Does Fiberglass Insulation Work Well?

by Erik North on December 29, 2010

fiberglass insulation


Why doesn’t fiberglass insulation work as well as other types of insulation? What is the best type of insulation for your home? How would you best insulate your home – and does fiberglass meet that requirement? Does fiberglass insulation save more money than other types of insulation?

Before I became an energy auditor, when I heard the word “insulation,” I automatically pictured the pink fluffy stuff – the fiberglass insulation that may be purchased in long fuzzy rolls or thick squares at Home Depot, Lowe’s, or even Wal-Mart. I didn’t know anything about the other types of insulation, nor did I know the benefits/drawbacks of those types.

It’s a confusing subject!

Insulating your home means slowing the heat loss through your attic, walls and basement. Improve the thermal barrier, the insulation and your house retains more heat. It’s similar to the layer of blubber around a whale, protecting it from cold waters.

To be clear I love almost any insulation. The more insulation, generally the less it takes to heat you home. And I’m all about keeping you comfortable and saving money.

However, not all insulation materials are alike (or equal).

Why Doesn’t Fiberglass Insulation Work Well?

Fiberglass insulation is made by melting sand and recycled glass together. It is spun out like cotton candy, collected as extraordinarily long glass threads. The threads are then matted into the pink blanket we’re familiar with. Fiberglass can come in a blanket (long roll) or batts (pre-cut rectangles) or loose fiberglass fill can be blown into a cavity. Fiberglass’s R-value is approximately 3 per inch.

As an insulation, it is cost effective, temperature resistant and equally effective in most climates. The main drawback is that it does not stop air flow in any way. A building shell consists of a thermal barrier (that’d be the insulation), air barrier and vapor barrier. Unless paired with an effective air barrier, fiberglass insulation allows air to flow past it.

The most common attic insulation found in the Northeast is fiberglass batts. Modern attic systems must cope with air flow from chimney bypasses, plumbing stacks, electrical and light fixtures and recessed lights. There’s an energy auditor joke that as an insulation, fiberglass is a great filter.

Check out this photo:

This is fiberglass from an energy audit where the customer was complaining of a cold bathroom. Why? In part because air was leaking out the bath fan, the attic hatch (located in the bathroom) and plumbing chase. And the fiberglass covering these ceiling penetrations was filthy from all the dust and dirt particles filtered out of the air.

Air flowing through the fiberglass leaves dust and dirt trapped in it. Dirty fiberglass is a tell tale sign of air transported heat loss. The air flow also hampers its insulating properties, preventing fiberglass from functioning at the advertised R-value. Finally, the warm, moist conditioned interior air is transporting moisture through the insultion which can lead to moisture, mold and ice dam issues.

Fiberglass Insulation’s Benefits (And Yes, It Does Have Them):

Yes, fiberglass insulation does have a couple significant benefits over other insulation. These benefits have lead to its ubiquitous use: it is less costly and easy to install. Any weekend warrior worth their tool belt can competently install fiberglass. Spray foam, cellulose, XPS and polyisocyanurate foam board insulation are more expensive and should be installed by a professional.

Is Fiberglass Worth The Convenience?

Only you can decide what type of insulation you’ll put in your home, but the decision is imperative – it lowers heating costs, can prevent air leakage, reduces the amount of heat needed, and makes your home a more comfortable place to live.

Despite its convenience and low cost, I usually don’t recommend fiberglass insulation to my customers. Nor did I insulate my own home with fiberglass. I insulated with a combination of spray foam, cellulose and polyisocyanurate foam board in the attic and extruded polystyrene (XPS) and spray foam in the basement.

{ 1 comment… read it below or add one }

Russell G. Namie February 7, 2015 at 11:24 pm

About 3 years ago we had the fiberglass insulation removed from the attic floor and replaced with Demilec open cell foam. Where I used to see icicles from heat loss now I see none. The attic, which we use for some storage, is much cleaner now. We lifted the osb flooring and replaced it in the process.

We are going to build a 20 x 22′ detached garage. It will be all brick to match the all brick house. Even though we are not going to be out there that much I feel that I should insulate it to avoid regrets later. Any heat source would be temporary ( space heaters ).

Please give your recommendation for insulating and finishing the walls. The spray foam guy also does cellulose dense packing.



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