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

Derek Stearns Roff derek at unm.edu
Sat Jul 12 15:24:38 CDT 2014


Thank you, Martin.  I always welcome your knowledge and information, and I’m grateful that you share them.

I’d like to expand a bit on the “reverse condition” which Martin mentions in his last sentence.  But first, a little preamble.  Vapor barriers were postulated as valuable beginning almost a 90 years ago, in the belief that moisture moving from inside a house would condense inside the wall during the winter, and cause problems.  This led to widespread use of polyethylene film vapor barriers, in the 60s and 70s.  We’ve seen a lot of failures in houses with those vapor barriers, and we’ve learned a lot since then.  The vapor retarders that Martin refers to constitute a much more nuanced and effective approach to matching a moisture strategy to a specific building and location.

One of the things I’ve learned in my research is that although the outer portion of insulation is likely to be below freezing on a cold winter night, there may not be any condensation of water vapor in the insulation.  A vapor-open structure can transport moisture from a warm inside space to the outside, sub-freezing air, without ever condensing into liquid water droplets or forming ice crystals.  The chances of condensation within the wall are smaller than our intuition suggests, in most cases.  With no specific vapor retarder, many houses in many climates don’t ever see condensation in the walls.  Risks increase with interior and exterior humidity levels, and with decreasing exterior temperatures.  Correctly chosen and installed vapor retarders are a useful strategy for dealing with winter moisture movement, especially in cold climates.

Winter offers a couple of elements that help protect marginal structures, when there are migrating moisture issues.  Cold air can’t contain much water.  While this is what leads to condensation, it also means that sub-freezing outdoor air is not a source of significant water.  Furthermore, mold will not grow in frozen or very cold water.  In many cases, small amounts of water in building can evaporate as the season warms, before microbes have a chance to become active.

All of this sets up our understanding of the reverse, or summer situation.  Summer air is much warmer.  It can and does hold a large amount of moisture, which can become massive in hot, humid climates.  In the summer, condensation frequently occurs at temperatures that are in the range where molds and other microbes thrive.  If a building is air conditioned, there is a possibility that the inside surface of the wall will be below the dew point/condensation temperature.  If a vapor retarder, or much worse, a vapor barrier exists near the inside surface of the wall, it is possible to condense large amounts of water inside the wall, during the summer.  This is the “reverse condition” that Martin referred to.  Many, many air conditioned buildings have had significant mold and rot problems, because of this summer moisture condensation against an air conditioned surface.

In the case of Misha’s building at 4,500’ in the California Sierra, there is a fair chance that air conditioning will be a feature of the building.  The climate may be dry enough to avoid problems, but some analysis of the summer condensation risk is worthwhile, if air conditioning and a vapor retarder are being considered.

Derek


On Jul 12, 2014, at 11:40 AM, martin hammer <mfhammer at pacbell.net<mailto:mfhammer at pacbell.net>> wrote:

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

This could get complicated fast, but I’ll 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 won’t 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 Zones.

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 stands.)

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.  It’s likely that at 4500’, Misha’s project is in Climate Zone 4, which means the IRC would not require the vapor barrier I mentioned.  It’s 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 “reverse” condition I described.

Martin



On 7/11/14 5:34 PM, "Derek Roff" <derek at unm.edu<x-msg://75/derek@unm.edu>> wrote:

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?

Derek

On Jul 11, 2014, at 4:57 PM, martin hammer <mfhammer at pacbell.net<x-msg://75/mfhammer@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 “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?

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://75/misha.rauchwerger@gmail.com> <x-msg://57/misha.rauchwerger@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/>>  <http://builtinbliss.com<http://builtinbliss.com/> <http://<http:/>builtinbliss.com/<http://builtinbliss.com/>> >




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Derek Roff
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