We have seasonal cottages in Parry Sound. We were wondering if there is a by-law stating that we are required to insulate and vapor barrier all exterior walls?
I would think you would want a vapor barrier, laws or not. A vapor barrier helps keep moisture out of your structure
A vapor barrier or vapor diffusion retarder is a material that reduces the rate at which water vapor can move through a material. The older term "vapor barrier" is still used even though "vapor diffusion retarder" is more accurate.
This is why it's very difficult to find unfaced rolls of fiberglass for walls. The kraft paper acts as your vapor barrier. If you don't have one, humid air can get into your structure much more easily, where it can then condense. This might not cause water damage, but it can cause the growth of mold, which can make you very sick. Since it seems that Parry Sound, Ontario, Canada is listed as being a humid area, you really need a vapor barrier.
If the main issue is wondering if you have to insulate, that's a different ball of wax. Assuming these are summer structures that are unused in the winter, you might be able to get away with that. Depends a lot on your local codes. If you don't have to insulate, you can probably get away with just using plastic sheeting. 6 mil plastic is sturdy enough to withstand construction, and you should be able to find some pretty easily (here's a good example on Amazon, and it adds termite resistance too). Just put it between your wall and the outside sheeting you're putting up. But I wouldn't skip the vapor barrier, or you'll regret it for sure, especially on a seasonal use cabin where the air is stagnant for long periods.