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Wood Stove Thermal Mass Update

Thermal MassFirst, a bit of background for those not familiar with the construction details of my cabin. It is standard 2x4 walls with R-13 insulation, R-19 in the ceiling and inside walls finished with plywood beadboard. While the floor is not properly insulated I did very carefully stuff MANY layers of bubble wrap in this fall with rolled wrap tightly stuffed into each end to block the wind. It’s not real insulation but I’m certain that there is FAR less wind and air movement under the space that had previously been open. The bubble wrap was not purchased but re-used from Greg’s shutter business. I’ve also got stacked rock along the base of the cabin from ground up a couple inches past the outer 2x8 rafter.

For this winter I stacked concrete blocks around my wood stove with excellent results thus far. I’ve got a total of 24 solid blocks (3.5" x 7.5" x 15.5"). They’re stacked on the the two long sides and behind the stove and up about 2.5 feet on the back side of the stove pipe. On the sides I’ve got them stacked two thick (about 7"). On top I’ve got a big enamel canning pot full of water which leaves just enough room on the stove top to put my coffee pot. I also reinforced the floor deck under this corner of the cabin using a couple concrete blocks placed snuggly under the floor rafters.

I’m finding that I can do two very distinct fires, morning and late evening. Thus far each fire is 3-5 logs for a fairly hot burn of 1.5 to 2.5 hours. The result is that the concrete blocks moderate the hottest peak of the burn because they are of course absorbing lots of heat. About an hour after the fire has burned out the heat finally really makes it’s way to the outer edges of the concrete. They are hot to the touch but by no means hot enough to burn anything. I type this at 3:15pm and the blocks and pot of water are still noticeably warm. My morning fire was over at 8:15am -that’s seven hours of steady, slow warmth. I expect that they’ll radiate heat for another hour, maybe two before diminishing. A huge improvement. Rather than peaking at 85 (or higher!) and fairly quickly dropping to 60 I’m peaking at about 80 and VERY slowly dropping. In fact, there is a moderation of temps even past the time that the blocks feel warm. I’m going out this evening and won’t be back till 9pm to rekindle the fire but if the past week is any indication the cabin will still be at 60 or above at that time… 12 hours past the morning fire. Outside temps today: 30 at sunrise, 40 at 3:30pm. Inside temps today: 60 at sunrise, 68 at 3:30pm. I’ve just started keeping track 9 days ago and in that time I’m seeing an average difference of about 22 degrees at sunrise and sunset before the morning or evening fire is built.

My guess is that in the colder part of winter when nights regularly dip to 20 or less and highs only in the lower 30s that I’ll be burning my morning and evening fires longer with more logs but I’m hoping that each fire will still be fewer than 10 logs. Based on what I’ve seen thus far I don’t think it is unrealistic to estimate that I’ll burn about 40-50% less wood than last year. I wish I’d thought to keep track last year with no blocks so that I could compare by numbers rather than memory of numbers. I routinely heated myself out of the cabin. It would warm very quickly but also cool fairly quickly, especially at night. Each day I’d try to get the fire up then let it go to very low coals and re-ignite. At night I’d try to keep the fire going till bed at midnight when I’d stock it up as much as I could without getting it too hot to sleep. If I failed to wake up at 2 or 3 am to get it going again I regularly woke to 40 degrees, sometimes less on really cold nights. Constantly up and down.

Regardless of how much wood I save I know for certain that the less extreme temperatures and warmer mornings will greatly increase my comfort level as well as the time I spend tending the fire. Well worth the $52 spent on concrete blocks! This is not even close to an original idea. There are many variations on the concept. Masonry stoves, cob…. the important thing is to have as much thermal mass around your stove as you can afford and safely place on the floor. If I had planned better I would have built this section of floor much stronger and would have 40 or 50 blocks rather than 24. In that case I’d often be able to get by with just one fire a day, burning it a bit hotter and longer and coasting for longer. The more mass the better the moderating of temps. The greenest choice would be a cob covered rocket stove. If I’d known of those when we started I probably would have gone that route.

Update: Last night got cold! Outside temp at 7am was 18 which I consider the first real test. Inside the bricks and water were still quite warm and it was 62 in the cabin. The fire did go late though as I got in late. Fire from 10pm with a big bed of coals at 1am, 7 logs burned. I’m VERY happy with this. I know from last year that a fire ending at 1am, with 18 degrees outside would have meant a morning just above 40 with NO residual heat from the stove. On a typical night though I’ll probably start my evening fire 2-3 hours earlier which will likely mean that the fire dies down at 11pm and the morning temp will be closer to 58ish. Still, a fantastic improvement!

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