GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Horizontal wing insulation on full basement

cragged | Posted in Energy Efficiency and Durability on

I live in northern North Dakota which is one of the coldest climates in the country. My house has a full basement with the footings down eight feet. Would horizontal wing insulation with R value 20 sloped  out to eight feet away from the foundation a foot below grade be a good idea to keep the basement wall warmer and drier? Thanks.  Craig McCormack

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    Is your basement floor at or near the level of the footing? Usually you get nearly all of the benefit of basement insulation by insulating the basement wall, insulating the floor doesn’t gain you much more. If it was a very shallow floor, then horizontal insulation may help.


  2. 730d | | #2

    Yes. I would think R10 4 ft out would be 3/4 as good as as R20 8 ft out.
    I would go closer to grade near the house and down as much as six inches four feet out.
    Assuming you have minimal insulation on your basement walls and rims maybe that's a place to put money first. You must have minimal foundation landscaping?

    1. cragged | | #3

      Insulating the basement wall would require a lot of deep digging. I am insulating the rim on the outside and down another foot. I will have to work around or remove landscaping. The existing basement wall is bare concrete the whole way down with no insulation inside but finished inside. Thanks for your replies.

      1. Expert Member
        Dana Dorsett | | #4

        Is digging a foot+ in an 8' wide perimeter around the house more difficult than digging 6"-8" wide to 4' below grade?

        Current IRC code min is R15 continuous insulation, but 4" of EPS (R16-R17) down to 4' below grade is almost as good.

      2. Deleted | | #7


    2. 730d | | #8

      I meant insulate on the inside of your foundation walls and rim.

  3. onslow | | #5


    If you want another opinion, I would suggest you stick with the wings of foam extending outward on a slope over digging down beside the foundation for half depth. While the effective insulation value of foam against the wall will be better than the wings, you risk altering the soil in such a way as to increase the likelyhood of making the wall wet.

    Once you disturb the soil as Dana suggests, it will likely take two years or more for the dug area to re-solidify to the same prior state. Even with careful grading and spout control on gutters, you risk creating a new easier path for water to get close to the unfoamed part of the wall. If the inside of the basement is finished, you might find a new problem due to wetting the foundation.

    By example, even though I packed the backfill adjacent to my walls in lifts, settling still occurred for the next two years. Fortunately for me, rainfall is precious here, so I was not alarmed. It will be a balancing act for you to decide if the projected heat loss adds up to a bigger cost/risk relative to potentially damp walls.

    I also think attempting to work in a small a slot as Dana suggests is unrealistic.

    1. Expert Member
      Dana Dorsett | | #6

      While it's true that a 4' deep trench will be prone to settling, backfilling the trench with 3/4" screenings and installing a 2'-3' wide EPDM wing apron & French drain approach would improve the moisture situation beyond where it is today, even before the soil is disturbed.

  4. onslow | | #9


    Thermally speaking, Mike is right. Insulating the inside of the basement will be far more useful if energy savings are the goal. Going with a version of frost protected foundation techniques will eventually yield a change in ground temperatures that will help, but to nowhere near the degree as insulating the basement walls. You wish to make the walls drier, by which I am inferring some level of dampness. Insulating over walls that are damp beyond the levels associated with interior humidity effects requires another discussion thread.

    I will leave it to your own best sense of what manner of contours and drainage issues you do or don't have currently. The article Dana references does not encourage me to recommend digging down the side of your foundation. The first photo shows a swath almost as wide as you suggest, only covered in membrane rather than foam board. The rest of the story shows why one can unintentionally create new pathways for water to visit your basement. At the very least it shows a level of effort that is quite amazing.

    I would also suggest thinking very hard about where your ground slope will be displacing water to. Hopefully you have a distinct collection low point in your yard that would get water away from the foundation and yet not dump the collected water on the neighbors. If safe to do so, consider adding a perimeter drain at the edge of the insulation wings. This will catch what will now be rapid water flow from the thin layer of soil over the insulation wings. You could also tie in under surface downspout connections. A large holding pit might be in order if the lot is tight.

    You should consider using fabric wrapped drain pipe. Set the pipe in a stone bed also protected with fabric before topping off to final graded profile. I recommend a full fabric wrap of the stone bed, some only fabric over the top side. If you do not protect the stone with fabric (as with the pipe), soil will infiltrate the stone bed fairly quickly and degrade its value.

    It sounds as if you do not currently have or need a sump pump, although you infer mild dampness in the basement walls. If you are on a relatively flat property or have heavy clay soils, digging down next to the foundation will almost certainly aggravate your dampness situation.

    The area of the midwest I left behind has fairly dense housing on predominately clay soils. Digging a foundation hole was much akin to digging a self lining swimming pool. Many homes needed multiple sump pumps. Worse, the newest regulations for dispersal of runoff and sump water made for very interesting landscape choices to say the least. I cannot say for certain how the builders were backfilling or managing the gutter drops, but I can say for certain that soggy basements from sump pump disasters much discussed at barbecues. Battery backed up sumps were big sellers.

    Hope this helps guide the choices you have ahead of you.

  5. cragged | | #10

    I may have made an inaccurate impression of my situation. I don't have a major moisture problem in my basement. I am interested in reducing the moisture level in the soil surrounding the house because I understand that dry soil is a much better insulator than wet, with wet soil drawing more heat out of the concrete foundation. I have dug down about 18" along the foundation and digging a narrow band by hand down 4' seems like quite a job. I do realize there is digging with the wings too. I just had the house resided and insulated the rim on the outside at that time with R 6 comfort board. There is no way to get at the rim on the inside. Craig

    1. Expert Member
      Dana Dorsett | | #11

      If doing wing insulation only, DO install EPDM under the insulation, with a French drain along the outer edge. Sheet foam insulation with the seams taped isn't likely to remain moisture tight over the intermediate & long term.

  6. cragged | | #12

    I'm going to try to bridge some leftover ribbed steel I have on hand over the joints in the foamular to tie everything together, overlapping just as you would on a roof. I am planning on gluing the steel to the top of the foamular with fuze it. Then six inches pea gravel above and below the assembly with no dirt over the top of the pea gravel.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |