Foamboard vs. Plywood Sheathing - House Exterior Walls
Several years ago my grandparents sold the extra lot adjacent to the one their house is built on, to a local builder. My grandpa (a carpenter) observed how the house was constructed and tsked and shook his head. "You can break into that house with a knife," he said, referring to the Styrofoam sheathing covered by vinyl siding.
I agreed with him, that was no good way to build a house.
So you think that in buying a home built around that same era, I would have considered checking the construction of the walls. Yet it wasn't until we opened up the drywall and pulled off the vinyl siding of this house as we are remodeling the first floor, that we discovered our house's exterior is sheathed in foam board, reinforced only by some diagonal metal strapping at either end.
I guess this was a common enough construction method, though I've noticed that all the new construction being done around here includes plywood or particle board sheathing with Tyvec house wrap or similar around it. I did a little research and found an article that verified my suspicions that plywood provides better structural integrity to a home than Styrofoam insulation panels. This is important, because our house shakes and shudders when a train goes by... and the nearest train tracks are over 1/4 mile away.
I also came across mentions of Styrofoam insulation when researching carpenter ants (another plague we've suffered) and it turns out insects of many varieties love tunneling into the foam insulation and making a home between it and the walls of your house.
Add to this that our handymen found a recently deceased mouse above the drywall ceiling we had them take down in the basement room underneath the location of the new kitchen. We've been dealing with mice in the basement since we moved here, and in spite of a few patches in the foundation and packets of poison bait above the suspended ceiling in the main part of the basement, this was evidence that the mice are still here.
So I called a wildlife control guy, who came to do a $45 mouse proofing inspection. The first thing he asked me is, "Is your house sheathed in that pink foam board, or is there plywood under that?"
I told him it was just the foam board. "That's bad isn't it," I asked.
"It can be," he confirmed. He said if it was done right it was okay, but he'd let me know.
In the end, besides a multitude of other holes and vents and such that needed mouse proofing, he said that the foam board was installed correctly with some type of plate at the bottom of the walls to keep animals from coming through, however if the mice went under this they could easily chew through the foam in the basement and up into the walls of the house. He found holes in the foam around the basement windows. He suggested that we consider replacing the foam board sheathing with plywood in the areas we had exposed for remodeling, and when we re-side the rest of the house in the future, to replace the rest of the sheathing with plywood (and house wrap) as well.
So there you have my three good reasons to side a house in plywood rather than foam board:
1) Better structural integrity
2) Less inviting to insects
3) Less easily penetrated by mice
