I blanket my mare. However, she is 26 and she's of the opinion she "will" die if she's not babied (aka, if she's not blanketed, she mopes about and becomes a whole lot less active).
Since it does get pretty cold in your area, I'd probably get a blanket of the appropriate weight -which is most likely a heavy one, or a heavier midweight, in your case- just to have on hand or I'd find someone with an extra in the correct size that I could have dibs on if my horse needs it, then not blanket the horse unless he shows signs of being uncomfortable with the cold.
For my mare, I started out with just a midweight blanket that she wore all the time since the coolest it generally gets here is about 15F (or about -9C) and that's only for a day or two here and there. I've since added to my collection and she now has a heavy weight blanket for those really cold days, a midweight blanket (and an extra to switch to if the first one gets wet through from rain or something) for average days and a sheet for warmer days with rain.
If your horse is sweating from being blanketed, that means that the blanket is too heavy. So, for instance, if your horse is sweating in a heavy weight blanket, depending on the amount of sweat, it means that he would be better off in a midweight blanket, a light weight blanket, or no blanket at all (depending on the temperature around him).
I've found that overblanketing is hard to do, for me at least. For instance, in my case, layering of blankets, at any time, would probably be overblanketing and cause my horse to sweat. So I don't layer blankets. But for people in colder climates, layering blankets is necessary and great.
I guess what I'm trying to say is that if you don't get carried away by the idea of making your horse snuggly warm and if you use a blanketing chart (they're all over if you google it), you shouldn't be at risk for overblanketing.
Another thing I do is whenever I go check on my mare (at least once daily), and she's wearing a blanket, before I have her move around at all, I stick my hand under her blanket at the withers area to check the temperature under there. If it's warm but not hot, I'm happy. Even a little cool is ok - just not cold because then that means that your horse is cold since the blanket is holding down all the hair he'd otherwise be using to warm himself. Hot means that you probably need to go down a weight (or take the blanket off completely, depending on the outside temperature) because your horse could be getting too hot.
Basically, when you stick your arm under the blanket, what you feel is what your horse is feeling. So, how I like to think about is "would I like to be the temperature I'm feeling currently, in the weather we're having?" If the answer is "yes" and she's not sweating, I leave the blanket alone. If the answer is "no", you get the drill. It's not super precise, but it's worked for my mare and I for a good three years so I'm sticking with it. :)
Good luck! And sorry for the novel. I apparently had lots of thoughts on this matter. Haha
Fabio - 13 year old Arabian/Lipizzaner gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.
Last edited by Wallaby; 10-03-2011 at 12:42 AM.