Insulating fireplace foundation walls in a basement
I will be insulating the concrete foundation walls of a home with a 3″ thickness of polyiso with an approved covering, 3/4″ plywood specifically. There are two fireplaces in the house that have block foundations with cleanouts in plane with the concrete walls. Obviously, I’d like to insulate this fireplace foundation masonry but the building code is not clear about required clearances. In my preliminary discussion with the local building commissioner he said a 2″ clearance to combustible material is required which most people make with steel studs. This air bypass is less than ideal, but I’m fine with it if needed. However, it leaves me with several questions for which I am hoping somebody may have insight.
1. Is a fireplace “foundation” a fireplace requiring fireplace clearances? R1001.2.1 describes a “foundation”. “R1001.2.1 Ash dump cleanout. . Cleanout openings located within foundation walls below fireboxes, when provided, shall be equipped with ferrous metal or masonry doors and frames constructed to remain tightly closed except when in use. Cleanouts shall be accessible and located so that ash removal will not create a hazard to combustible materials.”
2. Does there need to be a 2″ air space to combustibles or, for example, can rigid mineral wool be used against the fireplace foundation? Seems like air is required, but colleagues have used mineral wool in contact accepted by the building department. “R 1001.11 Fireplace clearance. Wood beams, joists, studs and other combustible material shall have a clearance of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The airspace shall not be filled, except to provide fireblocking in accordance with Section R1001.12.”
3. What are the horizontal boundaries of a fireplace foundation? Wouldn’t they relate to the cleanout which could potentially have a fire in it? Is the full 5 foot wide fireplace foundation within a fire hazard zone? That seems unreasonable. There is an exception in R1001.11 that comes close to describing a 12″ clearance, but does not include a cleanout:
“Exception 2. Where masonry fireplaces are part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete walls less than 12 inches (306 mm) from the inside surface of the nearest firebox lining.”
4. In my case, and shown in the attached photo, both fireplaces have inserts and the cleanouts are now not functional. In addition, power wires are run to the inserts for their fans through the cleanouts which is allowed in the electrical code [I am told by my electrician]. It is allowed because the cleanout are no longer operational. Would the building department treat it the same way? I would think that the insulation method would have to be treated like the fireplace inserts may be removed one day and wood could be burned in the original firebox.
5. Can the cleanouts be permanently disabled to help with the insulation plan? If there were no cleanouts is this foundation still a “fireplace” requiring clearances?
6. So what about the power wire running through the clean out door? Does that have to be relocated because there isn’t a true way to keep that “tightly closed”, even though the wire should be removed along with the inserts should someone oddly do that in this less than 1 ACH50 renovated home.
7. Does the hearth concrete overhead require a 2″ airspace / 2″ non combustible in contact?
8. Finally, the old chimney-vented boiler has been replaced with a direct vent, wall hung gas boiler for DHW and back up heat. Now that chimney that has two flues and 2 basement cleanouts, one for the old boiler and one for a fireplace. Should both cleanouts be considered potentially operational in the future? Or does/ will building code not allow atmospherically vented equipment anyway?
So many questions, but I think the answers are very applicable to many home renovations for many high performance contractors. Thank you!
-Kerry
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Replies
Mineral wool isn't considered a "combustible material", and the clearances don't apply to it. Note that this is NOT the case with fiberglass insulation.
I wouldn't trust that goofy power cable run. That looks like something that would fail inspection to me. You can see how the corner of that door was bent to clear the cable, so you've now "modified" that door, which would void any listing if it's part of a listed assembly.
I can't really help you on those other questions. You may want to call a contractor who specilizes in fireplaces and get their opinions.
Bill
The 2" gap needs to remain open, to dissipate heat. Mineral wool would not combust but it would transfer the heat to the wood framing.
Your other questions are not clear in the IRC; this is a situation where you should either engage a local pro familiar with local rules, or talk directly with the code enforcement officer.
My view is that there is a chance for hot embers to make their way to the cleanout, so combustible products should be at least 2" away from the opening. Fireplaces are always energy hogs so the thermal bridging is just part of the penalty of having such an outdated device in your home.
Bill and Michael,
Thank you for your input! I had a conversation with an inspector in the town yesterday who understood the situation, but actually couldn't answer these questions with specifics. I need to email/ talk to the building commissioner who has the authority to make the decisions about what is legal. The things that aren't obvious are the electrical wire . cleanout door legality and what the specific boundaries are for combustibles. I plan to share what I learn.
I've just gotten an official statement from the building department. They were kind enough to stomach a long document with photos, drawings and this multitude of questions and respond. I did visit in person first, but the inspector on duty agreed it is a gray area and asked me to elaborate by email for the building commissioner. I offered a plan of attack using MW in steel studs touching the fireplace foundation with some clearance around the cleanout for future access, but also wanted to know if there was a specific dimensional boundary. The answer is :
"We have reviewed all your scenarios. In all instances we are ok with mineral wool and steel studs up against the foundation, chimney, or cleanout shaft. We would require that the cleanout opening is accessible by way of an access panel or something similar so in the future folks are aware that it still exists."
Very reasonable. They are inferring that the fireplace inserts and wires going to them are permanent installations and therefore that the fireplaces won't likely be burning wood in them again. Which seems true unless some apocalyptic EMF blackout should happen. And then that homeowner will have a non-combustible fireplace foundation and know where to shovel out ashes.
Thanks for the update!