Thermopane windows are filled with vapor-free gas and sealed. If the seal fails, even slightly, you end up with moisture between the panes. I don't think that foil sealing tape on polycarbonate will hold up over a long period of time to prevent moisture from getting in. Personally, I wouldn't do it.

well my other option is to leave them open to breath .....

We have a cold frame that is double-walled polycarbonate that is open at the edges, held in place by an aluminum frame. So air (and presumably moisture) must get in between the "sandwiches" of the polycarbonate.

It's been in operation for about six years--maybe longer. There has been zero, zilch, nada, NO algea growth in there. The little "microclimate" formed by the little sandwiches of polycarbonate seems to me a quite inhospitable place for algal growth. In the night it will get quite cool--near or equal to ambient temperatures. On a sunny day it'll be as hot as Hades between the walls of those cells. Not a nice place to live!

I'm not sure about condensation, but if it did have some, I believe it would be short-term. But I need to think about how you are planning to orient the poly...

Are you running the "lines" parallel to the horizon, or up and down? I can mess with my cold frame out back and see what condensate does. Are you concerned about it dripping into the shed, or just about fogging?

Are these the hurricane panels from Home Depot (asking because I need to buy some poly for doors....)

-K

I am planning to use a Homedepot hurricane panel, arrives Friday. It is going to be in the middle of the roof as a big skylight with shingles around it. I'm now thinking of not enclosing the ends with foil tape and a leaving 1/8" space between it and the plywoord sheeting on each end, then apply a line of sealent on the top side and use Ice and Rain shield on the roof, overlapping 1" onto the poly , roof, and finish that with calking around the roof/poly border. This would leave the ends "exposed" underneath. I want to run the lines horizontal but maybe that is a flaw in my design. I could cut the panel in half and run vertical but need a few more parts to join them.

Yes, I think that it'll be safest to orient them vertically. I found the following on a site that talks about using polycarbonate in greenhouses. I am pretty sure that the vertical orientation is to make sure that water drains, instead of collects in the cells. All the panels of my cold frame are in vertically-sloped orientations, too.

Installing Twin wall Polycarbonate on Your Greenhouse

Taking care of some simple techniques will make your projects successful.
Always install the sheets with the flutes oriented vertically. When used as a roof covering, design with at least a 5 degree slope, never horizontal.
Remember to allow for expansion and contraction in every installation. Bronze and Gray panels change more in size. Use extrusions that allow for sufficient edge engagement plus the expansion allowance.
Panels can be cut with protective masking in place with a fine tooth saw or a sharp knife. Remember to remove the masking prior to final fixing in place. Chips from cutting should be blown out of the flutes.
The specially treated weather side is identified on the masking. This side must always face the exterior.
In vertical installations, the top flutes should be capped with an extrusion or tape and the bottom left open for drainage. Vented tape can be used on the bottom.

Here is a link that might be useful: Using polycarbonate in greenhouses

Another consideration with leaving both ends (or sides in this case) open, the heat insulating property of the polycarbonate panel will be significantly reduced. It is the dead air space that provides the insulation.

I don't think I'm too concerned with the heat insulation properties too much in this case. The polycarbonate panels are meant to breath and not be sealed air tight. tough choice on how to install. Probable leaning to putting the anti dust bottom tape on both ends and running vetical to easy the installation. Will let yoy know how it turns out.