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Does having EIFS siding affect your basement insulation strategy?

Shawn Hawkins | Posted in Energy Efficiency and Durability on

I own a home built in 1998 with a barrier type Exterior Insulation and Finishing System (EIFS). It is in Zone 4, near Knoxville, TN, I had the home inspected by an EIFS inspection specialist before purchase. There were no moisture problems discovered; they did a very thorough job. FYI – This house has huge eaves, in most locations 36″, which gives siding some protection. I am now interested in finishing the basement, which after a year carefully watching seems perfectly dry. I have examined my options to insulate, but I am confused and worried about what I should and should not do given the EIFS?

The basement has three sides concrete masonry block walls. Most of the CMU walls are below grade. I don’t believe the CMU walls are insulated below grade. CMU wall that is above grade appears to be covered with 1″ expanded polystyrene EIFS. On the basement interior, the CMU walls are un-insulated. I plan to have professional apply 1.5″ of closed cell spray foam (R10) to CMUs on interior . I will stud out the walls beforehand, setting studs back from wall about 3/4″ to avoid thermal bridging but preserve space.

Most of the rim joist area (22″ high) above the CMU walls is covered with 3/4″ expanded polystyrene EIFS on exterior side. This makes me nervous. Rather than spray foam, I thought I would “cut-and-cobble” 2″ XPS, air sealing perimeter with spray foam. That way I can ultimately get to the OSB to inspect. The remaining rim joist area (about 38ft) is covered by the lower garage wall (2×6) that is filled with fiberglass batts, paper facing inward toward basement. Not sure I need to insulate here? If I do, I will use the same “cut and cobble” technique, but don’t want to cause problems by sandwiching the OSB with a “double” vapor barrier?

The remaining basement side is a “daylight” wall that is a 2×6 stud and currently un-insulated. The OBS sheathing on this wall is again covered with 3/4″ expanded polystyrene EIFS, but most of it is protected from any chance of rainfall wetting by porch roof extension. Where protected, I’m thinking of filling the bays with 2-2″ layers of XPS, sealing perimeter with foam. The parts of the daylight wall that are left have a small eave overhang, so I’m nervous about continuing with 2-2″ layers of XPS. Perhaps just 2″ XPS, perimeter sealing with foam, and then fiberglass with no paper?

I want to make the basement well insulated but don’t want to overinsulate given the circumstances. I worry that my strategy may create problems in rim joist/daylight walls with EIFS?

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  1. GBA Editor
    Martin Holladay | | #1

    I think that you are worrying too much.

    There are two ways that your wall sheathing might get wet:

    1. If your house has defective exterior flashing, rain can enter from the exterior. This was one of the mechanisms that doomed many EIFS homes. But if your house has good roof overhangs, and if a thorough inspection showed that your house is in good shape, it sounds like you have no particular worries in this area.

    2. Interior moisture can accumulate on exterior wall sheathing if the sheathing gets too cold. But in your climate zone, the 1 inch of exterior EPS is enough rigid foam to keep your wall sheathing warm during the winter.

    Your insulation plan sounds fine to me.

  2. Shawn Hawkins | | #2


    First, please let me thank you, I appreciate what I have learned from this website, and you in particular, about how to make my home more energy efficient. It is fantastic information.

    I took the time to very carefully measure the barrier EIFS today. The total thickness is 1-1/2 to 1-3/4", including EPS and mesh/base-top coats. Thus, the EPS is at least 1" thick - a continuous R4.

    You may be correct, that I am overthinking this for my climate. I should forget about condensation on a cold exterior wall with the 1" EPS - thus flow of moisture out is not a problem.

    But I can't help but worry about the future of the EIFS and bulk water intrusion. In Ted Cushman's article "Robust Walls" he states that "a wall should not have two vapor barriers, because that could trap moisture inside the wall." I have seen this warning many times, and in my original post the thinner XPS (2") and certainly the double layer (4") for the rim joist/daylight wall will serve as vapor barriers. The EPS is now 16 years old, and while I will maintain it, failures will eventually happen.

    I will of course air seal everything with can spray foam. I'm now thinking I will reduce the XPS to 1/2" or 1", thus no vapor barrier, and fill the bays with fiberglass. I would appreciate your thoughts on the bulk water/EIFS concern and the plan to seal but allow drying to the inside.

  3. GBA Editor
    Martin Holladay | | #3

    You (and Ted Cushman) are both correct: in general, it's a good idea to avoid a "foam sandwich" that traps a wood-fiber product like OSB or plywood between two layers of rigid foam or spray foam.

    I was providing some practical advice. (Sometimes, when doing remodeling work, we have to choose which rules to follow, and which rules to break.) Although there is always a chance that bad flashing will direct rain water to your sheathing or rim joist, you have to evaluate how likely that failure is.

    If you want to be conservative, you can certainly insulate on the interior side of wood components like wall sheathing or rim joists with mineral wool or fiberglass batts -- insulation materials that allow inward drying -- especially if the components are covered on the exterior with a layer of rigid foam.

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