab applications to see how they stack up. One thing of note, however, is that the Comfortboard is NOT commodity pricing. EPS is sold for the same price per board foot, no matter the dimensions ordered. Roxul gets wildly more expensive as the thickness increases. For example, when I priced out two different thicknesses from our local supplier, the 1.5" Comfortboard IS was $8.58 per cubic foot, and the 2" product was $12.24 per cubic foot. Quite a difference, so beware! It may be less expensive to install 2 layers of a thinner spec, and actually get a higher r-value than one thicker layer, but for less money! (Comfortboard IS comes in 1.25", 1.5", 2" and 3" thicknesses, all in 2' x 4' pieces.)

  34. Expert Member
    MALCOLM TAYLOR | | #34

    Thanks - that was both interesting and very helpful.

  35. BCinVT | | #35

    I had always found rotten wood in association with any carpenter ant problem, but I know they will use adjacent foam as well. Thinking from the ant's view, I wonder how they happen to find suitable substrates to nest in. It may be that they are able to follow a chemical (odor) trail to mold or fungus colonizing wood. They definitely hunt during swarming behavior, and it seems it would be a great advantage over wandering randomly across a landscape.

    I've found nests in stacked, covered lumber that got wet and punky, yet they didn't bother a pile of Iso stacked nearby. I believe using XPS at ground level is not too risky if it is covered or encapsulated to below grade, Carpenter ants definitely do not tunnel into soil.

  36. rvaluehomes | | #36

    ICF and pests
    I have been building ICF homes in Michigan for 12 years. About 80 completed homes now. We have not experienced a problem with ants, termites, birds, or rodents (yet.) All foam must be completely covered from footing to truss. Below grade, we use a combination of Delta-MS and the manufacturer's own peel and stick. Then a synthetic stucco laps that and extends just under the siding. Siding covers it from that point on. If an air gap is used under the siding, the bottom of course must not be left open. Annual monitoring of the transition area between grade and siding is necessary.
    At least one manufacturer now has a metal shield that can be installed while stacking blocks to prevent insects from migrating upward into the foam.
    I think the point made that all homes are susceptible to damage and must have a plan to mitigate the risks cannot be overstated.
    Nobody mentioned the more certain risks of what happens when you don't use exterior insulation--namely interior cavity condensation--this isn't dependent on the whims of critter--it will happen every winter in many climates. Perfect interior vapor barriers are rarely seen.

  37. StollerB | | #37

    Great points!

  38. JC72 | | #38

    Mineral Wool and SE U.S (Zone 1-3) mixed humid and humid
    Since these climate zones are considered termite-land and some of us opined mineral wool as a substitute for foam I wanted to put this out there. Linked is BSC study on moisture durability of sheathing with vapor permeable exterior insulation (mineral wool) and a reservoir cladding (Brick, Stucco, Cement board, etc).

    Good stuff!!!

  39. user-1044645 | | #39

    Ants in a SIPs house
    We have a SIPs house - small 550 sq. ft structure, 10 years old. We have battled ants constantly with borates, DE, and most recently insecticides - don't really love the poison, but they certainly do the trick. Right now we have a handful of ants roaming through the house - so they're clearly around, but are they a problem? We're at the point where the population seems under control, but hard to fight them any further since there is not enough activity to trace back to where the nest is.

    Would love to chat about/swap carpenter ant war stories w/ anyone off list. [email protected]

  40. charlie_sullivan | | #40

    ants moving in after less than a year
    I found today that carpenter ants have moved in to EPS just below grade on my foundation, after only a year of having it there. It's covered by cement boards and stucco, but there was a crack between boards that was hidden by a rock, and the ants found it before I did. Fortunately, they quickly found the boric acid sugar syrup bait put out and are industriously carrying it back to the nest.

    It is near one of the damper corners of the house, where I had been planning on improving the grade.

    We used a foot of mineral wool on the top foot of the exterior of the foundation partly in hopes of having the foundation wall dry better, but I am also hoping that will help keep the ants away from the structural wood. Maybe I should tear open this section to see where they got to, and whether the roxul successfully blocked them.

  41. Expert Member
    MALCOLM TAYLOR | | #41

    Just met a couple who moved into my neighbourhood last year. They found their house was infested with carpenter ants - something that was missed by the pre-sale inspection. The remediation costs topped 100Gs! This one wasn't foam related.

  42. user-6250932 | | #42

    I have been experimenting with ant killers and find that Splenda works really well. I sprinkle it in their path and the ants take it to their nests. Presumably they die. I'm found a new trail in my adobe building. Hopefully they didn't just move to another building.

  43. GBA Editor
    Martin Holladay | | #43

    Response to Sherry Litasi
    Scientific studies have disproved the urban myth that Splenda (aspartame) kills ants. Here is a link to a relevant paper: Aspartame-based Sweetener as a Strong Ant Poison?

    The researchers wrote, "Our current results strongly oppose the rumor that aspartame-based sweeteners are effective lethal poisons for ants."

  44. DeanClarke | | #44

    Well it is now spring 2019 and I am starting to design our new dream home on Vancouver island. There are definitely carpenter ants around here. Has the understanding of how to insulate to minimise ant infestation improved at all?

  45. GBA Editor
    Martin Holladay | | #45

    Nothing has really changed since this article was written. Ants are still ants.

  46. Deleted | | #46


  47. draperd | | #47

    This is an old thread, but recently relevant to me :-)
    I live in a 70’s home in the Pacific Northwest, under and surrounded by fir and cedar, and the previous owners didn’t take care of it, so there have been plenty of issues with leaks, rot, and carpenter ants. Last week I had a swarm come through the house, and investigation showed carpenter ant damage in some places I hadn’t noticed before.

    I have a question, and an observation+anecdote.

    The question is: are the crack-sealing expanding foam products you can get at home stores the same kind of foam as discussed above? I was thinking of using this type of foam to seal off some old damaged areas to prevent new infestations there, but if the ants like the stuff, that won’t help much.

    The observation: you don’t necessarily need insecticide in the foam—a repellant could work as well, if it was effective. When the swarm hit the house last week, they were just boiling out of an opening about 5 feet from my bed. I physically blocked that off, and they started coming through a crack a few feet away. I thought I was going to have to go crash at a friends house! But I had read that carpenter ants don’t like castor oil, or tar. I happened to have some tar shampoo, so I tried squirting some in the second crack. They stopped coming through that crack, and started coming in a bit further on. There I wouldn’t have been able to get the shampoo in the crack, so I just drew a line on the floor. Sure enough, whenever an ant touched the line, it ran in the opposite direction. A few more lines in key places (I felt like a witch drawing wards), and I was able to sleep in my own bed — I had a few visitors, but not the dozens upon dozens I would have had otherwise. Obviously an anecdote isn’t enough to go on, but it seems worth some investigation as to whether a repellent like tar could be added after or incorporated into foam solutions...

  48. Expert Member
    MALCOLM TAYLOR | | #48

    My current project got me thinking about this discussion again. I'm replacing the roof on an older house built with 2"x6" t&g, an inch of foam, OSB sheathing, felt, and asphalt shingles. The shingles were left beyond their usable lifespan and leaked in places. Carpenter ants entered by the plumbing stacks and nested in the foam. There is damage throughout the roof, but more extensively in the areas where it leaked, and around the two chimneys where the ants clustered for warmth. Luckily they preferred the foam to the t&g, but it still meant removing all the foam, and about 1/3rd of the OSB, which couldn't be salvaged.

    This is what foam looks like when ants decide to nest in it:

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