Insulation & Air Sealing

A well-insulated, airtight building envelope is essential for comfort, energy efficiency, and moisture management. Insulation slows the transfer of heat through the building envelope — keeping the house warm in winter and cool in summer. Air sealing prevents uncontrolled air leakage, which causes drafts, carries moisture into wall and roof cavities (leading to condensation and mold), and can account for 25–40% of a home's heating and cooling energy loss. Together, insulation and air sealing form the building's thermal envelope.

Heat moves through building assemblies in three ways:

• Conduction: Heat transfer through solid materials (wood, steel, glass, concrete). Insulation resists conductive heat transfer with materials that have low thermal conductivity.
• Convection: Heat transfer through moving air. Air leaks carry heated (or cooled) air through cracks and gaps, bypassing the insulation entirely.
• Radiation: Heat transfer through electromagnetic waves. Low-E coatings on windows and reflective insulation (radiant barriers) reduce radiative heat transfer.

Insulation addresses conduction; air sealing addresses convection. Both are necessary — insulation without air sealing is like wearing a sweater with holes.

R-value measures a material's resistance to conductive heat transfer. Higher R-value = better insulation. R-values are additive — the total R-value of a wall is the sum of the R-values of each layer (sheathing, insulation, drywall, air films).

The International Energy Conservation Code (IECC) specifies minimum R-values by climate zone (zones 1–8, with 1 being the hottest and 8 being the coldest):

("ci" = continuous insulation — rigid foam or spray foam covering the entire wall exterior, not interrupted by framing.)

Fiberglass Batt Insulation

The most common residential insulation. Pink, yellow, or white blankets of spun glass fibers, available in pre-cut widths (15" for 16" o.c. framing, 23" for 24" o.c.).

Installation quality is critical. Batts must be cut to fit snugly around wiring, plumbing, and electrical boxes without compression or gaps. Compressed insulation loses R-value; gaps allow convective bypasses. Studies show that even 5% void area in batt insulation can reduce its effective R-value by 25% or more.

Advantages: Low cost, widely available, DIY-friendly, non-combustible. Disadvantages: Sensitive to installation quality; does not air seal; loses effectiveness when wet; can cause skin and respiratory irritation during installation.

Blown-In (Loose-Fill) Insulation

Loose insulation material blown into attics and wall cavities using a blowing machine.

• Blown fiberglass: Fine glass fibers. R-value approximately R-2.5 per inch. Good for attics; adequate for enclosed wall cavities (dense-pack).
• Blown cellulose: Recycled newspaper treated with fire retardants (borate). R-value approximately R-3.5 per inch. Better thermal performance than fiberglass, fills cavities more completely, and the borate treatment provides mold and pest resistance. Dense-pack cellulose (3.5 lb/ft³ density) blown into enclosed wall cavities provides both insulation and significant air sealing.

Advantages: Fills irregular cavities, good coverage in attics, cost-effective. Disadvantages: Requires specialized equipment; can settle over time in walls (cellulose more than fiberglass); can absorb moisture.

Spray Foam Insulation

Liquid chemicals sprayed into cavities where they expand and harden into rigid foam. Two types:

• Open-cell spray foam: Soft, flexible foam. R-value approximately R-3.7 per inch. Fills cavities completely and provides excellent air sealing. Vapor permeable (allows drying). Less expensive than closed-cell. Common for interior wall and roof cavities.
• Closed-cell spray foam: Dense, rigid foam. R-value approximately R-6.5 per inch — the highest R-value per inch of any common insulation. Also provides:Air barrierVapor barrier (at 2+ inches)Structural reinforcement (adds racking strength to walls)Water resistance (does not absorb water)

Closed-cell spray foam is used in rim joists, crawl spaces, basement walls, and any location where maximum R-value in minimum thickness is needed. It is also excellent for cathedral ceilings where the insulation must fit within the rafter depth.

Disadvantages of spray foam: Higher cost (2–3 times batt insulation), requires professional installation, off-gassing during application (occupants must vacate), quality depends on correct mixing ratios and application technique.

Rigid Board Insulation

Panels of foam insulation used on the exterior of walls and foundations, under slabs, and in cathedral ceilings.

• Expanded polystyrene (EPS): R-3.8 to R-4.4 per inch. White beaded foam. Least expensive rigid foam. Absorbs some moisture.
• Extruded polystyrene (XPS): R-5 per inch. Pink, blue, or green smooth foam. Moderate cost. Good moisture resistance. Common for below-grade and under-slab applications.
• Polyisocyanurate (polyiso): R-5.7 to R-6.5 per inch (at 75°F; performance drops in cold temperatures). Foil-faced. Highest R-value per inch among rigid boards. Used primarily above-grade on walls and roofs.

Continuous insulation (ci): Rigid foam installed over the entire exterior of the wall sheathing (outside the studs) creates a continuous thermal barrier that eliminates thermal bridging through the studs. This is the most effective way to improve wall thermal performance and is increasingly required by energy codes.

Air sealing targets the specific locations where air leaks through the building envelope. The most significant air leakage locations in a typical home:

1. Rim joist area: The junction of the foundation and framing is one of the leakiest areas. Seal with caulk and spray foam.
2. Electrical and plumbing penetrations: Every wire, pipe, and duct that penetrates the envelope is a potential leak. Seal with caulk, foam, or fire-rated sealant.
3. Recessed light fixtures: Standard recessed lights are huge air leak sources. Use IC-rated (insulation contact) airtight fixtures.
4. Attic access hatches: Weatherstrip and insulate attic access doors and pull-down stairs.
5. Top plates of walls: The double top plate has gaps at splices and intersections. Seal with caulk or gasket.
6. Window and door rough openings: The gap between the window frame and the rough opening is filled with low-expansion foam (not high-expansion, which can bow the frame).
7. Duct penetrations: HVAC ducts passing through unconditioned spaces must be sealed at the penetrations.
8. Chimney and flue chases: Use fire-rated metal flashing and high-temperature sealant (not spray foam, which is combustible).

Blower door test: A diagnostic test that measures a home's air tightness. A calibrated fan is mounted in an exterior doorway and depressurizes the house to 50 Pascals. The airflow required to maintain this pressure (measured in cubic feet per minute at 50 Pascals, CFM50) indicates how leaky the house is. Results can also be expressed as air changes per hour at 50 Pascals (ACH50):

• A typical existing home: 8–15 ACH50
• A code-built new home (2021 IECC): 3–5 ACH50
• An Energy Star home: 3–4 ACH50
• A Passive House: ≤ 0.6 ACH50

Moisture control in walls requires understanding the direction of vapor drive:

• In heating climates (winter): Moisture drives from the warm, humid interior toward the cold, dry exterior. A vapor retarder on the warm side (interior) of the wall slows this drive, preventing condensation within the wall cavity.
• In cooling climates (summer): Moisture drives from the hot, humid exterior toward the cool, dry interior. A vapor retarder should be on the exterior side — or omitted entirely to allow drying in both directions.

Classes of vapor retarders:

• Class I (vapor barrier): ≤ 0.1 perms. Polyethylene sheeting, foil-faced insulation, glass. Blocks almost all vapor. Appropriate only in very cold climates (zones 7–8 Marine) on the interior side. Caution: A Class I barrier on the wrong side traps moisture and causes mold and rot.
• Class II (vapor retarder): 0.1 to 1.0 perms. Kraft-faced batts, some latex paints. Slows but does not block vapor.
• Class III (vapor permeable): 1.0 to 10 perms. Latex paint, housewrap. Allows significant drying.

The current trend in building science is toward vapor-open assemblies that can dry in both directions, rather than relying on vapor barriers. Continuous exterior insulation (rigid foam or mineral wool) keeps the sheathing above the dew point, eliminating the condensation risk without needing an interior vapor barrier.