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What millimeter polycarbonate? Help

 
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Hello all,
My wife and I are planning on converting an old 31’x25’ chicken house on our farm into a greenhouse and I’m having a hard time deciding on what millimeter twinwall to use.
The build has a concrete slab with a short, block foundation wall as well as three 4’x8’ windows on both the north and south side of the house for cross ventilation.
My plan is to replace the tin roof as well as the windows with twinwall poly, then insulate the rest of the house.
I do not plan on heating during the winter but would obviously like as much season extension as I can get with the multiwall poly.
We live in southwest va so we don’t have really “harsh” winters but it certainly gets cold enough to hit the brakes on most crops each year.
The price difference between the 6mm and 10mm isn’t more than a couple/few hundred, I just don’t know if it’s the right way to go, overkill or wasteful because I won’t get much extension with out secondary heating.
I thought that this group where the perfect people to ask.
Any feedback on the glazing, ventilation or overall design would be very much appreciated.
Happy trails and happy vibes to all 🤙✌️
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Maybe 8mm?

8mm twinwall
 
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If you aren't insulating the remaining walls, I'd go with the thinnest I could get that would handle the conditions (rafter spacing, snow load, etc).  The thermal difference between the various polycarbonates are likely outweighed by the R1 or less of the walls.

Have you thought through ventilation?  Once there's a clear roof on that puppy, it will really heat up.
 
Jw Williams
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I have thought about a solar powered attic fan for ventilation. Maybe incorporate a few windows on the east and west side as well.
The reason I want to insulate and thought about going thicker, 10mm tips, is that I originally wanted to do a sunken passive solar greenhouse but after weighing out the expense and workload of that verses converting the building I decided to go with the chicken house.
I’m kind of holding onto the idea of passive solar if possible. I guess I just wonder if it’s worth the expense for the possibility
 
Mike Haasl
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Location: Northern WI (zone 4)
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I'm thinking you'll need much more ventilation than that to keep it survivable in the summer.  I could certainly be wrong though...

From the passive solar greenhouse research I did, it suggested ventilation equal to 20% of the footprint of the greenhouse.  I took that to mean 10% low down to let air in and 10% up high to let hot air out.  My greenhouse is pretty tall (18') so I think that helps me a lot.  On hot summer days (hot for me is 90 degrees), I get by with about 21' by 3' of vents near the ground on the south side and 24' by 4' of vents up high.  Those are hinged vents that are only open about 30 degrees but they let a lot of air through.  Greenhouse is 20x40.

I'm guessing it gets a bit hotter in VA so your ventilation needs would likely be greater than mine.  But then again, mine are passive (no fan).

Retrofitting an existing building is a great idea.  One option is to do the roof work this year and then see how it handles this winter.  If you want it warmer, add more thermal mass and insulation for next year.

Welcome to Permies by the way
 
Jw Williams
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Thanks for the warm welcome Mike. It’s nice to be able to talk about these things with people who speak the language. My internet research has been limited and saturated with advertisements.

Do you think that I would need additional ventilation with the three 4’x8’ hinged windows on the north and south open during the summer?
The heavy insulation and poly makes sense during the winter but I certainly don’t want to cook everything to death in the summer either.
I had thought about the windows and attic fan to help with the heat up top but hadn’t fine any research on the insulation factors.
The building is old and has no insulation whatsoever so I was concerned about all my heat getting sucked out in the winter.
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Mike Haasl
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Location: Northern WI (zone 4)
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That's what we're here for :)  

Those windows look a lot more impressive from that angle.  Would the crops be on benches or the floor?  I'm imagining the air would be noticeably cooler at or below the windows.  With them all open it would help a lot.

You could consider making part of the steeper pitched roof into some openable vents.  The trick is making it so that the rain doesn't just sneak in through them when they're open.  

I think insulation will do more to help in the winter than it hurts in the summer.  
 
Jw Williams
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🤙
You the man Mike. Much obliged
 
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