Chase insulation questions

[nervouz]

My house was build in 2016 and has a chase (doghouse) bumpout that contains a horizontally vented DV gas fireplace. The chase has a horrible draft coming through it so I'm in the process of fixing it. The chase is about 5' wide by ~10' tall and about 2' deep. The bottom half is the cavity that houses the DV FP and was somewhat enclosed by the builder using 2"x6" for framing, fiberglass insulation and vapor barrier with staples to secure insulation to the 3 exterior walls. They also placed a piece of plywood on top of the 2x6 frame as a draftstop to seal off the top half of the chase from the bottom. The plywood does NOT have any insulation sitting on top of it and the upper half of the chase is open to fresh cold air via some soffit vents.

I have found quite a few leaky holes in the DV piping & firestop but my main question here is about sealing up the lower half of the chanse where the FP resides. My plan of attack is:

1) Lay insulation on top of the plywood draftstop. I think I can slide some faced insulation batts through the stud bay up into the top half of the chase and get it to lay flat.
2) Reinstall fiberglass insulation into the lower half of the chase where the builder originally had it installed
3) Install vapour barrier over insulation and staple back in place
4) Cover entire lower half of the chase with drywall, caulk and tape every seam
5) During reinstallation of the FP I was thinking that I should apply some high temp silicone sealant between the firestop and the 2x6 wood framing that it sits up against. It is definitely not an airtight seal and I can actively feel cold air coming through it.

I'm open to any suggestions on my plan of attack.

This is the top of the lower portion of the chase. That's the plywood draftstop that has no insulation on top of it.

Same but left side of the chase

How the builder insulated the lower half of the chase. The top half is insulated in the same way.

Firestop attached to the 2"x6" framing. I can feel air coming through the sides of this. I want to seal this with high-temp silicone sealant


The stud by that I can reach my arm up through and access to the top of the draftstop. I'm thinking I can slide insulation up through there and lay it on top.

 

[begreen]

If you can reach around to the top of the firestop through a stud cavity, then it is not a firestop. Sounds like each stud cavity may need a block too. Don't worry too much about trying to randomly stuff insulation on top of the fireblock. It's not supposed to touch the chimney pipe. Just stop the leaks.

You can use regular silicone for sealing, no need for high temp in this location.

 

[nervouz]

I can reach up through the stud cavity and reach the top of the draftstop. The firestop is sealed to the back of the chase and allows the DV pipe to pass through.

The installation manual recommended a silicone rated for 300* of continuous exposure for sealing the firestop to the rear of the chase.

 

[DAKSY]

This is how the company I work for does it.
We prep the fireplace with aluminum tape at all seams & any holes except the one for the gasline.
We install Kraft-faced insulation all around up as far as necessary (4' min) in the stud cavities.
We install sheet rock all the way around the back & sides of the doghouse 4' high for
MOST gas fireplaces, but some are taller..
We use caulk in all the seams & use aluminum tape to seal the firestop to the sheetrock & use 3/4" tape gasket
around the OD of the pipe where it passes thru the firestop.
Then we build a frame work of 2x4s, with a cross brace in the middle, above the height of the top fireplace stand offs.
We fabricate the firedeck out of one piece of 1/2" sheetrock & lay it on the bracing.
Then the deck is covered with Kraft-faced insulation.
The gas line is run thru & connected & then unfaced insulation is wadded into any gaps.
Obviously, there are other ways, but that's the way Best Fire hearth & Patio in Troy, NY does it.

 

[begreen]

The installation manual recommended a silicone rated for 300* of continuous exposure for sealing the firestop to the rear of the chase.

Most clear silicone sealants are rated at 400º.

 

[nervouz]

We use caulk in all the seams & use aluminum tape to seal the firestop to the sheetrock & use 3/4" tape gasket
around the OD of the pipe where it passes thru the firestop.

1) Any particular caulking I should be looking for? Do you use this rather than using drywall tape to seal up the seams or do both?

2) Do you recommend a specific type of drywall (fire rated) or thickness I should use?

3) Should the drywall butt up against the metal firestop on the top and sides pictured in pic 5 above? I should use aluminum tape to seal the drywall -> firestop at this point right? I don't know what 3/4" tape gasket is. I'll do some search and try to find some.

Then we build a frame work of 2x4s, with a cross brace in the middle, above the height of the top fireplace stand offs.
We fabricate the firedeck out of one piece of 1/2" sheetrock & lay it on the bracing.
Then the deck is covered with Kraft-faced insulation.

4) I'm not sure I completely understand this part. The existing deck above the fireplace (about 2' above it currently) is not a firestop if I'm understanding you guys correctly. I think what I have is just a draft stop sheet of plywood up there to separate the top & bottom half of the chase. Are your instructions to build a firedeck and sandwich it between the existing draft stop?

5) Are "fireplace stand offs" the metal triangular on top of the FP?

My original idea of sliding insulation through the stud cavity was to just get some insulation above the FP cavity in hopes of reducing the cold air from above falling down into the FP cavity.

Thanks for your replies!

 

[DAKSY]

1) Any particular caulking I should be looking for? Do you use this rather than using drywall tape to seal up the seams or do both?

Actually, we use acoustic caulk, because it comes in GIANT tubes
& the cost compared anything else is a wash.

2) Do you recommend a specific type of drywall (fire rated) or thickness I should use?

1/2" regular stuff.

3) Should the drywall butt up against the metal firestop on the top and sides pictured in pic 5 above? I should use aluminum tape to seal the drywall -> firestop at this point right? I don't know what 3/4" tape gasket is. I'll do some search and try to find some.

The firestop should be OVER the sheetrock. The sheetrock opening should be the same size a the framed in opening between the studs in the st

DAKSY said:
Then we build a frame work of 2x4s, with a cross brace in the middle, above the height of the top fireplace stand offs.
We fabricate the firedeck out of one piece of 1/2" sheetrock & lay it on the bracing.
Then the deck is covered with Kraft-faced insulation.
Click to expand...

4) I'm not sure I completely understand this part. The existing deck above the fireplace (about 2' above it currently) is not a firestop if I'm understanding you guys correctly. I think what I have is just a draft stop sheet of plywood up there to separate the top & bottom half of the chase. Are your instructions to build a firedeck and sandwich it between the existing draft stop?

Ok, we call it a firedeck, but more properly it's just a sheetrocked deck.
2' above the unit is fine, but not necessary as long as the insulation IN,
& the sheetrock ON the wall studs, also goes up past that deck...

5) Are "fireplace stand offs" the metal triangular on top of the FP?

Yep.

My original idea of sliding insulation through the stud cavity was to just get some insulation above the FP cavity in hopes of reducing the cold air from above falling down into the FP cavity.

That's why we build & insulate that deck.