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Q&A Spotlight

Coping With Termites and Carpenter Ants

Does rigid foam insulation aid and abet these destructive insects?

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Home for carpenter ants. This rigid foam insulation in a roof became a nesting spot for carpenter ants. A homeowner weighing the use of rigid foam panels against his foundation walls worries it might disguise the passage of ants and termites.
Image Credit: Cathy Rust at BEC Green
Home for carpenter ants. This rigid foam insulation in a roof became a nesting spot for carpenter ants. A homeowner weighing the use of rigid foam panels against his foundation walls worries it might disguise the passage of ants and termites.
Image Credit: Cathy Rust at BEC Green
Ant highway Tom Richey, a home inspector in Nebraska, took this photograph of rigid foam on an exterior wall that was riddled with what look like ant tunnels.

Ralph’s new home will be in Cleveland, Tennessee, not far from Chattanooga and solidly in termite country. And that’s the problem.

“I want to use [rigid foam] on the exterior of the foundation (full height),” Ralph writes in a post at Green Building Advisor’s Q&A forum, “but I have been personally plagued by termites and carpenter ants in every home I have lived in. Short of soaking everything in dieldrin (hard to come by these days but worked beautifully in my opinion and has some nasty side effects), what is the current recommendation for stopping the critters outside the foam/concrete interface?”

He’s hoping to start construction this spring. How does he balance his plans for energy-efficient design with the practical necessity of controlling these destructive pests?

That’s the topic for this Q&A Spotlight.

Make sure your local code allows the use of rigid foam

Building codes in some parts of the country don’t permit the use of foam on exterior foundation walls, GBA senior editor Martin Holladay points out. So the first step would be to check with the local building department to see if that’s an option at all.

“Regardless of your local code,” Holladay writes, “here’s my recommendation: If you’re worried about termites, install the rigid foam on the interior of your foundation wall (whether you are building a crawl space or a basement). Leave a strip of your foundation wall uninsulated at the top of the wall as an inspection strip; that’s where you will look for signs of termite activity.”

Although that strip of bare concrete will leak heat, he says, “that’s the price you pay to live in a climate that is warm enough for termites.”

Holladay adds that expanded polystyrene foam (EPS) treated with borate may offer some protection. Perform Guard is one brand; another is manufactured by the Poly Molding Corp..

Sand or ground stone will discourage termites

Termites also can be stopped with the right backfill around the foundation, says Dana Dorsett. “Backfilling the exterior with clean compacted sand of the right granular size can mitigate termite intrusions fairly well when surrounded by stable soils that won’t settle or frost-heave creating gaps in the sand,” Dorsett writes.

“Using copper flashing as the capillary break and termite shield for the foundation sill at the top of an [insulated concrete form] wall is another measure worth taking,” Dorsett says. “The copper flashing is also a thermal bridge, true, but worth it in high-risk areas. (As I understand it, like borates, copper compounds are toxic to the gut flora of wood boring insects, protozoans required for the host insect to digest wood.)”

Lucy Foxworth says stone material ground to the right size is an effective termite barrier. “The types I researched online are made from rock and ground to a size that is too big for the termites to move, with spaces between that are too small for the termites and too hard to chew,” she writes.

Foxworth says this material is called BTB. Another variety is sold in Australia under the trade name GranitGard.

Another type of physical barrier, writes Hein Bloed, is stainless-steel mesh. “It is wrapped around the entire insulation,” Bloed says. “Laid out on the hard core, the EPS placed down and then the mesh is being folded up and over the sides. Like wrapping a parcel.”

Two-part flashing detail in Indiana

Tony Fleming, writing about a deep energy retrofit at his home in Indiana (“land of many lakes, wetlands, and subterranean termites”), says a two-part flashing system was the key to keeping termites from re-infesting the house.

“The key element is a J-flashing that wraps around the top of the foundation foam panel at about the sill elevation,” Fleming says. “The long leg of the ‘J’ is adhered to the concrete with a continuous double bead of silicon adhesive (the pest control operator said that termites will not go through the silicon), while the short leg extends back down the front of the foam panel. Any termite tunnels in the foam or on the foam-concrete interface are literally redirected back to the outside where they are visible.”

A modified “Z” flashing sits directly on top of the “J” flashing, he adds, and wraps around the rigid foam that is applied to the house sheathing.

“This process sounds like a lot of work but it was actually rather easy and took about a day to make and install,” Fleming says. “Our flashing consists of standard white aluminum flashing and was bent on site using a regular metal brake.”

The first step, Fleming says, is to identify the type of termite that’s likely to be a problem. “Subterranean termites are the most common in temperate latitudes,” he says, “but other species, such as Formosan termites, come into play in the south and near south. Different species have different requirements [for] moisture, light/darkness, wood, etc.”

And then there are those pesky carpenter ants

Carpenter ants are another unfortunate fact of life, although they probably don’t represent as serious a threat to the structural integrity of wood-framed buildings as termites do. Still, they can be a problem, and they apparently like both damp wood and rigid foam insulation. Or maybe it’s dry wood, as a conflicting post suggests.

Ants will sometimes bore through dry wood, Dorsett says, but they won’t nest in any wood with moisture content of less than 15%, typically wood that’s been attacked by fungus.

“Some species of ants seem to favor picking apart EPS beads (I’ve observed this with picnic-coolers), not just carpenter ants, but I’m not quite sure what that’s about,” Dorsett says. “I’ve seen EPS well-tunneled and traveled, completely riddled by carpenter ants, but never seen an actual nest in EPS — only found the queens and quantities of egg in wood.”

Malcolm Taylor, writing from the Pacific Northwest, says carpenter ants are locally ubiquitous, most active in the hottest part of the summer when they swarm.

“I often find their nests in the foam sheets insulating tongue-and-groove ceilings which are somewhere between warm and hot with no moisture present,” Taylor says. “Perhaps at some point in the year there was a higher level to attract them to the location in the first place, but they sure don’t move if things are dry during the majority of their tenure. To expand their nests I have seen them completely remove the double top plates between two studs, which at the time must have been under 6%.”

[In fact, the photo that accompanies this column (provided by Cathy Rust of BEC Green) shows foam insulation in a roof.]

“My experience has been the opposite of yours,” Taylor continued, “that they will tunnel through sound wood but usually nest in the foam.”

All of the nests he’s found have been in Styrofoam SM extruded polystyrene.

Our expert’s point of view

Here’s what GBA technical director Peter Yost had to say:

In my experience, both termites and carpenter ants love any material that is easy to chew through to get to a nice place to nest: warm, with access to moisture. Carpenter ants don’t actually eat wood, they just chew it to get to where they are going but termites chew wood for food.

Some good suggestions have been made already: inspection zones at the top of foundation walls, Termi-Mesh, the stainless-steel mesh, and the original Basalt Termite Barrier from Hawaii. All are completely non-toxic systems. Bait systems do use the latest and least toxic termiticides, and the bait traps confine the substances to the traps and target the termites. Termite baiting is not simple; use this University of Kentucky resource to evaluate bait systems.

But maybe the best option is to consider using a rigid foam material that can’t be chewed: rigid mineral wool (Thermafiber, Roxul and Knauf) or Foamglas. Availability and price are definitely considerations with these alternative rigid insulation boards, but they are all just too tough to chew.


  1. Andrew C | | #1

    Bugs and exterior foam in a warming climate
    The issue of how to deal with bugs in rigid foam seems to be a big deal to me, and I rarely see any mention of dealing with it in the plethora of articles expounding on the virtues of exterior foam. Ants or termites or both appear in much of the USA, and their ranges will likely expand as the winters get milder. If you're aiming for a durable, resilient home, it seems you should use strategies to account for bugs. I would really like to see this explicitly addressed in articles that use foam. If you show a cross-sectional detail drawing of an insulated slab/foundation wall/exterior wall, show the anti-bug provisions.

    In the north (I'm from Michigan, Canadians, Alaskans, et al forgive me), I'm even concerned about ants in roofs, as I've seen them myself. Many advise that you should always try to build so that materials stay dry, but plan for them to get wet. If you've got wet foam in roof or walls, what provisions are taken to keep the ants away?

    I'm a fan of the exterior foam insulation concept as a potential alternative to double walls, assuming you can get rid of formaldehyde and use less damaging blowing agents. But insects remains a bugaboo for me.

  2. Jenny Belman | | #2

    Termites live in colonies
    Termites live in colonies that exists in wood structures in your home. When they get into the foundation and walls of your house, they can cause extensive damage and cost you thousands of dollars. Be careful of them.

  3. Sean Wiens | | #3

    Mineral Wool
    Sure seams like a lot of effort to go through when there are better options. Exterior insulate above and below grade with mineral wool and you make most of these problems go away. Roxul makes great products for this application. Now you are also vapour open which will benefit your design in many climates.

  4. Derek Roff | | #4

    Price of foam vs. Roxul?
    The final paragraph of the article implies that products like Roxul rigid mineral wool insulation might impose a price penalty over XPS foam. However, comments on various other GBA articles have indicated that Roxul is price competitive with, and perhaps even cheaper than foam for a given R-value. Prices vary with time and region, but I hope Scott will offer a few more details on his perspective on the cost question, including current comparisons in at least one location.

  5. Aaron Gatzke | | #5

    Info on termites
    The Canadian Wood Council has a lot of information on termites and the information is for North America.

  6. Anita Brosius-Scott | | #6

    bug-proofing exterior foam
    Interesting to finally find the issue of bug tunneling in exterior foam addressed!
    Short version: try a layer of aluminum window screen over the foam. Buy it by the roll.
    Long version: Bug tunneling seems an obvious issue with the exterior foam approach to foundation insulation, and I was surprised to not see it addressed when I researched this insulation method. Exterior foam is used a lot here in Maine to retro-insulate concrete foundations - and insulate new ones. I saw plenty of evidence of the tunneling on another house I am working on (nothing was coated over that foam). I also did not see a satisfactory answer anywhere for the protective parging of the foam. When builders apply exterior foam here in Maine to concrete foundations, they apply a layer of structural skin (fiber reinforced cement coating used like stucco) over the abraded/textured foam surface and call it good. Sure, that's fine, until you bump it with the lawnmower or whack it with the weed whacker. Inevitably the obvious happens and the parging begins to flake off hither and yon. I see it everywhere. The builder doesn't, 'cause he's off working on the next job and probably not mowing your lawn.
    Undaunted by both the bug and the parging issue, we still decided to insulate the OUTSIDE of the foundation of our house rather than the inside (that decision is another story, and relates to concern about possible foundation failure in winter in these dense clay soils; I asked some local concrete contractors, if they'd seen or had to fix such failure after interior foundation insulation was applied. One "yep" was enough). We took the most complicated approach possible, which arose from our own tortured minds. After the 2" XPS foam boards were applied against the sealed foundation, we applied 4' wide aluminum mesh window screen (bought by the roll) over the foam, folding it over the top edge of the foam boards, so that now the foam boards had an outer skin covering of wire mesh screening, from the top edge, extending down 4' which generally means 3' deep in the sandy backfill. Hopefully the screen mesh size is smaller than a fat carpenter ant's behind (like those munching away in the garage), and maybe goes deeper into the soil than they want to. (That particular scientific method is called "hopeful.") We "attached" the mesh to the foam with small galvanized nails hammered into plastic mollys pressed through the screen and into the foam. For the foam panels out of sight under the porch and deck, we're hoping that's enough bug tunneling protection. No parging.
    For anywhere that would experience rain and water draining off the siding, drip edge was installed, up and under the bottom edge of the shingle siding, then out and over the top of the foam, which now of course extends beyond the surface of the siding.
    For the foam exposed to the weather, sun & lawn implements etc, we decided to just plain stucco it, good and proper. (Rats, which we'd thought it through before applying the screening. Oh well.) So, next, we applied expanded metal stucco mesh sheets, attaching the mesh sheets by using those long 3" nail-and-plastic-molly-with-the-big-button-washer thingys ("insulation fixings"). Of course this gave us the joyful excuse of buying another tool for the collection: a serious hammer drill - to pre-drill into the concrete (did I say we chose the complicated approach??). Then a layer of structural skin parging (OK, maybe that was overkill. Shoulda just used stucco). Then, because that was ugly, showing seam lines at each application batch, next came a slathered-on layer of concrete sealant (sloppy concrete-like mixture from a bag), which, with neat vertical brush strokes, put a nice fake-concrete "foundation" look to the surface. Done. Bet that surface will last longer than my neighbor's.
    OK, so it was a nightmare of work. But the "new"-old, now dry dry dry sweet-smelling warm basement beats the heck out of the moldy stinky wet horrible former basement. And I'm hoping we're keeping buggy foam tunnelers out of that exterior foam.
    Right, we were supposed to be talking about bugs. Sorry. Got carried away.

  7. Jin Kazama | | #7

    What about using a solid
    What about using a solid dimpled membrane over foam like the one from Cosella-Dorken ?

    That should keep the bugs out, can be surimposed to push it up to the finish and then some
    finish panels could be screwed on it etc.. ???

    At least it serves another purpose compared to screen method ( what is the life of alumin screen in the ground ? )

    Anita Brosius-Scott : for carpenter ants, i don't believe one would need to install screen more than 2-3ft of depth ..they do not dig deep within fill sand
    what was your sq ft price for the alum mesh ?

  8. Jin Kazama | | #8

    peel stick is still probably
    peel stick is still probably much cheaper than the Delta membrane tough ...

  9. Damon Gray | | #9

    EPS Foundation
    We have been using water base damp proofing and peel and stick.
    Damp proof all styro. Then use 3m Hold Fast 70 to glue the peel and stick on. Tedious job but figuring it will work well. Also sprinkled borax everywhere under the footing styro.
    Thoughts appreciated if there is any fails to this process.
    Do like the aluminium mesh approach. Maybe we should use that before the damp proofing.

  10. David Lucas | | #10

    rigid foam versus Roxul insulation
    I was just making my shopping list today for the roof at the cottage (inside). I removed all the old ceiling tiles and now I'm ready to insulate. I was thinking of using the rigid foam with foil to make the baffles for ventilation first and then add insulation before installing a tongue and groove wood product on the vaulted ceiling. But now after reading your comments about carpenter ants and termites (which I sprayed last summer) I am worried that they may come back again and nest in the foam insulation. Any comments or advice would be much appreciated. If I use the rigid foam for baffles do I still need to add Roxul to increase the R-value? Thanks!

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

    Response to David Lucas
    First of all, if you don't want to use rigid foam to create a site-built ventilation baffle, don't do it. There are lots of other materials to choose from, including thin plywood, OSB, or fiberboard. You could also use a manufactured vent baffle if you want to.

    Here is a link to an article that lists a lot of options: Site-Built Ventilation Baffles for Roofs.

    Second, if you plan to use an air-permeable insulation like mineral wool (Roxul) or fiberglass to insulate your rafter bays, it's essential to include an air barrier on the interior side of the insulation. Tongue-and-groove boards don't make an air barrier -- they leak like a sieve.

    The usual solution in a case like yours is to install taped drywall on the interior side of your rafters before you install the tongue-and-groove boards. The drywall is your air barrier.

    Here is a link to an article with more information: How to Build an Insulated Cathedral Ceiling.

  12. Wesley Manning | | #12

    Durock B2000
    For my home in Halifax area I insulated with 4 inches of type 3 (Canadian rating) EPS on the exterior foundation wall. The insulation is adhered with generous amount of Lepage C300 adhesive.

    I used Durock B2000 which is a synthetic stucco meant to go below grade. I did that down to 12" below grade and up over the top of the insulation.. Like all synthetic stucco it goes on with fiberglass mesh. I used two layers of 5oz fiberglass mesh with a third layer around corners. I find it is rock hard and no problem with grass trimmers or any other unintentional damage. You can even get high impact mesh (11oz, 15oz, 20oz) that is even tougher. I kinda wished I did a heaver mesh just for that extra overkill but I see no issues with durability.

    I sloped the EPS at the top to shed water. Durock has guidelines in their various stucco specs. That way I didn't have to do bulky Z-flashing. I then used a liquid applied flash on the foam to wood wall transition and a 2" L flashing with drip edge over that.

    I then used an elastomeric paint - Dulux Decraflex.

    Survived two winters still looking good. I think it provides a good barrier against ants.

    Thought might be useful to someone else.

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