I was a roofer years ago. We did a lot of commercial buildings with torch-down membranes. When it got down to near 0, We couldn't get enough pressure out of the 30lb bottles we used to keep the torches hot enough to do the job. We fixed that by occasionally using a torch to warm up the bottles a little. On some jobs where we had good access, we used a 500 gal portable tank. We'd throw a tarp over it with a space heater under it. It kept it warm enough so we had good pressure.
