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GSBN:RE: 5 perms / no ventilation; and top-of-wall

Mr. Korn,

I am honored that you would include me among those who might have some
insights into your questions.  However, I am only answering so that you
know I am not ignoring you.  While I would weigh in if the questions were
about the thermal properties, I clearly can only defer to Mssrs. Straube
and King on matters of moisture, permeance, and vapor barriers.

Nehemiah Stone

> [Original Message]
> From: Mark Piepkorn duckchow@...
> To: jfstraube@...; ecobruce@...;
stoneandstraw@...; GSBN@...
> Date: 3/7/2007 12:54:55 PM
> Subject: 5 perms / no ventilation; and top-of-wall
> Ahoy Mr Straube,
>          I'm afraid I won't be seeing you in Boston this year, as I
> already used my 12-month exposure allotment at the Ontario shindig.
> Luckily for me, and perhaps unfortunately for you, email doesn't
> count. With your indulgence:
>          We had a meeting of northeast-US natural building
> practitioners over the weekend, and there was some stumbling over
> your statement in Design of Straw Bale Buildings that the interior
> finish in climates with four months or more of cold shouldn't be more
> than 5 or 6 perms. There was good discussion about the need to
> control moisture through ventilation (in which case you state that 10
> perms is fine), but there are any number of strawbale houses that
> have gone up around here in recent years that don't have any active
> ventilation, presumably are reasonably tight ("we don' need no
> steenking blower door"), and have interior finishes with perm ratings
> running 9 or better, sometimes with less permeable exteriors. Now
> there's a builder or two wondering if they should be a worried about
> callbacks or worse due to interstitial vapor migration. Er,
> diffusion. (There was a moisture failure in the region last year,
> apparently almost certainly interior-sourced; the owner pulled the
> walls out before any forensics could happen. I don't have the full
> story - I suspect that you might have more details than I do - but
> there's a feeling that something somewhere somehow was fishy.)
>          My gut reaction is that if the house has a good interior
> envelope, and if the occupants' general habits are pretty sane, try
> not to lose sleep. Maybe offer to put in an inexpensive-but-nice
> Panasonic fan on a timer if it will help everyone feel safer. And
> design in ventilation going forward. In the meantime, I'm wondering
> if you'd care to share any thoughts about the approximate level of
> freakout that people might reasonably allow themselves.
>          I've suddenly decided to CC Bruce and Nehemiah, though this
> second question is also right up your alley.
>          Illustration 10C in the Details and Design chapter says that
> vapor in the wall assembly tends to rise, and can condense on the
> bottom of a roof bearing assembly. We know that air movement within
> the wall assembly can be a bad actor, as implied by Mister Stone in
> 7.3; we know (through modeling) what happens to the thermal
> resistance of a bale wall when there are gaps between the cladding
> and the bales. Given the cold winters here, it seems that having an
> air-permeable top-seal could reduce the thermal resistance of the
> assembly, possibly by a fair amount, especially if there's a leaky
> interior envelope. And piggybacking on the air movement, any moisture
> in the assembly will dump into places that likely have less capacity
> to deal with it. (With that there's material-dependency: air and/or
> vapor might flow more readily from the top of a bale wall into
> low-density cellulose than high-density, for instance.)
>          So, assuming that an air-permeable top seal is not a good
> idea in this climate... how vapor permeable should the air barrier
> across the top of the wall be? When there's no box beam or other
> full-width plate creating a condensation nightmare, people generally
> seem to lean toward smearing on some of whatever they're plastering with.
>          I've suddenly decided to CC GSBN, feeling like RT could have
> hisself a field day with this.
> Mark Piepkorn
> www.potkettleblack.com
> Captain, if I ever come to grief by forgetting
> your advice, I hope my last moments will
> be cheered by the sound of your beloved
> voice breathing into my ear the blessed
> words, "I told you so."
>   - Ambrose Bierce