Insulation Guide: Fiberglass, Blown-In, Spray Foam, and R-Value Calculations

Understanding R-Value

R-value measures resistance to heat flow. Higher R-value means better insulation. The R-value you need depends on your climate zone and the part of the building. Attics need the highest R-value (R-38 to R-60 in most climates) because heat rises and escapes through the roof. Walls typically need R-13 to R-21. Floors over unconditioned spaces need R-19 to R-30.

R-values are additive. Two layers of R-13 batts equal R-26. Adding R-19 attic insulation over existing R-19 gives you R-38. This means you can upgrade insulation incrementally — you do not have to rip out existing insulation to add more. In attics, blowing additional insulation over existing batts is the most cost-effective energy upgrade most homeowners can make.

The US Department of Energy publishes recommended R-values by climate zone. Zone 1 (hot climates like south Florida) needs less insulation than Zone 7 (cold climates like Minnesota). Check your zone at energy.gov before buying insulation — under-insulating wastes energy, and over-insulating wastes money with diminishing returns.

Insulation only works when it fills the cavity completely without compression. Compressing R-19 insulation into a 2x4 wall cavity does not give you R-19 — it gives you about R-13 because the compressed fibers lose their trapped air space. Match the insulation thickness to the cavity depth for the labeled R-value.

Fiberglass Batts

Fiberglass batts are the most common insulation in residential construction. Pre-cut rolls or panels fit between standard-spaced studs (16 or 24 inches on center) and joists. They are inexpensive, widely available, and a straightforward DIY installation. R-13 fits 2x4 walls (3.5 inches deep). R-19 fits 2x6 walls (5.5 inches deep).

Installation quality determines effectiveness. Batts must fit snugly in the cavity with no gaps, no compression, and no voids behind electrical boxes or pipes. A batt that is too wide buckles and creates air gaps. A batt that is too narrow leaves strips of uninsulated wall. Cut batts to fit around obstacles rather than stuffing or compressing them.

Faced batts have a paper or foil facing that serves as a vapor retarder. In cold climates, the facing goes toward the heated side (interior) to prevent moisture from condensing in the wall cavity. In hot, humid climates, the facing goes toward the exterior. Using unfaced batts with a separate poly vapor barrier gives you more control over vapor management.

Wear a long-sleeved shirt, gloves, safety glasses, and a dust mask when handling fiberglass. The tiny glass fibers irritate skin, eyes, and lungs. Work in a well-ventilated area. Fiberglass itching stops after washing — it is annoying, not dangerous, but avoiding it is better than enduring it.

Blown-In Insulation

Blown-in cellulose (recycled paper treated with fire retardant) fills attic spaces and wall cavities more completely than batts. It flows around obstacles, fills irregular cavities, and does not leave the gaps that batts create around wiring, pipes, and framing irregularities. For attic floors, it is the most cost-effective way to reach R-38 or higher.

A blowing machine (available for free rental at most home improvement stores with insulation purchase) breaks up the compressed cellulose bales and blows them through a hose. Attic installation is a two-person job — one person feeds the machine, the other directs the hose. Fill to a consistent depth measured with a ruler or depth markers.

Dense-pack cellulose fills enclosed wall cavities through small holes drilled in the sheathing or interior finish. This retrofit technique insulates existing walls without removing drywall. A professional with a dense-pack blower fills the cavity to about 3.5 pounds per cubic foot — dense enough to resist settling and air infiltration. This is not a DIY technique; it requires specialized equipment and experience.

Blown-in fiberglass is an alternative to cellulose. It does not absorb moisture and does not settle as much over time. The R-value per inch is slightly lower than cellulose, so you need more depth for the same R-value. Both products work well — cellulose is cheaper, fiberglass is more moisture-resistant.

Spray Foam Insulation

Closed-cell spray foam provides the highest R-value per inch (R-6 to R-7) and acts as both insulation and air barrier. It adheres to surfaces, fills every gap, and creates a rigid, moisture-resistant layer. It is the best choice for rim joists, cathedral ceilings, and any location where air sealing is as important as insulation.

Open-cell spray foam is softer, less expensive, and provides about R-3.5 per inch. It is a good air barrier but not a moisture barrier — it allows water vapor to pass through. Use open-cell in interior wall cavities where moisture management is handled by other building components. It is quieter than closed-cell (better sound dampening) and costs about half as much.

Spray foam installation is almost always a professional job. The chemicals require specialized equipment, proper mixing ratios, correct substrate temperature, and adequate ventilation during application. Improper mixing creates foam that does not cure properly and can off-gas harmful chemicals. The cost is higher than batts or blown-in, but the performance in air sealing justifies it for specific applications.

DIY spray foam kits exist for small jobs — rim joists, small gaps, and sealing penetrations. These two-part kits produce 15 to 200 board feet of foam and are adequate for targeted air sealing. They are not practical for insulating entire walls or attics — the cost per board foot is much higher than professional spray foam.

Frequently Asked Questions