Chase Ventilation - Defeats Purpose of Insulation?

[Gazerbeam]

I'm reading through the installation manual for the DuraPlus Chimney and it states that a 1/2" gap is needed inorder to ventilate the chase. Would this negate the benefits of insulating the chase walls above the firestop? I'm asking myself why I would do that if I'm going to have a 1/2" air gap all the way arounds the top that will allow cold air to flow into the chase volume above the firestop?

Up untill this point, I was just going to use the DuraPlus Ventilated Flat Roof Flashing, screw it down to the chase top and use silcone caulk to seal it.

 

[begreen]

The class A pipe is already insulated in this case.

 

[Gazerbeam]

Thanks for the quick reply.

I understand that the DuraPlus has the 1/2" ceramic blanket insulation, but I've read through several threads debating about whether one should insulate the chase above the firestop or not, with many saying it's better to have the whole chase insulated. I'm probably over thinking this whole project, but I don't want to put this thing in and then end up having issues with cold draft issues or weak flue draft issues, or basically any of the other nightmare scenarios I read about because I took a short cut by not insulating all the way up.

Is this ventilation of the chase important to condensation issues or is it more related to venting heat build up inside the chase?

 

[southbalto]

My old ZC Fireplace chimney was inside an exterior chase. It was insulated up to the first firestop, roughly 8' up. After that no insulation.

There is a 1/2 inch gap around the chimney up top to ventilate.

 

[begreen]

This is not an interior attached, ZC fireplace chase, correct? Perhaps there is some confusing different types of installations and flue pipes in the chase?

 

[Gazerbeam]

This is a Quadrafire 7100 ZC fireplace that I'm putting in. I have an existing exterior chase that is framed with 2x4's sheathed with OSB and then covered with bricks all the way up. It originally had a direct vent NG fireplace installed in it. So far I have insulated it up to the ceilings at 9ft and sheetrocked it with 5/8" Type X.

 

[Renovation]

Hey Gazerbeam,

I appreciate your question, because I'm pondering doing the same--enclosing a class A pipe in an exterior chase.

And I may have a solution.

Someone please correct me if I'm wrong, but class A pipe's installation spec is 2" clearance to combustibles, no insulation touching it, with no additional specification for air circulation around the pipe--E.G. it can be sealed in an airtight interior chase, as long as proper clearances are maintained. Correct?

If that's true, then the air gap is a nod to good building practices--venting the chase like venting an unheated attic, to avoid moisture problems.

And I believe you're right--venting the chase puts it, like most attics, outside of the house's heated envelope.

From what I've read, it is best to have a chimney "interior", because it stays hotter and therefore draws better, especially on startup where at first natural draft is all you have. So how to get that, while still venting your chase? I suggest sealing and insulating your chase thoroughly, while making sure to maintain minimum clearances to the pipe. Then vent the chase to the inside of the house, through the wall between it and the house.

This would make the chase part of the house's heated envelope, giving you the draft of an interior chimney. At the very least it would be warmer and draw better than if your chase were directly vented to exterior air.

Comments?

All the Best,

George

PS Nice brickwork!

 

[begreen]

GB, I think you are fine stopping the insulation at the firestop 9' level. Follow the stove mfg. recommendations.

 

[Renovation]

Okay, so rereading GB's original post, I see that he has a fireplace, and now understand that the space up to the false ceiling is already part of the conditioned space, a good thing. Most interior chimney retrofits are bare class A above the roofline anyway, so he'd be in good company leaving that space uninsulated.

I'd probably insulate the space above the roofline too, but that's my problem. ;-) I'm imagining what that upper section, vented per the manufacturer, would be like insulated with a fire burning in 20* weather. I'm thinking the insulation would raise the temps in there a bit and improve draft, but my thoughts and $2 will get you a cup of coffee at Starbucks.

Good luck!