On Tue, 20 Jan 2004 12:31:04 -0500, Dwight Sipler wrote:

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.

Note that the original poster is from northern Florida. I think we are
picking nits here. As moisture laden air heats up, it rises.