Insulating Basements…Hey, It’s Simpler Than Long Division

by Erik North on August 31, 2011

Insulating basements


Actually, no is isn’t. Sorry.

The basement (or cellar, as we usually call it here in Maine) is one of the most important locations in your home. Yes, the cobweb and mildew infested hole under your house may not seem like it, but it is. How do you understand your basement from an energy auditor’s point of view? How can you improve your basement so that it benefits your home (many, many ways)?

Basements served a variety of purposes in older construction. While rare in wet, marshy climates like much of the UK or American South, they have an indelible presence in American construction. They’ve served as food storage (root cellars), tornado protection (in the American Midwest) or as a stable footing for buildings during the winter/spring freeze thaw cycle.

In the 1950s, U.S. construction shifted to more suburban settings and storage space became a premium. That’s when homeowners, especially those in topographies like Maine, began facing the issues with digging an 8 foot hole into wet clay and porous granite then storing your belonging there. Moisture and water infiltration becomes a big fat problem.

Insulating Basements Is Simple, Right?

Homeowners are increasingly looking to use their basement space as an expansion of the main house and to make their home more comfortable. The downside is that many homeowners (and some contractors) use the same building materials and approaches in the basement as in the rest of the house.

That ain’t gonna cut it.

Wait, wait, wait, I hear you say…It’s as simple as framing up a few stud walls, add your faced fiberglass insulation (or cellulose or rockwool or…) and add a polyethylene vapor barrier over the fiberglass, right? (right?!?).

Actually, using standard techniques and materials against a foundation wall is like asking nature to slow roast your basement into a mold stew. I know, I know. Most folks avoid their basements unless at gunpoint. However, unless you’re hankering to brew some homemade penicillin, you need to understand a bit of what’s happening in your personal giant concrete ground hole.

Insulating Basements – What Problems Does Your Basement Face?

Let’s see…moisture, pests, moisture, mold caused by moisture, impact damage and getting wet.

However you decide to insulate, your basement is going to have to deal with some Mother Nature. Your concrete (or brick or rubble) foundation is essentially a giant concrete sponge and constantly deals with moisture:

  • Your foundation contains a few thousand pounds of water (if freshly poured, make that a few tons of water).
  • The traditional approach to basement water/damp proofing was to apply an asphalt exterior coating and allow the concrete to dry to the interior.
  • Rain and groundwater diffuses into the concrete (or if there are cracks, it just flows right in).
  • Water moves up the foundation wall through capillary action (and if there isn’t a capillary break between the foundation and sill plate, up into the building walls).
  • Lastly, interior moisture can condense on the portion near or above grade. Hooray.
  • Finally…How to Insulate Your Basement

    The two main approaches to basement insulation are exterior and interior.

    As an aside, I suppose there are four total approaches; the other options are either wildly impractical unless in new construction (insulating in the middle of the foundation wall) or redundant (both sides of the foundation wall…all the problems of exterior insulation at just twice the cost). So let’s stick to one side or the other.

    Exterior Insulation
    Exterior insulation performs exactly how you’d want with no more than 5 or 6 drawbacks. Having the insulation on the exterior of the concrete provides a strong thermal barrier, keeps the foundation warm and allows it to dry to the interior. From a building science standpoint, perfect!

    From a practical standpoint, lousy! Physical wear and tear plus the degrading effects of UV rays on most foam insulation necessitate protecting the foam board (typically XPS or EPS foam board but occasionally polyiso or spray foam). Stucco finishes, cementitious coatings or cement board can be used but this adds a very considerable expense to the exterior insulation option.

    And what do you get for that added expense? A straight shot for insects and pests to access the tasty wood of your building frame. Super. There’s no structural way to reliably prevent insect problems and chemical solutions carry their own secondary drawbacks. So…

    Interior Insulation
    Interior insulation avoids problems with insects and unless you’re engaging in particularly vigorous Foosball games, shouldn’t require cement board shielding. Interior insulation is the easiest to install and least expensive approach, providing the best investment of your money. However…moisture. Insulating the interior impedes the natural inward drying of the foundation.

    Remember the standard stud wall construction I mentioned earlier? No? Well, here it is: 2″x4″ stud walls, faced fiberglass insulation (paper face in) and a polyethylene vapor barrier over the fiberglass.

    With this construction, the plastic vapor barrier would stop all moisture and the minute glass fibers of the insulation would trap it, creating the perfect medium for mold. Excellent.

    So…What to do?

  • The insulation should not be water sensitive.
  • It should act as an air barrier, preventing interior moist air from contacting (and condensing on) the cool above grade building enclosure elements (infrared cameras to a pretty bangup job spotting problem areas).
  • The insulation should be vapor semi-impermeable or semi-permeable, allowing some inward drying.
  • There should be capillary breaks between the foundation and any wood building elements (the sill plate, the bottom plate of any basement walls)
  • And since this is interior space, it’ll need a code rated ignition barrier.
  • Again…what to do?

    Unfaced XPS and EPS foam board and medium density closed cell spray foam meet the bill. The insulation would be applied directly to the foundation wall, and if the space is finished, add a wood or metal stud wall with finish drywall. Don’t forget capillary breaks at the wall-sill connection and stud wall bottom plate and DO NOT finish with any wall system that’d act as a vapor barrier (vinyl wall paper, most paints).

    There are more details to basement insulation than most folks can imagine but as my marathon writing sessions go, it is 3am and time to shut it down.

    Additional Reading:
    For waaaaaaay more detail on this see Building Science’s article on ‘Understanding Basements’

    { 8 comments… read them below or add one }

    mrhilton January 25, 2012 at 6:49 pm

    I have been working through the internal vs. external basement insulation decision for an addition I am planning to build in Maine.

    After much research (including your article) I think interior is the way to go.

    Thanks for the great info.


    Erik North January 26, 2012 at 1:04 am

    Thanks, it’s great that it was helpful. Just make sure you include a capillary break (usually a foam sill seal) between the sill plate and the foundation wall.


    Tom November 3, 2012 at 11:51 am


    I’m a new owner of a property in southeast Wisconsin. There have been three additions put on over time, but the basement is only under the section built in the early 50’s. I believe it was finished sometime in the 70’s (and looks it!). Just before I bought the house in march the circulator pump for the boiler cracked and dropped 2-3 inches of water across the entire basement floor. It all dried quite well, but there is water damage and a little mold on the wood paneling and drywall behind, so I’m taking the opportunity to strip that out and make updates.

    When finished the basement originally had a thick poly liner against the concrete (what I’ve seen of it looks to be in great shape) followed by 2×2 framing (water damaged, non-pressure treated sole plates) and EPS press-fit between the 2x2s before drywall and wood panel. I’d like to pull that out and put continuous XPS or Polyiso against the wall, followed by a 2×2 frame (probably the existing framing with new PT soles) and then drywall.

    I’ve been reading a lot of conflicting opinions on whether to put a vapor barrier against the wall. My inclination is that if there hasn’t been a mold problem in 30-50 years, and even the mold stemming from the flooding hasn’t made any real progress between March and now, then whatever is there seems to be working. I do believe that my basement has perimeter drainage into a sump (which is actually out in the well pit immediately adjacent to the foundation).

    What do you think? Does past history override contemporary theory? I grew up in a 200 year old farmhouse with combination dirt/mud basement, so all this newfangled stuff from the 50s is like magic.



    Tom November 3, 2012 at 11:56 am

    Oh, additionally the space itself is not conditioned but the boiler room (water, not steam) is down there and there’s quite a bit of (not undesirable) heat transmission to the rest of the basement from that equipment and the hot main circulating line running around the perimeter of the basement. We use window air conditioners upstairs which have no effect on the basement.


    Erik North November 19, 2012 at 9:53 pm


    I haven’t forgotten about you…here’s the email I sent a couple weeks back:

    >In a nutshell, if it works, it works. How you describe your basement is fundamentally the same as current thinking. I insulated my basement with 2″ XPS foam board directly over the foundation wall, framed up with 2×4 studs with a pressure-treated footer, capillary break between the floor and footer. I then spray-foamed the sills, put fiberglass batts in the cavities and drywalled.

    Mold needs moisture and plant fiber cellulose to grow. The only massive red flag would be DO NOT put an impermeable vapor barrier over any plant-fiber products (wood studs, paper faced drywall, kraft paper faced fiberglass, cellulose insulation, etc.) It would trap the mold food in an high moisture/high >growth environment.

    I had an audit last week that illustrated this point. A previous owner had insulated the sills with dense pack cellulose. The foundation had a lot of moisture, and it transformed every sill into a blob of mold.

    There’s nothing wrong with the existing installation other than not having a capillary break under the wood framing. However, updating with more modern materials and techniques would produce a better result (for example, a continual foam installation would stop the thermal bridging that’s likely happening with the 2x2s).


    Janet July 29, 2013 at 10:02 pm

    What type of paint or finish would be acceptable?


    Erik North August 4, 2013 at 11:38 am


    Check the ‘Perm Rating’ on the paint or finish you are considering. This is the vapor permeability of the product. If the Perm rating is 1.0 or below, it is considered a vapor retarder and would prevent movement of moisture.

    Here’s a (slightly technical) info link:


    Ted December 3, 2013 at 11:45 pm

    Hi Erik, thanks for your excellent blog posts. I don’t see any posts here about bubble foil, which I’m considering for my basement, perhaps you can advise me on this.

    I live in a circa 1900 house located near the Hudson River in New York, and the unfinished basement has a foundation made of concrete with large stones that leak water when it rains. There is fiberglass insulation between about 50% of the joists of the basement ceiling which are now deteriorating, installed by a previous owner. Because of the moisture in the basement (a sump pump and dehumidifier keep it under control) the fiberglass was probably not a good idea but I think the idea was to keep a couple of the rooms warmer without sealing the whole basement off.

    I want to have the fiberglass insulation removed, and it is falling off in places already — but afterwards I feel that I need to do something to keep some of the heat in the house. I don’t think I can do much about the walls right now, keeping the water out is going to be a complicated process if it can be done at all. What do you think of my installing radiant heat insulating bubble foil over the joists? Maybe just some of the joists so I don’t make the basement so cold that the pipes freeze?

    I was told by a company that sells the bubble foil that by installing this it will create a vapor barrier and could achieve an R rating of 17. Their website seems to indicate that the downward R rating is really 10.5 but either way it’s better than nothing, plus water isn’t going to be a problem. Then later maybe I can do something about the walls. What do you think?


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