Can Fiberglass Insulation Grow Mold?

The Science Behind Fiberglass Insulation & Mold

Fiberglass batt insulation installed in the joists over a basement

Fiberglass batt insulation (glass wool) installed inside the floor joists in a basement.

There’s been a great debate in recent years over how effective fiberglass insulation is when installed in a moist environment.

Old school contractors swear by it, claiming that it is moldproof and appropriate for installation in basement and crawl space environments.

Newer building science thinking pins fiberglass insulation as a terrible idea for below-grade spaces, citing its being ruined by damage by mold, rot, and humidity.

Then old-school contractors retort that it’s the lack of ventilation, not the fiberglass itself that is the culprit.  And the debate rages on and on.

So What’s The Scoop?  Can Fiberglass Support Mold?

Let’s take a practical look at this. Fiberglass is made with:

  • Glass (Usually 20%-30% recycled industrial waste and post-consumer product)
  • Dyes (Fiberglass is not naturally pink!)
  • Paper Backing (Optional, but very common)
  • Resins (Optional — binds the fiberglass to the paper backing)

Of these, I think we can agree that at least two (the paper backing and resins) can support mold growth.

The proof, friends, is in the pudding.  See below:

Mold growing on the paper backing of fiberglass insulation

OK, so fair enough.  But what about fiberglass without paper backing?  That should work, right?  Except that fiberglass is porous and absorbent.

Consider these two points:

  1. Fiberglass can capture dust and dirt, holding it in its fibers, where mold can grow.
  2. Fiberglass can soak up moisture like a sponge and hold it against structural wood.

Exhibit A:  Fiberglass Filled With Dust, Dirt, & Debris

Dust filling fiberglass insulation from a leaky air ventIn this case, a leaking air duct from an old HVAC system has been blowing air into this fiberglass insulation for quite some time.  And unless the HVAC system has been properly maintained (it hasn’t), then you can be sure there’s plenty of mold spores in that dust as well.   The fiberglass has essentially served as an air filter — one that needs to be replaced.

Exhibit B:  Wet Fiberglass Leading To Wood Damage

Structural wood that's been damaged by damp fiberglass pressed against itIn this case, a slow plumbing leak several feet away had soaked the fiberglass insulation for several feet around it with light moisture.  In some areas, the fiberglass insulation had sunk and fallen off the ceiling, while it remained moist but in place in other spots.

In the places where it held in place, it sponged up the moisture and held it in place against the structural wood for months.  Eventually, the slow leak was discovered (a difficult challenge in an already wet basement that is rarely used) and was  removed.

This is what we found behind it:

Damp flooring that was made worse by fiberglass insulation.The floor above is springy and weak, sagging down underfoot.

Now, of course, the plumbing leak was the real issue in this case.  But the fiberglass, which held the moisture in and hid the problem for the homeowner, certainly didn’t help.

What Kind Of Insulation Should I Use Instead?

An image of a large open gap around an air duct in a basement

Air gaps are built into your home as it's built. Contractors create these gaps as they install air ducts, wires, and pipes in your home.

Instead of installing fiberglass insulation on your floor joists, we suggest that you consider rigid foam insulation on the basement walls.

While it’s a bit more expensive than installing fiberglass, the savings are definitely there. In fact, the US Department of Energy reports that as much as $400/year can be saved simply by insulating your basement walls!

The problem with fiberglass insulation installed in the floor joists is this:  Insulation is only effective when it can be installed in an unbroken sheet.

However, when fiberglass is installed in the joists, there are boundless opportunities for air to circumvent the insulation.

Pathways for insulation include around gaps in the basement door, up through laundry chutes, and through open gaps around air vents, pipes, wires, and more.

Transforming the basement into insulated space will also provide the benefit of protecting utilities stored in the space (HVAC systems, water heaters, furnaces, hot water pipes, and hot air ducts) from the colder environment.  And if your basement DOES flood, foam insulation will not soak up moisture, grow rot, or be ruined and need replacement.

Since you’re protecting the materials in the basement from water anyways, you should consider sealing the floors will also help to protect carpets from mold by holding back water vapor that would otherwise pass through the concrete slab.

The Moral Of This Story

If you’re installing fiberglass batt insulation in a finished basement, it can support mold growth.  It holds dirt, dust, and debris inside the fibers, and it soaks up moisture like a sponge.  Wet insulation sags, opening holes in the insular sheet.  And, really, fiberglass insulation has little or no insular value when it’s soaked with moisture anyways!

Moisture can enter a basement through the walls in many ways, including:

  • Water leakage through existing and newly-created cracks in the wall
  • Water leakage around pipe penetrations in the walls
  • Water vapor passing directly through the pores of the concrete
  • Moisture soaked upward during a flood in the basement

Before finishing your basement, be sure to address all sources of leakage and waterproof the basement.  You should seal the concrete and install a plastic vapor barrier on the walls to prevent water vapor from building up behind the walls and creating a moisture issue.  We do not recommend coating the walls with waterproof paints or installing fiberglass batt insulation.  Instead, install rigid foam insulation on all foundation wall surfaces in the basement.

To seal concrete in a basement, we recommend Concrete Treat:  Concrete Sealer & Blanket.  Our sealant is ideal for do-it-yourselfers — it applies safely and easily, and it dries in just 2-4 hours.  There’s no odor, and your concrete will not change in appearance.  You can even paint over it if you’d like!

You can buy our concrete sealer directly online, or you can give us a call at (203) 376-9180!

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