Condensation in twinwall polycarbonate channels

[Trent-Lion]

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 weather comes.

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.

[David J Bockman]

The manufacturer, Poly-Tex, recommends *not* sealing the ends of the cut sheets, for the very reason that you're describing.

Dave

[C]

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.

[Trent-Lion]

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.

[C]

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.

[Dwight Sipler]

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 sunlight).

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.

[C]

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.

[Dwight Sipler]

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.

[C]

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

[Dwight Sipler]

So noted. However, others are reading these posts so I thought I'd make it more general.

[C]

Your posts are fine. I don't mean anything by mine.

[dr-solo]

air is trapped in twinwall acting as insulation, but the greatest insulation benefit is there are two walls of nonconducting surfaces separated by nonconducting spacers.

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 create 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 air to both seep into the twinwall and evaporate out. but there is still the dead space between. the channel is not filled with water which is a better conductor of heat. there is little to no movement of the air thru the channel that would create convection. heat (kinetic energy) moves from high gradient to low, cold (or the lack of kinetic energy) DOES NOT move. So the main point is to keep the heat (hot air) inside the 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 the 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, and, spreading fungal and bacterial spores. which is why there are drip channels in most greenhouses. Ingrid

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