[Strawbale] 5 perms / no ventilation; and top-of-wall

[Mark Piepkorn]

A series of exchanges (long even after editing) about wall 
permeability and top-of-wall details. Additional input is most 
welcome. It all ends with another question.

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From: Mark Piepkorn

	We had a meeting of northeast-US natural building practitioners, and 
there was some stumbling over the 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 the book states that 10 perms is fine), but there are any 
number of strawbale houses that have gone up recent years that don't 
have any active ventilation, presumably are reasonably tight, and 
have interior finishes with perm ratings running 9 or better, 
sometimes with less permeable exteriors.

	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 over it. Maybe 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, we're wondering about the 
approximate level of freakout that people might reasonably allow themselves.

	Also, 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. 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.

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


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From: Rene Dalmeijer

The general practice in Scandinavian countries is to have less 
permeable interior finishes than outer. Meaning for instance that 
wooden window frames tend to have less coats of paint or thinner on 
the outer surfaces than inner. Another practice is that air barriers 
including caulking when made are on the inner side of the structure.

The Scandinavian practice is boiling over to the Netherlands and I 
suppose many other Northern European countries. I have also seen a 
very applaudable practice at some German strawbale buildings of 
taping all seams that intersect with interior plaster surfaces.

Besides measures regarding air tightness, avoiding cold bridges is 
also of great importance. The top wall plate has serious potential 
for creating a cold bridge. It would be good practice to apply an 
insulating facia to the outer surface of the wall plate before 
plastering. A good material would be something like pavatherm which 
is a woodfibre insulating board, Lambda .045 (metric value). An added 
bonus of this type of board is that plasters adhere readily to its surface.

Should the builders get ulcers laying awake at night imagining 
moisture disasters in the making? Maybe, it depends on the diurnal 
temperature swing. If it is small, ie almost steady state, they could 
be in for trouble. If the swing is substantial (5-10 degrees C) I 
don't expect may problems. The other factor is again if there are 
cold bridges in the structure. Problems, if they occur, will first 
appear in the vicinity of the cold bridges. They could function as an 
early warning system.


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From: Mark Piepkorn

         The diurnal difference is something I think none of us even 
considered. Even during the coldest times, most of this region will 
generally see close to a 10C or better daily spread. I'll pass this 
info back along the line, with props to you.

Mark Piepkorn
www.potkettleblack.com

Most people would succeed in small things,
if they were not troubled with great ambitions.
   - Henry Wadsworth Longfellow


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From: John Straube

Bruce, being the super editor he is, prodded me to provide 
quantitative recommendations in the book. So, I did. These are pretty 
general however and the permeance of the interior layer will depend 
on numerous factors, the primary ones are:
    1. how cold is the climate and how much sun does the wall see
    2. the interior temperature and RH during the winter months,
    3. the exterior plaster skin permeance, and
    4. the rain wetting and drying capacity needed.

1. The recommendation for 5 or 6 perms applies to the North East. 
However, in the Green Mountains, 5 perms is likely necessary, whereas 
a house south of Boston (much warmer) could work fine with 10 perms.

2. if the house is operated at 65-68 F and 25-30% RH during cold 
(less than 30 F) weather, you can pretty much do what ever you want 
with the interior permeance. If the conditions are 72-75 and 35-40% 
things are very different. Just to let you see how important this 
point is: 65 F and 30% RH has a dewpoint temperature of 32 F; 75 F 
and 40% RH the dewpoint is 49 F (the temp at which condensation will 
occur, and a direct measure of the air's moisture content). The 
higher temp/humidity air contains almost 30% more vapor. But, given 
the physics of diffusion through a wall, this means that the 
likliehood of wetting the back of the exterior plaster/straw 
interface will be about 4 times higher.

3. The higher the exterior plaster's permeance, the less the interior 
matters. The Scandinavian values of permeance ratios are another 
attempt to simplify advice. The ratios they use (5:1 or 10:1) are 
valid for layers that have low permeance, such as polyethylene on the 
interior (0.1 perm) and thus 0.5 to 1 perm on the outside. When the 
permeances are higher this ratio does not work as well. For example, 
with a painted drywall (say 5 perm) interior then a 10 perm exterior 
is usually fine, even though the ratio is just 2:1.  However, it is 
critical NOT to apply Scandinavian rules to much of the North East. 
Except for coastal Maine and similar climates, the rest of the NE 
actually gets warm humid summer weather, in which case the vapor flow 
reverses.  This is not something the Danes or Swedes understand, and 
applying their rules in, say, Boston can result in disaster.

4.  The reason we want a high permeance of the interior skin is to 
allow for inward drying during sunny days and after rain in summer. 
If you don't have much rain to dry, or if that rain can dry to the 
exterior, the interior permeance can be much lower with success. This 
is certainly the experience with cement plasters (which have much 
lower permeances). They only seem to fail when [a word seems to have 
been omitted here; probably "water"] gets in somehow.

Given all these variables, and the fact that each can vary by a 
factor of 2 or 3 or more, changing the interior permeance from 5 to 9 
perms cannot be seen as likely to result in failure.  The 5 or 6 perm 
recommendation was a pretty conservative and pretty general 
recommendation only. The reason for up to 10 perms is that with 
planned and controlled ventilation I know that the interior RH levels 
will be controlled, and the difference in interior conditions is dominant.

I do not, however, believe the contention that SB homes are 
inherently airtight. The few blower door tests I have seen have shown 
that some are tight, and some are loose. Depends on the details, not 
the stucco wall!!!! Realise that drywall which lines most wood framed 
homes allows no more airflow through than cement-lime plaster that 
lines SB homes. The reason both leak are joints at rim joists, around 
windows, at baseboard, at roof intersections and penetrations, etc. 
These joints exist in both types of homes and so leakage varies in a 
similar manner. In fact, SB walls are often built with pretty air 
leaky windows and doors, and so DO have more than normal accidental 
and unplanned ventilation.

So, I would not freak out in the least. I would try hard to get the 
outer permeance greater than the interior (since during winter this 
is the predominant drying direction) but I would try harder to 
control the interior RH (since this is the source of both diffusion 
and air leakage concerns).

I would be very concerned (meaning something like a greater than 10% 
chance of a serious failure) if the exterior skin was cement stucco 
(low perm) the interior was gypsum (high permeance, over 15 perms) 
and the interior was occupied by numerous people, in a cold place 
(e.g, Burlington Vermont) with no mechanical ventilation.  If this 
extreme case were already built, I would go back at about this time 
of year and measure the interior RH over several days and cut some 
cores into the concrete plaster to measure the moisture content of 
the plaster and straw.  If the RH was low, and the MC reasonable, I 
would provide a written direction to the occupants that they need to 
continue to control interior RH and then go home and sleep very well. 
If I did see high interior RH problems and moderate to high MC, I 
would install an exhaust fan and wire it up for continuous 
operation.  If the RH was not too high, and MC was high, I would 
first check for rain, and then add two layers of good latex paint to 
the interior. Then sleep less well and come back next winter.

RE: The Detail. The top seal of the wall needs to be an air barrier, 
and not a vapor barrier. This air barrier can best be formed by straw 
reinforced earth, or a weak mixture of thick lime plaster (applied in 
several layers to avoid cracking). This not only stops air movement, 
it accepts and redistributes any short term condensation that might 
occur. A sheet of peel and stick or poly would be a bad thing. 
Building paper over a thin plaster skin would be darn fine.

Hope this helps . .. .

Dr John Straube
Associate Professor
Dept of Civil Engineering & School of Architecture
University of Waterloo
www.johnstraube.ca
Waterloo, Ont., Canada
www.civil.uwaterloo.ca/beg
John Straube
Building Science Consulting
www.buildingscience.com
519 741 7920


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From: Aaron Dennis

Dr. Straube says air barrier, no vapor barrier - but why? That's what 
I was most wanting to know, why?

My question assumes that attic insulation is resting on top of the 
bale wall. Is it safe to allow vapor to migrate into, say an 
insulated ceiling/ roof? These insulated roofs seem to be problem 
areas for moisture. A vapor barrier at the top of the wall would not 
be like having a vapor barrier on the outside since there is 
insulation piled on top, or perhaps it may be a cold enough spot to 
cause condensation?

Aaron


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