Okay to insulate cavity of pre-fab fireplace with Roxul?

Thanks to all that read this. I'm a long time lurker and a first time member/ poster. This site is full of great information. I've read through every post I could on insulating behind wood stove inserts with Roxul and making and installing block off plates. I may just be dense, but I feel like I'm unclear as to what I can and can not do in my particular scenario.

My new insert is a Regency H2100 hearth heater. So it sits outside of the fireplace. The only part that actually inserts into the pre-fab fireplace is the stove pipe connection. This was installed inside of an old Superior fireplace. The guys that installed seemed very good, but after the install I realized that there was a freezing draft on my floor by the insert. They used a double walled SS liner, but now because of the draft I'm pretty sure they didn't put in a block off plate. I intend to do that as many of the members here have already done. Galvanized steel from some ductwork, Roxul, stove cement, etc. There are some really great examples on this site.

Besides the block off plate I also want to insulate the fireplace with Roxul. From what I've gathered on this forum it's ok to line the sides and back of fireplaces with Roxul before setting a wood burning insert inside. But is that only for masonry fireplaces or can you do it with prefabs as well??

If it is ok to insulate the prefab fireplace can I essentially fill the majority of the pre-fab fireplace with Roxul since I don't need to leave space for the stove to be inserted since my insert doesn't "INSERT" at all??? I wouldn't have the Roxul press right against the stove, but could I have it fairly close?

My goal is to reflect as much heat as possible into the living space, stop the freezing cold breeze coming around the fireplace, and improve burn times by insulating.

Most importantly I want to make sure I do it right. Which is why I'm here.

bholler said:

as long as you dont cover any of the air vents used to circulate air around the fire box and through the outer layers of the chimney it should not be a problem

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Thanks for the response. If I can fill that area with Roxul and maybe cut some sheet metal to sit in front of it I'll really be able to reflect the heat and stop the damned drafts. I just wasn't sure if there was a reason to not do that in a prefab box.

bholler said:

None that i can think of as long as you maintain the required air circulation that means you cant push the plate of that hearth stove back tight against those vents you need to leave a little room. With some zc units you will always have a little draft because of those vents though. I dont know about yours though

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I'm sorry. Now I'm a little lost. Are we talking about the vents at the bottom of the ZC Superior Fireplace or vents on the Regency Hearth Heater. Another goal of mine was to disconnect and remove the fan from the old prefab unit and stuff the area beneath the ZC with Roxul. That empty area underneath of the Superior seems to be where most of the draft is coming from and I'd like to stop that. Would it be unsafe for me to do so?

bholler said:

Because there is still a hot pipe in there and the air that circulates through the outer wall of the chimney to keep it cool comes through those vents in many units as well. That and every set of instructions i have ever seen for running anything through a zc unit says you can not modify the unit in any way and cannot obstruct any air vents. I typically dont like installing anything in zc units but that hearth stove is probably the best option. But you need to be carful

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Well that's awful news. I really appreciate the info but the draft coming from those vents was the main thing I wanted to eliminate.

So to sum up and clarify. I can insulate the block off plate and around the stove pipe with sheet metal and roxul. I can pretty much fill the old fire place full of roxul since it's a hearth heater. But I can't close off the vents underneath the old ZC stove with roxul or sheet metal. I suppose I can hope that once the block off plate is installed the draft will be significantly reduced.

Anyone have any suggestions for anything else I can do to reduce or eliminate the draft from the vents under the old fireplace?

Grisu said:

Do you have a model number or maybe the manual?

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The old Superior Box is HCE-4550A. Currently installed in front is Regency h2100.

bholler said:

Yeah the only thing i know that can help is make sure the chase is sealed up good some of that draft could be coming from there. Also if there is a fresh air intake you can close that off as well that could contribute to the draft also

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Thanks for the suggestions. I did have an old fresh air intake but I've since totally sealed that off. Thanks for all the replies bholler. I admit I really don't know much about this stuff, but I'm eager to learn. Why is it that the new hearth heater would be pulling air from the vents of the old fireplace? I thought it would pull from the room.

Oh and Grisu, I am staring at the manual at the moment. Anything I should look for?

I did see that and I know that there is a hot debate with regards to whether or not inserts should be installed in ZC prefab. That is a debate I'd rather not touch with a ten foot pole. I was actually just asking from a science/ physics stand point. I want to understand better. Will the vents from the old fireplace cool the pipe from the new stove or do they aid in drafting somehow?

bholler said:

they cool the pipe for the old fireplace and make it so that it can contain your new liner which is meant to go into a masonry chimney

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Thanks for that. I think this is starting to make sense to me. So I see why I can't shove roxul in there. Would it be possible to just lay sheet metal in front of where the vent openings of the old fireplace enter my living room to slow the draft? I wouldn't be filling the void, and it would still be filled with cool air from the chase. It just wouldn't be constantly leaking into the living room. Or is that a no-go as well?

mellow said:

Yep, roxul only in the fireplace itself around the insert, need the air barrier to help keep the outer firebox cool since it is zero clearance to combustibles, stuffing that area with roxul could transfer heat to the 2x4's and other wood that frame the zc box. Once it is heated that draft will stop, I know that doesn't mean much for the other times but it is what it is.

$10 says the air leak underneath is from the OAK to the ZC.

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Thanks for the reply. What is OAK? I'm guessing outside air......something?

bholler said:

Out side air kit

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One last thing. I'm wondering now if it's ok to seal off that outside

mellow said:

Air leaks on the bottom of ZC's is not a new issue, some of the time I see it with the air screen outside for the OAK not working correctly, your draft is reversing and cold air is coming in, with heat that will help correct it, if you had a gas zc unit they say to leave the pilot going so it is always warm and stops those drafts.

The main culprits are either the air kit or the chase is not built as good as it should be.

Daksy said it best in this old thread: Insulation in Chase

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Thanks for that link, Mellow. Is there anything I can do to the outside air kit to ensure that it works correctly? From what I bholler was saying I don't think I can do anything to change the existing Superior firebox so getting behind it to insulate and drywall seems impossible. I realize my situation is far from ideal and unlikely to be made perfect, but anything I can do to improve it would be great. I've got two toddlers and would love for them to be able to play on the living floor without a cold draft. I also don't have enough wood to burn 24-7.

Here's something that might help somebody figure out what needs to be done. When I moved into this house a year ago the Superior fireplace was super drafty. Like a strong breeze. Since it was incredibly inefficient we never used it and I stuffed the damper with insulation. That pretty much stopped the draft. I didn't do anything else to seal it and the insulation blocking the flue seemed to kill the draft. Now I have a more efficient heater installed and I'm right back to a big ol' draft. Could that all be from no block-off plate being put in with the hearth heater? When I sealed off the OAK from the Superior box it actually made my fires die down to the point I had to open a window to get my temps up. With that OAK sealed I still have drafts and no have less efficient fires.