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Insulation behind a gas fire place?

Have a client that has a high level of air infiltration from behind their gas fireplaces. The house is insulated but behind the fireplaces the brick is exposed and the area along the floor is unsealed.

Can I add ridged foam to the back wall of the fireplace, seal with foam around the edges and then cover with drywall?

Do I need to work about a vapor barrier in this space?

This area seemed to be missed during inspections when the house was renovated and the draft will extinguish a candle outside of the fireplace.


Asked by Charles Juris
Posted Oct 28, 2010 2:27 PM ET


4 Answers

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More information, please! Does this fireplace have a traditional brick chimney? Or are you describing some sort of decorative brickwork?

Is it a gas-fired metal fireplace insert that has been retrofit into a wood hearth? Or some other configuration?

Answered by Martin Holladay
Posted Oct 28, 2010 3:44 PM ET


Martin thanks for getting back to me

No brick chimney just a chase that the former builder built out to look like a chimney - the chase is made out of drywall and 2 x 4 framing

The brick is on the back side of the chase and is the exterior wall of the unit which is no longer attached to another unit as they tore that other unit down and now it is an exterior outside wall not just a party wall

It is a gas fired insert that has been inserted into a wood frame with marble surround and wood mantel. Walls on both sides of the chase have been insulated and are dry-walled but the space behind the fireplace insert and the floor have never been sealed. This condition is on two floors in the living room and in the master bedroom.

The chase runs up to the attic - almost like there was a chimney in that space but there are no other signs that it was actually there. The chase therefore works like a chimney and draws nice cold air from the attic down into the house.

The inserts do have proper vent lines running up to the roof line where they are vented.

Another thing is that the shut-offs are not accessible and if they were needed to shut the gas you would have to break into the drywall on the side that form the chase.

Felt we could provide a hatch in the side of the chase to access the shut-offs and at the same time that we opened that area for the hatches we could open the wall enough to insulate and seal behind the fireplace inserts

Hope this helps - thanks

Answered by Charles Juris
Posted Oct 28, 2010 4:34 PM ET


If exterior brick veneer is exposed behind your metal fireplace, and if there is no insulation on the interior side of the brick veneer, than you obviously have a problem.

All areas of your home's thermal envelope need insulation and an air barrier. If the builder left out insulation and air barrier details, you have a lot of work facing you.

You'll have to remove as much of the fireplace as necessary to gain access to the areas that need work. The first step is air sealing, using the usual bag of tricks: spray foam, caulk, panels of plywood, rigid foam, drywall, sheet metal -- different materials for different applications.

Then you need to install insulation. Consult your local code to determine the minimum insulation thickness for your climate.

Finally, the interior side of the insulation usually needs to be protected by an air barrier (for example, drywall). It's also important to meet all local requirements for thermal barriers, as well as all clearance requirements for air spaces between wood framing and certain fireplace components.

If you are unsure of any of these details, consult with your local building official.

Answered by Martin Holladay
Posted Oct 29, 2010 5:28 AM ET


I suggest you take a look at the detail found in the Strategies and Details page of GBA (you may have to purchase a GBA membership to view the detail):
-This is what your builder should have done.

Answered by Brett Moyer
Posted Oct 29, 2010 12:45 PM ET

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