Foam sheathing: inside or out?

Bruce Harley, author of Build Like a Pro: Insulate and Weatherize, replies: If you build a house this way, you probably will end up with a fine wall system, but the reflective barrier and airspace do almost nothing for you once you have a decent amount of conventional insulation. Radiant-heat transfer depends on a big temperature difference, and unfortunately, there will be almost no temperature difference between the inside surface of the foil-faced insulation and the back side of the drywall in your proposed assembly. No temperature difference, no radiant-heat transfer, no benefit.

Exterior foam is better. You can cover a wall with fewer obstructions. You also get better durability with exterior foam because the framing is always warm, which means less condensation inside the framing. Photo by: Daniel S. Morrison

I prefer walls with continuous rigid foam on the exterior for a couple of reasons. First, it’s easier to make continuous. Although exterior details around porch roofs, entryways, and garages can be tricky, interior foam has to be worked around every partition wall and floor joist that intersects the exterior wall; the interruptions diminish its value.

Second, although either approach reduces the risk of moisture damage from interior water vapor in all seasons and climates, only exterior foam also can enhance the rain-shedding and drying potential of the wall. Interior foam keeps the framing cooler for most of the year (particularly in cold, wet climates), which reduces the drying potential and requires perfect flashing and siding details. Exterior foam keeps the framing warm and dry, which makes the wall more tolerant to occasional leaks because the wall can dry.

My final suggestion for your walls: Instead of a 2x6 wall with 1-in. foam, build a 2x4 wall with 2-in. exterior foam and unfaced R-13 batts. You’ll end up with a slightly higher net R-value; you’ll buy less wood; and you will better control the condensing temperature of the exterior sheathing, which is important in a cold climate.

You don’t need an interior vapor retarder, and depending on the siding, trim, and furring strips, you might be able to use standard window and door extension jambs designed for 2x6 walls. (If not, you might be able to make it work with 1.5-in. rigid foam.)

By the way, consider using sprayed-foam insulation for the insulated rafter areas. It is more moisture-tolerant than fiber insulation, and it does not require venting (according to section 806.4 of the 2006 IRC), which can be difficult to do with some roof shapes.