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The Weakest Link: Attic Sealing and Insulation Before Winter

Snow falls, while heat rises. Put these two together and you can easily determine one of the biggest sources of heat loss in your house: the attic.

While figures vary, most experts suggest that up to 25% of heat loss occurs in the attic. Since hot air rises, it can leak out of your home and into the cold, causing higher bills at the expense of comfort. It also causes your furnace, thermostat and heaters to work harder, lowering their efficiency and lifespan. 

In contrast, a properly sealed and insulated attic can save as much as 50% off your heating bill, according to the Department of Energy.

Here’s how you can seal your attic in preparation for winter.

Seal Air Leaks

Before you can start on insulation, it’s necessary to plug all the air holes first. No amount of batts or fill will help if the hot air finds a way to escape through cracks and seams to the outdoors.

Check the following areas:

Attic windows

  • Make sure the weatherstripping is good. Brittle, cracked or deformed rubber should be replaced, if it can’t be mended.
  • To seal the window casing, use a can of minimally expanding spray foam. Make sure to do this before the cold sets in, so the foam has enough time to settle in. 
  • For the window sash and jambs, apply foam weatherstripping as necessary.

Exhaust vents and ducts

  • Check if the metal is level and flush against the wall. Tighten screws and realign as necessary.
  • Apply caulk on noticeable gaps, taking care not to get any inside the vent or duct itself so as not to obstruct the airflow.

Pipes and wiring

  • Use fire-blocking caulk to seal gaps less than ¼ inches
  • For bigger gaps, use fire-blocking spray foam. Make sure the foam is rated for use with wires, and check if the wiring is sound before applying the foam.

Chimneys and Flues

If your fireplace passes through the attic, this may be another weak point. Since the attic is less visible in daily life, any damage may not be evident compared to the living area or during rooftop check.

  • Look for gaps, cracks or misaligned parts in the mortar and base.
  • To fix gaps, use metal flashing combined with high-temperature caulk rated specifically for chimneys.
  • Larger areas of damage can be fixed with furnace cement, which can both metal-to-metal and metal-to-masonry.

Pre-Insulation Steps

With air holes plugged, you can move on to insulation prep. Before rolling out the batts, make sure the areas they will go in are ready.

Light fixtures

  • Recessed lights may be a fire hazard if they come in contact with insulation, unless you’re using mineral wool or the lights are rated safe for insulation contact.
  • Box out suspect fixtures using plywood, metal flashing or hardware cloth.
  • Leave a 3 inch gap around the fixture for safety.

Exhaust fans

  • It’s a code violation to vent any exhaust to the attic, however older houses may not have been built with this rule in mind.
  • Make sure that all exhaust fans and vents are directed away from the attic and safely out of the house.
  • Trapped exhaust, and the humid air that comes with it, is detrimental to insulation and may lead to premature replacement.

Water leaks

  • Like humidity, water is a mortal enemy of insulation. It destroys insulation, creates mold and mildew, and makes air pockets useless.
  • Watch out for water stains, pooling, or mold formation, which are sure signs of a water leak.
  • Vulnerable spots include corners, joists, and pipe bases.


With the initial prep out of the way, you can finally start on attic insulation. Insulation materials are graded by R-values, which is a measure of how well it can resist heat flow. The higher the value, the more effective it is at its job.

There are two main choices, both of which can be layered over existing insulation, or used as a base for uninsulated attics:

Loose Fill

These are bags of insulation fibers that can be blown to the required depth and density. They are ideal for irregularly shaped or highly cluttered attics, low headroom spaces, and nonstandard joists.

There are 3 types:

Fill Type

R-Value per Inch Composition


Fiberglass 2.2 to 2.7 Silica or glass Lightest weight, but requires thicker layers
Mineral wool 3.0 to 3.3 Recycled slag or rock fibers Naturally fire resistant, but most expensive
Cellulose 3.2 to 3.8 Recycled paper Highest R-value, but most susceptible to moisture damage


These are flexible blanket-like rolls that come in different widths and thicknesses. They can be layered on top of each other to attain the desired insulation level. Batts are ideal for regularly shaped attics with sufficient headroom and standard joists.

There are 4 types:

Fill Type

R-Value per Inch Composition


Fiberglass 2.9 to 3.3 Silica or glass Least expensive but less effective at airflow blocking
Mineral wool 3.0 to 3.3 Recycled slag or rock fibers Naturally fire resistant, but more expensive
Cotton 3.7 to 3.8 Recycled cloth Good airflow and sound insulation, but more expensive
Cellulose 3.7 to 3.8 Recycled paper Limited selection, vulnerable to moisture damage

The Department of Energy has a more comprehensive list of the differences, advantages and requirements for each insulation type.

As for putting in your insulation, refer to our guide on insulation installation.

Good luck, and happy sealing!

Mickey Luongo

Mickey is the resident heating and air conditioning expert with over 15 years of experience in the industry.

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