Canada, heat???? our vehicles can hit 60c if left in the sun with windows up in summer, good for blowing a board up with vent closed, but I've never heard of any sail damage. I guess sailing in 0c conditions some materials may get brittle but I've no idea really.

I store my sails permanently in my van. After use I dry them by standing them vertical in a shaded area out side for at least 24 hrs. When dry I return them to the van. I only rinse them if they are muddy at the end of the session.

so far have ONLY damaged sails in the cold - below freezing
if wet when it goes in the cold - gets brittle - logical

What about the rider? Does he turn brittle and snap in those conditions? Perhaps it is time to emigrate to warmer water.

During the winter, the temps are below freezing where I live. I keep my sails in the garage with the sails inside the sail bags. Never had an issue. When I go windskiing in the winter, I use my sails in the snow and they handle fine. I do have a separate set of sails I use for the snow and keep the sails for the water stored in the garage until windsurfing season starts again. I do bring my board and fins in the house for the winter, but all my other gear stays in the garage.

They're made of mylar aren't they?


usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Physical_Properties.pdf
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<div class="column">Mylar® polyester film retains good physical properties over a wide temperature range (–70 to 150°C [–94 to 302°F]), and it is also used at temperatures from –250 to 200°C (–418 to 392°F) when the physical requirements are not as demanding.

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Seems like temperature alone is not too bad for them. The information doesn't mention the effects of solar radiation though.

There is a graph there that shows the effect of temperature exposure at 150 deg C. It stays OK for 2,500 hrs and then takes a dive. The eventual decrease in strength with high temperatures appears to be something to do with driving out water from the film. Drying of the film is linked with brittleness in that information sheet.

From what I can see, don't let them dry out. Keep them wet in the bag. 2,500 hrs, at 150deg C, it's got to be a lot more hours at 75 deg C, let's say 5,000 hrs before loss of strength. (though probably not linear relative to 0 deg ) The car's hot for 8 hrs a day. 5000/8 = 500 summer days = 4 or 5 seasons. Don't worry about it. It's the UV.