12.3 Thermal & Moisture Protection

Insulation Types and R-Values

Thermal insulation resists heat flow and is rated by R-value (thermal resistance per inch). Higher R-value means greater insulating value. The 2018 IECC (International Energy Conservation Code) specifies minimum R-values by climate zone, adopted alongside the 2018 IBC. Arizona spans climate zones 2 (south) through 4 (north, higher elevation), with zone 2B (hot-dry) covering most of the Phoenix and Tucson metros.

Quick Answer: R-value is thermal resistance per inch of material; closed-cell spray foam offers the highest R-value per inch (about R-6.0 to R-6.5) while fiberglass batts deliver R-3.0 to R-3.8 per inch.

Vapor Retarders and Placement

A vapor retarder (also called vapor barrier) slows moisture diffusion through wall, floor, and ceiling assemblies. Building codes class vapor retarders by perm rating: Class I (0.1 perm or less, e.g., polyethylene sheet), Class II (0.1 to 1.0 perm, e.g., kraft-faced batt), Class III (1.0 to 10 perm, e.g., latex paint on gypsum).

Placement depends on climate. In hot-humid and hot-dry climates (most of Arizona, climate zones 2 and 3), the vapor retarder goes on the exterior side of the assembly, or is omitted in favor of a permeable assembly that allows inward drying. Putting a Class I vapor barrier on the interior in a cooling-dominated climate traps moisture inside the wall and can cause condensation on the back of the interior finish. The code permits omitting the vapor retarder entirely in climate zones 1, 2, and 3.

The physics behind this rule: in a cooling-dominated climate, the warm moist side is the exterior. Vapor drive is primarily inward, from hot humid exterior toward air-conditioned interior. An interior vapor barrier stops that inward drive at the cool interior surface, which is exactly where condensation risk is highest. A permeable interior finish (Class III) allows any vapor that enters the assembly to dry inward, while a properly installed exterior WRB and flashing manage liquid water from rain.

Weatherproofing and Flashing

Flashing redirects water away from openings and back to the exterior. The principle is shingled lap — each layer sheds water to the one below. Window and door flashing sequences: sill pan flashing first (with end dams), then side flashing extending over the sill, then head flashing extending over the sides. Self-adhered membrane (SAM) flashing tape bonds to the building wrap and sheathing. The building wrap or weather-resistive barrier (WRB) is installed behind cladding and flashed at every penetration.

Flashing types used in wall and roof assemblies include step flashing (interleaved with shingles at a roof-to-wall intersection), counter flashing (installed over base flashing to direct water over, not behind, the base), base flashing (turns up a vertical surface from a horizontal plane), drip edge (metal flashing at roof edges that directs water away from the fascia), and kick-out flashing (transitions roof-to-wall step flashing to the wall, diverting water into the gutter). Each flashing type serves a specific water-shedding function, and omitting any of them creates a leak path.

Roofing Underlayment

Underlayment is the water-shedding layer between roof sheathing and roofing material. The 2018 IBC and IRC require a minimum of one layer of No. 15 asphalt-saturated felt or an equivalent self-adhered or synthetic underlayment for sloped roofs. In ice-dam regions, a self-adhered membrane extends from the eave edge a minimum of 24 inches inside the exterior wall line. Arizona's hot climate can degrade asphalt underlayment faster; synthetic underlayment offers higher UV and heat tolerance during construction exposure.

Dampproofing vs Waterproofing Foundations

Dampproofing resists soil moisture and water vapor, typically an unmodified asphalt coating applied to foundation walls below grade where hydrostatic pressure is not expected. Waterproofing resists liquid water under hydrostatic pressure and uses modified bitumen, elastomeric membranes, or bentonite systems. The 2018 IBC (Chapter 18, Soils and Foundations) and IRC require dampproofing from the top of the footing to finished grade for concrete and masonry foundations. Waterproofing is required where the groundwater table is above the floor slab or where interior drainage is not provided.

Quick Answer: Dampproofing resists moisture vapor (asphalt coating); waterproofing resists liquid water under hydrostatic pressure (membrane or bentonite). Waterproofing is required where the water table is above the slab.

Air Sealing

Air leakage is a major source of energy loss and moisture movement. The 2018 IECC requires a continuous air barrier around the thermal envelope. Common air-sealing points: top and bottom plates, penetrations for wiring and plumbing, recessed light fixtures (use IC-rated airtight fixtures), attic hatch, and rim joists. Spray foam and caulking at plate-to-sheathing joints, gaskets at the top of walls, and sealed electrical boxes all contribute to a continuous air barrier. Air sealing is most effective when coordinated with insulation and ventilation; sealing without ventilation can trap moisture.

On the B-1/B-2 commercial general building trade exam, Thermal & Moisture Protection carries 10 of 100 items.