Insulation is the foundation of home energy efficiency. No amount of air sealing, smart thermostats, or HVAC upgrades compensates for inadequate insulation. This guide covers every insulation type, where to use each one, and how much you should expect to pay.
Insulation Types Compared
Fiberglass Batts
The most common and affordable insulation type, fiberglass batts come in pre-cut sheets that fit between standard wall studs and ceiling joists.
- R-value per inch: R-3.0 to R-4.3
- Cost: $0.40-$0.70/sq ft installed
- Best for: Attics (unfaced batts), walls during new construction, floors over unconditioned spaces
- Pros: Low cost, easy DIY installation in accessible attics, non-combustible, doesn't absorb moisture
- Cons: Loses effectiveness if compressed or wet, doesn't seal air leaks, requires precise installation around obstacles
Blown-In Fiberglass
Loose fiberglass blown into cavities using pneumatic equipment, filling irregular spaces more completely than batts.
- R-value per inch: R-2.2 to R-4.3
- Cost: $0.50-$1.00/sq ft installed
- Best for: Attic floors (over existing insulation), enclosed wall cavities
- Pros: Better coverage than batts, fills irregular spaces, can be added over existing insulation
- Cons: Can settle over time (5-10% settling in attics), doesn't air-seal, requires professional equipment
Cellulose
Made from recycled newspaper treated with fire retardants, cellulose is blown into cavities as loose-fill or dense-pack.
- R-value per inch: R-3.2 to R-3.8
- Cost: $0.50-$1.00/sq ft installed
- Best for: Existing wall cavities (dense-pack), attic floors, retrofits
- Pros: Eco-friendly (80-85% recycled content), excellent at filling gaps and voids, provides some air sealing when dense-packed, treated for fire and pest resistance
- Cons: Absorbs moisture (can lose effectiveness if wet), heavier than fiberglass (ceiling weight considerations), may settle 10-20% in attic applications
Open-Cell Spray Foam
Expands to fill cavities completely, providing both insulation and air sealing in one application.
- R-value per inch: R-3.5 to R-3.7
- Cost: $1.50-$3.50/sq ft installed
- Best for: Cathedral ceilings, rim joists, irregular cavities, areas needing air sealing
- Pros: Air seals while insulating, expands to fill all gaps and voids, good sound dampening, flexible after curing
- Cons: Higher cost, requires professional installation, must be protected from UV exposure, lower R-value per inch than closed-cell
Closed-Cell Spray Foam
The highest-performing insulation type, providing superior R-value, air sealing, and moisture resistance.
- R-value per inch: R-5.0 to R-7.0
- Cost: $3.00-$5.00/sq ft installed
- Best for: Basements, crawlspaces, rim joists, areas with moisture concerns, thin wall cavities needing maximum R-value
- Pros: Highest R-value per inch, air and moisture barrier, adds structural strength, won't absorb water
- Cons: Most expensive option, requires professional installation, rigid after curing, environmental concerns (blowing agents)
Mineral Wool (Rockwool)
Dense batts made from volcanic rock or slag, offering excellent fire resistance and sound attenuation.
- R-value per inch: R-3.3 to R-4.2
- Cost: $0.80-$1.50/sq ft installed
- Best for: Fire-rated walls, sound attenuation, exterior continuous insulation, areas prone to moisture
- Pros: Fire-resistant (melting point >2,000 degrees F), excellent sound dampening, water-resistant, doesn't support mold growth, retains R-value when wet
- Cons: More expensive than fiberglass, heavier and denser, requires cutting tools for installation
Rigid Foam Board
Rigid panels of polystyrene (EPS or XPS) or polyisocyanurate used for continuous insulation on exteriors or basement walls.
- R-value per inch: R-3.8 (EPS) to R-6.5 (polyiso)
- Cost: $0.75-$2.50/sq ft installed
- Best for: Basement walls, exterior continuous insulation, under-slab insulation, cathedral ceilings
- Pros: Continuous insulation without thermal bridging, moisture-resistant (XPS), high R-value per inch (polyiso)
- Cons: Must be covered with fire-rated material (drywall) when used indoors, can be damaged by UV exposure, joints must be sealed
Where to Insulate and Target R-Values
Attic
The attic is typically the highest-priority area because heat rises and uninsulated attics allow the most heat loss.
Target R-values by climate zone:
- Zones 1-3 (Southern US): R-30 to R-49
- Zones 4-5 (Central US): R-38 to R-60
- Zones 6-8 (Northern US): R-49 to R-60
Best insulation types: Blown-in fiberglass or cellulose over existing insulation, fiberglass batts for new installation
Typical cost: $1,500-$3,500 for a 1,500 sq ft attic
Exterior Walls
Wall insulation prevents heat transfer through the largest surface area of your home.
Target R-values:
- 2x4 walls: R-13 to R-15
- 2x6 walls: R-19 to R-21
- Continuous exterior insulation adds R-5 to R-10 on top
Best insulation types: Dense-pack cellulose (retrofit without removing walls), fiberglass batts (new construction), spray foam (premium option)
Typical cost: $2,000-$6,000 for blown-in retrofit; $4,000-$10,000 for spray foam
Floors and Crawlspaces
Uninsulated floors over crawlspaces or unconditioned basements lose significant heat and create cold floors.
Target R-values: R-25 to R-30
Best insulation types: Fiberglass batts between floor joists, spray foam on crawlspace walls (encapsulated crawlspace), rigid foam board
Typical cost: $1,500-$4,000
Basement
Basement insulation prevents heat loss through below-grade walls and improves comfort in finished basements.
Target R-values: R-10 to R-19 for walls
Best insulation types: Rigid foam board against concrete, spray foam (closed-cell for moisture resistance)
Typical cost: $2,000-$5,000 for walls
Rim Joists
The rim joist area (where the floor framing meets the foundation) is a major source of air leakage and heat loss.
Target R-values: R-10 to R-19
Best insulation types: Closed-cell spray foam (best for air sealing), rigid foam board with caulk
Typical cost: $500-$1,500
Insulation Cost Summary
| Type | Cost/sq ft | R-value/inch | Best Application |
|---|---|---|---|
| Fiberglass batts | $0.40-$0.70 | R-3.0-4.3 | Attic, walls (new construction) |
| Blown fiberglass | $0.50-$1.00 | R-2.2-4.3 | Attic over existing, enclosed walls |
| Cellulose | $0.50-$1.00 | R-3.2-3.8 | Existing walls, attic floors |
| Open-cell spray foam | $1.50-$3.50 | R-3.5-3.7 | Cathedral ceilings, rim joists |
| Closed-cell spray foam | $3.00-$5.00 | R-5.0-7.0 | Basements, crawlspaces, moisture areas |
| Mineral wool | $0.80-$1.50 | R-3.3-4.2 | Fire-rated walls, sound control |
| Rigid foam board | $0.75-$2.50 | R-3.8-6.5 | Basement walls, exterior continuous |
Frequently Asked Questions
What insulation has the best R-value per dollar?
Fiberglass batts and blown cellulose offer the most R-value per dollar spent. For $1 per square foot, you get approximately R-6 to R-10 with fiberglass or cellulose, compared to R-2 to R-3 with spray foam. However, spray foam's air sealing properties make it more cost-effective in specific applications like rim joists and crawlspaces.
Can I add new insulation over existing insulation?
Yes, in most cases. Adding blown-in fiberglass or cellulose over existing attic insulation is one of the most common and cost-effective upgrades. Don't compress existing insulation when adding new layers, and never add vapor-barrier-faced insulation over existing insulation (this can trap moisture).
How do I know if my home needs more insulation?
Signs include high energy bills, rooms that are hard to heat or cool, drafts, ice dams on the roof in winter, and visible insulation that appears thin, settled, or damaged. A professional energy audit with thermal imaging provides the most accurate assessment.
Is spray foam insulation worth the extra cost?
For general attic insulation, spray foam is rarely cost-effective compared to blown-in alternatives. For specific applications where air sealing is critical (rim joists, crawlspace walls, cathedral ceilings), closed-cell spray foam's combined insulation and air barrier properties often justify the premium.
Does the 25C tax credit cover insulation?
Yes. The Energy Efficient Home Improvement Credit (Section 25C) covers insulation materials and installation costs up to $1,200 per year (within the $1,200 annual cap for non-heat-pump improvements). This credit is available through 2032 and can be claimed every year for different improvements.
The Bottom Line
For most homeowners, the best insulation strategy is: blow cellulose or fiberglass into the attic to R-49 or higher, dense-pack cellulose into un-insulated walls, and use closed-cell spray foam on rim joists and crawlspace walls. This combination addresses the largest heat loss areas at the best cost-to-performance ratio. Budget $2,800-$6,500 for whole-house insulation improvements, and take advantage of the 25C tax credit to reduce your net cost by up to $1,200.
Related Reading
- DIY Attic Insulation Top-Up Guide
- Home Energy Audit Cost by State: 2026 Regional Guide
- How Much Does a Home Energy Audit Cost in 2026? State-by-State Guide
- Electric vs Gas: The Home Electrification Decision Guide
- Energy Audit Cost and Rebate Guide 2026
-- The Energy Audit Finder Team