If you have followed the previous posts about the insulation conflicts and moisture management issues, you may ask why not go simple – skip insulation altogether and just have brick wall exposed on the inside. A very tempting thought, isn’t it? It would look pretty good and we could avoid all these problems.
But we also would create a big problem. A three wythe (or 12”) thick brick wall may have a R-4 value. The air film on the wall would give me another R-1, totaling R-5. That is if the brick is dry. If it is wet, these values will drop. A decent window may have a better R-value than the brick wall! We need good insulation, if we want to have a decent shot at our zero-energy goals.
There is quite a variety of materials to pick from, starting with the very common fiberglass batts, the more expensive rigid foam boards, or materials with high recycled content such as blown-in cellulose or recycled cotton batts.
Understanding the limitations and opportunities that come with our masonry shell, and having distilled two key principles, the choice actually narrows to just one material: blown-in foam insulation.
“A low[…] risk approach to interior masonry retrofits that has been used for several years involves spraying an airtight insulating foam directly to the back of the existing masonry [shell].”
Also known as spray polyurethane foam (or in short: spray foam), it would eliminate air gaps and air leakage if applied across the entire interior shell, including the roof. Basically, it would give us an airtight building envelope and act as a moisture barrier (or vapor retarder) helping with the control of incidental rain penetration.
A Building Science publication (Building Science Digest 114) explains spray foam rationales and choice in detail and is worth while reading.
Because spray foam is semi-permeable (a vapor retarder but not a vapor barrier), it will allow moisture in the masonry shell to diffuse to the outside and in. I have to make sure that the perm rate of the entire interior wall assembly is greater than 1 – and we are all set!
There are two kinds of spray polyurethane foams out there:
As the name suggests, each little cell (or bubble) encloses an air pocket, forming a monolithic airtight layer at an R-value of around 6.5 per inch. Most closed cell spray foams have a density of about 2 pounds per cubic foot (pcf) and have a low diffusion or perm rate (around 1 to 2 at 1 inch thickness).
This foam is much lighter at a density of 0.5 pcf and forms more of a web structure. It is still considered airtight if applied at a depth of several inches. The R-value for open cell products hovers around 3.5 per inch. Water vapor can diffuse freely through the material.
The one disadvantage of spray polyurethane foam that is often mentioned is cost. And yes, it is much more expensive than your typical fiberglass batts – closed cell more so than open cell, because it requires more material. Plus, it needs to be installed by a trained professional.
- 6” of fiberglass batts (around R-19): $0.30 to $0.60 per square foot (material only)
- 6”of open cell spray foam (around R-21): around $2.50 per square foot (material and labor)
- 6”of closed cell spray foam (around R-39): $5.00 to $6.00 per square foot (material and labor)
We would pay more – and that is fine – because we will get more. With spray foam, we don’t have to worry about air leakage, condensation and potential mold problems, or diminished R-values. Instead, we get the airtight building envelope we need, and lasting R-values. If we would try to accomplish the same results with cheaper insulation materials, we probably would, in the end, pay as much.
A drawback that I still debate is that spray polyurethane foam is a petroleum based product. The good news is that most spray foams are now VOC (volatile organic compounds) free, using water as their blowing agent. Some products are marketed as green because of some soy based oil content. That overall content is, however, relatively small, plus I am not sure if I would accept soybean farming as a sustainable practice.
Another unanswered question that keeps me pondering has to do with the end-of-life use. There is no known recycling option or second use for this material. If the spray foam ever gets torn out, it is likely to end up in a landfill. The only conciliation I have is that it should serve and maintain its performance for several generations.
More info on spray foam:
Moisture barrier: See references below to vapor barrier and retarder.
Vapor barrier and vapor retarder
Additional resource: Consumer’s Guide to Vapor Barriers at the U.S. Department of Energy