[GSBN] Straw bale at high altitudes (and snow against bale walls)
Derek Stearns Roff
derek at unm.edu
Fri Jul 11 19:34:55 CDT 2014
No disagreement with what Martin wrote, but in Misha’s specific case, 4,500’ in California is not Canada, and not likely to fit the profile of the cold climate zone. John Straube and others have argued that vapor retarders have caused more problems than they have solved, when applied to buildings located outside of strongly cold climates. Martin, do you want to say more about vapor retarders in medium elevation California?
On Jul 11, 2014, at 4:57 PM, martin hammer <mfhammer at pacbell.net<mailto:mfhammer at pacbell.net>> wrote:
In terms of moisture concerns for strawbale walls, elevation in itself is not relevant. What matters is climate (precipitation, temperature, humidity, wind), and how it relates to interior “climate”. Higher elevations do mean lower relative temperatures for the region (in all seasons, 3-5 degrees F per 1000’), so indirectly it matters. But very low outdoor temperatures beg for strawbale walls (just ask our Canadian friends!). As long as you employ normal good practices to prevent relatively warm-moist interior air from condensing in the strawbale wall, by using a vapor retarder on the inside face of exterior walls (the IRC requires Class III vapor retarder (between 1.0 and 10 perms) in cold climate zones) and by sealing penetrations on the inside face of these walls.
I agree with what everyone else has said re: the foundation issues.
Also the concern was raised about snow sitting against the bale walls. I’ve never been convinced one way or the other whether this is a real or an imagined problem. Thoughts / questions include:
* When the snow-wall interface is below freezing, presumably nothing detrimental occurs.
* When snow at the snow-wall interface melts (because all snow is melting or because the snow at the interface melts because the wall surface is warm enough) is a space created that allows drying of the plaster, and thus no harm?
* When the snow at the snow-wall interface melts, does it saturate the plaster, which then wets adjacent straw causing degradation, and/or does moisture in the plaster sometimes freeze and damage the plaster?
Does anyone have experience with snow against bales walls that answers these questions?
Martin Hammer, Architect
1348 Hopkins St.
Berkeley, CA 94702
On 7/11/14 8:40 AM, "Misha Rauchwerger" <misha.rauchwerger at gmail.com<x-msg://firstname.lastname@example.org>> wrote:
I have a client that wants to build a straw bale house at 4000 feet in the Sierra Foothills. They have been getting conflicting information about the wisdom of building with straw at that elevation. In particular there is the concern about the effects of moisture in the wetter months, and possible condensation inside the walls. They know about the need for big eaves, and permeable plasters, but have been swayed against the idea from a local green architect in town. Please direct me to any research, or anecdotal evidence to support straw bale construction under these conditions, or maybe there is valid concern. I have only built in the lower/dryer elevations on flatter sites.
They also share these concerns:
Their lot is sloped, so they would likely have to build a full walk-out basement on the lower level, and the living space on the upper level. This means that the full lower level is built of concrete (probably Faswall or Durisol); will they would have to build out the lower walls to match the width of the straw bales? How is this disparity in wall thicknesses usually resolved with the least cost/impact?
- With a walk-out basement, is it possible/reasonable to do a stepped-foundation on the lower level to minimize the amount of concrete used? Or does the mixed use of concrete and post/beam and straw bale construction create unreasonable headaches in the building process?
Thanks everyone for your comments,
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