Here's a story for you. I'm an avid snowmobiler, have an uninsulated cabin in the Western Mountains of Maine. One weekend long ago it was -37F up there when I arrived, about -32F inside, freaking cold! The cabin is remote, wood heat and propane lights/stove. We went riding the next day when it warmed up to -20F outside ... after a couple hours we decided to head back because it was just too damned cold to ride. My moustache was white with frost from my breath, my eyelashes and eyebrows were white, eyelashes started sticking together from the frost on them ... all this while wearing a full face helmet! Try riding through the woods with your eyelashes frozen shut! I looked just like one of those Antarctic explorer pictures! Damn that a cold weekend. We were wearing long johns, a couple sweat shirts, flannel shirt, full leather snowmobile suits ... About 6-7 layers as I remember. Snowmobiles have heated grips and thumb warmer but they didn't help much. That was about 30 years ago. You could open the front door of the camp and watch the cold air pour in like a thick fog as it hit the warm moist air in the camp. The walls were board and batten and you could see frost forming on the inside between the boards! We has a barrel stove and it was so hot you could see the sides glowing red! A thermomether at about 7ft up on the wall was in triple digits but snow on your boots on the floor wouldnt melt. It took about 3-4 hours to thaw the place out upon arrival. Imagine using the outhouse ... it had a regular hard toilet seat but was so cold it felt like it burned your cheeks if you sat on it. Great memories!

An infrared heat lamp could do it too. They fit regular 120v light sockets, about the size of an incandescent flood light, pick up a couple contractor clamps with light sockets and clamp on the floor joist under the deck pointed at the plumbing.