Finally got around to insulating my fireplace

[mellow]

Just looking at the spec sheet for Durock NexGen at https://www.usg.com/content/dam/USG...ck-next-gen-cement-board-datasheet-en-can.pdf

Avoid exposure to sustained temperatures exceeding 93°C(200°F).

Hmm, wonder how warm it is where my pipe goes up through it?

Durock discussion

 

[Das Jugghead]

I recently started insulating my fireplace in an attempt to squeeze a little more efficiency out of my cheap Pleasant Heart wood stove. There were pre-existing threaded holes in the angle iron for the lintel where a set of brass fireplace doors used to be. I used the threaded holes to mount some angle iron I had in the shop to hold up the block off plate that I fabricated out of cement board.





I made a two piece cardboard template (I actually made three versions in order to get it as precise as possible. Then I reduced the size by taking a quarter inch off the perimeter to make it easier to fit and then stuffed rock wool in all the seams. The reason for a two piece block off plate was because I just did not want to fool with pulling the stove and reinstalling it - I will do that in the spring when I pull it for inspection/cleaning.




Before installing the block off plate I blocked off the damper throat by stuffing rock wool into it but neglected to get pictures of it. I also precut rock wool insulation to go on top of the block off plate before installing the second half of it. After stuffing rock wool in the seams I trimmed it off with a stupidly sharp razor blade.



To cover the seams and reduce convection through the seams I installed aluminum flashing in several layers. I also placed rock wool along the walls of the fire place and then covered it with aluminum flashing. The flashing serves to cover the rock wool and also helps reflect a little heat out of the fireplace. I still need to finish the top part and integrate that into the overall set up. It has made a difference for us in that we gained almost 5 degrees from 70 to 75 with a few runs up to 82 in the room where the stove is installed. Mostly the burn cycles keep the room at 75. If I had to guess I would say most of that gain is from stopping the heat from going up the chimney. The fireplace and chimney are on an exterior wall of the house so the insulation and reflective aluminum do help isolate the heat from the heat sink.




I still need to do some fine tuning and adjustments to tidy things up.

 

[Crawdad]

Just looking at the spec sheet for Durock NexGen at https://www.usg.com/content/dam/USG...ck-next-gen-cement-board-datasheet-en-can.pdf

Avoid exposure to sustained temperatures exceeding 93°C(200°F).

Hmm, wonder how warm it is where my pipe goes up through it?

Cement Board has styrene beads within the core. Be careful.

 

[begreen]

Cement Board has styrene beads within the core. Be careful.

Some do. It's good to check. Some like Durock use pumice granules or perlite. USG's styrene pellet board has a different name and is recommended for wet areas like showers.

Wonderboard Lite has polystyrene pellets inside.

 

[Das Jugghead]

Cement Board has styrene beads within the core. Be careful.

Durock - I should have been more clear in that.

 

[Das Jugghead]

It does not look like much but I finally got around to fabricating and installing these deflectors at the back of the fireplace to help the convective air move out of the fireplace. It has made an amazing difference in how much heat we have and for how long. Right now we have been running on a bed of coals for almost two hours and it is still 72 degrees in the house. Not bad. In the spring I plan to fabricate a new and cleaner setup when I pull the stove for inspection and cleaning.

The real litmus test will be over the weekend when the temperatures drop to below zero.

 

[jetsam]

Hey, it's mellow's old fireplace thread. I have linked to this thing a jillion times.

 

[jimmol]

Finished my Durock/Rolux block off. With the help of a templet I put my Rolux up around my pipe and it filled the area completely. I had purchased 1/4" Durock so it would be easier for me to put up the top and it went up fine, but I noticed it flexed more than I had hoped so I added a 1/2" piece below it. On to the insulation of the back.
As you can see from the picture, I put in insulation in the back of my fireplace as high as I could, then I put a 1/2" Durock as at back to square off the area. This piece provided part of support for my top sheet. I added more Rolux up to the top. In the front, the top piece was at a slight angle upward so it wedged just above the angle iron lip of the fireplace. Then I placed a second 1/4" Durock board to seal the Rolux in. This piece did not provide support for the top. Finally I added a 1/4 and 1/2" Durock on the right side and a 1/2" piece on the left side. These were also support for the top piece. There was not room for insulation on the side. I caulked the seams with 3M fire barrier sealant. I decided not to paint at this time as I wanted to see how the cement board handled the heat.
I will add the pictures as soon as they are approved.

1 Squaring it off

• jimmol
• Jan 19, 2019
• 1

Added insulation at the back up to where it would not fit with the Durock in in front of it...

Adding the second layer of insulation

• jimmol
• Jan 19, 2019

Insulation installed.

• jimmol
• Jan 19, 2019

1/4" Durock over the Rolux.

• jimmol
• Jan 19, 2019

Sides in place

• jimmol
• Jan 19, 2019

The sides are also used to support the top. Sealed with 3M Fire Barrier Sealant.

 

[SculptureOfSound]

Very nice install jimmol. That's what I wanted to do but didn't have enough room for the Roxul and durock. I originally decided to just put in the Roxul...it was touching the sides and back of the insert. Then I learned how potentially harmful the fibers are and realized with my install the way it was they could be getting into the blower path and ripped it all out.

One thing I did do and that you may want to do that would help is to put some sheet metal (cheap galvanised would work, it won't get hot enough for zinc off gassing) over the durock. This would reflect the radiant heat back towards the insert.

I don't know how much it helps but that's what I ended up doing after taking out the insulation. It did seem to make some difference...the side brick behind the sheet metal was about 85 degrees after a few hours of burning while some exposed brick next to it was 140.

 

[jimmol]

One thing I did do and that you may want to do that would help is to put some sheet metal (cheap galvanised would work, it won't get hot enough for zinc off gassing) over the durock. This would reflect the radiant heat back towards the insert.

I don't know how much it helps but that's what I ended up doing after taking out the insulation. It did seem to make some difference...the side brick behind the sheet metal was about 85 degrees after a few hours of burning while some exposed brick next to it was 140.

I will keep that in mind. I want to see how it holds up and if the cracks seems to get worse, I will get the metal. I have a lip about an inch lower than the Durock in the front so I could put it lower and then meet the top in the back.
I must admit, I wonder how the deflectors for the back might work.

Thanks to all for helping make this work.

 

[Das Jugghead]

I must admit, I wonder how the deflectors for the back might work.

I am not overly familiar with your set up - is it an insert? I have a small free-standing wood stove installed in my fireplace. I insulated the sides and back of the fireplace with rock wool and covered all of that with aluminum flashing. I also have a Durock block off plate in the top of the fireplace opening and more rock wool stuffed in above that and through the damper. These things made a huge difference in the amount of usable heat output from the stove. The last thing I did, about a week later, was add curved aluminum deflectors in the back to help the hot convective air from the back of the stove to make the transition from vertical to horizontal across the stove top and ultimately out the front of the fireplace. The additional deflectors at the back made at least as much of an improvement as the initial insulating of the fireplace - if not more. It is my entirely nonscientific opinion that the two improvements (insulating the fireplace and the curved aluminum deflectors at the rear) work in concert together to produce the better performance.

 

[mellow]

@jimmol nice install! You should see a marked difference in heat output and burn times with that insulation, will especially help with that smaller firebox.

 

[Brotatocrisp08]

Interesting idea. I have a similar set up with an exterior masonry fireplace, in which I have a small insert at the moment (looking to upgrade in the fall).

How did you go about attaching the durock/roxul to the back of the fireplace?

Yes good question. Tho I’m still looking into what durock/roxul is and where I can buy it or how to cut it. I was about to just go buy some insulation they’d wrap around hvac tubes in a crawl space. I can see it’s more complex than that

 

[begreen]

You can use ceramic wool (kaowool) blanket that is sold on Amazon.

 

[Brotatocrisp08]

You can use ceramic wool (kaowool) blanket that is sold on Amazon.

Thank you, begreen. My installer jackhammered my fireplace’s (fire bricks I think) out in the back so the matrix would sit flush. So, behind the insert is exposed to the cinder block that forms the chimney. Shoot maybe that’s where all my heat is going… Jeeesh I wish I’d found the forum b4 he installed it.

 

[mellow]

Yes good question. Tho I’m still looking into what durock/roxul is and where I can buy it or how to cut it. I was about to just go buy some insulation they’d wrap around hvac tubes in a crawl space. I can see it’s more complex than that

This whole thread is full of good info, your question is answered in it, I used tapcons to attach the cement board to the metal box, but could be used to attach to brick as well.

 

[Brotatocrisp08]

This whole thread is full of good info, your question is answered in it, I used tapcons to attach the cement board to the metal box, but could be used to attach to brick as well.

Thank you Mellow