[Strawbale] Revisiting top-of-wall details

[Mark Piepkorn]

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.

http://www.hugllc.com/index.php?option=com_content&task=blogsection&id=4&Itemid=15&limit=5&limitstart=170



Mark Piepkorn
www.potkettleblack.com

He can compress the most words into
the smallest idea of any man I know.
  - Abraham Lincoln