permaculture

[Stephen Sherman <spsherm@msn.com>]

I think you'll find that the commercial double glass pane units are air 
evacuated and filled with an inert gas. That makes the "middle" less 
conductive and convection loops less of an issue, which is why they can 
have 1" spacing without loosing insulating, I suspect. Unfortunately 
this is not really doable on a wood framed unit.

Look carefully at the total system cost of two 1/8" sheets of polycarb 
vs the commercial dual or triple wall polycarb GH extrusions. When I 
last compared prices (granted a few years ago) the roll your own 
polycarb solution was quite a bit more expensive; which makes sense 
since you end up using 1/4" thickness of plastic overall. The commercial 
GH extrusions are quite a bit thinner than that, and use less plastic 
per sq ft of coverage.

One can still put the multi-walled GH extrusions on a treated 2x frame, 
much the same way you would the single wall, you just are putting it on 
only one side of the wood structure. In CO, all new treated lumber is of 
the newer non-arsenic type for quite a few years now. I thought it was 
everywhere but you probably need to check. (The new stuff gives a 
reddish brown color to the wood, as compared to the old stuff which made 
the wood a greenish color.)

Whatever polycarb you go with, you should be sure to get some which has 
a UV protected surface coating, and be sure to put that towards the 
outside as you are installing. Failure to do so will mean your plastic 
will not last as long when exposed to the sun. No plastic (clear plastic 
at least) will last forever when exposed to direct sunlight. Up here at 
8200' elevation, the GH plastic I have has lasted 10 yrs so far and is 
still quite functional. Some say you can get 20 yrs out of it (it is 
guaranteed for 10). Interior (non UV protected) start whitening and 
cracking in as little as 5 yrs. No doubt all these times will be longer 
in places with less UV and sun.




On 1/18/2013 9:26 PM, venaurafarm wrote:
> On 1/18/2013 11:03 PM, Scott Pittman wrote:
> > 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
> There is plenty of glass double pane panels that have 3/4" airspace.
>
> > 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.
> My method just uses two Polycarbonate glazing attached to framing
> members, not air evacuated. My untested theory on prevention of thermal
> loops would be to use more framing members, blocking more light of
> course but creating a stronger panel that is more energy efficient. I
> would not hesitate to use this method on a skylight, in a sunspace or
> or a greenhouse. This by far the cheapest way to go plus extremely
> durable and lightweight; you get plenty of light with some degree of
> thermal efficiency.
>
> LL
>
>
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