What type of insulation has an R-value of about R-3.5 per inch?

The type of insulation that has an R-value of approximately R-3.5 per inch is loose fill cellulose. Cellulose insulation is made from recycled paper products and treated with fire retardants, making it a sustainable and effective option for thermal insulation. Its R-value indicates its effectiveness at resisting heat flow, meaning that a thickness of one inch of cellulose insulation will provide around R-3.5 of thermal resistance. This characteristic makes it a popular choice for both retrofitting existing buildings and new constructions, particularly in attics and walls where loose fill can be easily installed.

High density fiberglass can have various R-values, typically ranging higher than R-3.5 per inch, which makes it a different option for insulation. Loose fill mineral wool also typically has a higher R-value, generally around R-3.0 to R-4.4 per inch, depending on its density and manufacturer specifications. Expanded polystyrene, commonly known as foam board insulation, generally has an R-value ranging between R-3.6 to R-4.2 per inch, which again differs from the R-value provided for cellulose. Therefore, loose fill cellulose stands out with its specific R-value of about R-3.5 per inch.