Home Insulation Types Compared With R-Value and Cost Per Square Foot

I spent two weekends last fall crawling around my attic with a headlamp, pulling back old fiberglass batts and measuring what was underneath. The answer was not encouraging. Three inches of compressed, mouse-tunneled insulation that might have been R-11 when it was installed in 1987. My gas bill told the same story every January. So I started researching insulation types, and what I found is that the decision is less about picking the "best" product and more about understanding where your money goes the furthest.

R-Value in Plain Terms

R-value measures resistance to heat flow. A higher number means more resistance, which means less heat escaping in winter and less heat creeping in during summer. R-13 is a standard wall insulation. R-38 is a reasonable attic floor in a mild climate. R-60 is what the Department of Energy recommends for attics in the coldest parts of the country.

The thing most people miss is that R-value has diminishing returns. Going from R-11 to R-30 in your attic makes a huge difference on your heating bill. Going from R-30 to R-49 makes a smaller but still worthwhile difference. Going from R-49 to R-60 saves you maybe another 2 to 3 percent on heating costs. In most homes, aiming for R-49 in the attic hits the sweet spot between performance and cost. Going to R-60 makes sense if you're in climate zones 6 or 7 and plan to stay in the house long enough for the savings to compound.

The R-value printed on the bag assumes perfect installation. Gaps, compression, and voids all destroy real-world performance. A wall insulated to R-15 with 5 percent gaps performs closer to R-10 in practice. Installation quality matters as much as the number on the spec sheet.

The Main Insulation Types

Fiberglass batts are what most people picture when they think of insulation. Pink or yellow rolls that fit between studs, running about $0.50 to $1.50 per square foot installed. The catch is that batts need to fit perfectly between framing members with no gaps, no compression around wires or pipes, and full contact with the surface behind them. In practice, almost nobody achieves this. I've pulled back drywall in three different houses and never once seen batts installed without voids. They work fine in theory. In the real world, they underperform their rated R-value more often than any other type.

Blown cellulose is recycled newspaper treated with fire retardant, pumped into cavities or blown loose over attic floors. Cost runs $1 to $1.50 per square foot for professional installation. Cellulose fills gaps and irregular spaces much better than batts because it's loose fill. It settles over time, losing maybe 15 to 20 percent of its thickness in the first couple of years. Contractors account for this by overfilling. For attic floors especially, blown cellulose is one of the best bang-for-the-buck options available.

Spray foam comes in two varieties that perform very differently. Open-cell costs $1 to $1.50 per square foot at roughly R-3.7 per inch. It expands to fill cavities and blocks air movement, but doesn't stop moisture on its own. Closed-cell costs $2 to $3.50 per square foot and delivers about R-6.5 per inch. It also acts as a vapor barrier and adds structural rigidity. Closed-cell genuinely performs at a different level, but the cost means you need a clear reason to choose it. Basement rim joists, crawlspace walls, and areas where moisture control matters are where closed-cell earns its price. Spraying an entire attic floor with closed-cell when blown cellulose would do the job is spending an extra $3,000 to $5,000 for marginal benefit.

Mineral wool batts, sometimes called rock wool, cost $1.50 to $3 per square foot. They're denser than fiberglass, easier to cut precisely, and hold their shape in wall cavities better. They're also fire resistant up to about 2,000F and provide better sound dampening. Mineral wool doesn't absorb water, which makes it a good choice for basement walls. The higher cost keeps it from being the default for large areas, but for targeted applications it often makes more sense than fiberglass.

Where to Start for the Biggest Impact

If I could only insulate one part of a house, I would pick the attic floor every single time. Heat rises, and in an under-insulated attic the thermal stack effect pulls warm air up through every crack and penetration in the ceiling plane. An attic with R-11 that gets upgraded to R-38 or R-49 can reduce heating costs by 20 to 30 percent depending on the house. No other single improvement comes close.

Walls come second. Most older homes have either no wall insulation or thin R-11 batts in 2x4 framing. Retrofitting usually means drilling holes through exterior siding or interior drywall for dense-pack cellulose. It works, but it's a $3,000 to $7,000 project and the payback period is longer than attic work.

Basement and crawlspace come third. An uninsulated rim joist area is a significant source of drafts and heat loss, and it's relatively easy to address with rigid foam or closed-cell spray foam. Full basement wall insulation matters more in colder climates where the ground temperature stays well below your thermostat setting for months.

Air Sealing Matters More Than R-Value

This is the single most important thing I learned. You can pile insulation a foot deep in your attic, but if there are unsealed gaps around recessed lights, plumbing penetrations, the attic hatch, and ductwork chases, warm air will flow right through those openings and your expensive insulation will underperform badly.

In the attic, the big culprits are wire and pipe penetrations through top plates, recessed light housings, the chimney chase, bathroom fan ducts, and the attic access door. A can of fire-rated foam and a caulk gun can seal most of these in an afternoon. I sealed about 30 penetrations in my attic before adding new insulation. The difference was noticeable within the first week. Rooms that had always felt drafty near the ceiling suddenly held temperature more evenly. My furnace ran shorter cycles.

The Blower Door Test

A blower door test costs around $200 to $400, and it is the best diagnostic money you can spend before insulating anything. The technician mounts a calibrated fan in your front door, depressurizes the house, and measures exactly how much air leaks through the building envelope. The result is expressed in ACH50, air changes per hour at 50 pascals of pressure.

An older home might test at 8 to 12 ACH50. A well-sealed house will be around 3 to 5. New construction built to modern energy codes should hit 3 or below. Combined with a smoke pencil or infrared camera, the test reveals exactly where the leaks are. I had my house tested before and after air sealing. It went from 9.4 to 5.8 ACH50, a 38 percent reduction. The insulation I added on top of that sealed envelope is working the way the R-value on the bag says it should.

Climate Zone Recommendations

The Department of Energy breaks the US into climate zones 1 through 7, with 1 being the hottest and 7 the coldest. For attic floors, zones 1 through 3 should target R-38, about 10 to 12 inches of blown cellulose or fiberglass. Zones 4 and 5 should aim for R-49, around 14 to 16 inches of loose fill. Zones 6 and 7 benefit from R-60, roughly 18 to 20 inches.

For walls, R-13 to R-15 is standard in 2x4 framing and R-19 to R-21 in 2x6 framing, regardless of climate zone. The framing depth limits what you can fit without adding exterior rigid foam. The DOE has a zone map on their website. Generally, the Gulf Coast and desert Southwest are zones 2 and 3, the middle of the country is zone 4, and the northern states are zones 5 through 7.

DIY Attic Work vs Hiring It Out

Attic insulation is one of the few insulation jobs where DIY genuinely makes sense. Blown-in cellulose machines can be rented from most big-box stores for $50 to $100 per day, and the rental is sometimes free if you buy a minimum number of bags. You air seal first, lay depth markers across the joists, then blow cellulose until you hit the target depth everywhere. Two people can insulate a 1,200 square foot attic in a day.

Professional attic blow-in runs $1,500 to $3,000 for a typical house. Some contractors include a quick air seal of the obvious penetrations. Others won't touch it. Ask specifically, because skipping air sealing cuts the value of the whole project significantly.

Wall insulation and spray foam are not DIY-friendly. Dense-pack cellulose requires specialized equipment and training to get the density right. Spray foam requires professional equipment, precise temperature and humidity conditions, and protective gear. The cost of doing spray foam wrong far exceeds the cost of hiring a competent installer.

For my own house, I did the attic air sealing and blown cellulose myself over two weekends. Total material cost was about $650 for foam, caulk, and 40 bags of cellulose. A contractor quoted me $2,400 for the same scope without air sealing. The DIY route saved me roughly $1,750 and I got better air sealing in the bargain because I took the time to find and seal every penetration rather than just the easy ones.