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Community and Q&A

Insulating crawl space in a house on piers

lynelle | Posted in Green Building Techniques on

I have a house on piers. Height above natural ground varies from 1 foot to 4 feet. “Floor” is boulders and sand. House is skirted with vinyl siding with considerable gaps.
Climate is cool–I’m just at the 45th parallel in Bracebridge, Ontario Canada and am on the water. There are no issues with flooding at the house. I will be adding to the house in the spring as well as rewiring and changing plumbing, but need help on insulating the existing house now.  I would prefer not to have to dismantle stuff I  put in now when I add on.  Conversely, is it  better to wait and do it all when the addition goes on.  I should add that I will be doing a lot of the insulation myself.
The space has a “warm room” (pretty well insulated and with a baseboard heater) that houses the water line under the house (it’s buried until the crawl space), the pressure tank, the HWH, Iron and Tannin tanks, UB light and 2 filters. Septic piping is also here and exits well below ground.

Rockwool is between the joists, but joist ends are exposed.

I have no idea at present what (if any) subfloor insulation is present, but the exposed joist cavities in the warm room  indicate that the subfloor is plywood. I do have room to add rigid foam above it before I lay new floor. I could use foil backed foam (foil side to the interior) if it will net a benefit, but don’t know about vapour issues.

Crawl space is damp in the summer. I bought in May, so have no experience with winter conditions.

I had planned to insulate under the joists with Rockwool Comfortboard, then apply some sort of vapour barrier.  My neighbour, a contractor has suggested that I frame in the wall space between the piers and use closed cell spray foam to seal down to the boulders.

My gut says to isolate the living space.

I know that I will need a barrier of some sort on the ground, but poly seems to be too fragile if I’m using the space for outdoor furniture/kayak storage.

What say you all?

Many thanks!
Lynelle Hamilton

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  1. mr_reference_Hugh | | #1

    Hello Lynelle, I would suggest reading this very recent question in the GBA Q&A. It explains how one person managed a very similar situation in NS Canada. You will see how they did it and comments from some regular expert contributors to GBA.

    1. lynelle | | #3

      HI Hugh, I did see her post, but not until after I had posted. (The vagaries of search engines, I suppose.) NS has some different issues (wind being one of them), so I decided to leave my post up. The responses to Kirsten are helpful though.

  2. Expert Member
    BILL WICHERS | | #2

    If you have a house on piers, the usual way to insulate the floor would be batts in the joist bays and then some type of rigid insulation underneat the joists. You'd want something around the exterior too, similar to where the rim joist would be with a regular foundation wall, since you don't want the ends of the joists left uninsulated. You could use Comfortboard here, but I think you'd be fine with polyiso too. I don't see any need to frame down to the ground unless you want more solid walls to secure your storage space down there.

    You can get reinforced 20 mil poly which is pretty durable, so that may be an option if you want a liner on the crawlspace floor that is durable enough to store stuff on. I would recommend putting some plywood down there (which is better than OSB here, since plywood has a smoother surface), so that you can slide your kayaks and furniture in and out without risk of tearing the poly. With kayaks, I'd probably build a sort of rack with 2x4s to make some rails spaced to hold the kayak. You could even build a multilevel rack like this fairly easily if you need to store multiple kayaks.


    1. lynelle | | #4

      Hi Bill,
      Thanks. The 20mil poly may just do the trick for the floor, along with plywood strips (and possibly some fill to accommodate the varying 'floor' levels.
      I'll definitely insulate the exterior overall, and extend it down to the base when I strip the vinyl siding in the spring. I have some comfortboard I'd planned to use for the underside of the joists, so I think I'll proceed with that plan and use ISO for the walls in the spring.

  3. maine_tyler | | #5

    You may want a less vapor permeable insulation than comfortboard under the joists.


    It probably matters less with open piers and a less humid climate. So not positive if it's an issue but perhaps could be. Others here may know...

  4. rkymtnoffgrd | | #6

    1' to 4' slope is almost exactly what I had at my cabin in rural Colorado at 10K feet. Beautiful place, but totally wild. In the beginning, I installed batts between the bays, then pressure treated framing to the ground, batts, and skirt made from t11 panels. What a mistake... it became a rodent breeding den and a total mess... It was great for the first year, then mice found a way in. Once the mice got in, the rats soon followed, they would literally dig and chew their way in past all my sealing efforts to get back inside (or to get outside if inadvertantly sealed them in)... They set up the Rodent Empire under the house. I could manage the population with traps while I was there, but becasue it was a vacation cabin, it was to noavail. This was all due to the difficulty of effectively sealing to the ground around all the rocks, stumps, and ground inconsistencies. No matter how hard I tried, I couldnt keep them out with the 1/2ass way I sealed the skirt to the earth. Over 7 years they destroyed all the batts between the joists and the framing. I estimate at one point a population of 10 or rats and countless mice lived and nested in the framing under my house. I ended up ripping it all out, all that work was destroyed. I eventually created a small "Plumbing Central" 4x6 centerblock room with a proper concrete foundation directly under the water supply and plumbing stack. All the other plumbing drained outside the room sloped down to it and was not problem even in the coldest weather. No drain traps were exposed outside the little room. I then used 4" ridged foam tightly cut between the floor joists without any voids. I left this exposed, this way there was no voids and I could quickly inspect for issues and see if any rodents or damage existed. It completely solved my problem until I got the bright idea to install kyack, and lumber racks under the 4' side... the rats nested in the kyacks, destoyed the seats and floation, and mice nested inbetween the lumber and used the racks as way to access the ridgid foam between the joists. They began cheweing it once again above the storage... I ripped the storge out... fixed the foam and now keep it clean under there for the predators to move freely. If you are enclosing anything, make sure you are installing a proper subgrade foundation that is deep enough and ensure your details from your foundation to your sole plates are bullet proof... This is harder than it sounds after the fact...

    1. Expert Member
      BILL WICHERS | | #8

      Embed 1/2" mesh hardware cloth into your canned foam. Put in the hardware cloth into the gaps so that it makes springy contact with the entire perimeter of the gap, then spray in canned foam (or spray foam). You end up with spray foam with hardware cloth inside, so the hardward cloth provides a chew proof barrier to the critters while the foam keeps the hardware cloth in place and also provides an air seal. I have had very good luck with this method to keep out mice.


  5. Expert Member


    I agree. Skirts are really just a very poor crawlspace wall. My own feeling is that if you have a structure built on piers with no perimeter foundation, you are best off leaving things open and concentrating on sealing and insulating the underside of the floor.

    From both a practical and code perspective, adding a skirt makes it a crawlspace, along with all the requirements that come with that.

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