permaculture

["Scott Pittman" <scott@permaculture.org>]

I have always read that any airspace between the glass or poly should never
exceed 5/8 of an inch to prevent a thermal loop from developing which would
defeat the purpose of double glazing.  This is certainly the rule in
manufactured thermal pane windows and doors.  By evacuating the air space
between the glazing panels you can slow down this effect with over time the
vacuum gets lost.

Scott

"Another world is not only possible, she is on her way. 
On a quiet day, I can hear her breathing." 
~Arundhati Roy

-----Original Message-----
From: permaculture-bounces@lists.ibiblio.org
[mailto:permaculture-bounces@lists.ibiblio.org] On Behalf Of venaurafarm
Sent: Friday, January 18, 2013 7:37 PM
To: permaculture
Subject: Re: [permaculture] Double pane plastic windows?

On 1/15/2013 11:51 PM, Stephen Sherman wrote:
> I'd agree with the other posts regarding polycarb being the only way to
> go. How long polycarb will last varies with your UV exposure and
> weather, but most GH multi-wall panels are guaranteed against
> discoloration for 10 yrs. The GH polycarb extrusions have some other
> advantages, besides being multi-walled, such as uv coating on the
> outside, anti-condensation coating on the inside.
>
> Cost is probably the only disadvantage. Polycarb twinwall is about the
> same price per sq ft as the standard sized double pane glass panels.
> However it is not code (or safe IMO) to put glass, even tempered glass,
> on a GH roof. You should have tempered laminated glass there, to prevent
> glass pieces from falling on people if there is any breakage (such as in
> a hail storm).  Polycarb is much less than tempered-laminated glass. And
> plastics of all sorts are much lighter and can bend without breaking, so
> the framing for them is much simplier and less expensive than the
> framing for glass,

You could build your own double-glazed polycarbonate panels for 
greenhouses and shylights. Minimum would be 1/8" polycarbonate.
I think the next increment thicker is 3/16". Use full sized panels, i.e. 
4X8 or 5X10. Use silicone caulk and stainless screws (hex head only) 
with neoprene grommets to attach the panels to frames made of non toxic 
pressure treated framing members, 2X4 or 2X6. You will have
1 1/2" airspace. They will last forever. I would not attempt to use 2X2 
members as they will likely warp during any hot dry seasons and you will 
then lose your investment; i.e. use full dimension lumber and use NT PT 
rather than cedar or cypress.

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