[GSBN] Straw bale at high altitudes (and snow against bale walls)

martin hammer mfhammer at pacbell.net
Sat Jul 12 13:40:49 CDT 2014

Derek, thanks for pointing this out and asking for clarification.

This could get complicated fast, but I¹ll do my best.

For wood frame construction the International Residential Code (IRC)
requires a Class I or II vapor retarder (less than or equal to 1 perm) on
the interior side of exterior walls in Climate Zones 5,6,7,8 and Marine 4
(as it defines them). The higher the Climate Zone number, the colder the
winter climate.  There are also IRC climate classifications that relate to
moisture (A,B,C = Moist, Dry, Marine) that are important, but which I won¹t
address here.  And forgive me if I use the IRC as a measure of what is right
regarding this subject, but I think it does a good job categorizing climates
(per each county in the US) and tying various requirements to those Climate

In some wall venting or insulation situations the IRC allows a Class III
vapor barrier (between 1.0 and 10 perms), instead of Class I or II, to
satisfy this requirement.  The IRC Strawbale Construction appendix,
allows/requires a Class III vapor barrier on the inside face of exterior SB
walls for these same Climate Zones. (This requirement, like everything else
in the appendix, is open to debate, but that it is where it currently

To put a face on these Climate Zones, the interior US States that border
Canada are all in Zones 6 and/or 7. Canada, except ocean coastal areas,
would presumably be Climate Zones 6,7,8. California ranges from Zones 2-6.
It¹s likely that at 4500¹, Misha¹s project is in Climate Zone 4, which means
the IRC would not require the vapor barrier I mentioned.  It¹s possible that
in this situation, as Derek and John Straube suggest, installing such a
vapor retarder could cause problems. However, a vapor retarder in the upper
half of the perm range for a Class III vapor retarder (say between 5 and 10
perms) would probably be safe, and might have benefit reducing or
eliminating any risk of condensation in the walls during the coldest months.

One important related issue the IRC does not address, is possible
condensation in exterior walls during hot-humid summer months in these same
Climate Zones if the interior spaces are cooled.  The same vapor retarder it
requires to prevent interior moisture from entering the wall in winter
months (which I believe is an appropriate requirement), could trap moisture
in the wall in summer months in the ³reverse² condition I described.


On 7/11/14 5:34 PM, "Derek Roff" <derek at unm.edu> wrote:

> No disagreement with what Martin wrote, but in Misha¹s specific case, 4,500¹
> 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?
> Derek
> On Jul 11, 2014, at 4:57 PM, martin hammer <mfhammer at pacbell.net> wrote:
>> Re: [GSBN] Straw bale at high altitudes (and snow against bale walls)
>> Hi Misha,
>> 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 ³climate².  Higher elevations do mean
>> lower relative temperatures for the region (in all seasons, 3-5 degrees F per
>> 1000¹), 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.  I¹ve
>> 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?
>> Thanks
>> Martin 
>> Martin Hammer, Architect
>> 1348 Hopkins St.
>> Berkeley, CA  94702
>> 510-525-0525 (office)
>> 510-684-4488 (cell)
>> On 7/11/14 8:40 AM, "Misha Rauchwerger" <misha.rauchwerger at gmail.com
>> <x-msg://57/misha.rauchwerger@gmail.com> > 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,
>>> Misha Rauchwerger
>>> builtinbliss.com <http://builtinbliss.com>  <http://builtinbliss.com
>>> <http://builtinbliss.com/> >
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> Derek Roff
> derek at unm.edu
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