(Yeah, I typed this offline and then logged on just long enough to squirt it onto the blog)
People talk up thermal mass as if it’s the answer to keeping warm in winter. And it certainly has its charms – the Secret Lair has next to none, and while it’s easy to heat up it also easily goes back to cold. Winter mornings in the Lair are no big treat, it’s shiver in a coat until the fire builds up to heat the iron, and the iron heats the air. Truth is the more I insulate the Lair, the less unpleasant the winters become overall. Which is why I look back on winters in Michigan, where I grew up, with ironic nostalgia. Michigan has its flaws as a place to be from, but they do have central heat and lots of insulation there.
People have told me over and over that what I need is more “thermal mass.” That is, I should pile rocks or something around my woodstove to catch and hold the heat overnight. It’s not a bad idea at all, in fact I did a little of that last winter and plan to do it again now: I’ll take all my leftover tiles from the floor job and stack them neatly in the corner behind the stove. I can’t stack rocks around and under the stove, as someone suggested, because I have to be able to pull it out to clean the stovepipe – I’m not giving that up. But I can do a little of it, and will. I’ve got lots of heavy ceramic tiles left over, since I bartered D&L’s entire surplus and they had lots. I had enough tiles to floor three Lairs.
It’s of D&L I want to write because Monday L said something bearing directly on the subject of thermal mass. She said – on September 26 in a latitude not known for long, vicious winters – “We’ll probably need to start running the pellet stove this week.”
Regular readers know that D&L built a house with lots of thermal mass. 4000 square feet of thick adobe under the floor. Every interior wall – even the closets! – eighteen inches thick, made of earthbags. Exterior walls of strawbales coated inside and out with adobe. They’re lounging around in robes and slippers at five in the morning while I’m shivering in layered sweatshirts and my canvas coat, praying for the fire to catch. It takes a long time for all that mass to dump its stored heat, and it’s great while it lasts.
But there’s a downside. D&L burn fuel constantly in winter, not in spite of all that thermal mass but because of it. They don’t dare let their house get cold. It would be really hard to warm it up again.
I’m not saying this because I think thermal mass is a bad thing. It’s better in summer than winter. This last June I suffered through several 100+ degree days – indoors. D&L’s house never got warmer than 80, and rarely that warm. Oh, it was great – when I visited I didn’t want to leave. And in winter it certainly smooths out the temperature cycles. But somebody’s always got to be there to stoke the fire, or refill the pellet hopper or the propane tank. If you ever let it get cold, it would be the work of several days to warm it up again.
So I’m not saying thermal mass is a bad thing, not at all. But it’s also not an unmixed blessing. If I ever left during winter I could drain the pipes and let the cabin go to ambient temperature, knowing it would only take an hour or two to warm up. If they ever do it, they’ll need to hire me to run their stoves for them while they’re gone.
