You may be able to reduce the potential for wind, rain or snow getting in, but how will you prevent whatever the temp. is outside also being the temp. inside?
Take a styro which one would use to ship fish and put a thermometer inside of it in the dead of winter and leave it outside for a day or two. If the temp. outside is 20ºF (and this means ignore what the temp. is wind chill wise) and what do you think the thermometer inside the styro would read? My bet is 20ºF.
And this is why I say I am worried. While I did not leave them outside years round, I did tanks every summer on our screened terrace which had 3 sides screened and the 4th was the end of the house. I was doing warm water plecos which meant I needed the tank water to stay above about 77ºF. I started up the tanks at the end of May but all that ran at first was the bio-farm for cycling all the filters. Since the fish could handle over 90ºF, high temp. was not a problem, but at the start and end of the season the overnight temps. were.
I was able to keep the tanks heated OK at night as long as the temps stayed about 55ºF or above, By the end of my outdoor season I could protect the tanks from an overnight low in the low 50s and even the high 40s as long as it was worst overnight temp. as the sun would bring the temps back up into the high 50s to the low 70s. When I would see a lower number coming up for a single night, I could cover the tanks with towels and blankets.
I had major heating in the tanks - think 15 -20 wpg. A 20L had 150-200 wpg and the 40Bs got 375 or more. When the season temps needed more heat, it was time in the later spring to wait before fish went into the tanks. In the early fall that was when the fish came out and the tanks were shut down. Sometime for a very shrt term need for motre wattage I would ad another heater. Usually, this was after some of the tanks had become devoid of fish amd only a few tanks needed the extra heater.
I did not take any steps to use shades or anything else to help block wind etc. I also did not surround the tanks with styro or insulation as you are doing as I did not have outdoor tanks year round. I live about 35 miles NNE of NYC. We do get some really cold temps in winter. 20ºF is not unusual and I have seen it at 0º or few degrees lower as the worst it might get. But, mostly it is below freezing down to 20ºF. I assume it is similar where you are?
What I see as being your worst nightmare is deciding it is about to get colder than your set-up can handle and so it cannot keep the fish safe. How can you possibly rescue the fish if it is close to 0º when you might have to do so? Here is how my summer terrace tanks looked when I did weekly water changes, I pumped the water out the door in the 2nd pic. The end ff the house was both a sold wall and then a sliding door set-up to get onto the terrace from the house.
