Making a removable chimney plug is one of the quickest and cheapest projects you can do to improve your old home’s energy efficiency. You might even have all the materials you need already, making the project 100% benefit and 0% cost. And depending on how leaky your chimney currently is—a chimney flue can be the single largest hole in your thermal envelope—you could see an immediate difference in your monthly utility bills. Making a chimney plug is a great first step in the process of air sealing your house.
Your Chimney is Stealing from You
Your chimney is designed to quickly and efficiently remove hot air from your fireplace. That escaping air has to be replaced with something, so new air rushes towards your fireplace—fueling your fire with oxygen and keeping things burning nicely. A charcoal grill, or the aptly named “chimney starter,” is a beautiful illustration of this phenomenon.
But when you’re not using your chimney for a fire—which is probably the vast majority of the year—this process is still happening, just more slowly. In the winter, the air you’re paying to heat can rise up the chimney due to stack effect (also known as the “chimney effect”), which causes cold outdoor “replacement” air to be drawn into your house any way it can find. This replacement air is what you feel as a “draft.”
Believe it or not, this process takes place year-round. In fact, it can reverse itself in the summer when you’re running air conditioning, with humid outside air pushing its way downward through the chimney and forcing your conditioned air out of your house through gaps and cracks.
The upshot is that the same properties that make your chimney effective—for the tiny portion of the year that you have fires—are working against you the remainder of the time.
What about My Damper?
I can hear some of you asking: isn’t stopping drafts what my damper is for? Or maybe you use the term “flue,” as in “opening and closing the flue.” (Technically the flue is the opening in your chimney, and the damper is the flap that opens and closes).
Well, first of all: are you sure you have a damper? My house was built in the 1880s with two coal fireplaces and no dampers whatsoever. When we moved in, we just had two openings, each several square feet, leading straight up through the top of our thermal envelope. Lay on your back and see if you can see daylight out the top of your chimney (careful not to get your hair filthy). If you can’t, shine a flashlight and check to make sure you see a metal damper.
Even if you do have a damper, it probably doesn’t create an air-tight seal. You will probably be able to see daylight at the perimeter, or notice gaps when you shine your flashlight. If you can’t tell, light a match and hold it at the bottom of your flue. There’s a good chance you’ll notice the flame being drawn up by the natural draft of the chimney, despite your damper being closed.
By the way, the same is true if you have glass doors or another barrier in front of your fireplace. Unless they’re brand new and have gaskets or something similar to make them truly airtight, you shouldn’t rely on them for an air seal.
Buying a Chimney Balloon vs. Making a DIY Plug
There are products on the market called “chimney balloons” that are specially designed to inflate to fit the inside of your flue. In fact, the U.S. Department of Energy recommends using them to seal your chimney at the end of this article.
Chimney balloons are a great idea, but unfortunately the online reviews are mixed. Despite the price—around $50, depending on the size—a significant number of reviewers claim to have had issues with quality and durability. I always assume there’s a bias towards negative reviews on the internet (because happy customers don’t usually take the time to write), but I still haven’t tried one of these balloons myself because I haven’t wanted to risk wasting my money. I also think that although the balloons come in a variety of sizes, they’re probably not the best option for rectangular flue openings like mine. A chimney balloon may be a better option for a homeowner with a newer, circular flue opening.
Fortunately, there’s a more reliable and economical option for air sealing your chimney. The Department of Energy’s other advice, if you don’t want to buy a chimney balloon, is to just make a chimney plug yourself. Yes, folks—the cheap and effective DIY solution below is also an official recommendation from the United States Government!
What You’ll Need
There’s a decent chance you already have all the necessary supplies around the house. If so, this project is completely free and will probably take you less than 30 minutes including cleanup. Otherwise, you’ll need to spend a few bucks on some tools and supplies, all of which you’ll reuse for other projects. (Note that this list includes affiliate links.)
- A contractor’s bag. Unlike a chimney balloon, your plug will still work if the plastic tears a little bit. But it’s still a good idea to use a contractor’s bag, which has thicker plastic than a normal trash bag. If you need to use a normal trash bag, double it up.
- Scrap fiberglass, mineral wool, cotton, or other batt-type insulation.* The amount you’ll need depends on the size of your flue opening. You want to use a little “too much” for the opening so that you’re stuffing it—that way compression keeps it in place.
- A utility knife, if you need to cut a smaller piece of insulation. You’ll also want a cutting surface like a piece of cardboard.
- Safety glasses and a dust mask (or a respirator with a particulate filter if you’ve got one) are a good idea if you have to cut any insulation. You’ll also want them during installation, in case you dislodge any soot from your flue.
- Work gloves. If you’re using fiberglass or mineral wool, you’ll want to protect your hands. Otherwise you’ll be really itchy.
- Long-sleeved clothes. By the same token, you don’t want insulation on your arms or legs.
- Twine or another thick, strong thread or cord. You’ll use this to tie up your bag of insulation and give you something to pull out the plug with.
- A headlamp to help you with installation. Or just use the flashlight on your phone, but you’ll have your hands full.
*Regarding the scrap insulation, see if you have any left over from a previous project (yours, the previous owner’s, or a friend’s). If you strike out, take a look at your attic insulation—if you can see your joists easily, you probably don’t have enough up there and you’ll want to either add more or replace it soon—so go ahead and steal some for this project! If you actually do have to buy something, think about what other insulating projects you have to accomplish and buy the right product for that—no reason to buy something special for this chimney plug. Alternatively, you could really just stuff the bag for the chimney plug with anything compressible. It doesn’t have to be insulation. After all, the main goal here is to air seal your chimney, not to insulate it.
Making a DIY Chimney Plug
- Put on your long-sleeved clothes and work gloves. If you’re going to be handling fiberglass or mineral wool, you want to protect your skin from contact. If you need to cut any insulation, you’ll want to put on your safety glasses and dust mask or respirator too.
- Unroll, tear off, and open a contractor’s bag. If you’re using household trash bags, open two—one inside the other, to make a double-layer.
- Cut full batts down to size, if applicable. If you had to buy some insulation, the trick to cutting it is to lay it down (paper side up, if you bought “faced” insulation) on something sacrificial like a piece of cardboard. Put your knee(s) on it to compress it as much as possible, and use your utility knife to slice it all the way through. Depending on the size of your flue opening, you might want to start by cutting a batt in half across the short dimension.
- Stuff the bag full of scrap insulation. It doesn’t matter if you leave the paper or foil backing on. Your goal is to plug the opening—we’re not obsessing about insulation and condensation here.
- Loosely tie the bag shut and do a quick test fit. Check to see if you’ve got just a little bit “too much” insulation for the opening. If it slides in easily or is way too fat, you’ll have to add or remove insulation to compensate, so you don’t want to tie your bag too tight.
- Tie the bag tight once you’ve got it stuffed just right for your flue.
- Tie a piece of twine around the knot you made in the bag. Don’t cut it from the spool yet.
- Gently stuff your chimney plug up the flue, twine side down. Push it carefully and make sure it’s not catching on anything that could rip the bag. Remember that your goal is to block airflow, so it doesn’t matter how high up you push the plug as long as it fills the opening—no reason to go above the damper. Once you’ve filled the flue opening, only push it as far as you need to in order to keep from seeing it from the room.
- Check your seal. Look up the flue; can you see daylight around the edges of your plug? If not, use a flashlight to check for gaps around the perimeter. If you’ve filled the bag enough, you should be able to shift things around enough to fill any gaps. If not, you’ll have to remove the bag and add a little more insulation.
- Make sure the twine hangs down so that you can reach it—and see it—before cutting. The twine is there to help you remove the plug when necessary, but also to remind you to remove the plug! If you accidentally did start a fire with the plug in place, at least all the danger would be happening in your fireplace and chimney—but you’d be dealing with some lovely burning plastic smell at a minimum. Don’t assume you’ll remember: leave yourself the reminder.
That’s really all there is to it. When you decide to have a fire, gently remove the plug and try not to rip the plastic bag. But if you do rip it, just replace the bag. You can do that about 70 times before you’ve reached the cost of one chimney balloon!
Did you use any creative materials or methods to make your chimney plug? Please share!