Dave,

The ideal is to have a totally-sealed, "dead" air space, as that is the best
insulation. Obviously we cannot do that, so the blower maintains the
pressure while the film and its seams leak ever-so-little. There should be
so little flow, that the source of the air makes little-to-no difference as
to the temperature in between. it will be determined by the inside- and
outside conditions, not the source of the air used for pressurization.

Outside air is preferred because it is drier than the moist air from the GH,
and that has two implications. One, dry air, being less dense, will be a
better insulator, and secondly, if you use the moist, interior air supply,
you will get more condensation in between the layers of plastic film, and
the accumulated water will actually reduce the dead air volume and the
accompanying insulation.

Like Pat, most of the inflation systems I've ever seen were interior, but in
the last poly film GH I had, I tried the external blower - protecting it in
a plastic, bottomless "box" - and was quite pleased. There was no
temperature problem, and MUCH less liquid in between the layers. (In the
previous GH, it built up as much as 9" deep!)
--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!


"David Jewell" wrote in message
.. .
I put a greenhouse up this summer and am confused about a point and was
wondering if anyone could answer this one for me. I currently inflate the
greenhouse with air from inside the greenhouse but have been told I should
be drawing air in from the outside.

With drawing the air from the inside I am thinking that the bubble will be
warmer and give me better insulation but I am concerned with the issue of
water collecting in the bubble. With drawing air from the outside I am
thinking that the air will be a lot colder and thus not so good for my
orchids.

Any advice would be greatly appreciated.

Dave (Ontario, Canada)