What does it mean if your fiberglass insulation is black?

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.


20 thoughts on “What does it mean if your fiberglass insulation is black?

  1. Hi is there someone who can tell me what is going on in the attic of my building? There are dark spots all over the insulation in the attic. IMG_5491.HEIC
    I am trying to attach an image to here not sure if it will work.

  2. Pingback: When Insulation Turn Black: Explained - Steady Insulation

  3. My garage is dry walled, my home was built in 1979. The drywall between the garage and house is original as is the fiberglass insulation behind drywall. It is 1/2” drywall and I am replacing all drywall in garage with 5/8”. There are no visible signs of any water damage or staining to any of the drywall but when I started to remove the drywall I noticed sections of the fiberglass insulation are stained black and gray. I know without seeing it or testing that it cannot be a certain but would you believe with no signs of and water that it is mostly just old insulation and dirt and being that old I’m thinking of replacing it anyway

    • Those dirty spots are most likely indicating that there was some air flowing through cracks. That carries dust and the fiberglass picks it up, filtering the air. It’s nothing really to worry about but it is a good opportunity to seal cracks because it’s a good indicator of where to look for a little air sealing/caulking.

  4. ALL my attic insulation is black from top to bottom. How can I possibly have that many air leaks? The wiring and vents are caulked or spray foamed. Please help.

    • That’s bad! Is it possible that there’s a water leak or something else that could be saturating the insulation with moisture? Like, for example, bathroom fans or dryer vents that just go into the attic?
      If it’s that bad, you really should have a professional inspect it.

  5. Pingback: Joist A Minute | Courtyard House London

  6. With havin so much content do you ever run into any problems
    of plagorism or copyright violation? My blog has a lot of completely unique content I’ve either written myself or outsourced but it
    appears a lot of it is popping it up all over the
    web without my agreement. Do you know any techniques to help
    prevent content from being stolen? I’d genuinely appreciate it.

    • I’m sure it happens. I know the big blogs deal with this and there’s nothing they’ve been able to do to stop it. Once it’s on the web, it’s unfortunately free for anyone to copy.
      As far as I know, all you can do is do your best on your blog, create high quality content and be true to your readers.
      Good luck!

  7. My basement apartment has 3 walls of black loose insulation in the washroom. This was discovered after a flood, and a restoration crew had to cut the gyprock out. The floor had asbestos tiles, along with the wall tiles!! The homeowner is slowly correcting the problem, but I’ve been exposed to black mould and asbestos for almost one month now. I am scared. I can’t afford to move, is there anything I can do to protect myself?

    • I’d suggest a few things, but the most important is don’t panic

      Asbestos is not a danger when undisturbed. It’s the fine fibers that cause problems. This means that they’re usually only a health concern when the tiles are broken, like when a construction crew does a demolition.
      The black insulation isn’t necessarily black mould of the harmful kind. Insulation often turns black and there are numerous varieties of mould that we breathe every day without harm. You should note that if it’s a rental property, the owner could be held liable for a tenant living in unhealthy conditions. You certainly shouldn’t be forced to live in an apartment where the insulation is exposed. At the very least, the walls should be restored / covered.
      Depending on your relationship with the landlord, you could discuss your concerns with them and note that it might be a health risk. The last thing they’d want is a lawsuit.
      If you don’t feel that you can take it up with the landlord, then you’d have to go to a local building official/township office etc. and ask them for what recourse you might have.

      Good luck.

  8. My home has a bit of a weird lay out. I was in my basement and noticed black colorations on the insulation above so after further investigation i noticed as i went up the stairs which are bare with no insulation I looked up at sagging insulation which was laid to insulate the attic had the same color of black. This home as 2 fireplaces and no central air at all. I have had some handymen come an look and stated that it was nothing to worry about. This still does not give me any peace of mind. Although reading your article has definitely give me a bit of ease. I had seen mold show up on my home test kit and hoping that this isn’t black mold and maybe just a different type and that the black is from what you described in your article. Please let me know what you think thanks!

    • Very difficult / impossible to say without direct inspection, but, as you noted, often black insulation just means air flowing through it, not mold.
      I don’t know about the sensitivity of the mold test kit, but harmless mold is everywhere.

      If you have a real concern, I’d strongly recommend having it professionally tested. Better safe than sorry.

      • You can get home test kits from the hardware store or you can find a local contractor to do the testing. I don’t trust a lot of people who do mold work because they make money fixing mold problems so it’s in their interest to make it seem like the mold is going to kill you. I would ask a real estate agent that you trust for a recommendation. They’ll know the character of the mold contractors in your area.

  9. I have a black coating over the loose fiberglass in my attic. It is not just dirty. Is this not just mold but sulfate?
    From ehow.cm
    Soil and rock formations may naturally contain sulfates. As water moves through these, sulfates are picked up and dissolve in the groundwater, according to Minnesota’s Department of Health. Sulfur-oxidizing bacteria convert sulfides into sulfates and leave dark slime that blackens water and can stain the inside of toilet tanks. An article published by Wilkes University states that sulfur-reducing bacteria are more common than sulfur-oxidizing bacteria.

    Read more : http://www.ehow.com/about_5457669_sulfate.html

  10. It is not just ‘dirty’ fiberglass, I know the difference as my husband had a body shop building with fiberglass insulation exposed that got filthy, I am seeing black coating on attic fiberglass that had been used to vent the bathroom. This is not mold, I am thinking, but suflate:
    From ehow.com
    Soil and rock formations may naturally contain sulfates. As water moves through these, sulfates are picked up and dissolve in the groundwater, according to Minnesota’s Department of Health. Sulfur-oxidizing bacteria convert sulfides into sulfates and leave dark slime that blackens water and can stain the inside of toilet tanks. An article published by Wilkes University states that sulfur-reducing bacteria are more common than sulfur-oxidizing bacteria.

    Read more : http://www.ehow.com/about_5457669_sulfate.html

    Could this sulfate have been so extensive that it actually saturated the attic?

  11. 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!

  12. 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.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s