How Do Mineral Wool, Foam and Board Insulation Differ?

• Mineral wool: fibrous batt
• Spray foam: foam that expands to fill
• Rigid board: flat panel

What you will learn

• Recognise the form and installation of each type.
• Compare their R-value, moisture resistance and flammability.
• Pick the type that suits the place to be insulated.

Before you start

Know that a higher R-value means better insulation.

What are the traits of mineral wool?

Mineral wool is a fibrous material like fiberglass. A North Dakota State University publication says rock wool or slag wool is similar to fiberglass in the manufacturing process and properties. Fiberglass is made from glass heated and drawn out to form long fibers. It can cause skin irritation, but it is noncorrosive, has no objectionable odor and does not burn.

As for form, fiberglass and rock wool come in batts or blankets, and rock wool also comes as loose fill. According to University of Maryland Extension, the R-value of mineral wool is around 3.1 per inch. Batts, blankets and rolls suit open framing in attics, basements and crawl spaces, and must be installed without compression to maintain effectiveness.

• Framing studs
• Mineral wool batts between studs
• Open framing not yet filled

How does spray foam differ?

Spray foam expands to fill a cavity. According to Maryland Extension, spray foam, typically polyurethane, is applied by professionals and expands to fill cavities. It provides excellent air sealing and moisture resistance.

There are two basic types, based on how densely it is applied. The North Dakota publication says open-cell foam is vapor-permeable, has a lower R-value and costs less. Closed-cell foam has an R-value close to R-6 per inch and costs more than open-cell foam.

• Uneven cavity and spray nozzle
• Cavity filled with foam

What about rigid board?

Rigid board is flat, stiff panels. The North Dakota publication says it may be made from cellulose fiber, fiberglass, polystyrene or polyurethanes, and used on the inside or outside of walls.

Two forms of polystyrene are common. Expanded polystyrene (EPS), or "bead board", is less expensive but has a lower R-value per inch. Extruded polystyrene (XPS) is fine-textured and moisture resistant. In Maryland's quick-reference table, EPS board is about 4.0 to 4.5 per inch and XPS board about 5.0 to 5.5.

Fire needs attention. Polystyrene and polyurethane are flammable and release toxic substances while burning, so they need gypsum board or other fire protection. Maryland adds that rigid foam board must be covered with a fire-resistant barrier indoors.

• Uncovered rigid foam board
• Rigid foam board with a fire-resistant covering
• Fire risk

How do the three compare?

Real-world example

Picture three places needing insulation. In an open attic, mineral wool batts or rolls between the wooden framing suit open framing. In a hidden wall corner with many oddly shaped gaps, spray foam expands to fill the cavities. On a basement wall outdoors, XPS board is preferred for below-grade applications. These are illustrative examples, not advice for a specific building.

Common misunderstandings

All insulation handles fire alike. Fiberglass does not burn, while polystyrene and polyurethane are flammable and need covering.

All spray foam is the same. Open-cell foam is vapor-permeable, cheaper and has a lower R-value; closed-cell foam has a higher R-value and costs more.

EPS and XPS are the same board. XPS is fine-textured and moisture resistant, while EPS is cheaper but has a lower R-value per inch.

Frequently asked questions

What is mineral wool?

A fibrous material from rock or slag, similar to fiberglass in manufacture and properties, installed as batts, rolls or loose fill.

How does open-cell foam differ from closed-cell foam?

Open-cell foam is vapor-permeable, has a lower R-value and costs less. Closed-cell foam has a higher R-value and costs more.

Why must rigid foam board be covered indoors?

Because polystyrene and polyurethane are flammable and release toxic substances while burning.

Which type suits open attic framing?

Batts, blankets and rolls suit open framing in attics, basements and crawl spaces.

Summary

• Mineral wool is fibre in batts, rolls or loose fill; spray foam expands to fill cavities; rigid board is flat panels.
• R-value, moisture resistance and flammability differ by type.
• Polystyrene and polyurethane are flammable and need covering.