Fireplace chase frosty

[JUNEBUG 88]

I was up in the attic installing the fresh air intake for the fireplace and I noticed that inside the chase there was a layer of frost on the wood. Currently we are still waiting (months) for the fireplace to be completely installed due to numerous delays by installers. The chimney vent is not installed yet.
The contractors (not installers) stated that they usually do not insulate the chase. At this time there are only the wood walls and a wrap with a wood top. Vinyl siding will eventually cover the walls. The chase is approx. 6' tall.

NOTE: Inside the attic there is NO frost on any other roof sheathing only inside the chase.

If it should be insulated should it be on outside or inside walls of chase. There was no provision for the insulation prior to this so it may be to late to install on exterior prior to siding. Since, siding may extend out beyond the top edge although I am not sure. The chimney is 8" ASHT double wall and the chase is about 24" x24".

[JUNEBUG 88]

MORE INFO: we have been in the attic more often thus opening access to the attic from house and garage. We also added an access hatch from the attached garage so the access is easier, instead of crawling into a small closet and then up into a small hatch on top of the coat rack. We could seal the garage hatch door with weather-stripping to minimize the airflow from the occasionally heated garage to help with the frost? That access door is fire rated.
Can insulation (rigid foam) be added to the chase from the inside. The top of the chase does not have enough overhang to add insulation to outside surfaces under the siding.
Contractor said insulating the chase would be the same as insulating my roof sheathing in an insulated unheated attic.
1st picture of inside of chase at install
2nd picture of chase prior to siding waiting for the chimney

[adnadeau]

The frost probably results from warm moist air coming up through the chase from the attic. There shouldn't be any warm moist air in the attic, but if you've been accessing it a lot through a hatch in the house ceiling, some probably got in. Either that, or there's no vapor barrier in the ceiling. If you screw a piece of plywood up against the inside of the roof to cover the 2' x 2' hole where the chimney will go, you shouldn't get any more frost.

That said, it's not a bad idea to insulate the chase. Chimneys work best if they're warm. It may be too late to get back inside the chase; there is a little room left in there for rigid insulation, but it should be faced with a layer of drywall for fire protection. You also have to maintain the 2-inch clearance from the outside of the metal chimney. If you insulate the outside of the chase, you could build out the top overhang to protect it.

[JUNEBUG 88]

Originally Posted by adnadeau

The frost probably results from warm moist air coming up through the chase from the attic. There shouldn't be any warm moist air in the attic, but if you've been accessing it a lot through a hatch in the house ceiling, some probably got in. Either that, or there's no vapor barrier in the ceiling. If you screw a piece of plywood up against the inside of the roof to cover the 2' x 2' hole where the chimney will go, you shouldn't get any more frost.

That said, it's not a bad idea to insulate the chase. Chimneys work best if they're warm. It may be too late to get back inside the chase; there is a little room left in there for rigid insulation, but it should be faced with a layer of drywall for fire protection. You also have to maintain the 2-inch clearance from the outside of the metal chimney. If you insulate the outside of the chase, you could build out the top overhang to protect it.

Sorry for the delay in answering but it took this long to get the chimney vent installed!!
We have been accessing the attic numerous times during the installing of various parts to the system. Vapor barriers were also compromised where holes were cut for the chimney vents but only covered with wood.

The chimney vent has now been installed finally. Around the outer edges of the sheet metal base of the insulated firestop in the attic we applied a viqueen vaporbarrier. It is more than 4" away from the insulated part of the firestop that protects the chimney from insulation. The base is also sealed with hi temp caulk.

At this time I plan to put flashing around the chimney vent up against the bottom of the roof and seal that with hi temp caulk.

There was not enough room for insulation inside the chase with drywall. and the outside top was already pre-measured for a top to be applied so I could not expand the size to accomodate insulation.
Thanks for the help and suggestions