Should I Insulate My Basement With Fiberglass (Or Don’t Make Me Beat You With a Stick)?

by Erik North on September 14, 2011

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I thought this could use some expansion from when I wrote about basement insulation in a previous post.

Sometimes homeowners, visions of Man Caves dancing through their head, can be gung-ho about tackling their basement remodeling projects. Bombing back from Home Depot, they trundle back home with a mash of 2x4s, wiring, sockets and drywall in their Yukon’s trunk. Why, it’s so easy, just slap up the framing, add the fiberglass, add the drywall, slather on some Benjamin Moore and sit back and watch the Patriots.

This comes up less frequently with contractors who face liability issues and usually don’t want to make your basement into a mold petri dish.

So…should you finish your basement and insulate with fiberglass?

Short answer: No, don’t make me beat you with a stick.

Longer answer: No…You Shouldn’t Insulate with Fiberglass and Here’s Why.

To summarize a bit from the previous post, basements deal with a lot of moisture. Whether they’re built in the Mid-West, Great Plains or New England Northeast, basements follow the same basic forms and deal with the same problems. We dig an 8-10 foot hole into wet clay and loam, very possibly down to the water table and construct them from porous building materials (like brick, mortar or concrete).

Yes, the foundation exterior is usually damp-proofed with a tar or bitumen sealant, retarding moisture flow into the concrete. But foundations still deal with thousand and thousands of pounds of water which cannot dry to the exterior so traditionally basements are allowed dry inward.

Pulling this from the last post, here’s a little ‘moisture and your basement’ checklist:

  • Your concrete 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.
  • The Traditional Insulation Approach and Growing Penicillin In Your Basement

    Insulating the basement is a tremendous idea since concrete (or brick or slate) is such a lousy insulator. Concrete does a tremendous jobs sitting there and holding your house up, but is not far removed from an open window in terms of retaining heat.

    A homeowner looking to insulate their own basement can’t be faulted for falling to the Home Depot Fallacy (that’s just buying whatever is available at Home Depot/Lowe’s regardless if it’s the right material for the job). HDF afflicted folks hew toward the traditional insulated wall structure: A 2″x4″ stud walls, faced fiberglass insulation (paper face in) and a polyethylene vapor barrier over the fiberglass.

    So…What’s The Problem?

    Moisture from your foundation is drying to the interior. The standard construction adds a the plastic vapor barrier which would stop all moisture. And you’ve installed fiberglass insulation whose microscopic glass insulation fibers trap the moisture. Congratulations, you’re fiberglass wall has transformed into a mold incubation chamber. Fantastic for your child’s science project. Not so great for having healthy breathable air.

    So logically leave off the polyethylene vapor barrier and let the fiberglass breathe, right? Removing the vapor barrier won’t solve the problem because interior moisture can now contact the cold concrete, potentially condensing on the foundation wall.

    Another issue is that the 2”x4” stud walls are often installed without capillary breaks. Placing the walls bottom plate directly on unsealed concrete allows water to wick into the wood. Even if pressure treated lumber is used, over time the wood will wick moisture and rot.

    So…What’s The Solution?

    This was covered in more detail in in the previous insulation post. Go ahead…read it. I’ll wait *whistles*.

    There is too much, let me sum up…A sensible approach in interior basement insulation would be unfaced XPS foam board, EPS foam board or medium density closed cell spray foam applied directly to the foundation wall. If you’re finishing, 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).

    { 5 comments… read them below or add one }

    bob October 16, 2011 at 1:01 am

    I’d like to be beaten with a stick. Thanks.


    Erik North October 16, 2011 at 1:41 am

    *chuckle* funny.


    AJ Simkatu April 17, 2012 at 6:28 am

    So what do I finish the walls with if most paints are no good? Do you have a paint recommendation or some other sort of finishing system?


    Erik North April 25, 2012 at 3:18 am

    Any vapor permeable paint or finishing system would work.


    Phil June 22, 2014 at 6:44 pm

    I bought my house with a French drain already installed and a moisture barrier attached directly to the concrete walls.
    I’m finishing the space and am trying to decide on insulation. My original thought was fiberglass. I was going to remove the fiberglass that was in the basement ceiling and replace it with “Roxul safe and sound” and put the fiberglass in the walls.
    After reading your blog I’m not certain what would be right. I don’t have the money to used closed cell spray foam.
    What are your thoughts?


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