Insulation Guide: Fiberglass, Blown-In, Spray Foam, and R-Value Calculations
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Insulation is the invisible upgrade that pays for itself every month. Proper insulation reduces heating and cooling costs by 20 to 40 percent, makes rooms more comfortable year-round, and reduces noise transmission between spaces. But insulation is not one-size-fits-all. The right type depends on where you are insulating, what R-value your climate zone requires, and whether you are doing new construction or retrofitting existing walls and attics. This guide covers the main insulation types and helps you choose the right one for your project.
Understanding R-Value
R-value measures resistance to heat flow. Higher R-value means better insulation. Think of R-value as a rating for how hard it is for heat to pass through a material - a higher number means heat moves through more slowly, keeping your conditioned air where you want it. The R-value you need depends on your climate zone and the part of the building you are insulating.
Attics need the highest R-value (R-38 to R-60 in most climates) because heat rises and escapes through the roof. In winter, an under-insulated attic lets expensive heated air escape upward. In summer, the sun beating on the roof radiates heat downward into your living space. Walls typically need R-13 to R-21, depending on your climate zone and wall cavity depth. Floors over unconditioned spaces (crawl spaces, unheated garages) 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 without ripping out what is already there. In attics, blowing additional insulation over existing batts is the most cost-effective energy upgrade most homeowners can make. Many utility companies offer rebates for attic insulation upgrades, which can reduce the out-of-pocket cost by 20 to 50 percent.
The US Department of Energy publishes recommended R-values by climate zone. The country is divided into 7 zones, from Zone 1 (hot climates like south Florida and Hawaii) to Zone 7 (cold climates like northern Minnesota and Maine). Zone 1 needs less insulation than Zone 7 because the temperature difference between indoor and outdoor air is smaller for more of the year. Check your zone at energy.gov before buying insulation. Under-insulating wastes energy, and over-insulating wastes money with diminishing returns on each additional inch.
Insulation only works when it fills the cavity completely without compression. Compressing R-19 insulation into a 2x4 wall cavity (3.5 inches deep) does not give you R-19. The compressed fibers lose their trapped air space, which is what actually provides the insulating effect, and the result is about R-13. Always match the insulation thickness to the cavity depth for the labeled R-value. If your cavity is shallower than the insulation is thick, use a product rated for that cavity depth instead of cramming in a thicker product.
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 (typically $0.50 to $1.50 per square foot), widely available at every home improvement store, and a straightforward DIY installation. R-13 batts fit 2x4 walls (3.5 inches deep). R-19 batts fit 2x6 walls (5.5 inches deep). R-30 batts fit 2x10 floor joists (9.25 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 around pipes. A batt that is too wide buckles and creates air gaps along the sides. A batt that is too narrow leaves uninsulated strips of wall. Cut batts to fit around obstacles rather than stuffing or compressing them. Use a sharp utility knife and a straightedge to make clean cuts. Cut the batt slightly wider than the opening so it fits snugly by friction without buckling.
Faced batts have a paper or foil facing that serves as a vapor retarder. In cold climates (zones 4 through 7), the facing goes toward the heated side (interior) to prevent moisture from condensing inside the wall cavity. Moisture condensation inside walls leads to mold growth and wood rot. In hot, humid climates (zones 1 and 2), the facing goes toward the exterior because the moisture drive reverses - humid outdoor air pushes moisture inward. In mixed climates (zone 3), consult local building codes for facing requirements. Using unfaced batts with a separate polyethylene 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 and shower as soon as you finish. Fiberglass itching stops after washing with soap and water. It is annoying, not dangerous, but avoiding it entirely with proper protective clothing is far better than enduring it. Consider wearing a disposable Tyvek suit for attic insulation projects where you will be crawling through existing fiberglass.
Blown-In Insulation
Blown-in cellulose (made from 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, plumbing vents, and framing irregularities. For attic floors, blown-in cellulose is the most cost-effective way to reach R-38 or higher.
A blowing machine breaks up the compressed cellulose bales and blows them through a long flexible hose. Most home improvement stores offer free blowing machine rental when you purchase 20 or more bags of their insulation. Attic installation is a two-person job - one person feeds the machine at ground level while the other directs the hose in the attic. Fill to a consistent depth measured with a ruler or depth markers (many installers staple rulers to attic joists before blowing). For R-38, you need about 10 to 11 inches of settled cellulose.
Dense-pack cellulose fills enclosed wall cavities through small holes (typically 2 to 3 inches in diameter) drilled in the sheathing or interior drywall. 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, which is dense enough to resist settling and significantly reduce air infiltration through the wall. This is not a DIY technique because it requires specialized equipment, gauges to measure density, and experience to fill cavities completely without blowing out the drywall.
Blown-in fiberglass is an alternative to cellulose. It does not absorb moisture and does not settle as much over time, maintaining its depth and R-value longer. The R-value per inch is slightly lower than cellulose (about R-2.5 per inch versus R-3.2 to R-3.8 for cellulose), so you need more depth for the same R-value. Both products work well in attic applications. Cellulose is cheaper per square foot at the same R-value. Fiberglass is more moisture-resistant and better suited for humid climates or areas with roof leak risk.
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 in a single application. It adheres to surfaces, fills every gap and crack, and creates a rigid, moisture-resistant layer. Closed-cell foam is the best choice for rim joists (the wood framing between your foundation and first floor), cathedral ceilings where cavity depth is limited, and any location where air sealing is as important as insulation. Two inches of closed-cell foam provides R-13 and a complete air seal in a space where batts would leave gaps.
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, which can be an advantage in some wall assemblies where you want the wall to dry in both directions. Use open-cell foam in interior wall cavities where moisture management is handled by other building components (housewrap, vapor retarders). Open-cell foam is also quieter than closed-cell (better sound dampening) and costs about half as much per square foot installed, making it a good choice for soundproofing interior walls between bedrooms or between a living space and a noisy workshop.
Spray foam installation is almost always a professional job. The two-part chemicals require specialized heated proportioning equipment, proper mixing ratios (a 1:1 ratio by volume is critical), correct substrate temperature (above 60 degrees Fahrenheit for most products), and adequate ventilation during and after application. Improperly mixed foam does not cure properly and can off-gas harmful isocyanate chemicals for weeks. Professional spray foam installation costs $1.50 to $3 per square foot for open-cell and $3 to $6 per square foot for closed-cell, but the performance in air sealing and per-inch R-value justifies the cost for specific applications.
DIY spray foam kits exist for small, targeted jobs - rim joists, gaps around windows and doors, holes where pipes or wires penetrate walls, and small sections of band joist. These two-part kits produce 15 to 200 board feet of foam and are adequate for targeted air sealing work. They are not practical for insulating entire walls or attics because the cost per board foot is three to five times higher than professional spray foam. For anything larger than spot-sealing, hire a professional installer.
Rigid Foam Board
Rigid foam board insulation (XPS, EPS, and polyisocyanurate) provides continuous insulation on the exterior of walls, under slabs, and on foundation walls. Unlike cavity insulation (batts and blown-in), rigid foam covers the entire surface including the framing, eliminating thermal bridging through the studs. A 2x4 stud has an R-value of about R-4, far less than the R-13 batt between studs. Continuous exterior foam addresses this weakness.
XPS (extruded polystyrene, typically pink or blue boards) provides R-5 per inch and has good moisture resistance. It is the standard choice for below-grade foundation insulation and under-slab applications. EPS (expanded polystyrene, the white beaded foam) provides R-3.8 to R-4.2 per inch and is the most affordable rigid foam option. Polyiso (polyisocyanurate, foil-faced boards) provides the highest R-value at R-6 to R-6.5 per inch but loses performance in cold temperatures, making it best suited for above-grade wall sheathing rather than foundation use.
Cutting rigid foam is straightforward. Score and snap thin boards (1 inch or less) with a utility knife and straightedge. Use a table saw or circular saw with a fine-tooth blade for thicker boards. Seal all joints between boards with foil tape (for polyiso) or housewrap tape (for XPS and EPS) to maintain the air barrier. Any gap is a thermal bypass that reduces the effectiveness of the continuous insulation layer.
Air Sealing Before Insulating
Air leaks account for 25 to 40 percent of heating and cooling energy loss in a typical home, often more than the loss through under-insulated walls and ceilings. Before adding insulation, seal the major air leaks in your building envelope. In attics, this means sealing around plumbing vents, electrical penetrations, recessed light housings, the tops of interior walls where they meet the attic floor, and the attic hatch or door.
Use expanding spray foam (the minimal-expansion type for gaps up to 1 inch) to seal around pipes and wires. Use rigid foam and fire-rated caulk to seal larger openings like the tops of interior partition walls. Use weatherstripping and a rigid foam cover on the attic hatch. These air sealing measures often save as much energy as the insulation itself, and the materials cost is minimal - a few cans of spray foam, a tube of caulk, and some weatherstripping.
In existing walls being retrofitted with blown-in insulation, the dense-pack technique itself provides significant air sealing because the tightly packed cellulose fills gaps and slows air movement through the wall cavity. This dual benefit - insulation plus air sealing - is one of the main reasons dense-pack cellulose is the preferred retrofit method for older homes with uninsulated walls.
Frequently Asked Questions
What Is the Best Insulation for an Attic?
Blown-in cellulose or fiberglass is the most cost-effective choice for attic floors. It fills to any depth for the R-value you need, flows around obstacles, and covers existing insulation for an easy upgrade. For cathedral ceilings (no attic space), closed-cell spray foam is the best option because it provides insulation and air sealing in the limited cavity depth available between the rafters.
Can I Insulate My Walls Without Removing Drywall?
Yes, with dense-pack cellulose or injection foam. A professional drills small holes in each wall cavity (from inside or outside), injects insulation to fill the cavity, and patches the holes. The cost is moderate and the energy savings are significant. Uninsulated walls are a major source of heat loss in older homes, and this retrofit technique is far less disruptive and expensive than removing and replacing drywall.
How Much Does Insulation Cost to Install?
Fiberglass batts are the cheapest at $0.50 to $1.50 per square foot installed. Blown-in cellulose runs $1 to $2 per square foot. Open-cell spray foam costs $1.50 to $3 per square foot. Closed-cell spray foam costs $3 to $6 per square foot. Attic insulation upgrades typically pay for themselves in energy savings within 2 to 5 years depending on your climate and existing insulation level.