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Can Fiberglass Insulation Touch Stove Vents?


Is it okay for insulation to touch stove vents?

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Take caution when installing fiberglass insulation near stove vents. Yes, you want to ensure an energy-efficient home but this is a special circumstance. Can fiberglass insulation touch stove vents?

Fiberglass insulation can’t and should not touch your stove’s vent pipe. While fiberglass is not flammable, it can melt, causing fire risks. To get around this issue, you need to install a barrier of aluminum flashing between the vent and the insulation.

In the rest of this discussion, we’ll delve into why you shouldn’t install fiberglass insulation in direct contact with your stove vent. We’ll also take a step-by-step look at what you should do instead, so be sure to read to the very end.

What Is Fiberglass Insulation Made Of?

To understand why fiberglass insulation shouldn’t touch stove vents, you need to have a rough idea of what makes up this insulation material. 

Fiberglass is made by mixing silica sand, soda ash, and limestone. The silica sand makes the glass, while the soda ash and limestone help lower the melting temperature required to manufacture the material.

Other ingredients such as borax, magnesite, kaolin, and feldspar are also added, but these are usually meant to enhance the resulting material’s performance.

Once the above ingredients have been mixed, they’re heated to 2,500°F (1371.111°C). The resultant molten glass is then made into glass wool, fibers, or filaments. 

Often, fiberglass products require additional coatings to enhance performance. Examples of such additives include anti-static agents and lubricants to reduce their abrasive qualities.

Once all the additives have been added, the fiberglass is then shaped and packaged for shipping.

Why You Would Want the Fiberglass Insulation and Stove Vents to Touch

When insulating your home, there are two key reasons why you’d want the fiberglass insulation to sit flush with your stove vent.

Let’s discuss them below.

The Insulating Factor

Maintaining the thermal boundary in your house is an essential part of having an energy-efficient home. 

Ducts, plumbing, and vent pipes create small openings through which air can escape. If you’re familiar with thermal insulation basics, you know that this is exactly what we are needing to avoid.

When conditioned air leaves your building, any energy used to heat/cool it is wasted, increasing your heating costs.

Reduces Chances of Ice Damming

The second reason you’d want the fiberglass insulation to butt up to your stove’s vent pipe is to prevent ice dam damage. Ice damming often occurs when warm air accumulates in the attic. Warm air melts the underside of any snow that may have accumulated on your roof during winter.

The molten snow then flows down the roof’s surface until it finds a cold spot (such as the soffit or the eaves), where it freezes to form a dam. Since this is a continuous cycle, the snow keeps building upon this dam until it damages roof shingles, which causes water leakage.

Why the Fiberglass Insulation and Stove Vents Shouldn’t Touch

As we discussed earlier, fiberglass insulation is mainly glass, even though there are other additives. We all know that glass is a non-flammable material that won’t catch fire even when subjected to high temperatures.

So, what’s the problem with having fiberglass butting up to your stove’s vent when it won’t ignite?

There are actually several potential issues with such a set up: 

Other Materials in Fiberglass Insulation Aren’t Non-Flammable

While fiberglass doesn’t burn, the same can’t be said for the foil and paper backing you’ll find on this insulation material. These will ignite if the temperatures get high enough, and you can’t bet against that happening in your stove’s vent.

Fiberglass Can Melt

Fiberglass has a melting point of about 1000°F (537.778°C). Stove vents get extremely hot and can cause the fiberglass insulation to melt.

For more details on this, read Can Fiberglass Insulation Catch Fire? Know The Facts

Why Is Fiberglass Melting a Problem?

For starters, molten fiberglass can drip onto other flammable parts of your home and ignite them, which can start a fire in your home or turn an otherwise minor chimney fire into an inferno that’s harder to control.

Second, fiberglass typically releases several gasses as it melts and among them is oxygen (the oxygen is probably from the oxygen molecules in silica sand, whose chemical formula is SiO2).

If this is happening during a minor fire, expect it to speed up and spread more rapidly because oxygen typically speeds up combustion. 

So, while fiberglass may be non-flammable, it can speed up or even trigger a fire if it melts.

How to Insulate Your Home Using Fiberglass Without Risking Fire Hazards

Note that these are general guidelines – consult a professional in your area to ensure a safe install.

The fact that you shouldn’t let fiberglass insulation sit flush with your stove’s vent doesn’t mean you should ditch the insulation material altogether. It doesn’t mean you should leave air spaces between your stove’s vent and the insulation, either, because this can compromise your home’s insulation.

Instead, it means that you need to leave an inch (2.5 cm) of clearance between your fiberglass insulation and the metal flues (AKA the pipe that directs your furnace exhaust out of the building). If it’s a masonry chimney, this space should be two inches (Source).

You’ll then want to seal this gap with aluminum flashing and heat-resistant caulk to prevent air leakage. You’ll also want to build a metal dam before putting back your insulation. Doing this helps keep the fiberglass off the often hot flue pipe.

Here’s how to do that:

  1. Start by cutting aluminum flashing to a size that snugly fits around your flue. If your vent has a cylindrical shape, cut the flashing into two half-circles that overlap 3 inches (7.62cm) at the middle.
  2. Press the aluminum flashing into the bead of a heat resistant caulk before stapling or nailing it into place. If there isn’t any wood to nail or staple into, you can do that directly on the drywall; avoid nailing or stapling through it.
  3. Seal the space between the aluminum flashing and the flue using high-temperature caulk. Avoid using spray foam; it just isn’t ideal for this purpose.
  4. Build an insulation dam to keep the fiberglass insulation off the surface of the flue pipe. Cut a piece of aluminum long enough to wrap around the flue pipe and still leave an extra 6 inches (15.2 cm). Next, cut 1″-deep slots every other few inches along the top edge of the aluminum and bend in the tabs. Cut similar slots on the opposite edge, but this time with a depth of 2″ (5 cm). Bend out the tabs. You now have your dam.
  5. Finally, wrap your dam around the flue pipe before securing the bottom edge in place by stapling/nailing through the bent out tabs. Now you can put your fiberglass insulation back right up against the aluminum dam.

Summing Up

Fiberglass insulation shouldn’t touch stove vents because it may melt and trigger/speed up fires or cause the foil and the paper backing to ignite.

Remember that you need to leave a minimum 1″ (2.5 cm) space between the fiberglass insulation and your stove’s vent, but since that space would allow air leakage, you have to seal it using aluminum flashing and heat resistant caulk.

Paul

As a homeowner, I am constantly experimenting with making the structure of my house more energy-efficient, eliminating pests, and taking on DIY home improvement projects. Over the past two decades, my family has rehabbed houses and contracted new home builds and I've learned a lot along the way. I share my hard-learned lessons so that you can save time and money by not repeating my mistakes.

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