Block off plate versus Insulation?

[JoeRJGR]

OK, so let me just throw this out there.

Everyone pretty much recommends a block off plate for installation of an insert to keep the warm air in the room. Everyone also pretty much recommends insulating the SDS liner so that when it cools off there will be less moisture and cresote formed.

But if you dont put in a block off plate wont the air between the liner and the fireplace be warmer and thus less likely to have the cooling off issue described above???

 

[Eaglecraft]

OK, so let me just throw this out there.

Everyone pretty much recommends a block off plate for installation of an insert to keep the warm air in the room. Everyone also pretty much recommends insulating the SDS liner so that when it cools off there will be less moisture and cresote formed.

But if you dont put in a block off plate wont the air between the liner and the fireplace be warmer and thus less likely to have the cooling off issue described above???

Hi Joe:

Hot air rises, so the block plate will slow down the escape of hot air (being thrown off by your insert) up though the open space between the insert and the SS Flex pipe that goes all the way to the top of your chimney. The objective is to retain as much stove heat as possible and reflect that heat into the living space. Heating the masonry chimney above the insert just wastes heat.

To make the block plate even more effective at minimizing heat loss to the chimney , insulate the block-off plate - that side that is away from the stove using 2 inches of ROXUL, mineral fiber board. When you install the top plate - that's the SS assembly that sits on top of either your teracotta flue tile, or chimney crown wash - depending on your situation, you will form a dead air space between the bottom block-off plate and top plate. The dead air space minimizes the loss of heat from the SS Flex Pipe to the masonry chimney.

But to achieve maximum effectiveness with the bottom and top block plate, insulate the SS Flex Pipe with at least 1/2 inch insulation - the type of insulation that is sold with SS Flex pipe. This insulation will help minimize creosote formation in the pipe. To further achieve the best dead air space possible, I added 2 inches of ROXUL just below the level of my teracotta flue liner.

Below are two photos - one showing a "double" top plate with a) 2 inches of ROXUL and b) the SS top plate - which holds the liner in place and the rain cap.

Keep on trucking Joe.

 

[krex1010]

Creosote formation happens mostly in the time shortly after new wood has been loaded into the stove. Stove temps and heat output are generally low when we add wood so you can bet that the masonry chimney is cold, so the airspace around your liner will be cold until your stove has put out enough heat to warm up the masonry, but by then the fire is going to be past the phase where creosote formation usually occurs. So let the block off plate help the stove do it's job, which is heating your living space not your chimney. And let liner insulation and good burning practices help keep your liner clean.

 

[mhrischuk]

Thing is, with the top sealed there can't be any real air movement except for convection. A non-metallic, well insulated smoke chamber will stop the convection from occurring. A metal blockoff plate down below could even have the effect of transferring heat up into the smoke chamber.

 

[Vic99]

"A metal blockoff plate down below could even have the effect of transferring heat up into the smoke chamber."

Agreed. What would you build a block off plate out of then? When my liner was installed, the installer built the block off with a metal mesh and maybe one inch rock wool. He did a poor job and their are gaps in the rock wool.

I plan to rebuild the block off this month. Would mesh held rock wool by itself be enough?

Even though fiberglass blocks heat transfer, it does not block air movement as well as a material like polystyrene, which is obviously not rated for high heat. What could I combine with rock wool to make a well insulated block off plate?

 

[begreen]

By itself, a metal block-off plate may allow some transfer. Likewise, if you just stuff it with kaowool or roxul, there will be some air leakage. Neither is perfect by itself, though better than nothing. It's the combination of the two that does the best job. The metal block-off plate assures a clean, tight, air-sealed fit, closing off the damper chamber. Insulation on top of the block off plate prevents heat from transferring.

 

[EatenByLimestone]

Heat travels in 2 ways... convection and radiation. Insulation will slow down the radiation, but you can still have convection around the edges of, say, an insulation bat. The metal block off plate physically stops the convection at that point, but as pointed out, metal conducts heat so radiation can still occur. The insulation would slow that down. Do both.


Matt

 

[mhrischuk]

All sounds good. I was able to stuff a full batt up both sides of the damper opening. The last part got stuffed pretty good. Then I cut two more batts and wired them up. It's sealed really well. Also, Roxul seems to be a bit denser than fiberglass wool. So I have 4 full batts closing it off. The last two horizontal layers are fit tight around the edges. I agree a fully sealed plate would make it even better but in my case not really worth it.


This shot is just the smoke chamber stuffed.

This is after adding the two horizontal layers wired up with stainless wire wrapped around masonry nails on the sides.
That's about 6" thickness in itself.

 

[PLAYS WITH FIRE]

In Germany the word for this, I believe, is goodntight!