I started installing the foam insulation on the exterior walls this past weekend. I took a couple pictures this morning but it was difficult to get good shots. The glare from the sun reflecting off the foil facing was blinding!
There will be two and a half inches of polyisocyanurate foam insulation board on the outside. It’s predominantly in two layers. The first layer is an inch thick, and is tacked to the framing with cap nails. The second layer is an inch and a half thick, and is temporarily held in place with long screws and plywood washers until we install the 3/4″-thick plywood strapping. We use 6″ screws to secure the strapping to the framing.
Later, once all the strapping is installed, we’ll attach the siding to that. We will put corrugated steel siding on the north and south walls, and redwood lap siding on the east and west walls.
So far I’ve got the north wall insulation complete, most of the east wall, and the first layer on the south wall.
The foam insulation is fairly expensive, but there are many advantages of the foam insulation over conventional batts:
- The continuous foam on the exterior eliminates “thermal bridging”, which is what happens when you insulate only between wood studs. Wood studs are terrible insulators.
- The foam has a foil facing which serves as a radiant barrier. Put it in the sun and you can feel the heat reflected back at you. This is invaluable in a hot, desert climate.
- Foam has a higher insulation value per inch than batt insulation. If it’s thick enough, you can eliminate batt insulation entirely and leave the stud bays open, thereby facilitating utility maintenance in the eventual remodel.
- Batt insulation is difficult to install properly, especially if you have pipes or wires in your wall. To get the rated insulation value, batts have to be installed darn near perfectly.
- Foil-faced foam is waterproof and impermeable, so it serves as both a moisture barrier and a vapor barrier. If installed on only one side of a wall, it makes the house more resistant to moisture. Just be careful not to create a “moisture sandwich” by installing impermeable surfaces on both sides of the wall. By choosing to use polyiso on the exterior, I can’t use a waterproofing membrane on the walls of our shower.
- Foam insulation board doesn’t itch or release cancer-causing fibers into the air for you to breathe. It’s extremely easy to cut, handle, and install.
I chose this design because I wanted thin walls, and I wanted to be able to run utilities like pipes and wires in exterior walls. I am able to have a high-performance structure framed with 2X4s instead of 2X6s; in a bathroom, those inches really count.
This next weekend, Caleb and I will finish the foam install and the strapping.