I, a rank amateur and non-handy person, am trying to restore an old GH
with aluminum frame. I replaced the glass with twinwall polycarbonate
sheets. The frame has a groove in the bottom where the glass used to
fit and into which I fitted the poly. I could see no way to allow for
drainage at the bottom of the sheets. Both ends of the sheets are
tightly sealed with caulk, and I put more caulk at the bottom, to help
the sheets slide into the frame groove and to seal the juncture of
sheet and frame.
Now most channels of the poly look fine and clear, but a lot of them
stay cloudy from moisture (making for an unsightly appearance), and in
some channels several inches of water has collected at the bottom.
I don't understand why the channels are behaving differently. But more
importantly, am I stuck with this condensation/water problem forever,
because I sealed both ends of the channels with caulk? All I can see to
do to remove the water is drill a small hole at the bottom of the
affected channels and let the water drain out or perhaps pull it out
with a hypodermic syringe. Or perhaps the water will evaporate when hot
The weather in north FL where I am located has varied from the 30s to
about 70 since I installed the sheets. Will the problem get better or
worse when hot weather comes?
Thanks for help.
On Sun, 18 Jan 2004 10:11:59 -0500, Trent-Lion wrote:
Without knowing your setup, I am not so sure I would have been so eager to
seal the poly panels. Moisture has gotten in several with nowhere to go.
If you can, remove the cloudy panels and let them dry complete. Check
them for hairline cracks. Once dried, you could reseal. Since you are in
an area where temps aren't too extreme, I think you'd be safe not to seal.
You could also check the channels where they go and drill drain holes to
allow for drainage.
Greenhouses aren't necessarily supposed to be airtight. I'm not so sure
you really want to make the entire structure inflexible.
It is raining and I can now see that the moisture in my twinwall
chambers is getting in *from the bottom*. Some of the chambers are less
well sealed with caulk than the others, water is gathering in/near the
groove in which the panels sit, and capillary action is taking over
from there, causing columns of water to form and mist to rise off the
columns and up into the chambers.
If I can ever get the moisture out, by drilling small holes and sucking
the water out with a hypodermic or however, I can perhaps seal it all
up so no more can get in.
Thanks for comments received.
On Mon, 19 Jan 2004 11:11:05 -0500, Trent-Lion wrote:
Small vent holes near the top of each fogged channel would help. I'd put
the holes on the inside surface, that which is away from the weather. The
vents holes would allow trapped moisture a way to escape. The holes would
not need to be any wider than 1/32 or 1/16", I would think. It's worth
trying on one panel before committing to all of them.
When I put up my twinwall polycarbonate panels the manufacturer
recommended that the bottom of the panels be left open and the top be
sealed. This prevents air convection through the panels (decreasing
their insulation properties) while allowing moisture to get out through
the bottom. Sealing the panels can be done either with caulk or with
tape. The aluminium tape is weather resistant and will last a lot longer
than something like duct tape (which has about a 2-month lifetime in
In practice, there are times when moisture collects in the channels, but
the next sunny day clears it out.
If the caulk in the original poster's installation is too thick to
remove or if it is still unsightly after attempts at removal, you could
cut of an inch or two of the bottom of the panel and install a baseboard
(on the inside of the panel).
If you opt for the vent hole approach, I would put the holes at the
bottom, just above the caulk (or maybe even through it), so that they
could also act as a drain in the event that a large quantity of
condensate forms in the channels.
On Tue, 20 Jan 2004 08:10:44 -0500, Dwight Sipler 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
air is trapped in twinwall acting as insulation, but the greatest insulation
is there are two walls of nonconducting surfaces separated by nonconducting
think down coat. nylon on outside does not let wind thru stopped convection.
feathers create a dead air space ... the feathers dont act as insulation, they
the loft or space that is the insulation. squash the feathers and cold air is
conducted thru the material.
now the fact that there are holes on the inside of the twinwall may allow moist
to both seep into the twinwall and evaporate out. but there is still the dead
between. the channel is not filled with water which is a better conductor of
there is little to no movement of the air thru the channel that would create
heat (kinetic energy) moves from high gradient to low, cold (or the lack of
energy) DOES NOT move. So the main point is to keep the heat (hot air) inside
greenhouse rather than lose it to the outside.
water molecules are "heavier" than nitrogen. this has more to do with water
molecules interacting with other water molecules and precipitating out. inside
channel the water is going to cool and condense and leak back out the hole.
the biggest problem with condensation is rotting wood where the water drips,
spreading fungal and bacterial spores. which is why there are drip channels in
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