Fireplace Construction and Vapor Barrier

The Fireplace chase must be treated as being "inside the building" which makes sense since fireplace inserts are not listed as outdoor equipment.

The outer walls must be insulated, AND, a vapor sealing system applied or drafts will get in, and when the fireplace is used the chase will heat up and then will condense onto the outer walls if the vapor barrier and insulation are not effective.

The flue will often be quite cold in winter conditions, as there is nothing preventing cold air from entering the flue.   So of course the wall between the usable interior space and the chase must also be effectively insulated AND vapor barrier applied ALSO.  

If the vapor barrier is not effectively applied on the interior wall, moisture will move through and to the cold flue pipe, condensing and possibly rusting through, creating an inaccessible and non-inspectable fire hazard.


In all cases, the insulation must be positively controlled so that it can never fall down to the fireplace and create a house fire.  Cement board is a little expensive but it can be used to hold insulation in place, keeping in mind though that cement board is fireproof but not a fire barrier, and cement board is also not a vapor barrier.

http://www.fireplacesnow.com/cecinfo.asp

http://hvac-talk.com/vbb/showthread.php?t=99022

The link BELOW is a GREAT SITE for all types of Building Inspection, this particular link is for fireplace.

http://www.inspectionnews.net/home_inspection/fireplaces-chimneys-solid-fuel-burning-appliances-home-inspection-commercial-inspection/

Sealing the Chase

Sometimes, especially in cold climates, chases constructed inside the dwelling may need special considerations. In extremely tight structures, where air exchange rates are low and interior relative humidity is high, it may be beneficial to insulate, seal and vapor barrier the chase similar to treatment given outside wall construction in such dwellings. This will prevent moisture from penetrating the chase and condensing on the cold chimney during nonuse periods. If this condensation is allowed to occur, subsequent warming periods, especially during fireplace startup, can cause water to drip and leak into the chase space, often weeping through to stain and damage finished surfaces. The vapor barrier should be placed on the room side of the chase, just behind the finished wall sheathing. Some jurisdictions require that outside air be provided into the chase to provide cooling for the chimney.

The exterior of the chase may require a cricket, a peaked secondary roof section constructed on top of, and perpendicular to the primary roof to divert water away from the chase.

Firestopping the Chase

In chases, the firestop/spacers should be installed to correspond with all floor and ceiling levels in the dwelling, both for fire safety and for heat loss prevention. Firestop/spacers in the chase help the main lateral stability of the chimney to ensure that it stays in place. They slow the spread rate of an unfriendly fire from one level to another. They minimize heat loss from the fireplace system.

If the chase is to cut through the eave portion of the dwelling, it must not communicate with open attic spaces. Firestopping between the chase and the attic will prevent fire spread between the attic and the chase, and most importantly, will prevent insulation from the attic from falling or being blown into the chase and around the fireplace/chimney system. Insulation around the fireplace/chimney system would not allow required air clearances, would interfere with the system's heat dissipation, and could lead to a hazardous condition.

Chase Chimney Termination

The top of the chimney, its termination, can bring up questions of both function and design. The method of termination should be approached carefully, since safety issues are involved. The chimney cap must be the listed cap for the factory-build fireplace chimney system.

If a chimney termination is to be installed directly on a field-constructed chase, the chimney must be installed to within a specified clearance to the top of the chase. This allows the designated chimney termination to attach to both the chase flashing and connect to the chimney. The manufacturer's instructions will specify in detail how a chase termination is to be installed. Note here that chase tops must be made of noncombustible materials is within a specified distance from the termination.

The National Fireplace Assoc. prohibits the use of decorative shrouds unless they are listed for use with the specific factory-built chimney system. The addition of an untested cowling can change the aerodynamics and thermal characteristics of the chimney and fireplace system. Such an alteration can counteract the engineering of the cooling achieved by the connection of the air spaces around the firebox to the open air spaces between the chimney walls. The result can be higher than acceptable temperatures and possible ignition of combustibles at either the top of the chimney or elsewhere in the fireplace system Do NOT use decorative shroud unless it is specifically listed for use with the fireplace system.