Home Insulation Guide: R-Values, Types, and Best Practices

Why Insulation Matters

Heating and cooling account for 50-70% of the energy used in the average American home. Insulation is the primary defense against this energy waste. It slows the transfer of heat between the interior and exterior of your home, keeping it warmer in winter and cooler in summer. Upgrading insulation is one of the most cost-effective home improvements you can make, with a typical payback period of 2-6 years in reduced energy bills.

The effectiveness of insulation is measured in R-value, which indicates resistance to heat flow. Higher R-values mean better insulation. The R-value you need depends on your climate zone (determined by the US Department of Energy), the part of the house you are insulating, and the type of heating and cooling system you have. Use our Insulation Calculator to estimate material needs and costs.

Insulation Types

Fiberglass batts are the most common and least expensive insulation type. They come in pre-cut rolls sized for standard stud and joist cavities (16 or 24 inch on center) and R-values from R-11 to R-38. They are easy to install for DIYers but must be cut carefully to fit around obstacles and should not be compressed, which reduces their effective R-value.

Blown-in insulation (cellulose or fiberglass) is installed using a blowing machine that fills wall and attic cavities with loose material. It provides better coverage around obstacles than batts and fills gaps and voids more effectively. Cellulose is made from recycled newspaper treated with fire retardants and has an R-value of about 3.7 per inch. Blown fiberglass has an R-value of about 3.2 per inch.

Where to Insulate

The attic is typically the most cost-effective place to add insulation because heat rises. Most homes benefit from attic insulation levels of at least R-38 to R-60 depending on climate zone. Walls are the next priority. If your home was built before 1980, wall insulation levels may be below current standards. Exterior walls can be insulated by blowing cellulose or fiberglass into the wall cavities through small holes drilled from the exterior or interior.

Basement walls, crawl spaces, and floors over unconditioned spaces also benefit from insulation. Rim joists (the area where the floor framing meets the exterior wall) are often poorly insulated and are a major source of air infiltration. Rigid foam insulation or spray foam works well in these areas because it provides both insulation and an air barrier.

Common Mistakes

Compressing insulation reduces its R-value. A batt that is designed for a 5.5-inch cavity (R-19) compressed into a 3.5-inch cavity loses about 40% of its rated R-value. Always use the thickness rated for your cavity depth. Blocking air flow is as important as adding insulation. Even small air leaks can reduce the effective R-value of an insulated wall by 25-50%. Use caulk and spray foam to seal gaps around windows, doors, electrical boxes, plumbing penetrations, and any other openings.

Venting is critical in attic insulation. In cold climates, warm moist air from the living space that enters the attic can condense on the cold underside of the roof, causing mold and rot. Proper attic ventilation (soffit vents at the eaves and ridge vents or gable vents at the top) allows this moisture to escape. The insulation should not block the soffit vents. Use baffles to maintain a clear air channel from the soffit vents to the attic space.