Insulation filters the air leaks from your house, showing you signs of energy loss

You might have noticed some black insulation in your attic or maybe around the perimeter of your basement, where the house rests on the foundation. What does this mean? Is it moldy? Wet? Why is the insulation black?

In fact, black insulation is the energy auditor’s best friend because it tells us where the problems are. In just a few minutes of looking around the attic, you can find the most serious air leaks from the house. Here’s why…

When you have an air leak between the house and the attic, it is usually at an electrical wire or pipe that runs through the walls and into the attic. But sometimes, it’s at a bigger hole, like the duct chase in the photo above.

Fiberglass gets discolored when it filters the air leaking from your house. Over the years, the air leaking from the house, carrying dust and other particulates and moisture, turns the fiberglass black. So wherever you see black fiberglass, dig down and look for where the air is leaking out. Once you find the source, use some good quality caulk or canned foam to fill in the holes. If it’s a big hole, you might need to cut a piece of sheet-metal or drywall to cover the hole. You then want to seal it with caulk or foam to ensure that it’s air tight.

Out of sight, out of mind

These problems would have been easy to fix when the house was built or when the electric wire or plumbing was installed. Unfortunately, electricians and plumbers don’t usually want to be bothered with filling holes or doing carpentry (even though it’s required by building and fire code!) so when nobody is looking, they lay a piece of insulation over the hole. Done! Nobody will ever notice…

What's hiding underneath that fiberglass?

But what happens? As you’ve probably seen, every attic floor looks like swiss cheese. There’s electric wires drilled through the wall framing, plumbing vents, sometimes chimneys, recessed lights and bath fans… when you add up all these holes, you realize that they add up to something like leaving a window open in the house all year round. This is why all weatherization specialists attack these issues first. They’re easy to fix and they improve the comfort, health and safety of your home.

So next time you see some black insulation, you can smile, pull back the insulation and fill the hole. You’ll know that this small action is helping to make your home more energy efficient and safe for your family and every other person who lives in that house in the years to come. Since when was leaving a positive legacy this easy!


It’s important to note that you have to follow codes when you’re sealing holes. For example, don’t use a flammable material around chimneys and hot stove pipes. Here’s a great common sense article that tells you how to seal these hot penetrations.

High quality chemical and dust respirator

Also, when working in your attic, I always recommend wearing a high-quality respirator. Not those crappy paper ones, but good ones that use a rubber seal around your mouth and nose.

I use something like the one shown here because it’s awesome at filtering fiberglass and dust and it has chemical filters so I can use it when spray painting or using other nasty substances. However, you don’t really need to go crazy with it. Just make sure that you get a respirator with a good seal.

Additional reading:

Energy Star – Sealing Air Leaks: Basement

Fine Homebuilding – sealing air leaks around chimneys

Home Energy Magazine – Air Sealing in Occupied Homes. Classic article from 1995

Oikos building library – Advanced Air Sealing – Chimneys

The Family Handyman – How to Seal Attic Air Leaks. Great how-to article

Additional photos

Duct chases like this are huge energy holes. Cap and seal them after tightening the duct

Sometimes insulation hides bigger problems.

If you see insulation that looks like this, there’s definitely a BIG problem. Pull back the insulation to see what’s going on behind it. In this case, there’s a bathtub just sitting behind the insulation, with no air or moisture barrier to prevent the bathroom air from leaking into the attic. When this happens,  it can lead to mold growth and rotten roofs.

  1. mig says:

    Thanks a lot! I started working on this then got worried that it was some kind of old asbestos insulation. You put my mind at ease and put the project back on track!

  2. Tish says:

    This helped tremendously. I am a new homeowner and noticed black insulation in my attic and went nuts. Now at least I know, I have to not only change the insulation, but caulk as well. Thanks.

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