Best Insulation Types: Cost, R-Value, and Where to Use Each
Fiberglass, cellulose, spray foam, mineral wool, and rigid foam compared by performance and price
Key Takeaways
- Fiberglass batts are the cheapest at $0.50-$1.50/sq ft but provide R-3.2 per inch. Closed-cell spray foam costs $1.50-$3.50/sq ft but delivers R-6.5 per inch - double the performance
- For most attics, blown-in cellulose at $1-$2/sq ft offers the best value - R-3.7 per inch with excellent air-sealing properties
- Spray foam is worth the premium in basements, crawl spaces, and rim joists where both insulation and air sealing matter
Master Comparison: Every Insulation Type at a Glance
There are six main insulation types used in residential construction. Each has a different R-value per inch, cost, and ideal use case. No single type is best for every situation, but this table gives you the full picture so you can match the right insulation to each part of your home.
R-value measures thermal resistance per inch of thickness. Higher is better. Cost is for materials and professional installation.
| Insulation Type | R-Value/Inch | Cost/Sq Ft (Installed) | Air Sealing | Moisture Resistance | Fire Rating | DIY Friendly | Best For |
|---|---|---|---|---|---|---|---|
| Fiberglass Batts | R-3.2 | $0.50-$1.50 | Poor | Poor (absorbs water) | Non-combustible | Yes | Wall cavities, attic floors |
| Blown-In Fiberglass | R-2.5 | $0.75-$1.75 | Moderate | Poor | Non-combustible | Rental machine | Attics, enclosed walls |
| Blown-In Cellulose | R-3.7 | $1.00-$2.00 | Good | Moderate (treated) | Treated for fire resistance | Rental machine | Attics, retrofit walls |
| Open-Cell Spray Foam | R-3.7 | $1.00-$2.00 | Excellent | Moderate | Combustible (needs barrier) | No | Wall cavities, attic underside |
| Closed-Cell Spray Foam | R-6.5 | $1.50-$3.50 | Excellent | Excellent (vapor barrier) | Combustible (needs barrier) | No | Basements, crawl spaces, rim joists |
| Mineral Wool (Rockwool) | R-4.2 | $1.25-$2.50 | Moderate | Excellent (hydrophobic) | Non-combustible to 2,150F | Yes | Exterior walls, fire-rated assemblies |
| Rigid Foam (EPS) | R-3.8 | $0.75-$1.50 | Moderate | Good | Combustible (needs barrier) | Yes | Exterior continuous, basement walls |
| Rigid Foam (XPS) | R-5.0 | $1.00-$2.00 | Moderate | Good | Combustible (needs barrier) | Yes | Below-grade, basement walls |
| Rigid Foam (Polyiso) | R-6.0 | $1.25-$2.50 | Moderate | Good | Combustible (needs barrier) | Yes | Exterior continuous, roof decks |
R-value per inch matters because it determines how thick your insulation needs to be. In a standard 2x4 wall cavity (3.5 inches deep), fiberglass batts give you R-11. Closed-cell spray foam in the same cavity gives you R-23. That's a huge difference in the same space.
Fiberglass Batts: The Cheapest Option That Works Fine (If Installed Right)
Fiberglass batts are the pink fluffy rolls you see at every home improvement store. They've been the default residential insulation for 60+ years, and for good reason: they're cheap ($0.50-$1.50 per square foot), widely available, non-combustible, and easy enough for DIY installation.
The catch is that fiberglass batts are only as good as their installation. Any gap, compression, or void dramatically reduces performance. A batt that's compressed from R-19 thickness into an R-13 cavity doesn't give you something in between. It gives you poor performance everywhere. Studies show that poorly installed fiberglass batts can lose 30-50% of their rated R-value.
Fiberglass also does nothing for air sealing. Air passes right through it. In a leaky wall or attic, fiberglass batts are basically a filter for moving air rather than an insulator. This is why blown-in or spray foam options often outperform fiberglass in real-world conditions even when the rated R-value is similar.
- -Best for: Open wall cavities in new construction where you can install carefully, attic floors with no obstructions, budget-conscious projects
- -Skip it for: Tight spaces, irregular cavities, anywhere air sealing matters, retrofit projects where gaps are hard to avoid
- -DIY savings: Fiberglass batts cost $0.30-$0.80/sq ft for materials alone. If you're handy and patient, you can save 40-60% over professional installation.
- -Pro tip: Wear long sleeves, gloves, goggles, and a dust mask. Fiberglass fibers are irritating to skin and lungs.
- -Common mistake: Don't layer unfaced fiberglass batts over existing batts with kraft paper facing. The trapped vapor barrier can cause moisture problems. Remove the old facing or use unfaced batts for the second layer.
Blown-In Cellulose: The Best Value for Most Attics
Cellulose insulation is made from recycled newspaper treated with borate for fire and pest resistance. It's blown in using a machine, filling cavities completely and conforming to irregular shapes. This is what makes it better than batts in most real-world applications - it doesn't leave gaps.
At $1-$2 per square foot installed, cellulose costs slightly more than fiberglass batts but delivers R-3.7 per inch compared to R-3.2 for fiberglass. More importantly, cellulose is denser and does a better job of slowing air movement through the insulation. In attics especially, this makes a noticeable difference in comfort and energy bills.
For attic floors, cellulose is the contractor's go-to recommendation. A crew can blow in R-49 (about 14 inches) of cellulose in a typical attic in half a day. The machine fills around pipes, wires, and junction boxes automatically. You can also rent a blowing machine from most home improvement stores for about $50-$75/day if you want to DIY.
Cellulose is also the greenest insulation option. It's made from 80-85% recycled content and requires less energy to manufacture than fiberglass or spray foam. The borate treatment provides fire resistance and doubles as a pest deterrent - insects and rodents don't like the stuff. For retrofit wall cavities, dense-pack cellulose ($1.50-$2.50/sq ft) can be blown through small holes drilled in the exterior, filled in, and patched. It's one of the least disruptive ways to insulate existing walls.
Cellulose settles about 10-15% over time. Good installers account for this by blowing in extra thickness. If your attic needs R-49, they'll blow in enough for R-56 initially. Ask your installer about their settling allowance.
Open-Cell Spray Foam: When Air Sealing Matters More Than R-Value
Open-cell spray foam expands roughly 100 times its liquid volume when sprayed, filling every crack, gap, and void in a wall cavity or roof deck. It delivers R-3.7 per inch (same as cellulose) but its real advantage is air sealing. Open-cell foam creates an airtight barrier that stops convective heat loss completely.
Installed cost runs $1-$2 per square foot per inch of thickness. For a full wall cavity (3.5 inches), that's $3.50-$7 per square foot. That's 3-5 times the cost of fiberglass batts for the same wall. The energy savings from the air sealing often justify the premium, but it depends on how leaky your existing walls are.
Open-cell foam is permeable to moisture vapor, which is both a pro and a con. In cold climates, you typically need a separate vapor retarder (like painted drywall) on the interior side. In mixed and warm climates, the permeability is actually an advantage because it allows walls to dry in both directions.
- -Best for: Attic roof decks (turning a vented attic into conditioned space), wall cavities in new construction, noise reduction (open-cell is excellent for sound dampening)
- -Skip it for: Below-grade walls, anywhere exposed to water, tight budgets
- -Important: Open-cell spray foam must be covered with a 15-minute thermal barrier (like drywall) in occupied spaces. It cannot be left exposed.
Closed-Cell Spray Foam: The Premium Performer
Closed-cell spray foam is the highest-performing insulation you can buy. At R-6.5 per inch, it delivers nearly double the R-value of any other cavity insulation. It's also a vapor barrier, an air barrier, and it adds structural rigidity to walls. In a 2x4 wall, 3 inches of closed-cell foam gives you R-19.5 plus air sealing - better than a 2x6 wall filled with fiberglass batts.
The premium is real: $1.50-$3.50 per square foot per inch. A full 3.5-inch wall cavity costs $5.25-$12.25 per square foot. For a whole house, that can add $5,000-$15,000 compared to fiberglass batts. But in specific applications, closed-cell foam is worth every penny.
Basements and crawl spaces are where closed-cell spray foam shines brightest. These areas have moisture concerns that make fiberglass a terrible choice (wet fiberglass has near-zero R-value) and cellulose a risky one. Two inches of closed-cell foam on basement walls gives you R-13 plus a complete moisture and vapor barrier. No other product does both jobs at once.
- -Best for: Basement rim joists (the single highest-ROI insulation project in most homes), crawl spaces, basement walls, areas with height restrictions where you need max R-value in minimum thickness
- -Skip it for: Large open attics (too expensive when you could just pile on more cellulose), wall cavities where cost matters more than performance
- -Warning: Closed-cell spray foam uses blowing agents with higher global warming potential than other insulation types. If environmental impact matters to you, look for manufacturers using HFO blowing agents (Lapolla, Demilec) instead of older HFC formulations.
Mineral Wool (Rockwool): The Fire-Resistant Upgrade
Mineral wool - sold under the brand name Rockwool (formerly Roxul) - is made from basalt rock and recycite slag spun into fibers. It's denser than fiberglass, delivers R-4.2 per inch, and has one standout feature: it's non-combustible up to 2,150 degrees F. In a fire, mineral wool holds its shape and continues to insulate while fiberglass melts.
At $1.25-$2.50 per square foot installed, mineral wool costs 50-100% more than fiberglass batts. The higher price gets you better thermal performance, excellent sound dampening (it's the preferred choice for sound walls between rooms), and the fire resistance advantage. Mineral wool is also hydrophobic - water runs right through it without being absorbed, which means it doesn't lose R-value when exposed to moisture.
Mineral wool batts are rigid enough to friction-fit into wall cavities without stapling, which makes installation cleaner and reduces gaps. They also hold their shape over time and don't sag or settle like fiberglass can in wall cavities.
- -Best for: Exterior walls (especially as continuous exterior insulation), shared walls between rooms for sound control, areas near heat sources, fire-rated assemblies, anyone willing to pay a modest premium for better all-around performance
- -Skip it for: Attics where blown-in cellulose is cheaper and easier, budget projects where fiberglass does the job adequately
- -Pro tip: Rockwool ComfortBatt fits standard 2x4 and 2x6 cavities. Rockwool ComfortBoard is a rigid panel designed for exterior continuous insulation. They're different products for different applications - make sure you're buying the right one.
- -Sound control: If you're insulating interior walls between bedrooms, a home office, or a media room, mineral wool is the best choice. Its density blocks significantly more sound transmission than fiberglass at similar thickness. For the best results, combine mineral wool with resilient channel and double drywall.
Rigid Foam Board: EPS, XPS, and Polyiso Compared
Rigid foam boards serve a different purpose than cavity insulation. They're typically used as continuous insulation on the exterior of walls, under slab-on-grade foundations, on basement walls, and on roof decks. The key advantage is that they break the thermal bridging through wood studs and reduce heat loss by 15-25% compared to cavity insulation alone.
There are three types of rigid foam, and they're not interchangeable.
| Rigid Foam Type | R-Value/Inch | Cost/Sq Ft (1 inch) | Moisture Absorption | Best Application | Notes |
|---|---|---|---|---|---|
| EPS (Expanded Polystyrene) | R-3.8 | $0.75-$1.50 | Low (2-4%) | Under slabs, ICF forms, exterior walls | Cheapest option. R-value is stable over time. |
| XPS (Extruded Polystyrene) | R-5.0 | $1.00-$2.00 | Very low (0.3%) | Below-grade, basement walls, areas with moisture | Blue or pink boards. Better moisture resistance than EPS. |
| Polyiso (Polyisocyanurate) | R-6.0 | $1.25-$2.50 | Moderate (if foil facing is breached) | Exterior walls above grade, roof decks | Highest R-value. Performance drops in very cold temps. |
Polyiso loses R-value in cold temperatures. At 25 degrees F, polyiso drops from R-6.0 to roughly R-4.5 per inch. If you're in a cold climate and installing exterior continuous insulation, XPS or EPS may be the better choice despite lower rated R-values. Or use polyiso on the outer layer with EPS on the inner layer.
Recommended Insulation by Location: What to Put Where
Different parts of your home have different insulation needs based on accessibility, moisture conditions, and required R-values. Here's a practical guide to which insulation type works best in each location, organized by the Department of Energy's recommended R-values by climate zone.
| Location | Recommended R-Value (Zone 4-5) | Best Insulation Type | Estimated Cost | Notes |
|---|---|---|---|---|
| Attic floor | R-49 to R-60 | Blown-in cellulose | $1,500-$3,000 (whole attic) | Highest ROI insulation project |
| Exterior walls | R-13 to R-21 | Fiberglass or mineral wool batts | $1,500-$4,000 (whole house) | Add rigid foam outside for R-5 continuous |
| Basement walls | R-10 to R-15 | Closed-cell spray foam or rigid XPS | $2,000-$5,000 | Moisture resistance is critical |
| Crawl space walls | R-10 to R-15 | Closed-cell spray foam | $1,500-$4,000 | Encapsulate first, then insulate |
| Rim/band joists | R-13 to R-30 | Closed-cell spray foam | $500-$1,500 | Single highest ROI per dollar spent |
| Cathedral ceiling | R-38 to R-49 | Spray foam (underside of deck) | $3,000-$8,000 | Only option without venting the roof deck |
| Floor above garage | R-30 to R-38 | Mineral wool batts or spray foam | $1,000-$3,000 | Fire resistance matters here |
Federal Tax Credits for Insulation Under the IRA
The Inflation Reduction Act provides a tax credit of 30% of the cost for qualifying insulation improvements, up to $1,200 per year. This applies to materials and installation labor. On a $3,000 attic insulation project, you'd get $900 back on your taxes.
All insulation types qualify as long as they meet Energy Star requirements, which is essentially any insulation product installed to the recommended R-value for your climate zone. The credit applies to attic insulation, wall insulation, basement insulation, air sealing, and even the cost of the energy audit that identified the need.
You can combine the insulation credit with other energy efficiency credits (windows, doors, HVAC) up to the $1,200 annual cap. Heat pumps and heat pump water heaters have a separate $2,000 cap, so you could theoretically claim $3,200 in energy credits in a single year.
The $1,200 annual cap resets each year. If you're planning a major energy retrofit, consider spreading the work across two tax years to maximize your credits. Insulate the attic in December, then do the walls and basement in January.
Decision Framework: Which Insulation for Your Project
If you're still not sure which insulation to choose, work through these questions in order. Each answer narrows the field.
First, where are you insulating? Attic floors are almost always best served by blown-in cellulose. Basements and crawl spaces need closed-cell spray foam or rigid foam for moisture protection. Standard wall cavities work with fiberglass batts, mineral wool, or spray foam depending on budget and performance goals.
Second, is air sealing a priority? If your home is drafty, spray foam (open or closed cell) solves two problems at once. If your home is already reasonably airtight (built after 2000, or already air-sealed), cheaper options like cellulose or fiberglass will perform well.
Third, what's your budget? If you're insulating a whole house, fiberglass batts in the walls and cellulose in the attic gives you solid performance at the lowest cost. If you have a bigger budget, mineral wool in the walls and cellulose in the attic is a meaningful upgrade. Spray foam everywhere gives the best performance but costs 3-5x more.
- -Tightest budget: Fiberglass batts in walls, blown-in cellulose in attic. $2,000-$5,000 for a typical house.
- -Best value: Blown-in cellulose in attic, mineral wool batts in walls, closed-cell spray foam on rim joists only. $4,000-$9,000.
- -Maximum performance: Closed-cell spray foam everywhere, plus rigid foam continuous insulation on exterior. $15,000-$30,000.
- -One project to start with: If you can only do one thing, insulate your attic floor with blown-in cellulose to R-49 or higher. It's the cheapest, easiest, and highest-ROI insulation upgrade in most homes.
- -Biggest bang for the buck: Spray foam your rim joists. This small area (the perimeter of your foundation where the floor framing sits) is one of the leakiest spots in most homes. Two inches of closed-cell spray foam costs $500-$1,500 for the whole house and can cut heating bills by 5-10%.