What is the best insulation for this remodel? That is the question.
It is probably one of the most important decisions because not only is insulation expensive, it is a large part of the building envelope and has to meet several criteria to make the house energy efficient and to meet the Energy Star 3 guidelines. Part of the insulation layer should exclude air infiltration too. The only place that air should enter the building is where it is intended to enter. That is the reason for the ERV/HRV ventilation equipment that is recommended.
The spray foam insulation that we used under the slab forms an air barrier and provides about 6 R per inch. But it off-gasses and is flammable so it must be covered by drywall or concrete and it cannot be installed behind existing drywall. This was the best solution for under our concrete floor. We are not exposed to the off gassing and it was sprayed to mold around the wafflemat boxes. So it was a no brainer to choose spray there. While the Insulstar installer was at the house, he did an estimate for the rest of the remodel area for a 3″ depth. He said that is about the limit of what is required anyway and he would not put any more insulation in the cavity as a practical matter.
We not only have to insulate and provide an air barrier to the remodel area, we also may have to improve the insulation in the rear part of the house where we are not removing the drywall. I read that dense packed cellulose although not a complete air barrier, reduces air infiltration almost as well. So I got a quote for sprayed in dense pack for the entire house, not just the remodel area. This would require that we put up the drywall in the remodel area firstm otherwise we would have to pay extra for a membrane to hold the dense pack. The installers drill about 3″ holes in every cavity (with cross bracing in the roof that means extra cavities) and they pump essentially recycled paper treated with fireproofing into the spaces. The quote for the entire house without air sealing (this would have to be done before they pumped) was almost half of the cost of the 3″ of spray foam. Cellulose is considered a “green” material because it is recycled. It only provides about 3.5 r value per inch and the estimator said that the walls that crush the fiberglass would probably end up a bit less than that, given that the fiberglass batts would take up part of the space in the cavity. So the walls would be about R 19 which they probably are now but we seem to have a problem verifying the proper installation of the batts without removing the drywall!
A third option in the remodeled part of the house is to buy recycled polyiso board insulation (from Repurposed Materials where I bought the XPS for the slab perimeter) and cut it to fit after spraying an air barrier layer. This is advertised as very high R value per inch, about 6.5, although that degrades a bit over time to maybe 5.5. But testing has shown that this r value is measured at 72 degrees F. and the r value actually decreases as the outdoor temperature falls. So at zero degrees F. the insulation value can be as low as 17 for 4″ although this increases to 21 at 35 degrees. Building Science concludes that in cold climates the actual performance of polyiso is about 5 r value per inch. They recommend using a layer of insulation that is more cold tolerant in addition to the polyiso.
I would also have to cut the polyiso boards to fit between the rafters and studs. I could use a hot wire cutter and try to cut it outside to reduce exposure to the fumes that melting it would give off. The recycled stuff comes in bundles of 12 or 384 square ft. for $300. I could get some with aluminum sheeting that would improve the air barrier too. The ceiling in the remodel area alone would be about 1250 square ft. so one layer of foam would take about 3 1/3 bundles or just under $1200 to fit with 4″ of insulation. Just about the cheapest but the most labor intensive solution for R 20. Two layers in the 11.5 inch rafters would be double of course for about R40 where R38 is the recommended minimum. To fill the whole ceiling in the remodel area with used polyiso would cost about $3500 plus the labor of cutting and fitting for a conservative R value of 46 to 58 or so. That is not counting the stud walls which would be about 600 extra square ft. of 5.5 inches or another $1500 for R 22 to R 27.5 (at zero degrees) where the minimum is R 20. There is an extra 1″ of polyiso with the reflective coating on the exterior wall for another 4 or 5 R value.
This treatment could only be installed where the drywall has been removed or “wasted”. Adding the drywall to the waste stream, given that there is no way to recycle it, would detract from the LEED objectives for the building. This is a double-edged sword problem and I’m not sure how to solve it. And then there is the required air barrier issue to resolve, especially wherever the drywall is not being removed. The question is whether the existing wall structure will pass Energy Star and qualify the house for LEED.
There is a fourth choice for insulation. Rock wool batts from Roxul. This option is a bit more dense than fiberglass batt and costs about the same with a slightly higher R-value of 23/22. It has a reasonable recycled content and comes in 5 1/2 inch depths either 15 1/2″ or 23″ wide. The online calculator estimate is for 22 of the narrow batts for the walls and I added 68 for the ceiling for 1250 square ft. in a double layer for approximately $4500 in cost. The total R value would be about 28 in the walls and 46 in the ceiling where the drywall is being removed.
I am considering sheathing the gutted interior with either XPS or polyiso too. That would bridge the conduction through the rafters and studs, especially where there are no cavities to fill with insulation. We did this with EPS for the stud walls and ceiling of our home in Indiana and except for the many holes that renters seemed to need for TV cables, the walls have performed well.
There is still the possibility that we would not need to add any insulation to the rear of the house since the fiberglass/polyiso/plastic vapor-air barrier already establishes an R value of 24 or 25 which meets Energy Star guidelines. This area’s rating could be proven with Thermography following RESNET interim guidelines.