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Q.What kind of foam plastic insulation should be used on exterior foundation walls?

A. Bruce Nelson responds: We have found that many different materials can work, but that some require more expensive installation than others to give equal performance. There’s still much we don’t know, but our investigation of 59 foundations in Minnesota and discussions with others in the field suggest the following:

  • Extruded polystyrene (Amofoam, Certifoam, Foamular, and Styrofoam) always works well.
  • Although only two specimens of expanded polystyrene (EPS) were observed in our study, we feel EPS can be used successfully. However, use only the higher density products below grade, not the typical one-pound-per-cubic-foot variety found in most lumberyards.
  • If you use polyisocyanurate or spray urethane, include a protective coating below grade that is long-lasting and strong enough to avoid puncture from backfill. In some cases we observed higher water absorption, which may have been caused by damage to the below-grade protective coating.

Also, foam plastic is not your only option. Consider high-density fiberglass board for sites where foundation-wall drainage is a must. This material is available as a commercial roof insulation from Owens Corning. It is only available for residential use in Canada, as BaseClad (Fiberglas Canada, 4100 Yonge St., Suite 600, Willowdale, ONT, Canada; 416/733-1600). This product must always be installed with exterior perimeter drains.

No matter what type of material you use, two elements are essential to prevent moisture absorption and deterioration. First, insulate to the top of the foundation wall, making sure to leave no gap where the insulation meets the siding. Use mechanical fasteners or adhesive to prevent the insulation from slipping down the wall.

Second, protect the insulation above grade from physical abuse and sunlight. Since many of the installations we have seen have missing or damaged coatings, we recommend an above-grade covering material at least as durable as the siding. Examples include 1/2-inch pressure-treated plywood, high-quality stucco, and fiberglass panels.

Bruce Nelson is a senior engineer for the Minnesota Department of Public Service

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