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GSBN:Revisiting top-of-wall details

I came across the following experience on a big-bale building in
Minnesota. The interior was not plastered at the time. I haven't
found out whether their hypothesis was correct or if they came up
with a good fix.

- - - - -

Pete discovered that very cold air was showing up in many parts of
the North wall. That is the last place we expected to find a leak. It
is nowhere near doors or windows. It has three feet of fluffy
insulation blown on top of it.

Bob got diligent and recorded a bunch of surface temperatures and put
numbered tags on many spots on the wall. He wrote lists of the cold
spots and how they varied over time. Some places were substantially
colder than the outside air.

Finally a pattern emerged and we came up with a theory about what the
heck is going on.

The lower part of the building has a negative pressure, trying to
suck air in through any crack or crevice. We purposely didn't put a
vapor barrier over the top of the wall so the bales could breathe. As
planned, the air can't flow through the straw wall because the straw
is covered with cob on the outside, so the air gets drawn slowly down
through the insulation blown on top. Then it doesn't want to flow
through the bales themselves because they are so dense, so it flows
through the stuffing between the bales. Eventually the cold air
appears at the surface in the middle of the wall. It is still very
cold because the insulation is so effective.

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Mark Piepkorn

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