Insulation Materials Compared: R-Value, Cost and Best Uses

R-value in one paragraph

R-value measures how well a material resists heat flow. The higher it is, the better it insulates. R-values add up: two layers of R-19 make R-38. Metric countries use RSI (m²·K/W), where RSI = R × 0.176, or U-value, which is 1 ÷ RSI. A lower U-value means better insulation.

Fiberglass batts and blown fiberglass

Fiberglass is the most widely used insulation in North America. Batts give about R-3.1 to R-3.4 per inch and suit standard stud and joist cavities. Blown (loose-fill) fiberglass is ideal for topping up attics and gives about R-2.2 to R-2.7 per inch. It's cheap and non-combustible, but it only performs well when installed without gaps or compression.

Mineral wool (rock wool)

Mineral wool gives about R-4 per inch, is highly fire-resistant, repels water and deadens sound. It's the default loft insulation in the UK, and its rigid boards are popular for exterior continuous insulation. It costs a little more than fiberglass and is easier to cut accurately.

Cellulose

Cellulose is made from recycled paper treated with borates for fire and pest resistance. It's blown into attics or dense-packed into closed walls at about R-3.5 to R-3.8 per inch. Dense-packing also cuts air leakage noticeably, which makes cellulose a strong choice for retrofits.

Spray foam

Open-cell spray foam (about R-3.7 per inch) seals air leaks and is relatively affordable. Closed-cell foam (about R-6 to R-7 per inch) is denser, also acts as a vapour retarder and adds racking strength. It's ideal for rim joists, basements and unvented roof assemblies. Both must be installed by trained professionals.

Rigid foam boards

Polyisocyanurate (polyiso, PIR) offers about R-6 per inch. XPS offers about R-5 and EPS about R-4. Boards are used for continuous insulation over sheathing, under slabs and on basement walls. Polyiso loses some performance in very cold weather, while XPS and EPS tolerate ground contact well.

Quick comparison

Approximate R-value per inch and typical use:

• Fiberglass batts: R-3.2. Walls, floors, attics. Lowest cost.
• Blown fiberglass: R-2.5. Attic top-ups.
• Cellulose: R-3.6. Attics, retrofit walls.
• Mineral wool: R-4.0. Walls, lofts, fire separation, sound control.
• Open-cell spray foam: R-3.7. Walls, roof decks, air sealing.
• Closed-cell spray foam: R-6.5. Basements, rim joists, metal buildings.
• XPS / Polyiso boards: R-5 / R-6. Continuous exterior insulation, under slabs.

Working out if it pays off

The right material depends on the space, your budget and your climate. Use our insulation calculator to compare materials by the thickness needed to reach your target R-value, and to see how many years it will take for the energy savings to cover the cost.