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 Q&A Spotlight. (To read a previous Q&A Spotlight article on this topic, see “Coping With Termites and Carpenter Ants.”)
Does the foam have to be damp in order for ants to like it?
One person who has some experience with the problem is Building Science Corporation principal Joseph Lstiburek, who recalled his battle with ants in a 2012 article posted at the company’s website. Lstiburek had renovated a barn 15 years earlier, wrapping the building in a thick layer of EPS insulation. When the ants became an overwhelming problem, he took the barn apart to find out what had happened, and in the process found just two spots where they had made nests in the foam.
“The message here is obvious: keep things dry and you will not have ants,” Lstiburek wrote. “By the way, keep things dry and you won’t have a lot of other bad things as well. The dry/wet thing with ants is pretty much proving to be the case in all the ant foam infestation cases I (and others) have looked at over the years.”
To Chris M, who cited the article, Lstiburek’s story means that having a damp environment near the foam isn’t enough to attract ants: “Therefore one could deduce that if ‘having water nearby’ was the only requirement, then ants would’ve been found throughout the dry foam as well.”
So, dry foam means no ants?
Not necessarily, says Charlie Sullivan. “It might only mean that the ants prefer damp locations. If there were two ant colonies looking for homes, and no wet foam, they might have both moved into dry foam.”
“Agree,” replies Chris M, “but keep in mind that research shows carpenter ants will live in dry wood but prefer damp wood, so technically there’s nothing keeping them from setting up shop in any stick-built structure. So, yes, absent a preferred (i.e. moist) environment, carpenter ants ‘may’ take up residence in dry foam or wood — or they may just move on.”
Ants are a wily adversary
When Taylor built his own house 20 years ago, the only foam insulation he used was a strip covering the second-floor rim joist. Ants found it.
“The nest was on a sunny south wall which as bone dry when I opened it,” he said. “Unfortunately, by then they had also eaten a large hole in my top plates (which were not rotten or damp) and able to move down the wall to nest next to a baseboard heater. I surrounded the foundation with insecticides, but the tenacious and alarmingly intelligent colony found the one possible path out to forage by using my power lines and descending the pole on the street.”
Although foam seems to mimic the decaying stumps that ants prefer in the wild, Taylor says, they also can be found in wall cavities filled with fiberglass insulation, often next to a heat source such as a fireplace or a wall heater.
“I wonder if the seasonal swings in the moisture of exterior sheathing are enough to make them set up shop in usually dry and decay-free wood structures,” he says. “They sure like foam, though.”
Burke Stoller, another builder in the Pacific Northwest, has run into his share of ant infestations in exterior foam, and it makes him wonder just how deep these insects will live in the ground. The only place he typically uses foam insulation is underneath a slab foundation (full basements are uncommon in his area), and he wonders whether he should worry about subterranean ants.
“I am wondering to what depth we should be concerned about bugs tunneling under our footings and working their way back up to the underside of the slab, tunneling around in the EPS layer?” Stoller writes. “I share the concern Malcolm expressed, as on our Passivhaus projects we are beginning to see architects and engineers approve higher density EPS for structural load-bearing applications, and I, too, wonder if we are looking for trouble down the road.”
Is mineral wool insulation a better choicer?
Andrew C is concerned that the widespread use of rigid foam insulation, along with sloppy installation of flashing and other water-management details, will mean a backlash against green building and building techniques in which insulation goes on the outside of a building.
Chris M has the same worries, and suggests that mineral wool batts such as Roxul ComfortBoard are a better alternative than foam.
“Roxul has never made claims that insects won’t bore into their product,” he says, “but I’m not surprised, since in my opinion their product is generally sold in climate zones which don’t have termites.”
John Semmelhack writes that as far as he knows, neither ants nor termites will chew through rigid mineral wool boards, and the insulation is too dense to allow insects to crawl through it. For these reasons, he prefers mineral wool to foam insulation in Climate Zone 4 where he works.
Stoller is another believer in replacing rigid foam with mineral wool. “On almost any renovation project we do with rigid foam on the exterior of a foundation, or on an exterior wall near grade, we find an absolute ant farm,” he writes. “They love the stuff, and make incredible patterns to pull apart and view! So, we have completely switched (when we have our say on specifications and assembly design) to only using Roxul Comfortboard IS for exterior insulation detailing.
“It can be a pain for much of the detailing around windows and doors, and for ensuring that all of your rainscreen stays completely co-planar for exterior cladding,” Stoller continues, “but we have learned some tricks over the years to make life easier. Not only does it appear to be totally bug-resistant, but even better is the fact that it is extremely vapor-permeable (+/- 30 perms at 1 inch), meaning that assemblies will dry much better than their exterior foam counterparts.”
Other suggestions, and a warning
Among other suggestions from GBA readers for controlling ants and termites:
- From Andrew Bater: Install a removable trim system at the base of a stucco wall to allow inspections for ants, as well as an opportunity to apply insecticides if needed.
- From Lucy Foxworth: Cover soffit holes with metal screening so insects can’t crawl through, and trim trees around the house so they don’t form a “carpenter ant highway” into the house.
- From Christopher Peck: Tape the top and bottom edges of foil-faced polyiso insulation with aluminum tape, on the assumption that termites can’t bore through either the tape or the aluminum facing of the insulation.
Chemical insecticides are, of course, an option. But one obvious disadvantage of chemical treatment, as Andrew Bater points out, is the disconnect between building a healthy, green house and then having “to douse it with insecticide.”
Further, warns Robert Ohle, there’s the possibility that insecticides themselves can dissolve foam insulation. “Twice I have removed siding, where the homeowners had brought in exterminators to deal with ants, to find voids where the insecticide had been applied,” Ohle says. “In both cases small holes had been drilled through the wood siding, insecticide injected, then the holes sealed with a dab of caulk.”
Our expert’s opinion
GBA Technical Director Peter Yost added this:
I confirmed with two entomologists that carpenter ants can have satellite nests that are not damp, but that are in materials that are easy to chew and provide shelter and warmth. That is unnerving at best for all of us who have built with and recommended rigid foam on the exterior of wall assemblies.
There is a relatively new high-density EPS product by Atlas called ThermalSTAR, which contains Preventol, a Lanxness Corporation insecticide. The ICC Evaluation Service Report (ESR-2918) specifically states that Preventol TM-EPS “is used to treat expanded polystyrene foam (EPS) to provide protection against termites…”
If you look up Preventol in the PAN Pesticides Database, you’ll find more than one product listing. That is, Preventol has different forms used for different applications. While Preventol TM-EPS lists only termites under the pests that it can be used for, a form of Preventol called hs 100cs-50 is listed for use against termites, carpenter ants, and wood-boring beetles. Further, while the active ingredient in Preventol TM-EPS is listed as Imidacloprid in a concentration of 99.8%, the active ingredient in Preventol hs 100cs-50 is Deltamethrin in a concentration of 4.75% and formulated as a soluble concentrate. These two insecticides may share a brand name — Preventol — but they are very different chemicals.
I tried to contact a leading expert entomologist — Mike Potter at the University of Kentucky — who has proved really helpful in the past, in no small part because the guys at Atlas strongly suggested that I sort out the different behaviors and preventions for termites versus carpenter ants by talking to a bug expert. But I was not able to connect with Mike Potter this time around.
I think the final cut here is that we have one fairly widely available rigid insulation — Atlas ThermalSTAR — specifically formulated to be used below and above grade to resist termites but not specifically formulated to deal with carpenter ants.
It sure seems as though if you are building in a climate and wooded site that carpenter ants favor, rigid mineral wool is the safest bet.
Finally, I can find no evidence that carpenter ants are subterranean, as are some termites. Whether carpenter ants would be able to penetrate or be attracted to rigid foam under an at-grade or near-grade installation (for example, under a slab) remains to be seen.
I will continue to try and work this through with Mike Potter. Stay tuned.