Spray foam and fiberglass are the two most common ways to insulate a home, and they work differently. One seals and insulates; the other only insulates. Here is how they compare on the points that matter, so you can choose with confidence.
How Each One Works
Fiberglass is made of fine glass fibers, sold as batts, rolls, or loose fill. It slows heat transfer by trapping air in the material. Spray foam goes on as a liquid that expands into a solid. It both slows heat transfer and seals the cracks and gaps that air leaks through. That air seal is the core difference between the two.
R-Value per Inch
R-value measures resistance to heat flow, and higher is better. Fiberglass batts deliver about R-2.9 to R-3.8 per inch. Open-cell spray foam runs about R-3.5 to R-3.8 per inch. Closed-cell spray foam leads at about R-6 to R-7 per inch. The Department of Energy sets recommended R-values by climate zone, reaching R-49 to R-60 in the attic for the coldest regions, as covered in its guidance on where to insulate a home. Closed-cell foam hits a target R-value in less space, which helps in tight cavities.
Air Sealing
This is where the two split. Fiberglass does not stop air movement on its own. Air can pass through and around it, so a fiberglass job needs separate air sealing to perform well. Spray foam seals as it expands, closing the leaks in one step. The EPA estimates homeowners save an average of 15 percent on heating and cooling by air sealing and insulating together, according to ENERGY STAR, and spray foam does both at once.
Moisture
Closed-cell spray foam resists moisture and can act as a vapor barrier, which helps in basements, crawl spaces, and rim joists. Fiberglass absorbs and holds moisture, which lowers its performance and can lead to mold if it stays damp. In damp or below-grade spaces, that difference matters.
Cost
Fiberglass is the cheaper option upfront, which is why many homeowners start there. Spray foam costs more to install, and closed-cell costs more than open-cell. The trade is long-term. Spray foam’s air seal and higher R-value lower energy use over the life of the home. Fiberglass saves money on day one; spray foam saves money every month after.
Which to Choose
Neither one wins every job. Fiberglass works well in open, dry, easy-to-reach spaces on a budget. Spray foam earns its cost in attics, rim joists, crawl spaces, and anywhere air leaks and moisture are a concern. Many homes use both, matched to each area. A professional can map the right material to each space. Homeowners often ask a contractor like Northland Companies about spray foam insulation for the spots where sealing pays off, and keep fiberglass where it makes sense. A new insulation plan is one of the home improvement projects that add value and comfort at the same time.
