The biggest drawback would be light loss. How much light do your current panels transmit? Square it to get the new transmittance.

Of course, if you're currently using a solar pool cover + bubble wrap, ditching those for polycarbonate would probably be a very wise idea.

Is one inch foam board the only insulation you have on the north side? It's just polycarbonate, then foam board? The foam board isn't even aluminized, I assume? You can do better than that.

What do you do on the east and west walls?

I have about three quarters of the east wall with polycarb then foamboard I get a good bit of morning sun through the top quarter. The west wall have foamboard over polycarb as well except for the door which has bubble wrap. I couldn't find foil backed foam board around here I did buy those foil emergency blankets and hung them over the foam board with thumbtacks. I cant find any information on the light transmission of the 4 mil polycarb that comes with HFGH kit. The twinwall polycarb on this http://www.greenhouse-coverings.usgr.com/multiwall_polycarbonate_sheets.htmls site is is at 83% light transmission I do not know if this is the same as came with the HFGH kit but it is a ball park The 10 mil four wall polycarb is 76% light transmission so I am assuming That will close to what I get. I probably have less than that with the pool cover and the bubblewrap now. I think.

My source lists PAR transmissivity for twinwall PC at 78-83%...so all the numbers are pretty consistent. The figure that I have always heard for the clear pool cover is 80%. So your PAR transmissivity with double layer of twinwall PC should be about the same as one layer with a pool cover. I think you are on to something.

Here is a link that might be useful: GH energy efficiency link

the only problem is, how can you afford to buy it? have you priced twinwall lately...OUCH!

I have convinced myself this something I am going to do in the summer when the gh is empty. One question I have is would it be better to have a gap between the poly panels or should they be up tight to each other? Preferring to get most r value out of them.
sandy0225 I am not buying it I have it already on the north wall and roof of the gh's. I want to replace the twinall on the north with lumber and add that twinwall to the south.

Leave a gap for sure. The air will help insulate it.