Block Off Plate and Insulation Question

[brettdacosta]

Hey guys,

It's warmed up, and I'm getting everything ready to do a little work on the chimney, but I had a few questions first.

I have a roughly 22' dual flue chimney with two 11x11" square terracotta flues that is installed on an exterior wall of a house. The chimney is on the exterior wall so that it sits outside the house footprint (surrounded by air on 3 sides). The fireplace is downstairs in a 2 story home (1600 sq. ft. downstairs and 1600 sq. ft upstairs)

I currently have installed a 6" SS liner insulated all the way up one of the flues with 1/2" insulation. Houzz (which has really good deals on insulation kits) sent me close to 10 extra feet of insulation.

For the remainder of last season (probably around February through April), I only had the flue that the liner was installed through capped at the top. The other flue and the damper were left open. I felt the stove held it's own though. It's a High Valley 2500 and would last all night keeping the downstairs at 72 and the upstairs around 69 throughout about 6-8 hours of the night (on roughly 30 deg nights) when loaded up. Dual cats in it worked well (thermometer verified temps of 1100-1400 deg).

Obviously, I'm going to want to seal the flue the liner doesn't go down. But is this enough? I've kicked around the block off plate idea, but I'm not sure what kind of gains I could expect to see. Are we talking a 5% decrease in wood used or is it more like 30% increase? I felt like I saw the a ballpark number somewhere before, but I've searched for the past few days and found nothing.

My next question is about insulation. As I mentioned, I have some leftover insulation from my liner kit, and I'm thinking about using it on both the bottom-side of the flue seal plates and the top side of the damper plate (if necessary). I can't seem to find any r-value for this stuff though. Will it work well for this application or is Roxul the only way to go? How is it compared to Roxul insulation?

I really value you guys thoughts and comments. So please share what you're able.

Thanks in advance.

 

[jatoxico]

I don't know how anyone could predict how much of a difference you would see w/ a block-off but I say it's definitely worth doing. While the insulation on the liner keeps the flue gases hotter, the block-off keeps the heat radiating off the stove in the room.

I don't see why you couldn't use the extra insulation from the liner if you have enough though I needed a fair bit of Roxul to fill my smoke shelf and insulate down to the block-off.

Since you have an exterior chimney consider insulating the side walls and back if you have the room. You can lose a lot of heat through that brick work too.

 

[brettdacosta]

Could you explain where the "fair bit of Roxul to fill my smoke shelf and insulate down to the block off" is? Is a layer on the other side of the block off plate (shown in blue in the attached picture) not sufficient?

And are you suggesting putting insulation all around the fireplace's firebox in the locations I'm shown (same picture - also in blue)?

Thanks for the reply.

 

[jatoxico]

Insulate above the block-off as well as you can (area #3). Stuff insulation as far up around the liner as you can and fully fill that area. In my case it took just about a half bag of Roxul.

And yes if you have the room insulate the walls of the firebox. Member @mellow took the time to do a nice write up you can check out.

 

[brettdacosta]

What is the reason for loading up the smoke shelf with insulation? Wouldn't just putting a layer of insulation on the top of the damper plate be enough? I mean it would keep all the heat inside the firebox, right?

What's the temperature rating of that Roxul and what's a rough price for that stuff? @mellow made his block off plate a soft barrier...what's the advantage of that other a metal plate - ease of installation?

Thanks so much, guys. Anyone know an R value of the insulation for a liner kit? Just seems like it would be so much less with it only being 1/2" thick.

 

[yooper08]

Most of the comments so far have been about the flue with the liner. As for the flue you left the damper open with, you'll see benefits to sealing that up. As for how much, the estimates can be pretty high but hard to say.

Don't think you need to go the full block off plate route on that one, maybe just one of those removable wool flue blockers or chimney balloon.

 

[jatoxico]

Wouldn't just putting a layer of insulation on the top of the damper plate be enough? I mean it would keep all the heat inside the firebox, right?

Even if you insulate to the hilt you will not retain all the heat. A single layer of insulation will retain more than none and 4 layers will retain more than one. Roxul is cheap. A bag of Safe and sound is about $40 and is more than enough (I don't remember the temp rating (2000 F??) but it is well above anything you would see).

So as far as insulating the smoke shelf etc, this is a case where more is better. The stove top and first few feet of liner are very, very hot. Even with an insulated liner you would still give off some heat. So why wouldn't you want to retain as much of that heat as possible? It will improve heating while keeping the flue gases as warm as possible allowing you to run lower w/o forming creosote.

Use of a metal plate creates an actual air block where as insulation alone still allows air movement. Metal is a good reflector as well. Others have used Durock which I'm sure works just fine too but you want something that won't allow air movement.

To give you an idea I was recording temps of more than 80 F on my exterior brick before insulating and adding a block-off plate. That's heat that just going to the endless heat sink of the atmosphere. Now I'm just a few degrees above ambient.

 

[brettdacosta]

Even if you insulate to the hilt you will not retain all the heat. A single layer of insulation will retain more than none and 4 layers will retain more than one. Roxul is cheap. A bag of Safe and sound is about $40 and is more than enough (I don't remember the temp rating (2000 F??) but it is well above anything you would see).

So as far as insulating the smoke shelf etc, this is a case where more is better. The stove top and first few feet of liner are very, very hot. Even with an insulated liner you would still give off some heat. So why wouldn't you want to retain as much of that heat as possible? It will improve heating while keeping the flue gases as warm as possible allowing you to run lower w/o forming creosote.

Use of a metal plate creates an actual air block where as insulation alone still allows air movement. Metal is a good reflector as well. Others have used Durock which I'm sure works just fine too but you want something that won't allow air movement.

To give you an idea I was recording temps of more than 80 F on my exterior brick before insulating and adding a block-off plate. That's heat that just going to the endless heat sink of the atmosphere. Now I'm just a few degrees above ambient.

What a wonderful reply! Thanks so much! And I really appreciate you pointing me to @mellow's thread. Incredible read last night (although it did take close to 30 minutes - I'm a slow reader). If that doesn't make you a believer in insulating the firebox, I don't know what will.

Unfortunately, I won't have any 20-30 degree nights to get a baseline on the outside temp of my chimney (which is frustrating) since I plan to do this work before next year's season, but I'll take all you guys words for it. I have a pretty large firebox compared to the insert size, so I shouldn't have a problem insulating the back or the sides.

Speaking of space around the sides, if there was adequate room, what are your thoughts on insulation behind the Durock? Seems like it would be helpful, but since some of the sides still reach the inside of the house, would it be worth it?

Right now, I'm planning on a bag of Roxul to use on the back wall of the firebox, above the block off plate (inside the smoke shelf), and as far up as I can reach to stuff the unused flue at a minimum (maybe the sides). In addition to the Durock on all three sides and floor of the firebox. What's the benefit of Durock over a metal plate used for the block off plate?

Thanks so much for all the details.

 

[jatoxico]

Speaking of space around the sides, if there was adequate room, what are your thoughts on insulation behind the Durock? Seems like it would be helpful, but since some of the sides still reach the inside of the house, would it be worth it?

Maybe you could post a picture or two? I figure if you have the room and the motivation I would go ahead and add it. It's only going to make it better. You just want to be careful not to restrict air to secondaries etc so keep the final dimension such that you have an 1/2 inch or so around the unit.

What's the benefit of Durock over a metal plate used for the block off plate?

For those that used Durock I don't know that there was any special benefit, more the case that that was what was on hand though Durock does have an r factor. Could do the whole box in Roxul then cover with metal and achieve the same effect. I think the heat reflective qualities of metal may make it somewhat better choice than Durock but either will get you home.

 

[brettdacosta]

For those that used Durock I don't know that there was any special benefit, more the case that that was what was on hand though Durock does have an r factor. Could do the whole box in Roxul then cover with metal and achieve the same effect. I think the heat reflective qualities of metal may make it somewhat better choice than Durock but either will get you home.

I'll see if I can post a picture...

As far as @mellow's thread went though, he didn't see much gains from just the Roxul. It was only after he added the Durock that it got better, right? Is that because of the added R-value of the Durock or because of some other advantage of Durock. I just find it hard to believe that adding a sheet metal on the inside (side closest to the insert) would make the difference he achieved by adding Durock.

 

[jatoxico]

I'll see if I can post a picture...

As far as @mellow's thread went though, he didn't see much gains from just the Roxul. It was only after he added the Durock that it got better, right? Is that because of the added R-value of the Durock or because of some other advantage of Durock. I just find it hard to believe that adding a sheet metal on the inside (side closest to the insert) would make the difference he achieved by adding Durock.

You may right. On the other hand think how many times you've seen one sheet of metal used as a heat shield. Can do quite a lot. Maybe someone else has some data etc to support or refute.

Mellows system worked for him so I wouldn't criticize anyone who just followed that plan. Paints up nice too so there's that.

 

[mellow]

Concrete board is a harder material, bounces the heat back to the fireplace, I would recommend that over Roxul, and the both together if you have the room. Roxul has fibers that like to float in the air, I like to see that locked away behind the Concrete board for safety as well as extra insulation. The concrete board will also hold up longer.

 

[jatoxico]

Just for clarities sake, I was commenting on the difference between metal over Roxul or Durock over Roxul. Agree that Roxul alone is not the way to go and that if there is a difference it's probably negligible for this application.

 

[brettdacosta]

Mellow, nice to have you reply - and wonderful write-up on the other thread.

I'd like to hear your comments on why you went with Durock over sheet metal too if you get a chance?

I haven't compared the R-value of them (takes a bit of math for the sheetmetal), but the Durock wasn't that impressive when I looked it up.

Thanks,

 

[mellow]

My whole fireplace is thick metal, it did nothing in regards to stopping the heat from going out. That was my reason for going to concrete board, to see if it would stop the loss and it did when coupled with Roxul. I was going to upgrade to Micore 300 one day but honestly once I saw how well this worked I didn't feel like ripping it out for only small gains.

If I was starting from scratch I would use Micore 300 to see how it does.

 

[bholler]

My whole fireplace is thick metal, it did nothing in regards to stopping the heat from going out. That was my reason for going to concrete board, to see if it would stop the loss and it did when coupled with Roxul. I was going to upgrade to Micore 300 one day but honestly once I saw how well this worked I didn't feel like ripping it out for only small gains.

If I was starting from scratch I would use Micore 300 to see how it does.

For metal fireboxes like yours we typically just fill the void with thermix it is much easier.

 

[mellow]

Yea that would work, do you drill holes at the top and fill like they do insulation in a wall?

 

[brettdacosta]

Is there a way to insulate the bottom of the firebox or is it even necessary? And how strong is the Durock? Should I be worried to put my ~600 pound High Valley insert on it?

 

[bholler]

Yea that would work, do you drill holes at the top and fill like they do insulation in a wall?

We usually have to cut a notch for the liner to clear anyway so we fill through that you have to make sure you close off the vents though. We also fill through the top vents sometimes. It all depends on the situation.

 

[bholler]

Is there a way to insulate the bottom of the firebox or is it even necessary? And how strong is the Durock? Should I be worried to put my ~600 pound High Valley insert on it?

I have never worried about the floor you could put a piece of cement board or two under it but you would have a gap under the stove in the front then

 

[jatoxico]

My fans and all the wiring are directly under the insert firebox (shielded by a piece of sheet metal). The bottom never gets warm enough to worry about heat loss.

 

[Blazing]

My whole fireplace is thick metal, it did nothing in regards to stopping the heat from going out. That was my reason for going to concrete board, to see if it would stop the loss and it did when coupled with Roxul. I was going to upgrade to Micore 300 one day but honestly once I saw how well this worked I didn't feel like ripping it out for only small gains.

If I was starting from scratch I would use Micore 300 to see how it does.

I was inspired from your findings years ago and insulated my BK with both Roxul and Micore 300. In my opinion it should be a sticky for insert buyers.

 

[brettdacosta]

Hey guys... Here's the picture of the very loosely installed (it was temporary to finish out the year last year) high valley inside the huge firebox...

Also... I quickly looked up the Durock versus Micore. Help me understand the pros/cons. It appears to only be twice as good of insulation (r value of roughly 0.4 for Durock versus 1 for Micore), appears harder to find, and possibly more expensive. Where's the advantage?

 

[mellow]

Micore will be a better insulator, how much I really can't tell you as I would have to do a head to head and run the numbers. Micore is also used in cubicle separators for sound damping, so if you see a place that is going out of business you might be able to score some free Micore or maybe your work has some sitting around.

If money is tight then just use durock from Lowes while picking up the Roxul, honestly anything is better than nothing, Micore will be better, but trying to put that into a number will be tricky and vary on the install. You talking about single digit degree gains, how much is that worth to you?

 

[brettdacosta]

You talking about single digit degree gains, how much is that worth to you?

Exactly, if the Durock and Roxul combination will keep your losses to only 2 deg (41° to 43° - based on the test in your other thread), that really doesn't leave room for a lot of improvement.

Especially without insulating the sides in either case.

Micore is also used in cubicle separators for sound damping, so if you see a place that is going out of business you might be able to score some free Micore or maybe your work has some sitting around.

And also - thanks for this! I'll check with our maintenance guys. Within a year, they're talking about redoing all our cubes.