Two Carbon Smart Ideas in the AtticJune 2021
by Rachel White
A wide-open attic with no storage and no mechanical equipment. I can’t remember the last time we worked in an attic like this. Most of the attics Byggmeister retrofits already house an air handler. Or they will soon, as the majority of our projects include at least partial electrification, and attic space is prime real estate for ducted heat pumps. Most attics we work on also serve some storage function.
For a long time, we assumed that the only viable approach to attics like these was to bring them indoors, either by insulating the underside of the roof with spray foam or by adding board insulation on top of the roof sheathing.
But a couple of years ago, as we began to prioritize embodied carbon, we revisited this assumption. Maybe we could bring attics indoors using less foam. Maybe we could bring just enough attic space indoors to house mechanicals and use the remainder for carbon storing cellulose. And so, we developed two new, carbon smarter attic insulation strategies.
The first, which we call the low foam approach, is appropriate when there are significant attic storage needs. We build down the rafters; install 3” or R-20 closed cell foam against the underside of the roof sheathing for condensation control (in climate zone 5; See Table R806.5 of the 2015 IRC for other zones); install a smart membrane; and then dense pack the built-down rafters with cellulose for a total of R-49.
The second, which we call the head house approach, is appropriate when there are minimal attic storage needs. We build a small room or “head house” for the air handler; install the air barrier along the floor and then up and over the head house; install and seal the air handler and ductwork; and then insulate.
Assuming that we can ascribe carbon storage potential to the cellulose, our calculations indicate that the first strategy would be close to carbon neutral, with the carbon storage potential of cellulose mostly offsetting the emissions from spray foam. Our calculations indicate that the second strategy would be carbon positive, turning the attic into a carbon sink.
There are two reasons why we have focused on interior rather than exterior insulation approaches. First, outsulation is much more expensive and only makes financial sense if the roof is also due for replacement. Outsulation with polyisocyanurate is roughly 3.5 times the cost of spray foam, while outsulation with wood fiber board is roughly 4.5 times the cost. Second, outsulation can have significant adverse architectural impacts.
With the low-foam strategy, the biggest challenge we’ve faced is increased cost: it’s roughly 1.5 times more expensive than all spray foam. With the head house strategy, implementation has been trickier. It has taken some trial and error to get this right in the field.
One reason it’s considered best practice to put the air and thermal boundary at the roof is that it minimizes plane changes. With the head house approach, this boundary needs to transition from horizontal, to vertical, and then back to horizontal again. Each plane changes introduces possible discontinuities in the air barrier. The first time we built a head house, we didn’t fully think through sequencing and ended up spending a lot of time fixing the air barrier.
The head house approach also puts more pressure on duct sealing. While leaky ducts are never a good idea, outside the thermal boundary, they lead to higher energy use and can contribute to condensation and ice damming. With the head house approach, we are finding that we need to spend significantly more time on HVAC quality control.
We’ve also discovered that rebalancing the HVAC system is a pain when the dampers are buried in cellulose. It’s not so much of a pain that we wouldn’t recommend or follow this approach, but it is definitely something to be aware of.
It’s also worth emphasizing that the head house is most practical when storage needs are minimal. While it’s possible to build a larger head house to accommodate more stuff, a larger head house is more expensive. Our first approach with clients who have a lot of stuff is to gently suggest some purging. If that doesn’t work, our default is now the low-foam, build-down approach. If we can’t eliminate carbon-intensive spray foam, at least we can minimize it.
This is the second in a series of blog posts on reducing embodied carbon in residential retrofits. The first post is Rethinking Our Approach to Insulation. The third is Finished Attics: A Lower-Carbon and Lower-Cost Retrofit Approach