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using weedy straw in a strawbale greenhouse?

Posts: 105
Location: Ontario, Canada
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Welcome Jim,
I have some questions for you. I’m building a straw bale greenhouse in an existing barn. So the bales will stay dry when we build.  The straw we have is sourced from a neighbour who didn’t have time to harvest.  Plus he said there were too many weeds in the straw. We baled round bales and are now going to rebale everything into the size we need. Do you think the straw should still be okay weeds and all?  Also I want to have the windows on the south side double. We are using recycled everything.  So if I use patio doors as windows can I use the depth of the bale ? So window on outside of bale and on inside of bale. With a wooden sill in Between!  I’m thinking about using this greenhouse all winter and don’t want to lose any heat from the windows. Sorry if I’m not clear in my explaining.  Otherwise I’ll do a quick drawing.
Posts: 35
Location: Jacksonville, OR
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Hi Mary-Ellen,

There's a lot to unpack in designing a straw bale green house as you described--and I'm sure the time and space we have here won't do your project justice.  

But I’ll offer a few observations:

There are probably weeds in all harvested straw.  I don’t think the building code gets into whether some % can be weeds, but so long as it’s mostly straw I doubt if it’s a problem so long as the bales meet the minimum dry density requirements for code-level construction (see recent post about this, or review IRC Appendix S Strawbale Construction section on evaluating bales).

You might get good quality bales going from round to rectangular.  You might not.  My experience has been that a rectangular bale straight from the field will, when opened, break into distinct "flakes" and will be easier to re-size and notch for posts and beams.  Field bales have “integrity” in that they retain their shape somewhat. My limited experience with re-bales (rectangular bales made from loose straw processed from other bales) seem to have less integrity, so when restringing or notching they don't hold their shape as well.  When I cut the strings on a “field” bale it will expand but not disintegrate.  The strings held it in compression and I can, with some effort, retie those strings and achieve the same or similar compression.  When I have cut the strings on “re-bales” they seem to explode into millions of straw bits, and mostly lose that original shape.  
If you can mostly build with whole re-bales that meet the minimum dry-density standard they should work OK, but to the extent you need to shape and resize them, they might not cooperate.  Can you make a few, see what you think?

You are wise to recognize that windows are a major source of heat loss in a structure.  The walls will likely be in the R-20s, and the windows less than R-5, and probably only R-2 or R-3.   If I understand your double glass idea of using two glass doors separated by a bale width, you're trying to create an insulative air space between the panes, hoping that will boost the "double" window insulation value?  It might.  But I'm guessing it won't help that much.  This is getting a bit out of my wheelhouse, but there's going to be air between the panes; one side will be cold, the other somewhat warmer.  Perfect set up for a convective air loop that will circle around and around pulling heat from the interior and releasing it to the exterior. You will likely get condensation between the panes as well.  It may be better than one set of doors functioning as greenhouse windows, but you'll need to do an enormous amount of detailing to air seal the space.  If you can install thermal curtains, you'll probably get more insulation for the effort...but you need to raise and lower them every day.

Also, I believe that greenhouses are generally kept considerably more humid than most homes?  If the rest of the straw bale wall assembly isn’t air sealed, warm air leaking through gaps, cracks, and holes at wall joints, outlets, and plumbing openings will enter the wall, carrying lots of water vapor with it.  In cold climates the vapor could condense as it approaches the exterior walls surface, which causes problems for the bales.  

Good Luck!
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