15.2 Thermal & Moisture Protection: WRBs, Stucco, Insulation, Vapor Control & Roofing

Building Envelope, Weather-Resistive Barriers & Stucco

The exterior building envelope is a continuous system protecting conditioned interior spaces from liquid water, air infiltration, thermal bridging, and moisture vapor.

                   THREE-COAT STUCCO (PORTLAND CEMENT PLASTER)
  ◄────────────────────── Total Thickness: 7/8" ──────────────────────►
  ┌────────────────────┬────────────────────┬─────────────────────────┐
  │ 1. SCRATCH COAT    │ 2. BROWN COAT      │ 3. FINISH COAT          │
  │ 3/8" Thickness     │ 3/8" Thickness     │ 1/8" Thickness          │
  │ Horizontally scored│ Floated flat;      │ Acrylic or cementitious │
  │ to bond brown coat │ provides true plane│ texture and pigment     │
  └────────────────────┴────────────────────┴─────────────────────────┘
  (Installed over: 2 layers Grade D Paper / WRB + Expanded Metal Lath)

1. Water-Resistive Barriers (WRBs) & Flashing Integration

• WRB Materials: No. 15 asphalt-saturated felt (ASTM D226 Type I), Grade D building paper, polymeric housewraps, or fluid-applied air and water barriers. Over wood-based sheathing, IBC 2510.6 requires a stucco WRB with water resistance at least equal to two layers of 60-minute Grade D paper. One such layer may be used instead if a drainage space separates it from the stucco. The outer layer bonds to the plaster, and the inner layer stays free to drain.
• Lapping Rules: Must be installed shingle-fashion (bottom-to-top) with minimum $2\ \text{inch}$ horizontal laps and minimum $6\ \text{inch}$ vertical laps.
• Flashing Integration:
  • Sill Pan Flashing: Installed beneath all exterior window and door rough sills, sloped to the exterior with end dams and an interior back dam.
  • Step Flashing: Interwoven with shingle courses at roof-to-wall intersections; each step bends at 90 degrees (minimum 4" up wall and 4" onto roof deck).
  • Kick-Out Flashing (Diverter Flashing): Located at the lower terminus of a roof-wall intersection where the roof line ends. Diverts water runoff outward into the gutter, preventing water from penetrating behind wall cladding.

2. Three-Coat Exterior Stucco (ASTM C926)

Portland cement plaster (stucco) is the most widespread exterior cladding across Nevada commercial and residential construction. ASTM C926 specifies the three-coat system:

1. Scratch Coat ($3/8\ \text{inch}$ thick): Pressed firmly through self-furred galvanized expanded metal lath ($2.5\ \text{lb}$ or $3.4\ \text{lb/yd}^2$). Scored horizontally with a notched comb before setting to create mechanical bonding keys for the brown coat.
2. Brown Coat ($3/8\ \text{inch}$ thick): Applied over the damp scratch coat. Straightened with screeds and floated to a true, flat plane.
3. Finish Coat ($1/8\ \text{inch}$ thick): Decorative pigmented coat applied after curing, providing final color and texture.

• Total Assembly Thickness: A nominal $7/8\ \text{inch}$ ($3/8 + 3/8 + 1/8 = 7/8\ \text{inch}$) over metal lath. Two-coat work is permitted only over solid bases such as masonry or concrete.
• Weep Screed Standards (IBC § 2512.1.2): A perforated galvanized or PVC foundation weep screed must be installed at the base of framed walls. Must be positioned at least 4 inches above raw earth or 2 inches above paved walking surfaces. The WRB must lap over the vertical attachment flange of the weep screed to drain water outward.
• Control Joint Spacing (ASTM C1063): Control joints must divide wall surfaces into rectangular panels not exceeding $144\ \text{sq ft}$, with no single dimension exceeding $18\ \text{feet}$, and a maximum length-to-width ratio of $2.5:1$.

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3. Thermal Insulation & Vapor Retarder Physics across Nevada

Under the International Energy Conservation Code (IECC), Nevada spans multiple climate zones requiring distinct thermal and vapor control strategies:

• Southern Nevada (Clark County, Las Vegas): Climate Zone 3B (Warm-Dry): Cooling-dominated. High solar heat gain.
• Most of northern and central Nevada (Reno, Carson City, Elko): Climate Zone 5B (Cool-Dry): Heating-dominated, with significant winter freezing.

                      IBC VAPOR RETARDER CLASSES

  CLASS I VAPOR IMPERMEABLE:    Permeance ≤ 0.1 perm
                                Sheet polyethylene (visqueen), unperforated foil.
                                (Not required in Zone 3B; avoid on the interior)

  CLASS II VAPOR SEMI-PERMEABLE: 0.1 < Permeance ≤ 1.0 perm
                                Kraft paper facing on fiberglass batts.

  CLASS III VAPOR PERMEABLE:    1.0 < Permeance ≤ 10 perms
                                Standard latex or enamel wall paints on drywall.

Nevada vapor-retarder rule: The 2018 IRC (R702.7) and IBC require a Class I or II vapor retarder on the interior side of frame walls only in Climate Zones 5 through 8 and Marine 4. A typical Reno or Elko wall therefore gets one, often kraft-faced batts. A Las Vegas (Zone 3B) wall does not need one. In Zone 3B, building scientists also warn against interior polyethylene in air-conditioned buildings. Sun-heated stucco that is wet from rain or irrigation can drive vapor inward. The cool poly then traps that moisture inside the cavity, and the wall cannot dry toward the interior.

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Commercial & Residential Roofing Systems

Roofing assemblies are categorized by slope into low-slope systems and steep-slope systems.

                         ROOFING CLASSIFICATION BY SLOPE

       LOW-SLOPE ROOFING (Slope ≤ 2:12)         STEEP-SLOPE ROOFING (Slope > 2:12)
  ┌───────────────────────────────────────┐ ┌───────────────────────────────────────┐
  │ • Minimum slope: 1/4" per foot (2%)   │ │ • Asphalt Shingles:                   │
  │ • Single-Ply Thermoplastic (TPO, PVC) │ │   - Min 2:12 with double underlayment │
  │ • Single-Ply Thermoset (EPDM Rubber)  │ │   - Min 4:12 with standard underlay   │
  │ • Built-Up Roofing (BUR) & Mod-Bit    │ │ • Concrete & Clay Tile (heavy dead ld)│
  │ • Positive drainage to scuppers/drains│ │ • Standing-Seam Metal Roofing         │
  └───────────────────────────────────────┘ └───────────────────────────────────────┘

1. Low-Slope Roofing Systems (Slope $\le 2:12$)

• Positive Drainage Mandate: Low-slope commercial roofs must be engineered with a minimum slope of $1/4\ \text{inch}$ per foot ($2\%$) toward roof drains, scuppers, or gutters (achieved via sloped structural framing or tapered polyisocyanurate rigid foam insulation). Ponding water, commonly defined as water still standing 48 hours after rain, can void some manufacturer warranties. It also adds load that deflects the structure, which deepens the pond. The IBC sets the 1/4-inch-per-foot minimum for built-up (coal-tar built-up: 1/8 inch), modified bitumen, and single-ply membrane roofs.
• Single-Ply Thermoplastic Membranes (TPO & PVC): Reinforced single-ply sheets (typically 45, 60, or 80 mils thick). Seams are joined with hot-air welders, fusing the overlapping plies into a continuous seam; crews probe-test every seam. Highly reflective white surfaces deliver high Solar Reflectance Index (SRI) values, slashing commercial air-conditioning costs in Las Vegas.
• Single-Ply Thermoset Membranes (EPDM): Synthetic ethylene propylene diene monomer rubber. Chemically cured; seams cannot be heat-welded and must be joined using factory-applied splice tapes and liquid primers.
• Built-Up Roofing (BUR) & Modified Bitumen: Multiple plies of asphalt-coated glass fiber felts adhered with hot asphalt, topped with aggregate or mineral-surfaced cap sheets (SBS elastomeric or APP plastomeric torch-down).

2. Steep-Slope Roofing Systems (Slope $> 2:12$)

• Asphalt Fiberglass Shingles:
  • Minimum Slope Requirements: Absolute minimum slope is $2:12$. For slopes between $2:12$ and $4:12$ (low-slope application), building codes mandate a double layer of underlayment (Type 15 felt or synthetic) cemented together or self-adhering ice-and-water barrier. Slopes $\ge 4:12$ require a single layer of underlayment lapped 2 inches horizontally and 4 inches vertically.
  • Fastening: the number required by the manufacturer, but not fewer than four fasteners per strip shingle (IBC 1507.2.7). High-wind installations often need six. Nails penetrate at least $3/4\ \text{inch}$ into the deck, or through it if the deck is thinner.
• Concrete & Clay Roof Tiles: Widely specified across Nevada master-planned communities. Structural Weight Hazard: Tile roofing dead loads range from $900$ to $1,200\ \text{lbs per square}$ (100 sq ft), compared to only $220$ to $280\ \text{lbs/square}$ for asphalt shingles. Roof rafters and trusses must be specifically engineered to support this heavy permanent dead load. Tile requires a slope of at least $2\text{-}1/2:12$, with double underlayment from $2\text{-}1/2:12$ up to $4:12$ (IBC 1507.3). Fasten tile with corrosion-resistant fasteners, following the code and the manufacturer's or industry installation manual.

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Below-Grade Moisture Protection & Insulation Materials

1. Dampproofing vs. Waterproofing (IBC 1805)

• Dampproofing resists moisture where there is no hydrostatic pressure. Typical uses are basement and retaining walls that enclose interior space below grade, using bituminous or cementitious coatings.
• Waterproofing is a membrane system that resists water under hydrostatic pressure. It is required where the groundwater investigation shows a high water table that will not be permanently lowered by drainage. Laps and penetrations must be sealed.
• Foundation drains: Perforated pipe or other approved drainage at the footing carries water away so walls see less pressure. Discharge it to daylight or a sump, never into the sanitary sewer.

2. Insulation Materials

• Foam plastic thermal barrier: Foam plastic insulation inside a building must be separated from the interior by an approved thermal barrier, usually 1/2-inch gypsum board (IBC 2603.4; IRC R316.4). Limited exceptions apply, such as some attic and crawlspace cases. A bare spray-foam wall in an occupied room is a code violation and a fire hazard.
• Continuous insulation (ci): Rigid board installed outside the framing interrupts heat flow through the studs. Wood studs are thermal bridges, and steel studs are much worse. Energy codes often call for ci on steel-framed walls.
• Air sealing: The IECC requires a continuous air barrier and sealed penetrations. Many jurisdictions also require a blower-door test for new dwellings, so seal top plates, penetrations, and rim joists before the drywall goes up.

3. Sealants & Flashings at Joints

• Use sealant compatible with both substrates. Install backer rod so the sealant bonds on two sides only (an hourglass profile). A three-sided bond tears when the joint moves.
• Sealants are a secondary line of defense. Shingle-lapped flashing and WRB do the primary job of shedding water.