10.2 Insulation, R-Values, and Vapor Barriers
Key Takeaways
- R-values add in series for assemblies; U-factor = 1/R-total measures transmission.
- Thickness needed = target R divided by the material's R-per-inch (e.g., R-30 / 6 = 5 in. polyiso).
- IECC prescriptive minimums rise with colder climate zones; '+5ci' denotes continuous insulation against thermal bridging.
- Vapor retarders are Class I (<=0.1 perm), II (0.1-1.0), III (1.0-10) per ASTM E96.
- In cold climates place the Class I/II vapor retarder on the warm interior side.
Thermal Resistance Fundamentals
R-value measures resistance to conductive heat flow; higher R = better insulation. U-factor is the inverse (U = 1/R) and measures heat transmission for assemblies, windows, and doors. The exam mixes these intentionally.
For layered assemblies, R-values add in series: R-total = R1 + R2 + R3... A wall with R-13 batt + R-5 continuous foam sheathing + interior/exterior air films yields roughly R-18 to R-19 nominal.
Common R-Values per Inch
| Material | R-value per inch |
|---|---|
| Fiberglass batt | 3.1 - 3.4 |
| Cellulose (loose-fill) | 3.2 - 3.8 |
| Expanded polystyrene (EPS) | 3.6 - 4.0 |
| Extruded polystyrene (XPS) | 5.0 |
| Polyisocyanurate (polyiso) | 5.6 - 6.5 |
| Closed-cell spray polyurethane | 6.0 - 7.0 |
| Open-cell spray foam | 3.5 - 3.7 |
Worked example: A roof needs R-30 using polyiso at R-6/in. Required thickness = 30 / 6 = 5 in. If only R-5/in. XPS were used: 30 / 5 = 6 in.
Energy Code Minimums
The International Energy Conservation Code (IECC) sets prescriptive R-values by climate zone. Representative wood-frame wall and ceiling minimums:
| Climate Zone | Wall (cavity) | Ceiling/Attic |
|---|---|---|
| Zone 2 (hot) | R-13 | R-38 |
| Zone 4 (mixed) | R-20 or R-13+5ci | R-49 |
| Zone 6 (cold) | R-20+5ci | R-60 |
The notation R-13+5ci means R-13 cavity batt plus R-5 continuous insulation over the framing to interrupt thermal bridging through studs.
Vapor Retarders
IBC 1404.3 and IECC R702 classify vapor retarders by permeance (perms, measured per ASTM E96):
- Class I: 0.1 perm or less (sheet polyethylene, foil) - vapor barrier
- Class II: 0.1 to 1.0 perm (kraft-faced batt, vapor-retarder paint)
- Class III: 1.0 to 10 perms (latex paint)
Critical rule: In cold climates (Zones 5-8 and Marine 4), the Class I or II vapor retarder goes on the warm-in-winter (interior) side of the wall to keep moisture from condensing inside the cavity. Placing it on the cold side is a classic exam trap that traps moisture.
Condensation, Dew Point & Continuous Insulation
Vapor moves from warm-humid to cold-dry. If the interior vapor meets a surface below the dew point, it condenses. Continuous exterior insulation keeps the sheathing warmer, moving the dew point outward so condensation does not occur on the structural sheathing.
Scenario: In Zone 6, R-20 cavity + R-5ci satisfies code; but in a vapor-open wall, designers may use Class III (latex paint) interior only when sufficient continuous insulation keeps sheathing above dew point per IECC Table R702.7.1.
Takeoff Example
Insulate a 2,400 ft2 attic floor to R-49 with cellulose at R-3.5/in. Depth = 49 / 3.5 = 14 in. Volume = 2,400 ft2 x (14/12 ft) = 2,800 ft3. If a bag covers 40 ft2 at 14 in., bags = 2,400 / 40 = 60 bags. Always confirm coverage from the manufacturer's chart at the target depth, not a thinner reference depth.
A wall assembly has R-13 batt, R-5 continuous foam, and air films totaling R-1. What is the approximate U-factor of the assembly?
In a cold climate (IECC Zone 6), where should a Class I vapor retarder be installed in a wood-frame wall?
R-Value, U-Factor, and Adding Up Assemblies
R-value measures resistance to heat flow (higher = better insulator); U-factor is its inverse (U = 1/R, lower = better) and is how windows/assemblies are rated. R-values of layers in series add: a wall of R-13 batt + R-5 sheathing + interiors ≈ R-18+. Worked example: a window with U-0.30 has an effective R ≈ 1/0.30 = 3.3. The IECC sets minimum R/U by climate zone (e.g., zone 5 walls often R-20 cavity or R-13+5 continuous).
Insulation Types
- Fiberglass/mineral wool batts (~R-3.1–4.3/in) — common, must fill the cavity without compression (compressing lowers R).
- Rigid foam board — XPS (~R-5/in), polyiso (~R-6/in) — for continuous exterior insulation breaking thermal bridging.
- Spray polyurethane foam (SPF) — closed-cell (~R-6.5/in, also an air/vapor barrier) vs. open-cell.
- Loose-fill/blown cellulose for attics.
Vapor Retarders and the Warm-Side Rule
A vapor retarder slows moisture diffusion that would condense inside the wall. The rule: place it on the warm-in-winter side — interior in cold climates, sometimes omitted or reversed in hot-humid climates. A double vapor barrier traps moisture and rots the wall — a classic defect. Distinguish a vapor retarder (controls diffusion) from an air barrier (stops air leakage, which moves far more moisture).
Common Exam Traps
- Trap: Higher U-factor is better. Lower U (higher R) is better.
- Trap: Compressing batts keeps full R. Compression reduces R.
- Trap: Vapor barrier on the cold side in a cold climate (causes condensation).
- Trap: Two vapor barriers "for safety" — traps moisture.
A window is rated at U-0.25. What is its approximate R-value?
Thermal Bridging and Continuous Insulation
Heat short-circuits through conductive framing — a wood or steel stud bypasses the cavity batt, lowering the wall's effective (whole-assembly) R-value below the nominal cavity rating; steel studs are especially conductive. The IECC fix is continuous insulation (ci): rigid foam over the sheathing that is unbroken by framing, written as specs like R-13 + R-5ci. Detail the ci to stay continuous across slab edges, parapets, and rim joists, because the worst energy and condensation problems occur exactly where insulation is interrupted.
Fire and the Thermal Barrier over Foam
Most foam plastic insulation is combustible and must be separated from the interior by an approved thermal barrier — typically 1/2-in gypsum board (IBC/IRC) — so it cannot be left exposed in occupied space. Exceptions and ignition barriers apply in attics/crawl spaces with limited access. Spray foam and rigid board left exposed without the required gypsum cover is a frequent inspection failure, so coordinate the insulation with the drywall scope rather than treating thermal performance in isolation.