Worked in Northern Canada for years in 44's. If you have the cash then build it big enough to avoid "hangar Rash", cement floor , rails if you don't have the help. If you just want or can only afford a roof keep the following in mind...
- you need power for the battery blanket that you should have, especially if it's a Concorde. Also the 35 $ Tanis pad for your oil pan is a great investment...it's cheap and uses very little power. If you're not flying that much just plug it in the night before. For that matter just remove your battery and store in your nice warm house (Please not on concrete) and install as needed.
-Covers are relatively cheap compared to a full on hangar and work really really well when used in conjunction with what we call here in Canada "Buddy Heaters". If your machine is just in a barn this is a must and will put paid to any instruments going Kaput do to condensation. Put one by the pedals and the extra on the battery facing your dipstick if it's an Astro.
-The window cover is essential especially if you are looking to maintain re-sale value. Even if the machine is Timed Out that window is worth a fortune. And any dust, dung, or mud is just money out of your pocket.
If you fly quite a bit and have the money, then the Real Hangar with power, water (for the windows due to dust...never enough water), and a good clean floor is the way to go. Let's face it, DI's in the wind, rain , sub zero temps suck and you might miss something otherwise than if you were in a proper hangar.
If not then the Barn will work quite well with the above. Just remember...HEAT is the key. It will save your instruments, battery and engine a hell of alot of pain (ie cash) and will maintain performance. Not to mention your oil costs if you blow a seal when starting up with cold oil.
My 2 cents
Good luck, fly safe and have fun.
BWB