Fiberglass vs. Spray Foam: Which is Best?


When tackling an insulation project, one of your first decisions is likely “what material should I use?” Two common types of home insulation available today are fiberglass and spray foam. It’s important to know the key differences between the two — including cost, performance and safety — to determine which is right for your project. Let's get to the facts, so you can make informed decisions.

What is Fiberglass Insulation?

Fiberglass insulation is recycled glass “cullet” and other raw materials which are melted and spun into fibers. Fiberglass insulation comes in pre-cut batts, rolls or loose-fill forms.

What is Spray Foam Insulation?

Spray foam is a chemical-based insulation that is mixed on site by insulation contractors to create foam, and applied with a sprayer. It is available in either closed-cell or open-cell varieties. Closed-cell is denser and more expensive than open-cell.


Fiberglass insulation is easy to install with minimal training which allows most DIYers to complete the task. Recommended Personal Protection Equipment (PPE) are gloves, protective eyewear, long sleeves, and dust mask. Fiberglass insulation requires nearly zero downtime because there’s no drying or curing time required. Plus, other trades can safely continue working during installation.

Spray foam insulation requires precision and expertise. DIYing is typically not an option. Because spray foam insulation requires proper handling and mixing of two hazardous chemicals on the jobsite, installers must go through an extensive training program, such as through OSHA or the Spray Polyurethane Foam Alliance (SPFA). Extensive Personal Protective Equipment (PPE) is recommended including full-face supplied air respirators and chemical protective clothing.

Spray foam creates downtime as other trades are recommended to evacuate the jobsite during installation and wait up to  24-48 hours before re-entering since it takes time for the spray to cure and off-gas. Spray foam poses a risk of fire from spontaneous combustion if not applied correctly due to an exothermic reaction. The potential for off-gassing may cause adverse health issues if not mixed and applied properly.


Quality Control

Fiberglass insulation is manufactured at a facility with strict quality control measures in place. No special storage or handling of material is typically needed with this type of product.

Spray foam insulation is essentially manufactured on site during application. The two chemicals meet and mix at the spray gun causing a foaming reaction. Many factors can influence the quality of the resulting product including:

  • Air temperature and humidity during installation — if the relative humidity is too high or the air temperature too low, there can be problems with installation
  • Substrates also must be within a certain temperature range. If you apply 80°F foam on 40°F OSB, you can run into problems.
  • Age of the chemicals — which have a shelf life that can affect mixing and application
  • Temperature of chemical storage — optimal storage is between 65 and 85°F, depending on manufacturer guidance. If there are variances or chemicals are not stored properly, the product can be compromised


Health & Safety

Understanding the health and safety impacts of building materials is crucial for architects, contractors and the families who will inhabit the space.

The International Agency for Research on Cancer (IARC), the US National Toxicology Program (NTP) and the California Office of Environmental Health Hazard and Assessment have all stated that fiberglass insulations are not considered classifiable as carcinogens when proven to be biosoluble.

One of the main ingredients of spray foam insulation are isocyanates, which can cause skin, eye and lung irritation, asthma and “sensitization.” There is no recognized safe level of exposure to isocyanates for sensitized (responding to certain stimuli in a sensitive manner) individuals. Isocyanates have been reported to be the leading attributable chemical cause of work-related asthma; both dermal and respiratory exposures can trigger adverse health responses.1



The installed cost for R13 of spray foam insulation has been shown to be nearly 3-4 times as much for the same R-Value as fiberglass.1  Additionally, spray foam insulation has a high cost of entry with required training and equipment. A spray foam rig can cost upwards of $70,000.

Fiberglass insulation is the go-to product for most contractors where they need a fast, easy and efficient solution meeting building codes across many climate zones.



Now more than ever there is a desire for more sustainable building solutions for the good of the world that we share. Fiberglass insulation is made from renewable and abundant resources like recycled glass and sand. Knauf sustainable insulation contains at least 50 percent recycled content, putting billions of glass bottles back to work rather than into landfills. 

Spray foam insulation is strictly a chemical product and therefore contains very little recycled content.1


Insulation comparison: recap



Spare the spray

Fiberglass insulation is a safer, more cost-effective and sustainable choice. And it’s easier to install — with no training, heavy equipment, or drying/off-gassing time required. It’s no wonder fiberglass is still the solid preferred choice of insulation professionals with 71 percent of the new home market (Builder Magazine study).

More benefits of using Knauf fiberglass insulation for residential projects.




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