A Case of Shingles

Gosh, old houses are so charming.

The outside of the house is covered in shingles, as seen here. But the singles don't cover the outside of the house. They are the outside of the house.

There is no exterior wall surface behind the shingles. They're it. They're nailed directly to narrow horizontal laths that are, in turn, nailed to the studs. On the other side of the stud is the interior wall, generally lath and plaster.

There's no insulation in the walls, either. Just shingles on the outside and plaster on the inside, with a few inches of air in between. This really came home to me, as it were, when I was demolishing the walls in the bathroom. Once you take down the drywall you're looking at the back side of the shingles, and if there are any tiny gaps you see daylight.

So... the bathroom is getting insulated. The problem is that old houses like this are designed to "breathe" by allowing moisture to pass through the walls. Insulate incorrectly and you'll trap all that moisture inside the walls, where it will rot the timber. The trick is to prevent hot/cold airflow while still allowing moisture to pass through.

Pink fiberglass insulation is exactly the wrong stuff to use. It acts like a giant sponge, trapping moisture inside the walls. Instead, we'll be using expanded polystyrene (EPS), which looks like big chunks of white styrofoam with shiny foil on one side. If we install it with the foil side facing out it should reflect a lot of the sun's heat. And in this room, with the sloped roof/ceiling, that should help a lot.