Block Off Plate - Insulation and My Chimney

100% yes to the block off plate, search on here how to do one. If you are getting good draft and your flue tiles are not cracked then you should be ok with no insulation on the liner.

begreen said:

Yes, with an exterior chimney, you will get more heat and use less wood .

https://www.hearth.com/talk/wiki/why-damper-seal-is-needed/

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My installer who designs, builds, and repairs chimneys was adamant about not installing a blocking plate in a chimney with a clay flue. He said once the top is capped off it is best to allow the heat to rise up from the removed damper to eliminate moisture. Further, he stated that the heat also improves draft because the cavity does not cool the metal chimney pipe attached to the insert. He also said you may get additional BTUs by capping the damper but possibly pay a few thousand or more later when the chimney flue begins to crumble from excessive moisture. He has been doing this line of work for 40 years and is well respected in the field.

That is interesting point.....I would think though with all the hundreds or thousands of years experience collectively on here someone would have had problems like this....I mean sewage pipes used to be made form Terra-cotta and I've seen them dug up after many decades in the ground in near perfect condition.....granted not exposed to heat like a flue. I've been to Europe and have seen roofs made from TC that's 1200+ years old in near perfect condition. I'd image those have been though many expansion/contraction cycles and are original....that's very interesting point and I'm not arguing!

Where would the "excessive moisture" come from? If there is a blocking plate in the flue and it is sealed at both ends (at the blocking plate and the cap) there should not be a way for any "excessive moisture" to get into the clay liner. If there is a way for the moisture to get into the clay liner then there is most likely a way for it to get out! And further, if you are installing a sealed top plate, would that not effectively prevent the rising heated moisture from escaping at the top? I'm not an installer, mason, or a fireplace guru, but I do know my physics, and general engineering.

Maybe the installer was giving you a line of horse apples so he didn't have to make a blocking plate for each of his installs!

I think it would depend on what sort of chimney and stove/insert set up you have. In my case I have a 25' tall masonry chimney that is 4'x8' and contains over 6500 bricks. I have a 6" flex liner inside of an 11"x18" clay tile flue. When I changed out my old insert for a Jotul F 600 free standing stove I put in a metal block off plate with insulation above it. I also put insulation down my chimney about 4 - 5 feet around the liner to the roof line. Along with sealing off my top plate with silicone caulk. With this set up my chimney bricks around the top of the fireplace opening hit 250 F. Fifteen feet up the chimney is reading 95 F. I've got to believe a lot of heat is still getting to the clay tile liner driving out any moisture concern.

On the subject of insulation, based on the reading I have done on this forum, insulation will help reduce the build-up of creosote in your liner because it will help the liner stay warm once heated by your fire. Insulation will minimize the condensing of creosote in your liner that could be cold on the outside because you have an external chimney and you live in CT.

You might have some limitations on how to insulate in the 8x12 clay liner. You may need to do a poor in type insulation and that would almost necessitate the block-off plate.

mstoelton said:

Where would the "excessive moisture" come from? If there is a blocking plate in the flue and it is sealed at both ends (at the blocking plate and the cap) there should not be a way for any "excessive moisture" to get into the clay liner. If there is a way for the moisture to get into the clay liner then there is most likely a way for it to get out! And further, if you are installing a sealed top plate, would that not effectively prevent the rising heated moisture from escaping at the top? I'm not an installer, mason, or a fireplace guru, but I do know my physics, and general engineering.

Maybe the installer was giving you a line of horse apples so he didn't have to make a blocking plate for each of his installs!

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The way it was explained if you cap the damper then the cold chimney (brick and flue) exposed to the hot metal chimney creates condensation. If the chimney can't breath (like an attic that has the soffits blocked) moisture collects.

mstoelton said:

Where would the "excessive moisture" come from? If there is a blocking plate in the flue and it is sealed at both ends (at the blocking plate and the cap) there should not be a way for any "excessive moisture" to get into the clay liner. If there is a way for the moisture to get into the clay liner then there is most likely a way for it to get out! And further, if you are installing a sealed top plate, would that not effectively prevent the rising heated moisture from escaping at the top? I'm not an installer, mason, or a fireplace guru, but I do know my physics, and general engineering.

Maybe the installer was giving you a line of horse apples so he didn't have to make a blocking plate for each of his installs!

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Not that type of guy. He is well respected in the field and there is a waiting list to get his services. As I stated in my original post he designs, builds, and repairs chimneys. He has a website Fennerbuilders.com

PLAYS WITH FIRE said:

That is interesting point.....I would think though with all the hundreds or thousands of years experience collectively on here someone would have had problems like this....I mean sewage pipes used to be made form Terra-cotta and I've seen them dug up after many decades in the ground in near perfect condition.....granted not exposed to heat like a flue. I've been to Europe and have seen roofs made from TC that's 1200+ years old in near perfect condition. I'd image those have been though many expansion/contraction cycles and are original....that's very interesting point and I'm not arguing!

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I understand your comments. I too had a hard time wrapping my mind around this. But the more I thought about it the more sense it made. Once the cavity is heated or put another way - approaches equilibrium with the temperature of the stove then the stove will significantly reduce giving away BTUs to the cavity. The benefit is a hot cavity which starves off moisture and a hot temperature surrounding the chimney pipe keeping the smoke hot as it exits the chimney. This increases draft and allows the smoke to rise more efficiently. Cold smoke sinks like a hot air balloon when heat is removed.

Tronsilver

I understand your thinking with respect to the cold flue collecting condensation, however, this would only happen if there were air (containing water vapor) circulating in a chimney outside of the liner. If the circulation is stopped (sealed at top plate and at the flue blocking plate), then there is no opportunity to collect condensation. Water vapor is not going to come from the chimney liner and the top and bottom plate are sealed so were does the condensation come from - answer - it doesn't. The scenario you describe only happens if you don't have a flue blocking plate in place! If water vapor travels through the chimney walls - then it can exit the same way.

The attic scenario does not compare because you cannot have a sealed attic- there are always sources of airflow, and sources of humid air in an attic.

Without an air source (open flue plate) condensation can't happen in a chimney because there is not a source for water vapor if it is sealed at top and bottom. Condensation just doesn't happen - condensation is carried by circulating air.

On the other hand if you do not install a flue block, then humid air will cause a collection of condensation to collect in your chimney due to the air circulation from your living space entering the area between your liner and your chimney tile, and yes then you will have condensation in your chimney. with a blocking plate in place and sealed, this should be eliminated.

Note: I am talking about a blocking plate made of galvanized steel that is sealed between the appliance and the flue plate. It can be made airtight.

Brick and mortar are not impermeable. Rain soaked brick can absorb water and release vapor to the interior. Google effluourescence for examples.
The potential depends on installation specifics like exposure, if you have an adjacent flue and it's separation, presence and condition of clay liner, etc.

I could be wrong, but I would also think rain soaked brick will absorb water. I also have a 30 foot brick exterior wall chimney. There could very well be cracks as you get higher up the chimney in some of the mortar. Would this allow moisture to enter? I don't know but is sounds like it would. So, with the block off plate installed at the top of my chimney by my installers, and than the cap put on, would I be doing myself harm installing a plate on the inside bottom as well?

If the brick/mortar/flue liner allow water to enter due to cracks or just due to not being impermiable then they would also allow the water to exit in the same manner. The flue block will not stop the water from moving out in the same manner in entered.

It will reduce the amount of heat lost to the chimney.

Bagelboy said:

I could be wrong, but I would also think rain soaked brick will absorb water. I also have a 30 foot brick exterior wall chimney. There could very well be cracks as you get higher up the chimney in some of the mortar. Would this allow moisture to enter? I don't know but is sounds like it would. So, with the block off plate installed at the top of my chimney by my installers, and than the cap put on, would I be doing myself harm installing a plate on the inside bottom as well?

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I had the same doubts especially after reading numerous posts about the benefits of a blocking plate. And believe me I quizzed my installer on this topic because I was convinced I was right. He also stated that during his 40 years of working with chimneys the UL standards had changed numerous times and there is no longer a requirement for damper blocking plates because of the moisture issue. I suppose some insert manufacturers want you to have as much thermal efficiency as possible and are not concerned about damage that may take a number of years to show it's ugly face ( crumbling flue from moisture) so a damper blocking plate is recommended in their installation instructions. I was further convinced when I searched google for "chimney sweeps" and read info on their sites staring the number one enemy of masonry chimneys is moisture. So for me I'd rather error on the side of caution and listen to a guy with 40 years of experience running a successful business who has personally inspected a thousand chimneys and seen the damage firsthand. I completely understand that each person contemplating this issue will need to make a personal choice based on their own reasoning because there is nothing available that specifically states "don't install a damper blocking plate for these reasons." This topic reminds me of the argument about cigarette smoking, I'm sure you too heard the person espousing the reason for not quitting : " my grandfather smoked for 40 years and lived to be 80."

Meh, if the terracotta goes after 20 years I will just bust it out and it will give me an excuse to install rigid insulated pipe, of course who knows if we will still be able to burn in 20 years.

mellow said:

Meh, if the terracotta goes after 20 years I will just bust it out and it will give me an excuse to install rigid insulated pipe, of course who knows if we will still be able to burn in 20 years.

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I hear ya. In my instance the chimney is already 35 years old so I want to help the old girl make it a while longer. Don't want to spend another 3 - 5K on top of the 5k I just spent for the installed insert.

Mine was built in 1955, the terrcotta flue has cracks in it but the chimney overall is still structurally sound and should be good to go for another 50 years, just saying if the terracotta goes it isn't a deal breaker due to the block off plate.

I for one would rather have those BTU's going to my living space.

I thought I'd share the email my installer sent me concerning the damper blocking plate. Not trying to change anyone's mind just injecting information for consideration. As implied in the email I was pretty firm on my original opinion and as such he finally says OK if you're not going to listen to me then at least forgo the metal plate and just use insulation. He does though throw in the caveat that I still may be playing with fire (no pun). I also found interesting the part about UL and warranty.


"There are more factors than what you are considering, I have been a mason and fireplace installer for over forty years, I can tell you that most old sites on insert installations show a blocking plate, but those with a blocking plate don't have a seal at the top of the chimney. Blocking plates were required years ago when liners were not required, the clay flue liners have to breathe or they will deteriorate from moisture and sweat.

Especially when you have an outside chimney. Cold chimneys create poor performance and a cold chimney with a warm liner in it will not only cause the liner to sweat but also the empty spaces between the liner and the brick exterior. I have seen this many times. The chimney will crumble. Blocking plates also make the installation, cleaning and maintenance very difficult because the liner can't move inside the firebox, the only thing I would recommend in place of the blocking plate would be fiberglass insulation in the damper, which the manufacturer suggests as an alternative but even then, you trying to increase output, may wind up shooting yourself in the foot with future chimney repair that can be many thousands of dollars. I have seen many laws and codes come and go over the years because of their failures, the biggest problem with homes today is that they are made too tight and causing environmental hazards and home damage. At least with the insulation some moisture can wick out. I fix hundreds of fireplaces each year from one to two hundred years old and can tell you the history of each success and failure in design. These plates are not available with any ul listing any more and any item installed affecting the operation of the UL listing of the stove will void the warranty. I strongly recommend allowing the chimney to warm up with the fire."

I understand what he is saying now. You used to be able to run the liner to the first tile with a damper plate installed. I guess you could have run it without a chimney cap and I can see his point with no cap.

But I do not see his point with a full liner to the top of the chimney and a top plate covering the chimney.

Well that's what I had installed. They installed my insert with a liner going all the way to the top of my 30' exterior brick chimney. They put a blockoff plate at the top and of course a cap. So, if moisture gets in the chimney from cracked mortar or from the bricks, will it also cause moisture problems since the moisture has nowhere to go? Another words, with the blockoff and cap, it can't exit. And, if this is the case, would installing another blockoff plate near the damper not only keep the heat inside my insert, but also have no affect on moisture since it can't go anywhere with the blockoff plate up at the cap?

Going back and forth about my install in a couple of weeks. Interior chimney 15 foot with another 9 foot outside with a 8x8 terracotta flue. I am currently making a block plate and the insert installer will install for me. Usually they just do rock wool but will do it for extra $. Putting in a Jotul C550. After reading the C550 thread I am worried that my space will be too warm. We keep the house around 61-63. When I build my fires I can get it up to the high sixties maybe 70 but that is feeding constantly feeding the fire. My first plate I had 8" above the insert. I found a new location with it about 18" above. I have some time so I will be watching this thread.

Made a costly mistake about not putting in a big enough gas fireplace upstairs and do not want to make it again with the Jotul not heating the space. Keep on going back and forth. Better safe than sorry?

Bagelboy said:

Well that's what I had installed. They installed my insert with a liner going all the way to the top of my 30' exterior brick chimney. They put a blockoff plate at the top and of course a cap. So, if moisture gets in the chimney from cracked mortar or from the bricks, will it also cause moisture problems since the moisture has nowhere to go? Another words, with the blockoff and cap, it can't exit. And, if this is the case, would installing another blockoff plate near the damper not only keep the heat inside my insert, but also have no affect on moisture since it can't go anywhere with the blockoff plate up at the cap?

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What he's saying is the top cap needs to be installed to keep out weather but no bottom plate so the chimney cavity can stay warm and breath. Choking it off by using a damper blocking plate allows moisture to accumulate.

Bagelboy said:

Well that's what I had installed. They installed my insert with a liner going all the way to the top of my 30' exterior brick chimney. They put a blockoff plate at the top and of course a cap. So, if moisture gets in the chimney from cracked mortar or from the bricks, will it also cause moisture problems since the moisture has nowhere to go? Another words, with the blockoff and cap, it can't exit. And, if this is the case, would installing another blockoff plate near the damper not only keep the heat inside my insert, but also have no affect on moisture since it can't go anywhere with the blockoff plate up at the cap?

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The way I understand this is if the metal liner going up through the flue is warmer than the clay flue as a result of the ambient temperature then condensation can form. It doesn't matter if the flue is cracked or not because the brick, flue, mortar are permeable. Think about a cement floor in some basements where moisture will form at times on the surface because of the differences in ground temperature compared to the heat of your home. This is basically the same physics. By leaving the bottom of the chimney cavity open the heat from the stove will heat the cavity and keep the flue and chimney above the temperature needed for condensation to form. As stated in an earlier post moisture is the enemy of clay flues.