Masonry heater: Good investment or waste of time?
Now that my 150-year-old house in New Milford, Connecticut, is heated with ductless minisplits, I have an empty flue in the chimney running from the basement where the boiler used to be. I use my uninsulated, stone-walled basement as an occasional workshop for tinkering with small projects, and I thought putting a woodstove down there might be nice just to make it a little more comfortable in the winter (it can get into the low 40s down there on a really cold night) and to have back-up heat for power outages.
I’ve always been curious about masonry heaters, and the claim that you can burn them for a few hours a day and then just have the benefit of the residual heat throughout the day. I know they wouldn’t be great in well-insulated houses, but mine is far from that.
Would you just put a wood stove in the old flue, or would you consider investing in a masonry heater if you were me?
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Masonry heaters take up a lot of room, are very expensive to install, are inappropriate for a modern home, and are relatively unresponsive. (They often overheat a room, or leave the room cool, especially if weather changes suddenly.)
Install an ordinary wood stove.
We had a Tulikivi masonry heater installed when we built our well insulated timber frame home. We love it and absolutely consider it to be worth the cost. Used as designed, burning the recommended amount of firewood daily in two short burns, our Tulikivi is always warm, but never too hot to touch or lean against, and radiates a consistent amount of heat into the room throughout the day and night. It is also incredibly efficient - we almost never see smoke exiting our chimney during a burn.
That said, it would be inappropriate to install a masonry heater in a room that had uninsulated masonry walls and not consistently occupied. It is just not designed to quickly heat up a room when only used occasionally, or to immediately generate more heat if you throw another log in.
More information is available in the following two articles:
"Masonry Heaters Burn Hot and Clean"
"Are Masonry Heaters a Good Match for Superinsulated Houses?"
Wood stoves that are fully below grade are prone to backdrafting. The combustion air intake to the room is ideally be no higher than the firebox.
Insulating and air sealing the basement walls to current IRC code min (R15 continuous insulation) would keep it above 50F even during extended cold snaps, if the rooms above are heated. The subsoil temps in CT are north of 50F, and with a 65F floor above, 55F dirt below, it won't go much below 60F. If the room temp drops to 50F the floor will be emitting heat into the room.
I recommended a masonry heater for my good friend in Aspen Park, CO. The house was log walls but a standard roof and poured concrete foundation. I specified the insulation details for the foundation and the ceiling.
He used a TempCast unit and just loves it. He has some acreage so dead fall wood is available. Masonry heaters must be built be a craftsman and are somewhat costly. They use outside air for combustion and are typically designed and built for the individual characteristics of the house.
You said "occasional" use which is incompatible with a masonry heater that needs to be kept warm the whole heating season. Just my two cents.
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A masonry heater is going to function most effectively when it will be fired with regularity - at least once and as much as twice daily. Beyond that the efficiency decreases due to the energy loss in bringing the mass up to temperature. They are particularly appropriate in a modern and well insulated house where the heater's thermal mass and its slow and consistent transfer of heat best matches the heating requirements and characteristics of a well insulated home. I'm perplexed at the comments to the contrary. That said, there are many examples of masonry heaters in older, poorly insulated homes that still do the job albeit with more wood and work. The problem that arises is the classic regarding heaters and efficiency: it's not so much the efficiency of the heater but rather the efficiency of the structure that makes the difference; the most efficient heater in an inefficient structure isn't going to save you much due to your demand. The benefit of a masonry heater is somewhat lost in an inefficient structure. It would need to be oversized and fired excessively. As stated earlier the mass of masonry wall would also act as a heat sink absorbing the heat from the masonry heater's mass, although if you fired the heater with regularity and kept the temperature constant it would work out fine. If you want a heater that you can fire occasionally on those times when you'll be using the space then a woodstove would serve you well. The flue is a key element to proper draft. You'll have your best chance of good draft if the flue (I'm assuming in that old house that it's a masonry chimney and square or rectangular flue) is inside the house and isn't against the wall outside. If it is in the house and square or rectangular it would serve the draft well to have a round insulated liner installed. That's a decent investment though considerably less than a masonry heater, so you might want to consider the stove first and consult the manufacturer regarding flue requirements and max length. Stove manufacturers will give you specs regarding flue size requirements and also can offer input on functional length. Keep in mind, modern efficient stoves when fired properly release less heat to the chimney so that flue down in the basement needs to be optimal. I've seen a lot of basement stoves with good set ups that work beautifully.
For the cost of a typical masonry heater, you could insulate your basement (using low global warming potential, HFO-blown spray foam) and add a mini-split head for heating, cooling and dehumidification, and still have money left over.
If you want to use a masonry heater, which should stay warm enough from your home's primary heat source to draft well, you could consider one of the "Tigchelkachels" kits, intended for smaller spaces:
https://mainewoodheat.com/masonry-heaters/heater-products/imported-heater-kits/
Or just get a standard, EPA-certified wood stove. But it would be dumb to not insulate the basement first.
Thanks, everyone. Insulation would ideally be my first step, but I guess I've always been intimidated by the limestone walls. If I had concrete--or even granite or some other durable stone--I would have tackled that project a long time ago.
To those who pointed out that I should insulate and offered specific suggestions on how to do it (Dana and Michael):
- Would you be concerned about covering up the limestone and not being able to inspect it for deterioration?
- Or would the fact that it's covered actually make you feel better because it would be limiting the amount of moisture moving through the stone?
...if I do cover the walls with foam:
- Would you repoint the stones and add a lime wash or lime plaster to the wall first?
- Would you use some sort of membrane or dimple mat between the foam and the wall?
- Would you add a perimeter sub-slab drain as an extra layer of protection?
Only once in the 16 years I've been in the house have I seen water come in through the wall, and it was because of erosion around a loose downspout making the ground slope towards gaps between stones in the wall. But still it makes me nervous not to see the condition of the stones even though I've dealt with the drainage around the outside of the house.
How about this alternative? Adding some sort of exterior rigid insulation on the above-grade portion of my foundation (foam? mineral wool?) and extending it partially below grade. I know from using an IR camera that the 2 feet or so of brick that makes up the exposed portion of the wall is the biggest heat sink in my entire house. I would obviously cover it with something like cement board or stucco, and I would pull off some siding to flash it in properly. It just seems like a lot less work than insulating the whole basement.
Thanks for all of your advice!
Rob, there are legitimate concerns about covering over an actively leaking foundation, and one one occasion I had a client opt to excavate the entire perimeter of the foundation and install dimple mat over the stones, with a drain below. But if it's just an occasional leak, I wouldn't be concerned. More importantly, Dr. Joe Lstiburek has written about this and is also not concerned with minor seepage. There are enough small voids that a dimple mat on the interior should not be necessary, though it might help with an interior perimeter drain solution. A lime wash would be a good idea if you are worried about spalling, as a sacrificial layer, but if it's covered up with foam it will eventually degrade so I don't know if it's worth the effort.
If you want to stay on the outside, the choice of insulation depends on aesthetics, how smooth or rough the stones are, your concerns with environmental impact, and your budget. I would lean toward using rigid mineral wool, going as far below grade as you can afford (at least below the frost line), covering it with expanded metal lathe and parging it.
Sounds good. I've researched these topics a bunch over the years, so I'm pretty familiar with a lot of the practical details, Mike, but I guess I reach out to you guys to give me confidence that I'm doing the right thing. Thanks again for your advice.
I've got a rubble stone foundation here in Mystic CT. Pretty big stones, many of them cut, originally pointed with lime mortar. Timber frame construction. Someone poured a slab in the basement at one point, and thankfully put plastic under it, but no insulation.
After pricing out spray foam as per Joe Lstiburek's method, I decided to use rigid foam board instead.
The rocks were very uneven, but the sill is nice and straight handhewn 8x8 chesnut.
Can't put foam on the outside of a foundation like this, because the way it is built is that the rocks flare out away from the house and dirt was used as mortar. Once you start digging it is very hard to find the transition from random rocks in the dirt to rocks that support the wall. Another way to put it is that the house foundation is integrated into the surrounding soil structure. Pretty organic way to build!
So what I did was this: I had to repair a ton of rot in the sill anyway, so when I was lifting up the sills I snuck strips of heavy ice-and-water barrier between the wood and the rocks. This is a capillary break to keep water away from the newly replaced wooden sill. These strips are about 20" wide and I draped the extra width down the inside face of the foundation.
Next I excavated a trench using a jackhammer around the perimeter of the basement, busting through the slab about 8" all the way around. I dug it out and installed a perforated drain pipe bedded in pea gravel. I wrapped the pipe in landscape fabric to keep dirt from clogging the holes. The pipe is very slightly pitched toward my sump pump bucket.
I ran heavy sheet plastic around the whole basement and stapled it to the sill. I hung the plastic down into the trench, behind the drain pipe. The plastic catches any water that comes in through the rocks and directs it down into the pipe.
Finally the insulation. I attached strips of EPS rigid foam to the sill in layers until the surface of the foam was as high as the highest rock, or in other words, plumb with the proudest spot on each wall. Usually this meant at least three layers of 2" foam. I scribed it around the ends of joists (not necessary when the sill was parallel with the joists, although because it is a timber frame there are purlins and girts etc). I filled any gaps with spray foam as I went.
I discovered that my foundation walls are sloped away from the bottom slightly, which meant that the bottom of the walls was usually the highest point. This meant that each layer of foam could be slightly wider than the last, as it could hang lower on the wall. This is good because the higher you get in a foundation the greater difference between inner and outer temperatures. So more insulation high up. Fortunately for me the top layer of my foundation is made of beautiful rectangular cut pieces, which are thinner than the rest of the wall, so more room for insulation right where I need it most.
Finally, I hung full sheets (or nearly so, my basement is more like 7' high) all the way from the subfloor to the top of the drain pipe. I attached the top of the sheet to the sill insulation (all the way up to the subfloor) and let the sheet sit right on the drain pipe. I shimmed each sheet as it went in with scraps of foam and wedged it in front with temporary rocks until it was perfectly plumb. Then I hung the next one. I sealed between these with spray foam as I went. When all the sheets were hung I finally filled over the top of my trench with concrete- this locked in the base of the foam board so it can never move.
The result is perfectly plumb, well insulated walls. Below grade the insulation is R-10 because I went with 2" EPS; above grade, it's more like R30. Now if I want I could build a stud wall and insulate that, with no fear of condensation, but I probably won't.
This sounds pretty tricky, and I guess it was, but I think it's better than spray foam. First, the walls are vertical and not all bumpy and weird looking. Second, it's reversible, no damage to the stones if for some reason we want to expose them again. Third, the really hard parts were getting the capillary break in there and digging the drain trench, and you have to do that with spray foam too. Fourth, vertical walls are easier to flame-proof than bumpy ones (if you are putting a wood stove in your basement, code will require this, or any non-sealed combustion appliance for that matter). Fifth, it was way cheaper.
Because my renovation started with completely gutting my old house, I am insulating around the chimney. I also have an unused flue space. This basically allows cold air to blow down in there all winter and chill the house from the inside out. If you aren't using the flue I recommend capping it to prevent this. I'm using my flue as a vent for some tricky plumbing spots (probably not recommended but it works) which means I can't cap it. So instead I'm insulating all around the whole chimney stack as it is currently exposed, using phenolic board which is non-combustible (KoolTherm). Probably not up to code either. But warmer. The active side of the flue has a top mounted damper on it to keep cold air out- these are better than the usual ones right over the firebox, but not perfect. Old house renovations are all about compromise.
Usually, you load the fire every 12-24 hours (or even less often if you have a very well-insulated house). Most of this heat would simply escape from your chimney if you used a hearth or a metal stove. This makes masonry heaters much more efficient, and due to the thoughtful design of the firebox, they produce a minimum amount of harmful emissions. Therefore, I consider masonry heaters a good investment, and in order to buy it, I even took out part of my savings on https://www.moneyunder30.com/. I have accumulated a decent amount there over the year, and I have not regretted it for a second.
I'll take advantage of the spam bump to say that our BS + Beer Show topic on Oct. 6 will be revisiting masonry heaters, with experts Albie Barden and Rod Zander. Albie was recently in Finland where he learned about new developments in masonry heaters. The last time Albie was on the show he spoke convincingly about masonry heaters as a viable option for climate-resilient homes and he is excited about sharing what he has learned.