Coping With Termites and Carpenter Ants
Does rigid foam insulation aid and abet these destructive insects?
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.
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“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 (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) treated with borateBoron-containing chemical that provides fire resistance to materials such as cellulose insulation and provides decay and termite resistance to wood products. Borate is derived from the mineral borax and is benign, compared with most other wood treatments. 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 capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. 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.
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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. .
“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.
- Cathy Rust at
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