posted 6 years ago
Timothy is absolutely correct! I'm lucky to be working in an area with will over sixty permitted straw bale residences--all built over the past thirty years, most of them in the last ten or fifteen years. Since much of my work involves remodeling these older buildings I get to poke around inside the walls, see how the buildings have weathered, ask original and new owners about performance and energy costs, and have learned quite a lot.
First, there are plenty of energy in-efficient straw bale buildings out there! Just having plastered straw bale walls doesn't make the building efficient, even when the bales have been stacked tightly and the plaster applied well. What saves these structures from being uncomfortable is their incredible thermal mass--they may use more energy than necessary to heat and cool than if they had been built the way we know how to today, but the thermal mass delays abrupt temperature swings so they are, despite being full of holes, usually quite comfortable--just not energy efficient. Conversely, there are also plenty of super air tight energy efficient buildings out there as well.
Second, there are things a homeowner can do to improve the efficiency of an inefficient straw bale building, mostly by filling these gaps and cracks. Look at the joints between ceilings and straw bale walls, around windows and doors, and any plumbing or electrical outlets in the straw bale walls. If the gaps are large, you can use foam backer rod, followed by caulk (colored to match, more-or-less, the finish plaster or adjacent materials. If the gaps and cracks are small a bead of caulk or other bits of insulation stuffed into the space can seal it. You can also use gap filling expanding foam in many places. With clay plasters we can often wet and re-tool the area around the gap to close it off once the caulk or insulation has been installed. For electrical boxes, turn off power to the circuit, open the box covers, and if you can see straw through the little holes in the back of the box, air can come and go. If the straw you see is discolored (grey or black), that might be the moisture from the escaping through the wall, deposited in the wall on its way through. Openings like these account for five times more moisture in the wall than the highly vapor permeable plasters, and climates with very cold winters that water vapor can condense as it nears the exterior wall...so best to keep it out of the wall! caulk the holes and install outlet and switch plate cover gaskets.
Third, if building new, think about stopping air from passing through the wall as you build. Straw Bale Building Details: An Illustrated Guide for Design and Construction is loaded with details showing easy things you can do that cost next-to-nothing. One example: a single or double layer of building paper (I use 60 lb. Grade D because that's what I have around for separating wood from lime plaster) folded into the straw-bale wall's ceiling wall joint will be lapped by plaster on the wall, and lapped by sheetrock on the ceiling. Should the plaster shrink a small gap might open there, but it's "backed" by the paper air barrier. We do the same thing where partition walls about the straw bales.
I recommend getting a blower door test once the house is completed. A competently applied interior and exterior plaster is an effective air barrier, as are well flashed and sealed doors and windows. Anywhere plumbing enters or exists the building, or where wood stoves or range exhaust vents exit are potential leak areas. Seal them up!
One way to learn how effectively you have sealed gaps and holes is to have a blower door test done on the house. This test simulates the conditions of a windy day when air might be drawn (forced) out of your living envelop by the difference in air pressure between the inside and outside. The test doesn’t tell you how much mechanical ventilation the house should have—only how much the building envelope leaks. The lower the score, the less leakage.
You may be wondering, but won't an air-tight house be dangerous. Yes, if there isn’t some kind of ventilation system. A lot of building scientists believe that all residences should have automated mechanical ventilation systems that exhaust stale, moist air and bring in fresh, filtered air all while exchanging heat so there’s no net energy loss in the building. Some builders are aiming for a Passive House Air Change per Hour (ACH) of .6 or better, which absolutely requires a heat-recovery ventilation system. But a lot of people just don’t like the idea of living in a nearly sealed envelope, and prefer to open and close windows to keep fresh air flowing in—knowing that some energy is lost out the window. You can decide where on this scale you want to be, but know for certain that you don’t want a really leaky straw bale house!