R-Value Calculator

What Does R-Value Mean?

R-value stands for resistance value. It measures how much a material resists the movement of heat.

A low R-value means heat can move through the material more easily. A higher R-value means the material slows heat movement more effectively.

For example, if one attic has very little insulation and another attic has a properly installed layer of insulation with a much higher R-value, the better-insulated attic will usually do a better job slowing heat transfer between the attic and the living space below.

This matters in both summer and winter:

• In the summer, insulation helps slow heat from moving from the attic into the home.

• In the winter, insulation helps slow heated indoor air from escaping into colder areas.

Insulation does not “create cold air” or “create heat.” It simply slows the movement of heat, helping your HVAC system work in a more controlled environment, improving efficiency.

Typical R-Values by Insulation Type

Fiberglass Batts
Fiberglass batts commonly provide about R-2.9 to R-3.8 per inch, depending on product density. Standard wall batts are often rated around R-11 to R-15 in a 2x4 wall cavity, while higher-density products can achieve higher values in deeper framing cavities. The Department of Energy notes that high-density fiberglass batts can reach R-15 in 2x4 walls and about R-21 in 2x6 walls.

Blown-In Fiberglass
Blown-in fiberglass commonly provides about R-2.2 to R-2.7 per inch, although product specifications vary. It is often used in attics where additional insulation can be blown over existing material to increase the total R-value.

Blown-In Cellulose
Blown-in cellulose commonly provides about R-3.2 to R-3.8 per inch. Because it is dense and fills irregular spaces well, it is often used in attic insulation upgrades and retrofit projects.

Mineral Wool / Rock Wool
Mineral wool commonly provides about R-3.0 to R-4.2 per inch, depending on the product. It is often valued for thermal resistance, sound control, and fire resistance.

Open-Cell Spray Foam
Open-cell spray foam commonly provides about R-3.5 to R-3.9 per inch. It expands after application and can help reduce air movement when properly installed.

Closed-Cell Spray Foam
Closed-cell spray foam commonly provides about R-6.0 to R-7.0 per inch, making it one of the higher R-value-per-inch insulation options. It is denser than open-cell foam and may be used where higher R-value, air sealing, and moisture resistance are important.

Rigid Foam Board
Rigid foam board commonly provides about R-4.0 to R-6.5 per inch, depending on the material. Common types include expanded polystyrene, extruded polystyrene, and polyisocyanurate. Rigid foam is often used in walls, roofs, foundations, and certain commercial applications.

Radiant Barrier
Radiant barrier is different from traditional insulation. It is not usually rated the same way as fiberglass, cellulose, or spray foam because it is designed to reduce radiant heat transfer rather than resist conductive heat flow. The Department of Energy describes radiant barriers as especially useful in hot, sunny climates because they help keep homes cooler by reflecting heat.

Why is the calculator only an estimate

The R-value rating is specific to insulation and is a good guide; however, the calculator cannot factor in air movement through gaps or leaking ductwork. If air is allowed to move freely between an exterior wall or attic into a house, or ductwork allows cooled air to leak into the attic, some of the benefit of the insulation will be muted. It is important to have a professional review the existing level of insulation, discover gaps, and check ductwork to create a comprehensive insulation strategy.