Need quick advice re-insulating exterior of old B-Vent

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Jan 14, 2018
Denver, CO
I posted a few months ago but have a better idea of what was wrong. If I could delete the old post I would.

Yes, the installers did a terrible job sealing and insulating the B-Vent. My home has three decorative B-Vents (yuck), but the one in my basement is very leaky and keeps the basement freezing. My other two are very manageable in comparison. I have a wood stove but I can't afford to remove these B-Vents at this time.

The bump out was very leaky, no air-barrier, no caulk, loose fitted plywood, and big gaps pressed against the cedar siding (hidden by trim). Yes, this unit was an afterthought installed when previous owners finished the basement. Inside the bump out was insulation on the bottom half, nothing on the top half, and no drywall. (Was this the way these were installed back then for safety reasons?) I removed it, took off the siding, and removed the plywood sheathing.

The insulation was just stuffed in certain wall cavities, and one cavity had no insulation at all. I removed the old stuff and put in pink R-19. I'll seal it back up today, caulk any gaps, put siding back up and then tackle the bump out.

Per the manufacture the stove requires 1/2" clearance on all sides. I asked why it needed the clearance and they explained how B-Vents use the inside air for combustion. I asked why the exterior should matter and she explained it's for making up the air loss. But when I asked if it's okay I seal up the bump out they said that was fine. Wouldn't that do the same as removing the clearance (stopping the air exchange)? Or does the B-Vent need air to evenly come in from around its enclosure? This is the source of my cold air issues and I'd like to seal it as much as I can. Is the clearance a combustion issue?

The same person mentioned these bump outs are typically fully insulated on all sides and covered with drywall. Is the drywall important? I will be using Mineral Wool.

There are two parts to your question. The clearances that are specified are typically defined as requiring air space. This means that nothing, not even non-combustibles can be in the space.

As for the insulation, I have attached some pictures of how we insulate and draft stop an exterior bump out.

All factory built fireplaces usually have standoffs on the top to establish the minimum height that the header may be installed.
It looks as if yours have been removed. I would highly recommend finding the installation manual for that fireplace and adjusting the framing to where it is supposed to be.

Need quick advice re-insulating exterior of old B-Vent Need quick advice re-insulating exterior of old B-Vent Need quick advice re-insulating exterior of old B-Vent
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I have attached some pictures of how we insulate and draft stop an exterior bump out.
Hi Lennox - I can't thank you enough for sending these photos. I've been looking for an example like this for weeks! It looks like normal (non-fire rated) pink insulation behind the thermoply? I live in Colorado and it doesn't look like Home Depots out here have thermoply. All I see are Fire-Retardant Pressure-Treated Plywood which looks like normal plywood. Do you know names of other products I could use? Could I simply use Mineral Wool insulation or do I need to cover it with something? The exterior side is currently plywood which I plan to seal up best I can with caulk.

Speaking of is it okay to seal this bump out with caulk? Since your pointed out the clearance is for air space, does that include the need for air exchange (so the bump out shouldn't be tight)? If combustion isn't the issue I'm unsure what air space is needed for.

fireplaces usually have standoffs on the top
Are these those standoffs? They are on the other side of the fireplace. For some reason they are not on the side you probably saw in my other photos.

Also, I notice you have some foam insulation just outside the framing. Was that just to fill a tight space where the pink stuff didn't fit? Or is there something more to that? Would it need to be fire-rated?

Thank you very much for your insight!

The HeatnGlo fireplaces that we install do not specify anything other than insulating and draft stopping the exterior as one would any other part of the thermal envelope of the house. None of the products that we use for this have any special fire rating properties.
I am not familiar with any other brand names but Thermoply is just cardboard. We use it to keep the insulation in place and help with draft stopping.

The bump out should be as tight as you can get it. If you insulate it properly, the underside of the bump out, three walls, and roof will all be insulated as well as the rest of your house. The fireplace will then be inside the conditioned part of your house.

There will always be some cold air infiltration from the vent pipe because the vent can not be insulated, but you can minimize this by sealing each section of pipe and sealing around the firestop where the vent pipe goes through the wall.

I do see the stand offs now.
That is not foam insulation, it is the back side of the thermoply and once again, there is no need for fire rated material because all of the clearances are maintained.
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