We figured out that insulating the building from the inside with spray polyurethane foam (or, in short, spray foam) is the most suitable approach. It avoids potential conflicts with our masonry shell and will help with the moisture management in the brick walls.
The next question is: how much insulation do we need? We can look at it in terms of R-value (thermal resistance) or the depth of the spray foam layer, although both are somewhat proportionate to each other.
Here is what the building code says: R-49 for ceilings, R-19 for exterior walls and R-10 for basement foundations (Chicago Building Code, Chapter 18-13-102.1.1; Building thermal envelope insulation, Table 18-13-402.1.1). The Chicago Green Homes program requires R-52 for ceiling, R-21 for exterior walls and R-15 for basement foundations.
Having our eye on the zero-energy goal, it appears that more insulation or the highest possible thermal resistance is better. But there are limitations we have to wrestle with.
To keep the moisture management of the masonry shell intact, the whole interior wall assembly must have a perm rate of greater than 1. Closed cell spray foam has a better thermal resistance than open cell foam, but also lower perm rates. Limiting the closed cell foam to a 1 inch layer followed by open cell foam should yield the right perm rate and allow for the needed diffusion of water vapor through the wall assembly.
And then there is the space limitation. The building originally had no insulation. There was the outside masonry shell, a ¾ inch furring strip, followed by a ¾ inch wood lath and plaster assembly, which we removed.
Replacing the old 1 ½” interior wall assembly with 1 inch of closed cell foam plus dry wall, would only give us an R-value of around 6.5. Adding more insulation, beyond the 1 inch, would take away from the room size. Here are some scenarios:
My friend David Lemair knew about our effort to balance room size with R-value and pointed me to an article in Fine Homebuilding. I learned that spray polyurethane foam has a point of diminishing returns:
“… you would think that an R-40 wall full of spray foam would perform twice as well as a wall sprayed to R-20 with the same foam, but that is not the case.”
Source: Yagid, Rob; Spray Foam – What Do You Really Know?; Fine Homebuilding, June/July 2009
The article goes on to explain that the increased effectiveness from the R-20 to the R-40 wall is only about 2%. Open cell foam apparently reaches its point of diminishing return at 5 inches, closed cell foam already at 3 or 4 inches. No technical explanation is given to what causes that diminishing return, but I would really like to know!
The puzzle is coming together. We have determined that the closed cell foam must be limited to 1 inch to keep the perm rate greater than 1. It looks like open cell spray foam has its point of diminishing returns at 5 inches. That would give us a 6 inch insulation assembly with an R-value of about 24 that takes 4 ½ to 5 inches away from the room size. This is a good balance between R-value and room size.