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Retrofit external insulation for a brick foundation – ground/below-grade interface

itech20 | Posted in General Questions on

I’m exploring adding rigid insulation to the exterior of our brick foundation.  We’ve done a substantial amount of retrofit insulation/air sealing retrofit on the rest of our house and the foundation stands out as a serious remaining weak point.

To give a sense of the size, our footprint is roughly 20’ x 30’.  The exposed portion of the foundation wall is 4’ on three sides and 6.5’ on one of the 20’ sides, where the basement walks out into an excavated below grade area in our back yard.

In most cases the foundation wall descends directly into the local soil.  In others, it seems that a course of cinder blocks has been added against the brick.  These blocks are set just above grade and covered with a thin layer of concrete.  On the 6.5’ portion of the foundation along the yard the foundation meets up with a concrete sidewalk surface.

What are my options for dealing with these ground interface areas?

On the extreme side, I’ve seen this video from our friends in Minnesota, which demonstrates minimal exterior trenching that allows rigid foam to be installed all the way to the bottom of the foundation wall.

This looks fairly invasive/expensive, and I’ve no idea if the required excavation equipment and associated liquid spray foam is even available in our area.

On the flip side, I worry that simply ending the insulation layer at or just above the local ground level will leave a significant uninsulated or low-insulated area at and just below grade.

I haven’t attempted to get any of this work quoted yet.  If there’s reason to think a trench/excavation-free project would still provide some significant improvement, I’m open to doing it myself.

I’d be grateful for any guidance or inspiration here.


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Ian.

    A lot more heat loss happens through the above grade portion of the foundation wall, so insulating only above grade is not without merit. But of course, excavating and insulating below grade is better. Because you didn't mention it, my first question is, can you insulate the inside of the foundation walls?

  2. Charlie Sullivan | | #2

    The exposed portion of the foundation is by far the most important. The first foot below grade is more important than the next foot, and so on. Digging down one foot is not all that much work and might be a good compromise.

    In some cases, excavating deeper is useful for solving moisture problems: damproofing the wall and installing footing drains. In that case you can take care of both at once.

    I'm not sure what the concrete blocks are for or what to do with them.

    1. itech20 | | #3

      Many thanks for the replies. so far.

      On the question of "Can you insulate the inside?" - A few different things have me looking first at exterior options.

      1) The interior foundation wall surfaces are, in several places, covered with difficult to move equipment. These include our main electric panel, the whole-house tankless water heater and a stringer for the stairs leading into the basement. I can imagine a not-too-invasive scheme for moving the tankless heater, but the electric panel and stairs are likely to stay where they are, so I'd have some gaps.

      2) The received wisdom I've read on interior retrofit foundation insulation all points towards spray foam (after prepping and sealing the interior surface). I've struggled to find a low-GWP foam contractor in the Chicago area.

      3) Exterior rigid insulation opens the possibility of using reclaimed materials and even absent reclaimed materials has some options that avoid high GWP blowing agents. (It's also something I can imagine chipping away at on a DIY basis, unlike foam.)

  3. Roger Berry | | #4


    I moved from the Chicago area some years ago. I left a home with a block foundation that dated to the middle of the depression. None of the cores were filled with concrete, so the wall cores filled with water. Much of the mortar between blocks had degraded. I mention all this only to amplify on the need to do a thorough look at the foundation and water concerns.

    The blocks you describe sound like they may be laying on their sides as a sort of trim against the brick. Probably empty cores and of no interest beyond what is underneath them.

    Your terrain description suggests an older neighborhood outside the central city with drainage away from the house. Hopefully, not too close to a river or canal. If the foundation is truly brick all the way down to the footings, it is possibly a three brick wide system. That would have been a pretty lux method. It may be face brick over a cast wall if you are not seeing brick inside.

    General health questions for the foundation. How are the mortar lines holding up? Hairline cracks? Loose bricks? Receding mortar lines? Tuck pointing present? Is the basement notably damp feeling? Do you store things down there or have designs on finishing for living space?

    The relevance is for judging whether the wall has good water management at the footings and downspouts. For the likely era of construction, fired clay tile was often used for drainage, sometimes to a cistern located at the back of the house down to or below basement floor levels. If your downspouts drop straight down into a little mortar circle, it is a good bet that there is a tile elbow just below.

    Digging around the house to go below grade might be a very bad idea if there is risk of disturbing the drainage tiles. It would also be a very bad idea to have authorities find out that you have a cistern and they demand you fill it in. Trust me, the house I bought had both problems and the water management fixes were royal pains. If you are enjoying a relatively dry basement in a house that is 70-100 years old, lucky you. Don't mess with anything outside below grade if possible to avoid doing so. At least not without full understanding of what is down there.

    Another issue that may cause major anxiety is finding out that the drains are connected to the sewer system. Many localities over the last 25 years or so have been aggressively mandating that homes with footing and downspout drains connecting to the sewer system be separated, often with little help as to where to go with the water. Further problems can arise depending on what choices are available to you for where to send that water. Most solutions are not happy ones. Deep Tunnel is still not handling the mess that is Chicago's storm drains.

    I would suggest having a friend in an apartment make any inquiries so not have your address associated with any questions. Another reason to analyze your choices depends on how the neighbors might respond to seeing outside work.

    Hope this isn't too terrifying a take on what you want to do. I do miss the Field Museum.

    1. itech20 | | #5


      Many thanks for the lengthy reply, and apologies for taking so long to follow up.

      The foundation is two bricks wide. We had most of the above grade portion of the outer course replaced with new face brick about 10 years ago, and it has held up well. The interior brick has been extensively patched and pointed over time and the below grade portion is covered by dimple board that is part of an interior french drain we had installed around the same time as the exterior brick work. I've peeked behind the board in a few places and it all seems to be relatively stable. That is, in the full decade that we've lived here, both the new and the old brick seems to have held up just fine.

      The gutters do, indeed, feed into a buried tile drain in several places. This drain has _already_ partially failed, and our response has been to disconnect the affected downspouts, route them a reasonable distance horizontally away from the house and let them drain directly onto our lawn. The drainage grade is generally favorable and we have not actually had problems, particularly since installing the interior french drain.

      We had the interior drain installed, in part, with an eye towards finishing at least a portion of the basement space.

      Based on some earlier comments I have revisited the possibility of _interior_ spray foam. I've found at least one contractor who offers one of the newer formulations with a low GWP blowing agent.

      My tentative/sketch of a plan, should we go this route, is to foam over the dimple board, exposed brick and rim joist area as one continuous application.

      One possible add-on would be to add either more dimple board, or another standoff to fully cover the vertical brick up to the rim joist, so that I have a continuous drainage path down to the tile between the foam and the foundation.

  4. itech20 | | #6

    I thought I'd give a brief update.

    After looking a bit harder, we were able to find quite a few Chicago area contractors who offer a low GWP blowing agent closed cell product. The particular provider we went with used "ICYNENE ProSeal HFO". We opted to do a relatively modest ~1.5"/R10-15-ish application everywhere except the rim joist area, which we had more or less filled in/up.

    We coupled this with dense pack cellulose in several empty wall cavities on the first floor.

    The difference has been quite dramatic. My data so far this season suggests that our heat load at a given temperature is down by at least 1/3rd. What's more, the basement has gone from running about 10 degrees (or more) cooler than the first floor to being only a degree or two cooler. (Our forced air furnace and duct work for the first are in the basement). The absence of a "cold floor" is a very welcome development and we're now seriously considering creating a finished space in a portion of the basement.

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