View Full Version : basement wall insulation

09-25-2012, 07:28 AM
I have a bit of an oops moment.

I'm currently in the process of finishing my basement and after doing some research about insulating my basement walls (poured concrete), I came across a wonderful BuildingScience.com article regarding different insulation strategies. I was a bit over zealous to start the project and I THOUGHT I understood what I needed to do. So I bought 1" EPS foam board (foil on one side, plastic on the other) and attached them to the concrete wall. I never took off either the foil or the plastic 'skin' on the EPS board... effectively creating a vapor barrier and the exact opposite of what I wanted to do. :(

Is my only option right now to remove all the foam board I installed (yikes!)? Maybe I could leave the foam board as is on the upper 4' section of the wall and only replace the lower 4' section? Any thoughts/suggestions would be much appreciated.

09-25-2012, 09:06 AM
If you have a good sill gasket between the foundation sill and the concrete, some amount roof overhang, decent drainage, and at least ~12" of foundation exposed above grade on the exterior to dry to, don't sweat it- you'll be fine. The concrete will dry toward the exterior, even if it's wicking moisture all the way from the footing. Keep an eye on it for a couple of years- if the exposed exterior of the foundation shows a lot of efflorescence it may need a sacrificial parge of lime-mortar &/or improvements to the foundation drainage.

If you don't have a sill-gasket AND minimal roof overhangs AND poor drainage at the footing, cut the foam to about 12-14" off the slab to allow the footing & foundation to dry toward the interior.

No matter what the higher moisture/mold/rot risk is from bulk water reaching the foundation, either at the surface or from below the footing. grading the surface so that surface water moves away from the foundation is important. If you have minimal roof overhangs and no gutters redirecting water well away from the foundation, a buried sloped drain on the exterior (http://www.finehomebuilding.com/PDF/Free/021111098.pdf) of the foundation can make a big difference in the average moisture concrete. Concrete is quite tolerant of moisture, but minimizing it's moisture content where it contacts wood is most-important.

Even with a sill gasket air leakage at the band joist and sill is usually quite high- accounting for more air leakage than windows & doors in most houses, and a major entry point for stack-effect driven infiltration. When insulating a foundation from the inside it's important to detail the foundation foam and foundation sill/band-joist as part of a continuous air barrier. (Building Science Corp covers this a bit on their details, but don't ignore it.) Tape the seams of your faced EPS with appropriate housewrap tape or FSK, and use 1-part foam to seal the top edge to any rigid foam you're cutting in for the sill/joist insulation. If there's a lot of gap to fill it's sometimes better & cheaper to spring for a 2-part FrothPak kit (available at box stores) rather than 24 cans of 1-part foam.

If building an interior studwall and filling out the R with batts, make it tight to the wall foam, and put 1" of either EPS or XPS under the bottom plate of the studwall as a capillary and thermal break. If you're insulating the slab from above as well, just run the floor-foam all the way to the foundation wall, slipping both the subfloor and foam under the interior studwall. Use only unfaced batts (rock wool would be best, fiberglass 2nd choice)- avoid using hygroscopic insulation such as cotton or cellulose in this application.

In a Grafton, MA climate 1"/R4 of foam is a bit marginal for going high-R on fiber insulation to the interior of the above-grade sections- R11s may be less condensation-prone than R13s. If you have the budget for another inch of EPS bringing it to ~R8 you'll pretty much eliminated all risk of wintertime condensation within an insulated stud-bay. (Try the Insulation Depot in Framingham or the local bulletin-board list (http://worcester.craigslist.org/mat/3275256658.html) for local sources of reclaimed roofing foam at 1/4-1/3 the cost of virgin stock. If all they have in stock is polyisocyanurate that's fine, just don't let the bottom edge rest on the foundation. With 1" of EPS and 2" of iso glued to the interior you can build the studwall with the studs turned 90 degrees (it's not structural, after all), and end up with a higher R value (~R16 whole-wall) than a batt solution since you won't have any thermal bridging of the studs cutting into the performance. It means you'd have to do a bit of cutting to get electrical boxes etc, in place but that's still a very good solution.

09-25-2012, 04:02 PM
Dana - thank you for the quick and thoughtful response... you definitely made me feel better as the house meets all the criteria you had mentioned. Lots of great information in your post and it's taking me a little time to digest everything :)

In some areas of the basement I've already put down treated lumber for the bottom plate with no EPS or XPS underneath - is this a problem? I also do plan on insulating the slab with either EPS or XPS.... Is the same principal of not using a vapor barrier also applied to the concrete slab?

09-26-2012, 09:25 AM
The pressure-treated can still wick water up from the slab, but it's not a disaster as long as it can dry into/through the cavity-fill & gypsum into the room. (With the built-up foam + subfloor + finish floor you'll have to nail any kick-board to studs-only though.)

As long as the slab isn't in contact with susceptible wood it doesn't need to dry- the concrete itself is quite tolerant of high moisture content. Putting 10-mil poly between the floor & foam makes it nearly gas-tight, even if you tapcon the subflooring to the slab (staggering seams of the subflooring with those of the foam.) With the foam between subfloor & concrete the subfloor has no span to support, it's fully supported by the foam & concrete. It doesn't need to be supported by sleepers, and doesn't need to be the heavy stuff. Half-inch t & g sheathing works, so long as that is enough retention for the finish floor fasteners. TapCons 24" o.c. are usually enough to keep it flat.

Windows on Washington
10-01-2012, 05:17 PM
Typical Dana response...totally factual and rock solid.

Good to see the big brain isn't just over at GBT!!