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Community and Q&A

Exterior foundation insulation in termite zones

iamkarlp | Posted in Energy Efficiency and Durability on

Long time lurker, first time poster.

I have an existing structure (35~years old) which has a CMU basement in the SouthEast (Zone3).

The structure never had an exterior foundation french drain, and over the years the foundation has been compromised.

To resolve this we will be excavating all the foundation back out later this year, adding internal steel reinforcements, new water treatment, drainage mat and an appropriate foundation drainage system.

Being that we need to go through all this work, I would like to add outboard insulation while I have everything open.

I have worked on a detail to do this, and I believe I have one that I like. It is as follows:

Below Grade I would have a fluid applied membrane on the CMU over which is adhesively bound 2~4 inches of XPS over which is DELTA-MS tied into the french drain.

Above grade the XPS continues, directly over the CMU over which is Ground Contact plywood mechanically fastened through the XPS to the CMU. Over the plywood I would use polywall alumaflash plus, battens for rain screen with bug shields then cladding. To tie these above and below ground details together, there is tape between the plywood and DELTA-MS, then the alumaflash comes over the tape.

I was (am?) pretty happy with this, but over the past few days I have a bit of unease as it relates to termites. After (more than) a few hours of research I’m more confused than I started. If I look around this area, it seems like many people are doing outboard XPS, in many cases in direct contact with the ground – so people are doing it every day. But also plenty of stories online of the problems of it.

On one hand, most problems seem to stem from people with exposed foam, in my case it is entirely detailed and encapsulated by other products. On the other hand I don’t want to knowingly give myself any problems.

Does anyone here have enough experience with this to give me pointers on this?

Should I consider a different detail? I have considered trying to use comfortboard 80 instead, but the attachment methods require below-grade penetrations through the fluid applied, it is slower to work with, and the dimensions make my higher-floor tie-ins much more difficult to manage. Maybe it’s worth all the pain?

I could give up on exterior insulation entirely, but It’d make a huge difference if I could get it in here.

I have attached a model of my detail. Apologies for not creating a clean cut away but I got lazy.

Any help appreciated,

Karl P

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  1. onslow | | #1

    Mr. Peterson,

    I can't say too much about termites with any certainty, as I have fortunately never had to deal with them. Carpenter ants I can. Like termites, they are here on earth to remove fallen trees back to the soil, however slowly. Wet door and window sills seem to mimic the scent of a fallen rotting log which means a new nest possibility to those flying versions looking to set up a new frontier. Their arrival is usually noted by little piles of sawdust where they start entry, as carpenter ants do not eat the wood. Termites I guess would leave little piles of poo. For the most part, both need moisture to keep the nest a going enterprise, with termites using mud tunnels to keep moist enough to extend a current nest near or under your house. It has often been suggested to monitor the surface of ones foundation for these mud tunnels to spot potential nesting intrusion points. I pity people with homes built on piers in the south, not much fun inspecting.

    Most of what I have heard about termites and guarding against them involves breaking any paths of access to the tasty parts of your house. Never quite understood why the flying form during swarm season don't just land all over and under the house and start above any barriers. Prior postings have included such thoughts as copper flashing that would stop them from coming up higher than the concrete portion of a house. This would be located under the sill plate and extend outward past any insulation or drainage membrane attached to the foundation. Presumably the little buggers can't manage to crawl upside down long enough to get around the drip lip. I think the copper is also supposed to give them a bellyache if they contact the leachate.

    This anti-crawling theory has also been used to protect corn cribs of old from mice by placing large diameter metal discs on top of the piers supporting the crib feet. The equivalent Old World solution was to use large thin flat stones. You can find lots of videos of squirrels making a mockery of similar attempts to defeat their access to bird feeders. These will make you depressed about how bulletproof these ideas are.

    Ground injection of poisons seems to be the favored method of creating a barrier against termites with some claiming five year protection. How you feel about doing that to your soil and environment is up to you. It still doesn't answer why the swarm version of termites don't start directly on houses. Keeping all paper and wood building debris out of the trench around the house certainly will help at a basic level. I have heard that termites can tunnel in well below the surface, so leaving buried bits of box and 2x cut offs as feeding stations is not a good idea. Good drainage of the soil adjacent to the house may also help to slow them down by making tunneling that way unappealing. Termites like(some) humans won't persist in low reward endeavors. I don't think they can smell your house.

    Dry termites are horror that so far seem to be limited in distribution to Florida, southern California and some other random coastal spots. These don't need moisture and can fly into your home high up through vents. The reproduce slowly so they are even harder to spot in early years. I hope I won't regret any statements I have made about not having to experience termites. My freezing winters may be the only thing going for me.

    On other notes, your drawing would seem to indicate that at least part of the house is also CMU, unless the drawing is showing a portion of basement that is above grade. If the house is all CMU, then the frontlines for defense will be doors, windows and penetrations for water, sewer, etc. The very tough plastic of the dot drainage sheeting would seem to be sufficient deterrent to termite chewing, but the drawing provided doesn't seem to provide much of an air gap between ground surface and the start of the siding. Can you provide more? Will the drain mat survive extended UV exposure?

    You are looking to put liquid whatever on the previously buried block wall. Have you checked to see how clean or dry the block needs to be for good adhesion? Will the glues you plan for the insulation be compatible with both materials? If you were to use Roxul, wouldn't the fasteners defeat the liquid sealant on the block? How will the drain mat be attached? Only at the top edge? Fasteners in the field area? How do you close the edge overlap? Will the overlapped edges be termite tunnels to the foam? (Maybe the mat locks together like LEGO dots?)

    One last tip that may not bear up under close scrutiny. I recall a posting, maybe on this site, that termites can't stand digging through a very particular size of sand particle. Something to do with their body segments and a "beach sand in your crotch" effect. Where one finds this sand and verifies the size is beyond me. And it still doesn't answer why the (non-Dry Termite) flying versions don't just pick a window or door like carpenter ants.

    Hope this doesn't muddy the issue too much. Also hope that people with real experience with termites don't find my comments laughable.

  2. iamkarlp | | #2

    Thanks for the thoughts Roger. A few targeted responses to that which applies to my situation.

    - Part of the basement is exposed, hence the CMU above ground. It terminates into the wood structure out of scope of this conversation.
    - I understand the requirements of cleaning the existing foundation.
    - I understand the adhesive needs of my proposed stack
    - I will be only top-attaching the dimple mat, and will be ordering it in full height to avoid the need for hoizontal seams. I will only have 2 vertical seams and understand how to make them resilient to ingress for moisture (and thus insects as well) with proper overlap, adhesives, and tape per Dorken.
    - I dislike roxul exterior foam below grade due to the fastener needs. I don't understand how some people are ok with the fasteners (hammer drilled fasteners no less) piercing the membrane all over the place - fluid applied or otherwise.

    All that said, I came this morning to much the same thought W/R/T physical barriers. I have toyed with a few details and I think I like this detail, composed of multiple over-lapping metal flashings.

    I also changed the above ground detail to move the barrier back to a plane which is easier to deal with on the above floor portions. Nonetheless.

    The below grade stackup here is fluid applied membrane, XPS, and DELTA-MS capped with an inverted L flashing.

    The above grade stackup is GT plywood over the CMU tying into the OSB on the house itself, allumaflash over the plywood, XPS with insect screen over the bottom edge, rain screen battens with bug screen between, and finally cladding. There is an L flashing with bent drip edge sitting on and overlapping the bottom inverted L flashing.

    It would seem that this detail would force any potential visitors out and around the flashing detail where they could be easily inspected for. The inside is also inspectable.

    Any further thoughts by people is appreciable.

  3. Jon_R | | #3

    Keep in mind that basement waterproofing has much more to do with pressure relief and drainage than blocking bulk water. Angle the fasteners upward (outside to inside) and there shouldn't be an issue with Roxul. It might even eliminate the need for DELTA-MS.

    In this video, the Roxul was stuck on. Looks quite secure.

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