C wrote:

...I was thinking the top because heated air rises thus evacuating more
quickly. I doubt whether minute holes in the panels will affect their
insulation properites to any noticeable extent. The original poster's
outdoor temps were not so bad. There is still plenty of air trapped in
the panels to insulate.


The faster you get the air out of the channels, the less effective they
will be for insulation. If this is not a problem in your area, it might
help to put them high. Water vapor has a lower density than air, (H2O
has a molecular weight of 18, compared to 28 for Nitrogen) so it might
tend to rise. However, if the holes are very small, you will not get
much leakage, since you need to resupply the air in the channel to keep
the pressure constant. A 1/16" hole will allow the air to exit the
channel, but very very slowly. Two 1/16" holes will increase the flow.
Placing one hole high and one low will increase the flow further, since
it will then be driven by convective forces.

Again, based on empirical observation on one greenhouse, Sealing the top
and leaving the channel fully open at the bottom keeps the condensation
in the channels to a minimum. I have not seen a greenhouse yet that
solved the problem conpletely, but I don't travel around the country
looking at greenhouses, so my observations are limited to New England.

Of course, if your area's temperatures are moderate, you probably don't
need twinwall anyway and could make do with single layer polycarbonate
or fiberglass.