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 }