Fifth Wind Farms wanted to create a building that could reach the energy efficiency requirements of the Passive House standard without resorting to the use of foam, mineral wool or other materials with a high carbon footprint. While straw bale buildings can have excellent energy performance, typical straw bale construction does not meet the Passive House standard without the addition of an extra layer of insulation (see our “Straw-Cell” project for a different take on this idea).
The building owner proposed the use of “jumbo bales,” which are produced from the same local straw and by the same low-carbon machinery, but are of dimensions that greatly increase their thermal performance. While typical straw bales are 14″ x 18″ x 32″, the jumbo bales used for this project measure 32″ x 32″ x 60″! At a nominal R-value of 2.0 per inch, that would give a jumbo bale wall a rating in the range of R-60, more than enough to help the building meet any energy efficiency rating.
However, the jumbo bales provide some issues when it comes to window and door openings… with a wall that thick, window sills and returns are extremely deep, creating not just aesthetic concerns but also concerns about air flow in the deeply recessed bays and the likelihood of condensation forming on the windows in cold weather.
Our solution was to form the window sections at a wall depth of 16″ using double stud framing and hempcrete as the infill insulation. This would keep us on track as far as low carbon footprint is concerned, and the hempcrete would be used to create the tapered window returns to meet the full depth of the bale walls. As a bonus, the hempcrete would completely fill any voids at the ends of the jumbo bales.
One issue with using jumbo bales: they weigh over 500 pounds each! We used a boom truck to install them in the building. With the bales in place and the top plate secured over the bales, we then mixed our hempcrete (you can find our recipe here) and tamped it into the framing and around the jumbo bales. The two materials are very complimentary, with the easily-formed hempcrete able to compensate for the uneven ends of the jumbo bales and creating smooth window returns.