The choice of which type of home insulation to get is harder than I thought. I figured there would be a clear cut choice for green, energy-efficient houses in 2020. There's not.
Here is a current breakdown of the types of insulation going in to new Single Family Homes in 2018.
It's not surprising to me that fiberglass batts is going in to 52% of new homes. That is what I'd imagine production builders using. It's cheap and been around for a long time.
When I talk to custom home builders in the Austin area, it seems spray foam is what they like to use. But is that currently best practice for green, energy efficient homes?
One good resource is this list of 224 homes meeting the US Department of Energy's list of Zero Energy Ready homes. These are homes "are verified by a qualified third-party and are at least 40%-50% more energy efficient than a typical new home". You can scroll through the list of homes and read about details of how they were built.
I started with homes built in Texas to see if there was a clear trend in the type of insulation used. Not all of them list the type of insulation. There are only 12 homes listed in Texas from 8 builders. One builder with 3 of the homes on the list used Structural Insulated Panels (SIPs) for the walls. The insulation in those is typically foam. Another of the 8 builders used SIPs as well. Here is what the others used for walls where available:
- Preston Haven: 2x6 studs 24" OC, advanced framed, 7⁄8" OSB sheathing; 1" R-6.5 rigid polyiso with taped seams plus R-21 blown fiberglass for R-28 total wall insulation.
- Village Park: Advanced framed wall using 2x6 studs spaced at 24 inches on center instead of 2x4s at 16 inches on center to provide a deeper wall cavity providing more room for insulation. Other advanced framing techniques like 2-stud instead of 3-stud corners, ladder blocking at intersecting walls, insulated rather than solid wood headers over doors and windows, and single rather than double top plates at the tops of walls were employed to further reduce the amount of lumber in the walls, allowing more room for the open-cell spray foam that fills the wall cavities providing both insulation and air sealing. (Note: this description has really good tidbits on exterior sheathing. Come back to that.)
- Smartlux: The mostly 2x6 wood-framed walls are filled with 1.5 inches of closed-cell spray foam that insulates, air seals, and glues the wall components together, providing up to 300% additional racking strength, which can help the home withstand high winds. The remainder of the wall cavity is filled with blown cellulose.
- Green Heights: 2x6 16" o.c. advanced framing, R-15 total: ½" drywall, in cavity - 4" open-cell spray foam, on exterior - 5⁄8" coated OSB house wrap.
- McKinley Project: The 2x4 framed wall cavities were then filled with 3.5 inches of closed-cell spray foam for an R-value of R-20. In the attic, 5.5 inches (R-39) was sprayed over the ceiling deck, completely air sealing and insulating the ceiling. The insulation contractors then went into the crawlspace and sprayed an additional 5.5 inches of closed-cell spray foam along the underside of the floor, insulating it to R-39.
From the diversity of that list, it suggests to me that there is no one way to efficiently insulate a house. It probably matters more how good the installation job is. Installation quality is so important there has been an insulation installation grading system in place since 2006. In that rating system a grade 1 (lowest) installation job will lower the R value by 12%. So installation matters.
So finding a high-quality installer, that is really experienced in the type of insulation they prefer, and one that takes the time to do it right, is the most important point to consider.
My take is that SIP's would probably be the best option given no budget concerns and having a contractor that is experienced with SIP construction. But I think that is the hard part - finding an SIP experienced contractor. Update on SIPs: I've since talked to a few people about their experience and it's mixed results with SIPs. The common experience I heard from the three people I talked to was that they had to make modifications to the panels before installing. There seems to frequently be an issue of them not being cut right, particularly around windows. With the new options available, like insulated sheathing panels from Huber Zip, the extra expense of SIPs are not worth it.
My concern with the spray foams is the environmental impact. They measure this with a Global Warming Potential number. A GWP of 1 is the baseline - it's the equivalent of releasing carbon dioxide into the atmosphere. A lot of the spray foams have a GWP of 1000, which is really high. The good news is there are now low GWP spray foams on the market. Icyene HFO CW and Insulthane Extreme are two examples that have a GWP of 1.
Depending on the contractor I find and price, I will likely be deciding between low GWP closed-cell and open-cell foams, coupled with an insulated sheating panel on the outside.
One of the builders above said 'his company prefers to use closed-cell rather than open-cell foam due to the high humidity in central Texas. The spray foam air seals the structure, keeping humid air out and conditioned air in.'
I am lucky that Matt Risinger lives and builds home in Austin so I can piggy back on what he does. In 2016, said he is big fan of open-cell foam. "I’m a big fan of foam for this climate zone,” says Risinger, principal of Risinger Homes, in Austin, Texas. “It works really well, it’s got some air-sealing abilities, and it’s a total-fill product." In this blog post, Matt says:
I am a fan of using spray polyurethane foam (open-cell above grade; closed-cell below grade or wherever you can afford it). It’s a good choice because it can air-seal some potentially difficult areas, like band joists and rim joists
He also mentions some other tips on 'prefoaming' the house to seal all "breaks in the framing, penetrations, overhangs, mechanicals, and where framing meets the foundation. I initially used lots of caulking for sealing these areas, but now I use flexible flashing tapes where possible, especially where framing meets concrete."