The advantage of a slow moving project is that you don’t rush into decisions you may regret later on. This is certainly true for the decisions on insulation, which are rather significant.
I have researched and thought about the insulation for one and a half years and received numerous helpful comments and references. What once was a very exotic subject has become very familiar.
Insulation or outsulation?
We started with the dream of insulating the masonry shell from the outside (outsulation), leave the brick work exposed on the inside and use it as thermal mass. Whenever I advocated that idea, I was promptly discourage from doing so.
That said, it appears to be the favored option of some green industry professionals. The principle reasoning is that it protects the masonry structure from freeze-thaw cycles, which can be detrimental if the brick or stone work is saturated.
I bet there are existing masonry structures out there that lend themselves to outsulation, that have enough dimensional room around the building to accommodate the extra layers and have fire code requirements that would allow it.
We are not so lucky with the west side of our building partially located right along the property line and ornate architectural features on the front elevation that I will not dare to cover up.
So – insulation it is.
The two things I could not shake, though, were the issues that a) SPF is a petroleum based product, and b) the question of end-of-life use—or, what happens to the foam once it gets ripped out?
As it turns out, spray foam scraps don’t have to go into a landfill, but can be turned back into resins and reused in another SPF application. I hope that this is not one of those “too good to be true” stories.
With regard to the petroleum based content, the industry is now also marketing foam products with renewable resource content derived from soy or castor beans. But even with the addition of renewable resources, SPF remains largely a petroleum based product. The other remaining question is how sustainable soy farming is.
How about a high recycled content material?
An idea emerged while we were installing the perimeter wall framing. Rather than spraying the entire six to seven inch interior wall cross section with SPF, we could only use the foam in the three inch gap between the masonry wall and the framing. Then we could fill the three and a half inches of framing with a batt system.
There are several products that have a high recycled content. The first one I investigated was a cotton batt product with 90% recycled content. The recycled cotton batts have good thermal resistance and sound absorbing properties, plus they are more economical than SPF.
What worried me a little is that cotton is good at absorbing moisture (think of the good old sweatshirt). Moisture content in the batts may reduce their thermal resistance and begs the question about mold growth.
To reduce the risk of mold the batts are treated with a borate solution (boric acid). I am not sure how I feel about introducing yet another chemical into our indoor environment.
At one point I listened to a GreenBuildingAdvisor.com podcast in which rock wool was mentioned as an insulation material. The one thing that really stuck with me is that rock wool – made out of rocks and slag – won’t burn. That did sound awfully attractive and I researched this some more.
Rock wool insulation is manufactured in batts of all sort of sizes, has good thermal resistance and is easy to handle and cut. It is not treated with boric acid, is manufactured with a high recycled content and even more economical then the cotton batts. Bingo!
Updating our insulation plans
We need an effective air seal to meet our energy goals. SPF is the most effective product to achieve this objective. Plus, SPF also acts as the vapor and thermal control layer in the wall assembly. We pay more, but we also get more.
To reduce the environmental impact and drive cost down, we fill the wall cavity space in the framed section with rock wool batts.
This insulation assembly should give us an R-value of 26 to 28.