Insulation slows heat transfer through the building envelope, which keeps homes cooler in summer and warmer in winter while trimming energy bills. The material choices run from fiberglass batts and cellulose to foam boards, spray foam, mineral wool, and reflective barriers, and each one behaves differently. R-value, moisture resistance, cost, and installation method all factor into the decision, as does the location: attic, wall cavity, foundation, or garage. Contractors and homeowners who compare insulation materials for building envelopes before buying avoid the two most common mistakes, undersizing for the climate and installing the right material in the wrong place. Well-insulated homes cut heating and cooling demand by a meaningful share, and the payback on materials and labor usually lands within a few years.
Understand R-Value Before You Buy
R-value measures resistance to heat flow: the higher the number, the better the material slows conduction. R-values add up, so a wall with R-13 batts inside and R-5 foam sheathing outside performs at about R-18. Homes in mild climates need less insulation than homes in cold regions, but even warm-climate houses benefit from attic insulation, since heat pours through the roof in summer.
How R-Value Is Measured
Manufacturers rate each product at a set thickness, usually per inch. Fiberglass batt runs about R-3.1 to R-3.4 per inch, cellulose about R-3.2 to R-3.8, mineral wool about R-3.3 to R-4.2, and closed-cell spray foam about R-6 to R-7. Polyiso boards rate highest per inch at around R-5.6 to R-6.8, though the value drops in very cold temperatures.
Climate and Target R-Values
Building codes set minimums by climate zone. Typical targets call for attic insulation of R-38 to R-60 in cold regions, wall cavities of R-13 to R-21 depending on whether the frame is 2×4 or 2×6, and floor insulation of R-25 to R-30 over vented crawl spaces. Choosing wall insulation types starts with these targets, then narrows the field by cavity depth and budget. Local code officials publish the exact table for your zone, and energy auditors use it as the baseline when they recommend upgrades.
Fiber Insulation: Batts, Rolls, and Loose Fill
Fiber insulation covers most residential jobs because it is inexpensive, familiar to crews, and easy to retrofit. The category includes fiberglass, mineral wool, and cellulose, and each has a different strength. Contractors compare fibrous insulation types on density, fire resistance, moisture behavior, and how they settle over time.
Fiberglass and Mineral Wool
Fiberglass batts and rolls fit between studs and joists and come in widths for 16-inch and 24-inch framing. Standard batts run R-13 in a 2×4 cavity and R-19 or R-21 in a 2×6 cavity. Mineral wool is denser, resists water better, and blocks sound more effectively, which makes it a popular choice for shared walls and basements. Both materials come in batts, rolls, and dense boards, so crews can match the form to the cavity.
Faced vs. Unfaced Batts
Faced batts carry a kraft paper vapor retarder that points toward the warm side of the wall, while unfaced batts have no paper and work where a separate vapor barrier is planned. Installing faced batts backward traps moisture inside the cavity, so confirm the orientation before stapling.
Cellulose and Other Loose-Fill Options
Loose-fill cellulose is made from recycled paper treated for fire and pest resistance, and it is blown into attics and wall cavities with a hose. It settles slightly over time and delivers about R-3.5 per inch. Other loose fills include fiberglass and mineral wool versions, which settle less than cellulose.
- Blown-in attic fill: quick coverage over existing insulation
- Wall-cavity dense pack: tight fill for retrofit walls
- Specialty loose fills: fiberglass and mineral wool versions
Place Insulation Correctly in Roofs and Walls
Location beats material: the best batt performs poorly when gaps, compression, or moisture ruin the assembly. Proper insulation placement in roofs and walls starts with sealing the air barrier, because moving air carries heat around insulation no matter how thick it is.
Air Sealing Comes First
Caulk and foam the penetrations before insulation goes in: wire chases, plumbing stacks, electrical boxes, and top plates. A house with good air sealing and standard R-values outperforms a leaky house with an extra layer of insulation. Simple materials do the job: caulk for cracks, expanding foam for gaps up to an inch, and rigid blocking for larger openings.
Vapor Barriers and Climate
Vapor retarders belong on the warm side of the assembly in cold climates and are often omitted in hot, humid zones. The local code defines the right side, and getting it wrong leads to condensation and mold inside the wall.
Ventilation for Roof Assemblies
Attic insulation must not block the soffit vents. Install baffles at the eaves so air flows from the soffit to the ridge, and keep loose fill a few inches clear of can lights and other heat sources.
- Seal the air barrier and all penetrations first.
- Cut batts to fit tightly around wiring and boxes.
- Push batts fully into the cavity without compressing them.
- Add a second layer perpendicular to the joists in attics.
Insulate Slabs and Foundations
Floors over unconditioned spaces lose heat through the slab edge and the underside, so foundations deserve their own insulation strategy. Rigid foam boards are the standard choice because they resist moisture and carry load. Compare perimeter versus full under-slab insulation before pouring concrete, since retrofitting under a slab is expensive.
Perimeter vs. Full Under-Slab Insulation
Perimeter insulation wraps the slab edge with R-10 to R-15 foam and works for most houses in temperate climates. Full under-slab insulation places a continuous layer of R-10 or better beneath the entire floor, which suits heated slabs and very cold regions.
Rigid Board Under Concrete
Use closed-cell boards rated for ground contact, tape the seams, and protect the foam above grade with a parge coat or a protective membrane so it survives weather and impact.
Foam Boards, Spray Foam, and Reflective Barriers
Foam plastics deliver the highest R-value per inch and seal air when applied as spray. Rigid boards work on exterior walls, basements, and roofs, while spray foam fills irregular cavities and hard-to-reach gaps. Homeowners comparing types of insulation often weigh foam performance against higher cost and the need for a trained installer.
Open-Cell vs. Closed-Cell Spray Foam
Closed-cell spray foam is dense, water-resistant, and runs R-6 to R-7 per inch, but it costs more and must be installed by a licensed crew. Open-cell foam is softer, expands more, and runs about R-3.5 per inch, which makes it cheaper for large attic areas.
Spray Foam Cost and Coverage
Closed-cell jobs typically cost $1.00 to $2.00 per board foot, while open-cell runs $0.45 to $0.85 per board foot. A board foot covers one square foot at one inch thick, so a 1,000-square-foot attic at 2 inches of closed-cell foam runs $2,000 to $4,000 before any wall work.
Insulation Comparison at a Glance
| Material | R-value per inch | Best locations | Notes |
|---|---|---|---|
| Fiberglass batt | 3.1-3.4 | Walls, attics, floors | Cheap, familiar, easy retrofit |
| Cellulose | 3.2-3.8 | Attics, wall cavities | Recycled content, settles over time |
| Mineral wool | 3.3-4.2 | Walls, basements | Fireproof, sound damping |
| XPS board | 5.0 | Foundations, exterior walls | Moisture resistant, carries load |
| Polyiso board | 5.6-6.8 | Roofs, walls | Highest per inch, drops in cold |
| Closed-cell spray | 6.0-7.0 | Cavities, rims, roofs | Air seal plus insulation, pricier |
Radiant Barriers and Reflective Insulation
Radiant barriers use a shiny foil surface to reflect heat rather than slow conduction. They work best in hot climates, where crews staple them across attic rafters or drape them over the insulation to block the sun heat before it reaches the living space. Reflective insulation combines foil with a bubble or foam core and earns an R-value only when installed facing an air space.
Coordinate Insulation With Openings and the Whole Envelope
Windows, doors, and garage doors leak far more air per square foot than framed walls, so the envelope only performs as well as its openings. Sealing around window and door frames, weatherstripping, and an insulated garage door cut drafts that otherwise bypass wall insulation.
Garage Doors and Other Building Openings
An attached garage sits on the thermal envelope in most homes, yet many garage doors ship with little or no insulation. Upgrading to an insulated door assembly, or adding foam panels to an existing door, raises the comfort of rooms above and beside the garage. Garage door systems come with published R-values, and the same weatherstripping rules apply at the door perimeter.
Garage Door R-Values
Single-layer steel doors rate about R-0 to R-2, insulated doors with a foam core run R-6 to R-18, and the higher ratings matter most when a conditioned room sits directly above the garage.
Choose Rigid Foam for the Toughest Spots
For exterior sheathing, foundation walls, and continuous insulation layers, rigid foam insulation boards earn their higher price through moisture resistance and high R-value per inch. Pair the board selection with the climate: XPS for below-grade work, polyiso for roofs, and EPS for budget exterior jobs. The final check is simple: walk the house, list every assembly, and confirm each one has the right material at the right thickness for its climate zone.