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Community and Q&A

Carpenter Ants

Lindaloowho | Posted in General Questions on

Should I wait to resolve a carpenter ant problem before encapsulating/conditioning/insulating and air sealing the wood skirt around our pier foundation?

If I am unsure that the plywood skirt that was used was suitable for ground contact, what’s the best way to resolve the issue without removing the plywood? Can I flash with aluminum flashing on a roll? I would like to raise the height of grade that sits against the plywood skirt so it can be sloped down and away from the skirt. By flashing it, I wonder if there is any advantage by way of protecting the wood from the earth’s moisture.


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  1. walta100 | | #1

    It has been my experience that carpenter ants are attracted to wet wood more than likely you have a roof or flashing problem you need to deal with before you do anything else.

    Yes I know carpenter ants can and do infest dry wood only when no better options are available.


  2. Expert Member


    Make dealing with the carpenter ant infestation your first priority, whether or not you upgrade the skirt. The damage they can do is alarming.

    When you encapsulate, insulate and condition the area under your house, it becomes a crawlspace, with all its attributes and requirements. Neither building codes nor building science makes the distinction between one with a skirt and one with a foundation wall. The only significant difference between the two is the skirt, on which so much 0f the rest of the work is attached and depends on, is more fragile and subject to rot.

    If it were me I would first upgrade the skirt so that it will support the other upgrades you are proposing over time. That is essentially dealing with the vulnerable intersection between it and grade where rot and water infiltration is most likely. Covering the plywood, which may or may not be ground-contact rated, and raising the grade, is probably g0ing in the wrong direction. You need to make sure it is rot and water resistant before subjecting it to more moisture by burying it.

  3. Expert Member
    BILL WICHERS | | #3

    +1 for dealing with the ant problem first before doing your project.

    If you don't deal with ants first, you're essentially just remodeling their home. They'll burrow in the new wood if they want to, making a mess of your work. Once the ants are gone, you can seal any voids they've created. I have used canned foam with the small plastic tip gun attachment to inject foam into ant tunnels before to seal them.

    You could try using Copper Coat on non-treated wood near grade, but it's not as good as "real" pressure treated wood. A borate treatment would probably also be a good idea since you've had an ant problem before.

    When you say "protect the wood from the earth's moisture", do you mean moisture wicking up a foundation wall? You need a capillary break for that. I like to use thin (1/32") pieces of HDPE sheet for that purpose, but installing it is a pain since you have to jack up sections of the house a small amount to get it inserted where it needs to go.


    1. Lindaloowho | | #9

      I had to look up HDPE! I’ve only ever known it as “puck board”!

      Thanks for the suggestions!

  4. Lindaloowho | | #4

    Thanks everyone.

    The carpenter ant evidence was discovered upon removing the tile on the shower wall. The drywall next to the shower had started to show signs of water damage so we knew something was up. I expected mold, but was surprised to see the signs of burrowing tunnels in the wood and a whole pile of frass. I had to look it up, but as soon as I saw the size of a carpenter ant, I recognized it as a creature I had seen from time to time in the house. What has me worried is the fact that I saw a winged ant near the window today...this means they’re on the move? Mating and establishing new colonies, yes?

    I called one extermination company to ask for them to come out to identify the issue, but these days, with the pandemic, you just send a photo. No answer yet. I will start seeking attention more aggressively.

    Can anyone tell me what to expect as far as professional carpenter ant treatments? What do they entail? I’m guessing I’m going to need the works. Thanks Bill, I will indeed use Boracare with Mold care on the skirting and joists of the skirted crawlspace.

    I have a strong suspicion about where the ants are entering the shower area. A old plumbing hole in the subfloor. I’ve seen it under the cottage while I’ve been scuttling around down there in my Tyvek suit and mask, cleaning the area to get ready for the vapour barrier. I just thought it was some dried out wood, but more likely hollowed out by carpenter ants and left brittle. From the perspective of the subfloor, it appears that the only areas visibly affected appear to be the bathroom floor and only a few boards. When the entire shower was removed, the affected wood only went as high as the tap that was probably leaking. The ants habitat was probably made more comfortable by the fibreglass insulation bat with overlying vapour barrier in all the stud cavities adjacent to the shower stall...but incidentally, nothing at all in the cavities elsewhere in the washroom. 🙄

    I won’t rest on my laurels though because there is likely one or more areas of damage in the house if that winged ant is any sign.

    Bill I was thinking about the wicking up of water by the plywood skirt into the wood siding and walls above. We have sand under the cottage, but a good amount of soil has accumulated on the outside perimeter of the cottage due to previous owners planting beds around the perimeter. Of course, at that time, there was only lattice skirting.

    I’m concerned that we may have exacerbated the issue with the carpenter ants because we did have an insulated skirt built around the cottage perimeter without knowing, as we do now, that we had to control moisture with a sealed vapour barrier in the newly created skirted crawl. We had a deck building contractor do the work so this is why I don’t know if the plywood is rated for ground contact. There were no vents put in the skirt, but there was no dehumidification either.

    Malcolm, we know that the skirt plywood is pressure treated, just don’t know if it is “brown” pressure treated or the PWF grade plywood approved for ground contact. Should I worry too much as long as it’s one of these two? Any quick and dirty way to tell the difference?

    Thanks again. Your advice is so helpful!

  5. Expert Member
    BILL WICHERS | | #5

    If you have built up the ground around the structure, it would be a good idea to do some grading to bring that level down and slope it away from the house. My own personal rule is that you want at least one course of block between the ground and the wood structure.

    I would check the ends of joists and studs around the ant colony. I’ve found they like to burrow into wood from the ends to build their galleries. It’s actually interesting what they do, but it can damage the structure if it gets too extensive.

    I think you’re right about the winged ants being around breeding time. I’m sure you could find out for sure online. I’d check any exterior foam for signs of ant damage too since they like to burrow in moist rigid foam.


    1. Deleted | | #8


  6. user-2310254 | | #6


    Baits work well to control carpenter ants. It's also helpful to seal up any penetrations that are allowing ants to move in and out of the home. If you have tree branches or bushes that touch your home, you should trim them back.

  7. gusfhb | | #7

    I think personally I would trim the skirt to be above grade regardless of what wood it is made of.

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