StrawCell – 3rd Generation Strawbale Wall

First Published in Green Building Advisor Guest blog – edited by Martin Halloway

Straw-Bale Walls for Northern Climates

A ‘third generation’ straw-bale technique combines a straw-bale interior wall with an exterior stud wall insulated with cellulose

Posted on Nov 17 2014 by ben graham

 

An exterior stud wall provides room for more insulation. Without an exterior stud wall, most straw-bale walls have an R-value of R-33 or less.

The mechanical baler was invented in the 1850s (Reynolds, History of Hay Balers), and it’s been a while now since those folks in the Midwest put up a couple of bale houses. You would think that by now we would have very refined construction techniques for straw-bale construction, given that some of those original buildings are still standing. Well, we are getting there.

Let’s call those first bale houses the first generation. The bale houses that came out of the natural building boom in the Southwest during the 1990s I’m going to call the second generation. This was more of a reinvention (along with cob construction), as there was a big gap between the first and second generation, with little continuity or carryover of development.

From that point on, as straw-bale building has spread across the country and the world, there has been a steady development of technique and skill.

The Northeast gets a lot of rain

When straw-bale construction reached the Northeast, builders quickly realized the climate-specific designs from the Southwest and Midwest were not quite appropriate for northern climates. We soon began to adapt by adding bigger roof overhangs and “air fins” to make the walls tighter and more protected. (“Air fins” were developed as part of the second-generation airtight system; they consist of pieces of an airtight material such as drywall that are installed wherever there is a break in the interior plaster. Typical spots requiring air fins are at posts and around windows or doors. The air fin material should allow plaster to bond to it, and should be installed so that it runs continuously — for example, behind a post — to allow the plaster can act as an uninterrupted air barrier. Air fins can be seen in Images #2 and #3, below.)

The classic timber-framed straw-bale wrap kept us busy here in the Northeast for the first ten years of the millennium.

Then something happened to push straw-bale construction into what I am calling its third generation of design: Builders and designers got caught up in the Passivhaus movement. This spurred greater attention to building performance data, and in northern climates builders wanted more than just a bale’s worth of insulation.

New ideas emerge

One result of these developments was a technique pioneered by Mark Hoberecht of Harvestbuild in Columbia Station, Ohio. After getting his Passivhaus certification, Hoberecht began to combine straw-bale with cellulose-insulated wood-framed walls. This was not the first time that happened, but it was the first time I know of where the technique was marketed as a unique design.

Another development was the preassembled straw-bale panel. These panels are being constructed in controlled environments for better outcomes, with specific design features for connections and airtightness.

Adding a stud wall filled with cellulose

I am going to focus on the straw-bale and cellulose combination that we at New Frameworks have developed after engaging in early conversations with Hoberecht in 2012. We had just completed an energy audit of seven houses that we built between 2008 and 2011.

We used Kickstarter to fund a project aimed at understanding the results of our work in order to develop better straw wall details. The conclusion we came to was that we were making progress on developing second-generation straw walls to perform to our rising standards of energy performance, but that (given the constraints of the design) the curve of returns on investment were too steep. Even with our modifications to window details and air fins, we felt that the whole system needed to be rethought, in order to achieve the higher performance we wanted in an easier manner, as well as to make it more affordable.

As we began to think through the idea of adding a load-bearing wood frame to the exterior of the straw wall, we realized there were a host of beneficial opportunities that we wanted to make sure we capitalized on, based on our insights about what made the second-generation straw bale wall design difficult to build.

  • A coat of plaster could be eliminated. This was a bit of a paradigm shift for some of us here who had always instructed that bales needed to be fully encased in plaster to preserve and ensure optimal hygrothermal performance. One of the big questions was could the bale system maintain integrity with cellulose blown in against the exterior of the bale? And with the elimination of the exterior plaster coat which was definitely the more costly than the interior, we were looking the end of racing to complete exterior plasters for cold weather.
  • Bales could come down to the interior finish floor and we could eliminate the 18” pony wall needed to keep bales off the ground and protected from rain. By placing the stud frame wall to the exterior, the bales were now completely inside the structural shell and do not need to be protected from weather, though they still need to be kept dry.
  • There were many more places to secure the bale wall to the structural frame than the typical timber-frame structure.
  • Windows and doors are installed just like any other stud frame structure.
  • By incorporating interior strapping to sandwich the bales to the stud frame, we could cut the bale strings and eliminate the process of squaring bales and stuffing cracks.
  • We could easily add more insulation to achieve our increasing standards for superinsulated structures and make this wall system a candidate for Passivhaus designs.
  • Homeowners could choose more conventional wood siding with a rainscreen and a water-resistive barrier (WRB). You could even place your air barrier to the exterior if you wanted.

At this point we knew we were working with a whole new generation of the classic straw-bale wall. With a few projects under our belt and a blower door test result of 0.84 ach50 on a 1,200-square-foot addition — a project where we were aiming for 1.5 ach50 — we are finding it hard to imagine going back.

Add your comment:

Thanks very much for leaving a comment.