Wood Stove for Cabin - Concrete Block Walls

[Big Scioty]

Hi, looking for a woodstove recommendation for a cabin I just purchased:

• 30 x 25ft x 7ft ceilings (650sf inside) - 2 bedrooms off main
• All walls are concrete blocks (R1) no drywall
• Rubber roof w/paneled ceiling - no insulation
• 1 window on each outside wall. 2 doors
• Stone hearth installation - need rear vent
• Will be primary heat source

The cabin is in SW PA so worst case lows of around zero degrees (low 20s more normal). Winter stays will range from 2-7 days. Its a small place and my research is telling me something like a Jotul FC-3B or Morso 2110 or similar (1cu ft firebox) would normally be enough stove but I'm leary of the concrete blocks which may act like a big heat sink upon start-up and insulate about as well as single pane glass. Would like to get some more experienced perspectives!

 

[Simonkenton]

You are correct, sir! Those uninsulated block walls will be a massive heat sink. Couldn't you put up studs on the inside, and at least get 3 1/2 inches of fiberglass in that building?

 

[begreen]

More like a big heat suck than a heat sink. With virtually no insulation in the building whatever heat source is used, a large percentage of the BTUs will be heating outdoors. I would be looking at a stove at least double in size. That is 2 cu ft or larger. If the building is allowed to go cold in between stays then I would be looking even in the 2.5-3 cu ft range.

 

[Big Scioty]

You are correct, sir! Those uninsulated block walls will be a massive heat sink. Couldn't you put up studs on the inside, and at least get 3 1/2 inches of fiberglass in that building?

Yes, definitely doable - that would be my fallback if my stove can't keep up and I start living there more. Was hoping to size stove to account for losses but maybe that's not the right approach for the long term... Thanks for the comment.Helpful.

 

[xman23]

Yea those stoves sound small. What's the roof / ceiling design and insulation. The majority goes up and out. A friend insulated his basement with this thin foil / insulation. It had some unbelievable R value. A builder I know started using it when the code required basements to be insulated.

 

[Big Scioty]

More like a big heat suck than a heat sink. With virtually no insulation in the building whatever heat source is used, a large percentage of the BTUs will be heating outdoors. I would be looking at a stove at least double in size. That is 2 cu ft or larger. If the building is allowed to go cold in between stays then I would be looking even in the 2.5-3 cu ft range.

Thanks. I think this is a valuable recommendation. I was offered a returned Oslo (2.5 cu ft) model at half price but told the dealer I thought it was too big for the space..maybe not. Is it possible to run a big stove hot but not necessarily at full capacity on less cold days? My only experience is with Jotul 602 which I run pretty much full out in my home as auxiliary heat.

 

[Big Scioty]

Yea those stoves sound small. What's the roof / ceiling design and insulation. The majority goes up and out. A friend insulated his basement with this thin foil / insulation. It had some unbelievable R value. A builder I know started using it when the code required basements to be insulated.

Thanks for reading. Rubber membrane roof I'm guessing over plywood - air gap - then thin interior paneling - no fiberglass.

 

[EatenByLimestone]

The main concern I'd have is opening up the door to a cabin that's 10 degrees. It takes an awful lot of energy to bring that cabin up to temperature in a reasonable amount of time. I doubt 2.5 cu ft will do it quick enough. Is there any way to supplement the initial warmup with propane or kero?

 

[begreen]

Thanks. I think this is a valuable recommendation. I was offered a returned Oslo (2.5 cu ft) model at half price but told the dealer I thought it was too big for the space..maybe not. Is it possible to run a big stove hot but not necessarily at full capacity on less cold days? My only experience is with Jotul 602 which I run pretty much full out in my home as auxiliary heat.

The Oslo would be a good choice. It can be run with a partial load of fuel.

 

[xman23]

Thanks. I think this is a valuable recommendation. I was offered a returned Oslo (2.5 cu ft) model at half price but told the dealer I thought it was too big for the space..maybe not. Is it possible to run a big stove hot but not necessarily at full capacity on less cold days? My only experience is with Jotul 602 which I run pretty much full out in my home as auxiliary heat.

I'm heating about 1500 sq ft of well insulated cabin. After getting the place up to temp, most of the time my Oslo can't be run to hard or I overheat the house. After I get a bed of coals I can put in two small splits. The stove top temp runs up to 300 or so then it cycles thru the burn back down to 175 before reload or let it go out. Yes I would say it's running it at 1/3 or 1/4 capacity. It works for me without gunking up the chimney. I do have well seasoned wood.

I would look take a look at that Oslo . Half price, with low millage doesn't sound bad. Just figure out what chimney height you will end up with. You don't want to have draft issues.

 

[begreen]

Good point. The Oslo is going to want at least 14 ft for top exit and perhaps a few feet more for rear exit.

 

[Simonkenton]

I wonder if you could insulate the outside of the little cabin. Perhaps you could attach some 4x8 foot 3 1/2 inch thick foam panels to the outside with construction adhesive.
This company sells panels like this.

http://www.raycore.com/roof-panels.php

They are referred to as "roof panels" but would work fine for a wall. As you can see, these panels have 2x4s incorporated into them, so that, you could attach some T 111, or some kind of wood paneling on the outside.
If you insulated the exterior walls, then the concrete block would act as a heat bank. The massive concrete block walls would absorb heat from the wood stove, and slowly give it back into the room.
A lot of trouble, and money to do, it just depends on how much time you will spend in the cabin.

 

[begreen]

Good idea, though it would still take most of a day to bring all that mass up to temperature if it was as cold as outdoor temps.

 

[Big Scioty]

The main concern I'd have is opening up the door to a cabin that's 10 degrees. It takes an awful lot of energy to bring that cabin up to temperature in a reasonable amount of time. I doubt 2.5 cu ft will do it quick enough. Is there any way to supplement the initial warmup with propane or kero?

I agree. My first day in the cabin outside air was 50 degrees but inside low 30's due to previous week of low temps. Built a test fire in the fireplace and got very little increase in temperature.There is an old duo-therm oil burner (functional/vented w/blower) that I got working but it is really stinky to the point I'd rather not use it - but it is a fall back. Cabin has electric so I could throw in some baseboards but that kind of goes against the rustic experience. Thanks for the comments!

 

[Big Scioty]

I wonder if you could insulate the outside of the little cabin. Perhaps you could attach some 4x8 foot 3 1/2 inch thick foam panels to the outside with construction adhesive.
This company sells panels like this.

http://www.raycore.com/roof-panels.php

They are referred to as "roof panels" but would work fine for a wall. As you can see, these panels have 2x4s incorporated into them, so that, you could attach some T 111, or some kind of wood paneling on the outside.
If you insulated the exterior walls, then the concrete block would act as a heat bank. The massive concrete block walls would absorb heat from the wood stove, and slowly give it back into the room.
A lot of trouble, and money to do, it just depends on how much time you will spend in the cabin.

Great idea! Will keep that in mind as I move towards better efficiency, longer stays, etc. Interestingly, the interior walls are block also which I think means more warm up time required but at least these will retain most of the energy.

 

[Simonkenton]

Interior block walls! Those are a big plus for you.
What we are talking about is thermal mass. A conventional house made of 2x4s has a negligible thermal mass. My house has thermal mass coming and going.
First it is a log house and the massive weight of the logs gives this structure thermal mass. In other words, it doesn't want to change temperature. If you go into the house after an absence of a week, in January, and you forgot to leave the heat on, and it is 40 degrees inside the house, it will take 10 hours to bring the house up to temp.
On the other hand, if you have been running your wood stove for a couple days, and you let the stove go out, and it goes down to 35 degrees that night, you won't need any more heat, because the massive weight of the log walls has absorbed all that heat, and slowly gives it back into the house.
The big advantage of logs over concrete block is that, my walls insulate, while the concrete walls provide virtually no insulation.

Even better in my house is the fireplace pictured to the left. This fireplace weighs 18 tons and is entirely inside the house, so that, the back wall of the fireplace is the inside wall of the bathroom, and is entirely enclosed within the bedroom upstairs.
Here again, the thermal mass of the masonry changes temp very slowly. I can run my wood stove for a couple days, and those 18 tons of masonry are absorbing that heat the entire time, and if the heat is turned off, the chimney walls slowly give that heat back into the house.
Your interior concrete walls will do just the same as my log walls and my fireplace, you get the advantage of thermal mass.
However, your exterior block walls will be just the opposite. Concrete has a R value of about zero, in that, it does not insulate, but it transmits heat or cold. So you have the cold Pennsylvania temps coming in from the outside, while your wood stove drives heat into the block from the inside. Heat battling cold in your exterior walls.

Any way, if you are going to go to the trouble of insulating, insulate the outside of the exterior walls and take full advantage of the thermal mass. Your walls are probably heavier than my fireplace.

 

[Big Scioty]

I'm heating about 1500 sq ft of well insulated cabin. After getting the place up to temp, most of the time my Oslo can't be run to hard or I overheat the house. After I get a bed of coals I can put in two small splits. The stove top temp runs up to 300 or so then it cycles thru the burn back down to 175 before reload or let it go out. Yes I would say it's running it at 1/3 or 1/4 capacity. It works for me without gunking up the chimney. I do have well seasoned wood.

I would look take a look at that Oslo . Half price, with low millage doesn't sound bad. Just figure out what chimney height you will end up with. You don't want to have draft issues.

I'm heating about 1500 sq ft of well insulated cabin. After getting the place up to temp, most of the time my Oslo can't be run to hard or I overheat the house. After I get a bed of coals I can put in two small splits. The stove top temp runs up to 300 or so then it cycles thru the burn back down to 175 before reload or let it go out. Yes I would say it's running it at 1/3 or 1/4 capacity. It works for me without gunking up the chimney. I do have well seasoned wood.

I would look take a look at that Oslo . Half price, with low millage doesn't sound bad. Just figure out what chimney height you will end up with. You don't want to have draft issues.

Very helpful feedback on the Oslo, burn capacity, etc! I bet your place heats up relatively quickly given the insulation. I think the Oslo would work for me if I leave the cabin "as-is" but I might be able to go down a notch in stove size if I insulate (understanding it will still take a while to get the blocks up to temp). Thanks for sharing your experience!

 

[Big Scioty]

Interior block walls! Those are a big plus for you.
What we are talking about is thermal mass. A conventional house made of 2x4s has a negligible thermal mass. My house has thermal mass coming and going.
First it is a log house and the massive weight of the logs gives this structure thermal mass. In other words, it doesn't want to change temperature. If you go into the house after an absence of a week, in January, and you forgot to leave the heat on, and it is 40 degrees inside the house, it will take 10 hours to bring the house up to temp.
On the other hand, if you have been running your wood stove for a couple days, and you let the stove go out, and it goes down to 35 degrees that night, you won't need any more heat, because the massive weight of the log walls has absorbed all that heat, and slowly gives it back into the house.

Even better in my house is the fireplace pictured to the left. This fireplace weighs 18 tons and is entirely inside the house, so that, the back wall of the fireplace is the inside wall of the bathroom, and is entirely enclosed within the bedroom upstairs.
Here again, the thermal mass of the masonry changes temp very slowly. I can run my wood stove for a couple days, and those 18 tons of masonry are absorbing that heat the entire time, and if the heat is turned off, the chimney walls slowly give that heat back into the house.
Your interior concrete walls will do just the same as my log walls and my fireplace, you get the advantage of thermal mass.
However, your exterior block walls will be just the opposite. Concrete has a R value of about zero, in that, it does not insulate, but it transmits heat or cold. So you have the cold Pennsylvania temps coming in from the outside, while your wood stove drives heat into the block from the inside. Heat battling cold in your exterior walls.

Any way, if you are going to go to the trouble of insulating, insulate the outside of the exterior walls and take full advantage of the thermal mass. Your walls are probably heavier than my fireplace.

Totally agree. With the exterior insulation the house would function like a soapstone stove but with the heat radiating in. Thanks for your insights and for sharing details on your set-up! It sounds nice.

 

[xman23]

Very helpful feedback on the Oslo, burn capacity, etc! I bet your place heats up relatively quickly given the insulation. I think the Oslo would work for me if I leave the cabin "as-is" but I might be able to go down a notch in stove size if I insulate (understanding it will still take a while to get the blocks up to temp). Thanks for sharing your experience!

I does take a real lot to raise the temperature of a cold house. We keep the place at 45. When we get to the cabin we turn up all the electric heat and crank the stove at 700 degrees for hours. Maybe all that raises the temp 4 or 5 degrees an hour. You sit around in coats hovering over the stove. It takes a lot to heat the mass, but when you get the walls heated, it cools equally as slow.

We stopped that years ago. Now the heat comes on at noon. When we arrive the house is 65. The Oslo is in semi retirement. It rarely sees 500 degrees any more.