[Strawbale] Alternative to PVC for a greenhouse (was re: Sustainability of wood heat.)

[Robert Tom]

Sherwood Botsford wrote:

>> I'm considering a PVC pipe + PE film greenhouse this summer.  What
>> should I be using for the frame instead?

> Ok, why is PVC so evil?

Okay, I know we've played this game before, as much as 13 years ago in the  
very early days of the CREST SB List so I won't bother doing a long-winded  
rant on the evils of PVC.

For those who are not familiar with the evils of PVC I would suggest  
Googling "PVC environmental toxin" and there should be plenty of hits.

In particular, look for references WRT  
chlorine/methyl-mercury/mercury-cell chor-alkali facilities, vinyl  
chloride, dioxins, phthalates  (in particular diethylhexyl-phthalate  
(DEHP)).

Besides the risks to human health that are associated with the above,  
there is also the issue of PVC's inability to be recycled and how it is a  
contaminant in the plastics recycling stream that can render entire loads  
of otherwise recyclable plastics unusable.

Bottom line is that it is evil stuff in every phase of its existence, from  
production and use through to disposal -- perhaps about as far from being  
a Green material as one can possibly get, short of using spent fuel rods.

Quite frankly, I think that hoop greenhouses in Northern Climates are a  
waste of time/resources but if one must build them, I'd look at using 10M  
(#3 in Murrican) rebar in lieu of the PVC tubing.

Last time I bought a load of 10M rebar, the cost was about $6 per 6 metre  
length (available in 12 m lengths from the supplier if you have the means  
to transport 12m lengths to your site).

While it is true that steel is a high embodied-energy material, it would  
be easily salvaged and re-used at the end of the useful life of the  
greenhouse so that its high EE would be amortised over several  
incarnations, lowering its effective environmental impact and in the end,  
it can still be recycled into new steel.

That being said, greenhouses built in Northern Climates would do well to  
emulate the designs developed by the Brace Research Institute at McGill  
w-a-a-y back in the late '60s and early '70s of the previous millennium --  
essentially eliminating non-equator-facing glazing, earth  
berming/insulating on the non-equator elevations and incorporating thermal  
mass.

For the frame, very strong/durable frames can be made from small  
cross-section steel angle (preferably salvaged ... you'd be surprised how  
much gets tossed regularly),configured as trusses if necessary.

Anyone that can use a saw, drill and wrench can fabricate a frame using  
the stuff.
True, it may be a bit more work than simply bending some PVC tbing to make  
hoops but the resulting frame will be substantially stronger and more  
durable.

For the glazing, I would look at diverted-from-the-wastestream tempered  
patio door insulating glass units (IGUs) that have been discarded due to  
failed edge seals.

One condo or apartment building replacing them as part of their scheduled  
maintenance program would yield more IGUs than you could likely make use  
of.

Separate each lite into its constituent single panes and you've got twice  
the coverage.

Unlike polyethylene sheet which you'd be lucky to get more than one  
season's use from, tempered glass will last for as long as you want it to.


-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
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