Adding to existing foam insulation
While getting quotes to insulate our standalone, 2 story above grade garage (with studio on top), we thought we’d take the opportunity to also increase the insulation in the roof of our house. It’s located in VT.
The house was a circa 1960 cape in mostly original condition with full shed dormer along part of the back. A new System 2000 had been installed, but the rest of the house needed new windows, siding and roofing.
As part of a 2011 gut/remodel all the exterior walls – basement, 1st and 2nd floor and roof were insulated with open cell spray foam. The house has 2×6 walls, so R-19+ in the walls, and 8″ rafters, so R-28+ in the roof where the foam also covers the rafters. A Fantech ERV was also installed at the same time with the unit in the attic. For the time period, this all seemed appropriate and fortunately we have not had any issues with excess moisture.
We’d like to increase the R-value in the roof and it seems the only option is to add another coat of open cell foam. One recommendation is to add 4″ for an additional r-16, bringing the roof up to r-44. Adding closed cell seems out of the question as we’d be laying a vapor barrier on top of the open cell which at any point in time could have vapor passing through it. We’d essentially be trapping moisture if I understand the mechanisms correctly and that could lead to potential sheathing damage. I understand that building products and building science strategies and recommendations continue to evolve over time based on study and testing. I’d like to determine what’s the best approach to move forward (or not) in our efforts to increase the roof insulation.
I had been brushing up on the latest foam conundrums, reading the experts, and foam manufacturer websites to prepare for discussions with installers about the garage. Surprisingly, in the past week I have been told many contradictory things about open/closed cell foam in addition to getting different recommendations about the amounts of foam to install. I have also found that quite a few installers have abandoned working with open cell altogether.
For our upcoming garage project, we had settled on:
– a combination of 2″ closed cell flashing under the roof for vapor/air barrier followed by open cell for r-49
– open cell in the exterior walls for r-22
– 2″ closed cell on the garage ceiling/studio floor for R-14 so we get the benefit of both an air barrier and vapor barrier preventing garage odors from migrating to the living space above.
– 2″ closed cell on top of 5′ of exposed foundation along the one wall that’s partially built into the hillside.
So 2 questions:
1. What’s the best thing to do to improve the R-value in the house roof?
2. Does the insulation approach to the garage look reasonable? Do I need to go to an all closed cell approach?
From reading the building scientists and experts, there seems to be place for, and benefits to, both open and closed cell foam as long as it’s installed correctly and care is taken to provide a vapor barrier and ventilation (and it’s less expensive than all closed cell).
But in the field right now, it seems like open cell has become the boogie man, and installers just want to work with closed cell. I’ve also been getting recommendations to forgo code in the roof due to the diminishing returns as you go above R- 35 to R-38 where the incremental cost to go to R-49 is not worth the additional investment. It does seem to be a valid argument, but I’d like to stick to code especially where the upstairs will be living space and it should be comfortable.
We plan to keep the garage at 40+ during the winter and higher when working there. And eventually heat and finish the studio with a bath and kitchenette. We also plan on putting in spot HRVs in both the garage and studio.
Hopefully we can find one company that is still using both open and closed cell foam. Otherwise it’s just cost inefficient to bring out 2 different installers to apply the 2 different foam types.
Thanks in advance for reading and providing any insights/recommendations.
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