Would a Thermal Cinder Block Wall Transmit Heat well?

[Post Carbon]

Hi all,

DH is building an alcove in a cathedral ceiling room that will poke into the next-door garage/workshop by about three feet. The idea is that the stove sits mostly in the alcove out of the way of traffic in the living area of the big room, and the completely closed off alcove juts into the garage. He wants some heat transfer into the garage, but we don't want any ducts or holes. By code, we can't do that anyway because they worry about gasoline in cars, since it could be a garage.

Our idea, maybe insane, is to build the back wall of the alcove, from concrete floor to wooden ceiling, out of cinder block, and hope that enough residual heat will transmit thru the block into the garage to take any edge off the cold while he works in there. Mostly it's for woodworking. This is the Pacific NW so it doesn't get super cold. The sides of the alcove would be framed as normal, covered with cement board, and then sides and back rocked with flagstone or such around the stove for the aesthetics of it. The alcove wall is 9 feet high to the joists.

Has anyone heard of this being done? Are we completely out of our minds? We're pretty new to woodburning, but we think this alcove will trap some heat in the stone and block and go somewhere.....we thought cinderblock because then we wouldn't have to do anything to the wall on the garage side.

And then there's the question of the thinner 4" block or the regular 8" block with bigger air spaces.

I searched the archives for cinder block and thermal or heat transfer and come up with nothing.

Thanks for any wisdom.....

PCP

 

[1kzwoman]

Don't know about heat, but a cinder block wall will almost let a cold wind pass thru unimpeded at 35 below

 

[GENECOP]

Probably won't do it, search out Rocket Mass Heater or Rocket Stove....read, read, read....

 

[Simonkenton]

You have come up with a great idea. Concrete block has an R factor of about zero. That is, it has almost no insulation qualities and will transmit heat, or cold very well.
Put that block wall there and I assure you it will transmit lots of heat into the other room. Furthermore, due to the massive weight of the block, that wall will hold heat and continue to transmit it long after the fire has gone out.

 

[GENECOP]

You have come up with a great idea. Concrete block has an R factor of about zero. That is, it has almost no insulation qualities and will transmit heat, or cold very well.
Put that block wall there and I assure you it will transmit lots of heat into the other room. Furthermore, due to the massive weight of the block, that wall will hold heat and continue to transmit it long after the fire has gone out.

What happens during the winter when they don't have a fire going?

 

[Simonkenton]

In the winter when there is no fire going, the concrete wall will transmit cold back into the room with the wood stove.

 

[Nick Mystic]

My woodstove sits in front of our massive brick fireplace and chimney where it heats up the 6500 brick monster. I can tell you once all that thermal mass gets warm it gives back the heat for a good long time. In my setup the chimney is much more massive than what you are suggesting, but you would certainly get heat coming through your cinder block wall and into your garage. However, I doubt it will actually do much to heat the garage to the level where it would make a big difference to the point where it would become warm enough to work comfortably without an additional heat source. If you do go ahead with your proposed plan you would have to finish the garage with insulated walls and ceiling and seal all the air leak areas to get any hope of feeling the stove heat. Just heating the concrete floor is going to be a big challenge since it's unlikely you'll be able to insulate it in any meaningful way. Good luck.

 

[GENECOP]

What about the cold from the unheated garage side....The stove is not the primary heat source, won't they end up with a cold wall transmitting into the heated space...When they are not burning..

 

[BrotherBart]

My neighbor's chimney was exterior till he built the garage on that end of the house. Burning in his insert around the clock all winter does zip for heating the garage.

 

[Nick Mystic]

Our large two car garage is made of 12" cinder block walls on the two sides with four inch rock veneer on the outside. The front has two garage doors with single pane windows. The back wall is up against the unheated basement, which stays around 54 degrees during winter. However, the ceiling of the garage is the master bedroom floor upstairs. I don't have any insulation in the floor, but there is an inch and a half of plywood for the floor with a pad and carpet on top. The only heat in the garage is a propane hot water tank giving off some residual heat. The heat coming through the bedroom floor and the 54 degree heat coming through the back cinder block wall is enough to keep the garage above freezing all winter long, even on days when it is only in the single digits. So, you will get some heating from your proposed plan, but working in a shop that is only in the 40, let's say, isn't going to feel particularly warm. It will, however, keep the pipes from freezing if you have any plumbing running through the garage like we do.

 

[mstoelton]

heat will get sucked into the garage area even when there is no fire going in the stove. This will make the area the stove sits in very cold and "drafty" feeling when there is no fire in the stove. It will also reduce the felt heat coming out of the stove when the stove is fired. I suggest you don't do it.

 

[Simonkenton]

For a comparison, I have a masonry fireplace that sets entirely within my house. That is, the back rock wall of the fireplace is the interior wall of the bathroom.
The back wall of my fireplace is 2 feet thick, solid masonry. When you run the fireplace it heats up that back wall. It takes 5 hours for that back wall to begin throwing heat. That back, rock wall will get up to 105 degrees.
I will let the fire go out at midnight. The entire next day that bathroom is warm as toast. The bathroom will stay at about 80 degrees that entire next day, with no fire burning in the fireplace, and with the bathroom door open.

For your purpose, you could make some insulating panels to put up when the wood stove is not running. Get some 10 foot 2x4s and frame up some panels, 9 feet high. Put those sheets of foam insulation in the 3 1/2 inch space. That dense cell foam is about R 9 to the inch. So you could have foam panels with R 27. When the wood stove has gone out and the wall has cooled down, set your insulated panels in place.

 

[GENECOP]

Last week there was a post (how I make hot water) guy had a regular wood stove with an after market coil system that he installed into the stove, (he even linked the supplier of the coil)..Now if you did something like that and ran piping through your block wall and filled your block wall solid you would be on to something, small 12 V pump and you have your heated garage.....

 

[begreen]

This might work in a minor fashion if, and only if the garage is thoroughly sealed and insulated. Otherwise I suspect the heat loss of the garage would easily exceed the heat gain. I would also put insulated mat flooring down too.

 

[Kosmik]

Already built alcove?
May wish to reconsider the design. A stove will radiate only so much heat to heat up masonry (not enough for another room).
Masonry stoves operate on the principle that a 1000+ deg fires gases are being funnelled trough channels of specific types of stone to absorb heat and then radiate that heat. Kinda the idea you were going towards. Cinder blocks aren't 'massive' in the world of stone.
Large fireplaces (hearths) of old operate much like a masonry heater as you cooked and heated in the hearth and the resultant warming of the stone by the fire radiated the heat back.
Iron/steel stoves minimize that mass/surface area need with metal.

Any existing HVAC, or radiant sources?

 

[xman23]

If you have the stove at normal clearance's, the wall won't be much over 110 degrees. By the time it get to the other side of a cinderblock I don't think you will have any heat. Even if you did get it to work there would be no way to regulate it. What happens when you don't want to heat the garage, you will be wasting heat. Going where you don't want. I'd use something else.

 

[begreen]

A ducted EPA ZC fireplace in this situation might be interesting. That would allow you to insulate the behind the fireplace for isolation from the garage, yet dump heat into the garage when desired as long as the fireplace was burning.

 

[Simonkenton]

"If you have the stove at normal clearance's, the wall won't be much over 110 degrees. By the time it get to the other side of a cinderblock I don't think you will have any heat."

If the front side of the block wall is 110, the back wall will be about 108. Concrete transmits heat very well. 108 degrees is a lot of heat, coming from a massive block wall.

 

[byQ]

I kind of know of one case where someone did something like this. Thomas Elpel in Montana built a masonry heater with the front in the living room and the back in the garage. Moreover, he put a door on both the front and the back - so 2 doors. His logic for doing this was to be able to load the heater from the garage where the wood was, thus avoiding hauling firewood into the living room. This set up would heat an insulated garage.

You could even make the masonry heater such that for example 30% of the heat went into the garage and 70 % went into the living quarters. This could be done by using some insulating firebrick on the outer shell part of the heater that is in the garage. These insulating bricks would slow down some of the heat transfer into the garage.