I recently got a secondhand Old Town Discovery 169 canoe, which is at least 15 years old and made of some kind of polyethylene (I assume). The problem is, I have nowhere good to store it. I'm considering putting an access door in the ceiling of my one-car garage and keeping it in the attic. My main concern is temperature - will the high sustained summer temperatures in the unfinished attic age or damage the canoe material? Would I just need to be a bit more careful supporting the canoe properly to avoid deformation? Temperatures often climb into the 90Fs (32C) and occasionally 100s (38C +) during the summer, and the attic probably gets tens of degrees hotter than that. The alternatives are to store the canoe outside, or to park in the driveway and let the canoe have the garage itself, neither of which are ideal.
While it is possible that the canoe can take the summer heat it is also likely it will deform. And as you are not sure of the plastic used, I would not take the risk.
Storing it under the ceiling in the garage or out of sunlight in your house or outside space seems a better option.
If you want to move forwards with storing it in this area, insulating it and ensuring good ventilation would probably be enough to give reasonable certainty. You could also measure the temperature on hot sunny days, to see how much risk you're prepared to accept.