I don't know what issues you might encounter, but for my 2 cents worth, skinny dipping in a tropical jungle in the dead of winter sounds like a blast to me. Cheryl

LOL, good thought Cheryl! Now i KNOW i can get my hubby on board- thanks for the idea ;-)

Its a good idea but you may need to consider the condensation issues.
The hot tub will generate a lot of excess humidity which will condense as soon as it hits anything cold..like the glazing. The warm, moist conditions would then be ideal for mould and other nasty stuff.

Thank you for the input hex :-) i wonder if there's a way to counter the condensation, or reduce it. I am thinking it will be an issue mostly when it is open (in use), when it's closed not as much. Obviously i dont want to vent in the winter, it would defeat the whole purpose. I wonder if enough air movement within would suffice to keep things from getting mold and such. hmmmm....

Hi
I would be concerned about how much space the tub would take up even in summer. Great hot tub limited Gh lol
You will get a LOT of condensation for sure . What do you want to grow that can handle the humidity??
I use a circulating heated water system but I'm in 10 florida ,Very different conditions and problems I'm sure.
It works but not nearly as well as I'd hoped lol gary

OK, yeah condensation would be huge, maybe you could divide that GH in half and use the hot tub part for tropicals like the stuff that grows in Hawaii. The other half could have a small vent to connect to the drippy side to benefit from the moisture, and this would be the "serious" side where you do your seed starting and propagation activities. And of course it would serve as a dressing area after hot tubbing to prepare you for the mad scamper back to the house in -10 degree weather (I know I wouldn't wanna make that trip, warm, sweaty and nekkid). Cheryl

well, with the tub inside, it leaves me roughly 10x12 of GH space, which isnt as much as i'd like, so it's definitely a compromise, but if i dont have enough heat, then i can't use it in the winter anyways, so....hmph, lol.
As for the condensation, dividing it is a pretty simple option, but i wonder if i will be able to get enough heat if its divided. It would be nice to have tropicals on the tub side though.
I was trying to think of ways to get the heat without the steam, and divising like a 'cuff' that sits on the top, maybe 6" tall between the tub and the cover, so that a bit of heat comes off, without it being too draining on the bill (and hopefully partly offset by the fact that the tub is in an enclosed area).

I guess a hot tub is quite well insulated on the sides and base?
It looks likely the water surface would transfer mainly latent heat to the air. So the worst case situation is the humidity in the air increases without a corresponding increase in the air temperature.
The hot tub could act as a humidifier and not as a heater.