Block Off Plate and Insulation Question

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.

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.

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.

jatoxico said:

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.

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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 said:

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.

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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.

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,

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?

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 said:

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

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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.

mellow said:

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.

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