Cold Air Coming from Fireplace

Every year as the seasons change and the temps outside get colder, we always get calls from home owners who tell us "cold air is coming in from my fireplace, and I can't figure it out!"  Luckily, there are some relatively simple solutions homeowners can do to improve this situation.

Fireplace bump-out

First, let's talk about where this air could be coming from.  Is the fireplace on an outside wall of the house, or is it centrally-located?  Today we'll focus on fireplaces located on outside walls.  Look for an upcoming blog on fixing issues with centrally-located fireplaces soon!

If it's located along an exterior wall, many times the fireplace sits in a "bump-out" that extends away from the main body of the house.  This also creates an overhang underneath the fireplace, referred to as a "cantilever".  An example of this is shown on the right.

Often, this cantilever is not insulated and air-sealed properly, and cold air is allowed to freely move through the bottom of the cantilever into the house, which typically enters into the basement through the "rim joist".

If there is a basement present, the good news is it won't be hard to fix this issue.  

 The thermal image seen on the left here shows you what a centilver looks like from inside the basement in a new home.  This picture was taken in the summer time, so the bright yellow colors represent hot outdoor air that is coming into the house through the fiberglass insulation located inside the cantilever.

If the rim joist is accessible from inside, like in this example, all you need to do is completely fill each floor cavity of the cantilever with insulation, preferrably a densely-packed insulation like cellulose, or spray-foam.  You don't want any gaps or empty spaces inside the cavity, so pack it tight!

Side-shot of properly insulated & sealed cantileverNext, you'll need to get some rigid material, like foam board, OSB, or drywall.  This material will be used to create a vertical bock at the inside edge of each floor cavity - where it meets the top of the exterior wall below.  Cut the material into square pieces that are large enough to fill the majority of the square openings leading into the cantilever.  Then, use expandable foam (like the Great Stuff brand you buy at the local hardware store) to fill the gaps between the rigid material you installed and the wood framing around it.  Ideally, you also want to remove the bottom of the cantilever from outside and make sure there is a solild material underneath the insulation, which is also air-sealed to the surrounding framing. This will effectively block all air from moving through the cantilever going forward.

Example of a well-sealed rim joistThat's the first place you start if you have a fireplace inside a bump-out along an exterior wall, and you have a basement.  What else can be done to reduce air-movement around these fireplaces?  Well, the only other areas to consider are the exterior walls that surround the fireplace, along with the ceiling of the bump-out, and the place where the fireplace flue penetrates the exterior wall or ceiling

With these other options, it really depends on whether or not you're willing to pull out the fireplace and/or remove some drywall to locate and correct the issues.  Most of the time, the issues can't be corrected without doing this, so you'll have to decide whether the problem is worth the additional expense to fix.

What you SHOULD see, if you were to remove the drywall, is all the exterior walls surrounding the fireplace insert should be completely insulated and covered with an air-barrier, like you see in the picture to the left.  

This house was insulated with wet-spray cellulose insulation, and the air-barrier they chose to install over the insulation is a house-wrap material.  All seams in the house-wrapped have been taped and the material has been secured to the framing.  Ideally, we'd rather see drywall or a rigid material with a higher fire-rating than house-wrap, such as ThermoPly or Thermax insulation board, but house-wrap serves the purpose here too, if installed properly.

Many times what we find is that the insulation installer does not insulate the exterior walls surrounding the fireplace, and instead they simply insulate across the "face" of the fireplace wall.  This insulation does little to nothing to stop heat gain/loss and air-movement, and is a big reason many houses built in the last 30 years have leaky fireplaces.  If this is the case in your house, contact us to schedule an onsite inspection and we will develop a plan for you to repair and improve this problem area.

Lastly, if you do decide to open up the drywall to make repairs, make sure the ceiling of this bump-out is insulated properly (R30 or better), and wherever the fireplace flue exits the ceiling or exterior wall, the flue pipe should be air-sealed to a metal collar, which is also sealed to the exterior wall or ceiling with fire-caulk.  It will look similar to the picture on the right (before it's sealed).

Have questions or comments about this topic?  Please share with us below!