Protecting Foundation Insulation
A heavy gauge of metal flashing offers increased durability if you plan to let it contact the ground.
We are in Minnesota (Zone 6a) and building a 2200-sq.-ft. slab-on-grade home. It will have a 10-in.-thick double-stud wall that cantilevers 4 in. beyond the foundation to sit flush with the outside face of the 4-in. foam applied to the edge of the slab. I plan to protect the foundation insulation using painted aluminum flashing nailed to the sheathing and extending down below the backfill. All the wall finishes will lap the flashing. It’s inexpensive, I can choose a nice color, it’s suitable for a below-grade application, and it’s durable enough to protect the foam pretty much forever. Actually, it seems too easy, which makes me immediately wonder—what am I missing?
—Scott K., via greenbuildingadvisor.com
Martin Holladay: You will need some type of screening between the furring strips to prevent insects and rodents from entering your rainscreen gap. But except for that, your planned approach doesn’t raise any
If you go this route, be sure to choose a heavy gauge of metal flashing for increased durability. The possible disadvantages are that the metal flashing can get dinged up over the years, leaving the bottom of your house looking like a metal entry door that someone has tried to kick in. The thicker the flashing, the less likely it will look dinged up.
There is no perfect answer to the question of how to protect above-grade exterior foam. While metal flashing is a good solution, it’s not the only possibility. Here’s a list of materials that can be used to protect the above-grade portion of the exterior rigid foam used to insulate a foundation wall:
• A brush-applied cementitious coating or cementitious stucco (for example, Styro Industries Brush On ST), with or without metal lath
• A trowel-applied cementitious coating that includes chopped fiberglass (for example, Quikrete Foam Coating or other surface-bonding cement)
• An acrylic coating like Styro Industries FlexCoat or Styro Industries Tuff II
• An exterior insulation and finish system, or EIFS (synthetic stucco)
• Cement backerboard, with or without a layer of stucco
• Pressure-treated plywood
• A fiberglass panel like GroundBreaker from Nudo Products
• Styro Industries FP Ultra Lite panels (XPS with a coating of mineral granules adhered to one side)
• Protecto Wrap Protecto Bond (a flexible peel-and-stick membrane with a textured, gritty coating)
• ProGuard Concrete Insulated Sheathing