Is Spray Foam Insulation Better Than Fiberglass?

Choosing the right insulation is a big decision. You have to consider your budget, your climate zone, and how you plan to use a space. Fiberglass is one of the most popular insulation products available due to its low investment cost and availability. Spray foam has become more popular in the last decade despite a higher investment cost and entry barriers. Which insulation is better for you?

South Central Services has insulated hundreds of projects in our service area, which includes Camp Hill, PA, and Martinsburg, WV. The choice between spray foam and fiberglass depends on what you need. Our team installs both products, and we help homeowners determine the best insulation for them.

By the end of this article, you will understand:

• How R-values for spray foam and fiberglass compare
• What features spray foam offers that fiberglass does not
• Which insulation is better for your project
• When fiberglass is not the best option

Spray Foam And Fiberglass Can Have Comparable R-Values.

The traditional measurement of insulation performance is R-value. R-value, or resistance value, quantifies the amount of conductive heat transfer an insulation can resist. The higher the R-value, the better the insulation performs.

Spray foam insulation has a consistent R-value measured per inch. While the exact R-value varies slightly by manufacturer, spray foam R-values generally match these specifications:

• Closed cell spray foam: R-7 per inch
• Open cell spray foam: R-3.7 per inch

This means that 2 inches of closed cell spray foam have an R-value of R-14, while 10 inches of open cell spray foam have an R-value of R-37.

Fiberglass batt insulation has a range of R-values that vary from batt to batt. You can purchase fiberglass with an R-value as low as R-11 and as high as R-49.

These batts also vary in dimension and thickness, so purchasing a batt that fits the cavity and matches the necessary R-value is essential.

When selecting an insulation by R-value, either spray foam or fiberglass could fit the bill.

Unlike Spray Foam, Fiberglass Insulation Cannot Air-Seal.

While both spray foam and fiberglass resist conductive heat transfer, only spray foam resists convection and radiant heat. As building science has advanced, we have learned more about how heat travels through the air. R-value is an important measurement for insulation, but it is not the only measurement of insulation performance.

Fiberglass insulation functions by allowing air to pass through it. The design of the material is ideal for trapping sound waves, but it does nothing to stop air leakage.

Spray foam insulation, on the other hand, can create an airtight seal. When spray foam is installed, it is sprayed directly against a cavity. Like glue, spray foam adheres to that cavity. When the spray foam expands, it seals off any gaps or cracks where air could leak.

Since heat can travel through the air, air-sealing is critical in stopping energy waste. It doesn’t matter how high of an R-value your exterior wall insulation has if there’s an open window in the middle of the wall. Little gaps and cracks in the thermal envelope of a house contribute to high energy bills, drafts, poor air quality, and an uncomfortable home.

When you need high-performing insulation, spray foam is better than fiberglass. Spray foam stops more forms of heat transfer than fiberglass, making it an inherently better thermal insulator.

The Best Insulation Depends On Your Goals.

Not every insulation project is seeking maximum performance. For example, you may have a garage you want to keep more temperate in winter. However, the garage doesn’t need to be as comfortable as the space you live and sleep in.

Objectively, spray foam insulates better than fiberglass batts. However, in certain applications, fiberglass performance may be sufficient.

An example would be the interior walls of your home. Since the inside walls are not part of the thermal envelope, the performance of fiberglass is more than enough. Additionally, fiberglass traps sound and works well to dampen noise throughout the home.

Whenever you opt for fiberglass insulation in the thermal envelope, we recommend pairing it with an airtight seal. Your thermal envelope generally includes your:

• Attic
• Basement
• Crawl space
• Exterior walls

Let’s look at fiberglass for each of these areas in a bit more detail.

Fiberglass Batt In An Attic

Attics experience extreme temperature differences, so there are better options than fiberglass. However, an airtight seal will maximize its performance if you are determined to use fiberglass upstairs.

Fiberglass Batt In A Basement

Basements don’t deal with as significant temperature differences, but they struggle with drafts. Cold air leaks from your basement band boards, creating drafts. Fiberglass batt is a reasonable insulation for your basement ceiling, as long as the band boards are insulated with an air-sealing product.

Fiberglass Batt In A Crawl Space

Crawl spaces are one area where we do not recommend fiberglass batt. Encapsulation is the best method for dealing with musty crawl spaces in our climate zone. In our experience, there is no way to prevent fiberglass from absorbing moisture and becoming ineffective.

Fiberglass Batt In Exterior Walls

Exterior walls could be insulated with batt insulation alone. However, one of the best ways to utilize batts in exterior walls is through the hybrid insulation method. This method ensures an air-seal as well as a vapor barrier.

We Don’t Recommend Fiberglass For Metal In Our Climate Zone.

One application where spray foam is distinctly better than fiberglass batt is metal substrates. In our climate zone, temperatures can be extreme. The disparity between inside and outside during summer and winter is significant. These temperature differences pose real problems for metal substrates.

Metal buildings and pole barns often opt for fiberglass, since it is so inexpensive to invest in upfront. However, as air passes through the fiberglass, it risks condensing due to the extreme temperature differences. This condensation can lead to mold, mildew, and wood rot. Additionally, wet fiberglass is completely ineffective as an insulator.

For metal substrates, our recommendation is closed cell spray foam. Closed cell offers the necessary vapor barrier. It also offers a high R-value at a shallow thickness.

The Bottom Line About Spray Foam And Fiberglass Performance

Spray foam will always outperform fiberglass. Fiberglass is a budget-friendly insulation with a few key benefits and many drawbacks. Spray foam is an expensive insulation that can be used in almost any area of a home.

As long as insulation performance is not your primary concern, fiberglass can be a good choice. However, we often recommend pairing fiberglass batts with air-sealing techniques to maximize their effectiveness.

Now that you understand why spray foam is technically better than fiberglass, your next step is to:

• Estimate the cost of spray foam insulation for your project
• Discover if spray foam is really air impermeable
• Learn how spray foam insulation can save you money