Minimizing heat loss through fireplace framing
I had a Kozy Heat wood-burning fireplace installed on the first floor. It is a low-emission EPA-certified fireplace and all combustion air is provided via an outside air kit.
I have a similar unit in my current house and these units are very airtight, but the are made of metal and are connected to the exterior so they are thermal bridges. I realize I can’t do anything about that, but I do want to limit the heat losses through the framing.
The stated clearances to combustibles is 0″. The fireplace is installed “in the room” so there is no bump out in the exterior wall for the fireplace. I plan on insulating the 3 sides of the fireplace that face the conditioned space. On the left and right side, I am using ComfortBoard 80 held into place with 2×3’s (see picture) and then covered with backerboard, seams taped with fiberglass mesh and thinset, and finished with stone. The front will be similar except no furring strips, the 1.5″ Roxul will be held in place by the 1/2″ backerboard and 4″ GRK multi-purpose fasteners.
The rear wall which is the exterior wall, will not have any insulation between the studs (there is 3″ of Roxul on the exterior). I was planning on stuffing the area below the fireplace (it is raised off the floor) with ComfortBatt, as well as the ceiling space between the trusses, but leaving the minimum clearance around the chimney.
The room below the fireplace is the garage and there will be 2″ of CCSP applied the underside of the subfloor. The chimney extends through the 2nd floor which has a cathedral ceiling. The chase on the 2nd floor is roughly 20″ square and will be insulated in a similar manner. The flex duct in the picture is not the chimney but part of the heat duct kit that sends warm air to a 2nd floor register.
Anyone see any issues with this plan?
Thanks,
Jonathan
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Replies
What is the purpose of the 2" closed cell foam on the underside of the subfloor underneath the fireplace?
In climate zone 4 IRC code-min for floors over completely unconditioned space is R19. It takes at least 3" of ccSPF to meet/beat that even with HFO blown goods, and that is undercut by the shortness/low-R of the thermal bridge through the joists. R23 rock wool is greener, cheaper, considerably higher performance, and more fireproof than 2" closed cell foam.
I completely agree with Dana that mineral wool insulation is a better choice here than pretty much any other type of insulation. Mineral wool is fireproof, so fireproof that code allows its use as a firestop material to help fill holes in firewalls when also sealed with the proper caulk.
These fireplaces often have airflow from convection that puts a cold draft into the room. There are some companies that make magnetic covers for the indoor air vents to help with this.
Bill
Dana/Bill,
Thanks for the responses. Sorry for the confusion, but when I mention CCSP, I am talking about the garage ceiling which is below the room that the fireplace is in. The fireplace does not sit on the subfloor, but is raised about 12". In that space, I was going to place ComfortBatt. I posted some more pictures, one showing the fireplace raised and the other showing the garage ceiling. As an aside, the insulation plan for the garage ceiling is 2" CCSP (Lapolla 4G) and then 7 1/2" of dense pack cellulose. The garage ceiling joists are 1 3/4" x 9 1/2 LVL's so Roxul would be a bit tight on the sides and a bit shy in depth.
I still don't get what the point of 2" ccSPF is in the garage ceiling location. It's not really buying you anything.
Why not do it all with dense pack cellulose? At 9.5" the cellulose would hit ~R35, nearly 2x the IRC code minimum for zone 4. Depeding on spacing whole-assembly R of the all-cellulose solution would be pretty close to the R30 called out in Table 2 p10 of BA-1005 (still a pretty good go-to reference for finding the fuzzy line of what might be financially rational on a lifecycle basis):
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
During the summertime cooling season the average temp at the subfloor is going to be above the dew point of the outdoor / garage air, and the cellulose would buffer and redistribute any moisture that made it to the subfloor. In the winter the subfloor itself is a smart Class II vapor retarder protecting the finish ceiling below from moisture accumulation even if you went with something more vapor retardent than latex paint.
Dana,
I don't really have a good answer as to why except that this is what is in the details and what the subs quoted - so like I said, I don't really have a good answer. Based on your suggestion, I will change the insulation to all dense pack in the garage ceiling. The dense will be held in by Majrex and furring strips. The walls of the garage are concrete so I will still use spray foam in the rim joist just like the rest of the basement, yes?
Any concerns about thermal bridging through the LVL's or at nearly 10" is it not really an issue?
I also have a wine cellar and root cellar in the basement that had a combination of spray foam and dense pack. Instead of LVL's for joists, these two room have 11 7/8 deep TJI's. Should these rooms be all dense pack too?
Thanks,
Jonathan
Jonathan and Dana,
It's quite possible that the contractor is using the spray foam an an air barrier -- to prevent noxious fumes in the garage from entering the conditioned space above. I wouldn't eliminate this air barrier unless the contractor has an alternate plan for air sealing.
Martin,
I don’t recall if the contractor knew I was using Majrex so he might have assumed the foam was the air barrier. I am also wondering if he is concerned about the dense pack sagging, but from what I have read at GBA, an air space between the insulation and sub floor is a good thing albeit not to code.
An air space between sagging cellulose and the ccSPF would be pretty much the same thing.
It's pretty easy to detail the subfloor as an air barrier without using a continuous layer of 2" closed cell foam. Since it's harder to damage detailing the subfloor as the primary air barrier make more sense than ignoring it, relying solely on the Majrex below. Dense packing cellulose would plug most of the holes with cellulose even without sealing it ahead of time. If a continuous spray foam approach is deemed the most desirable method, 3" of open cell foam would get you there at something less than half the cost of 2" of HFO blown closed cell foam, maybe less than 1/3 the cost.
In your ocean tempered zone 4A climate even at the Polar Vortex disturbance temperature extremes, with 9.5-10" there isn't going to be temperature striping of any concern from the fatter LVL joists over a garage. Even if there was some temperature striping, ccSPF between the joists doesn't touch that. If the garage door stays closed most of the time the garage will remain well above the outdoor temperature during those extreme cold events.