8.3 Thermal/Moisture Protection, Roofing, Fenestration & Finishes
Key Takeaways
- Vapor retarders are classified by perm ratings (Class I ≤ 0.1 perm, Class II 0.1 to 1.0 perm, Class III 1.0 to 10 perm) and must be installed on the warm-in-winter interior side of insulation in West Virginia's heating-dominated climate.
- Asphalt shingle steep-slope roofing requires a minimum slope of 2:12 (with mandatory double underlayment between 2:12 and 4:12), while an ice and water barrier membrane must extend at least 24 inches inside the warm interior wall line.
- Attic ventilation mandates a minimum net free ventilating area of 1:150 of the attic floor area, reducible to 1:300 when a Class I/II vapor retarder is present or when 40%–50% of ventilation is balanced at the ridge and the remainder at eave/soffit vents.
- Window and door rough opening flashing must follow a strict shingle-lap drainage sequence: sloped sill pan flashing first, followed by the window unit, side jamb flashing, rigid head drip cap, head membrane tape, and upper WRB integration.
- Gypsum board finishes per GA-214 range from Level 0 (unfinished) to Level 5 (full-surface skim coat required under gloss paints or severe raking light), with 5/8-inch Type X gypsum board providing a code-mandated 1-hour fire resistance rating.
Last updated: August 2026
8.3 Thermal/Moisture Protection, Roofing, Fenestration & Finishes
The building envelope separates conditioned interior living space from the external environment, regulating heat flow, air movement, and bulk moisture. In West Virginia, where seasonal climate shifts bring sub-zero winter freezes, heavy mountain snowfall, and humid summer conditions (IECC Climate Zones 4 and 5), contractors must execute envelope details with rigorous technical precision. Flashing failures, missing vapor retarders, or unventilated attics lead to severe mold colonization, structural rot, and catastrophic warranty claims.
1. Building Envelope Science: Four Control Layers & Vapor Physics
Building science establishes four fundamental environmental control layers that must wrap a building in a continuous, unbroken barrier.
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| THE FOUR ENVELOPE CONTROL LAYERS |
| (Ranked by Failure Impact) |
| |
| PRIORITY 1: WATER CONTROL LAYER --> WRB, roof membranes, flashings, |
| sill pans, drainage plane. |
| |
| PRIORITY 2: AIR CONTROL LAYER --> Continuous air barrier, taped |
| sheathing, canned foam, gaskets. |
| |
| PRIORITY 3: THERMAL LAYER --> Batt, board, or spray foam |
| insulation (IECC R-values). |
| |
| PRIORITY 4: VAPOR CONTROL LAYER --> Class I, II, or III vapor retarders |
| controlling internal condensation. |
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Vapor Retarder Classifications (IRC R202 / IBC 202):
Permeance is measured in perms (1 perm = 1 grain of water vapor per hour per square foot per inch of mercury vapor pressure difference).
| Perm Class | Permeance Range | Material Examples | Permitted Climate Applications |
|---|---|---|---|
| Class I (Impermeable) | $\le 0.1$ Perm | 6-mil polyethylene sheet, unperforated foil facing, sheet metal | Warm interior side in cold climates; vapor barrier under slabs |
| Class II (Semi-Impermeable) | $0.1 < \text{perm} \le 1.0$ Perm | Kraft paper facing on fiberglass batts, smart vapor retarders | Standard warm-side retarder in Climate Zones 4 & 5 |
| Class III (Semi-Permeable) | $1.0 < \text{perm} \le 10$ Perm | Latex or enamel paint on $1/2"$ gypsum board | Permitted with continuous insulated exterior sheathing |
[!IMPORTANT] Vapor Retarder Placement Rule in West Virginia (Zones 4 & 5): In heating-dominated climates, water vapor flows outward from warm, humid interior living spaces toward cold exterior sheathing. Class I or Class II vapor retarders must be installed on the warm-in-winter interior side of the thermal insulation (directly behind the drywall) to prevent interior moisture from condensing inside the structural wall cavity.
2. Insulation Systems & IECC Climate Zone Requirements
Thermal resistance ($R$-value) measures a material's resistance to conductive heat flow ($R = \Delta T / Q$). The International Energy Conservation Code (IECC) and IRC Chapter 11 establish prescriptive insulation minimums for West Virginia.
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| INSULATION TYPES & CHARACTERISTICS |
| |
| TYPE R-VALUE / INCH KEY PROPERTIES |
| ----------------------------------------------------------------------- |
| Fiberglass Batts R-3.0 to R-3.8 Economical; air-permeable; |
| requires tight air barrier |
| |
| Mineral Wool R-3.3 to R-4.2 Non-combustible (> 2,150°F); |
| (Rockwool) hydrophobic; soundproofing |
| |
| Dense-Pack Cellulose R-3.5 to R-3.8 Recycled paper + borates; |
| reduces convective air loops |
| |
| Expanded Poly (EPS) R-3.8 to R-4.2 Beadboard; permeable foam; |
| under-slab & insulated forms |
| |
| Extruded Poly (XPS) R-5.0 Closed-cell pink/blue board; |
| moisture resistant; foundation|
| |
| Polyisocyanurate R-6.0 to R-6.5 Foil-faced thermoset foam; |
| (Polyiso) highest R/inch; low-slope roof|
| |
| Open-Cell SPF R-3.7 0.5 lb/cu ft; air barrier; |
| (Spray Foam) vapor permeable (breathable) |
| |
| Closed-Cell SPF R-6.5 to R-7.0 2.0 lb/cu ft; Class II vapor |
| (Spray Foam) retarder at 1.5"; structural |
+-----------------------------------------------------------------------------+
West Virginia IECC / IRC Prescriptive Insulation Matrix:
| Building Assembly | Climate Zone 4 (Central / Southern WV) | Climate Zone 5 (Mountain Counties) |
|---|---|---|
| Ceiling / Attic | R-49 to R-60 | R-60 |
| Wood Frame Wall | R-20 cavity or R-13 + R-5 continuous | R-20 cavity or R-13 + R-5 continuous |
| Basement / Crawlspace Wall | R-10 continuous or R-15 cavity | R-15 continuous or R-19 cavity |
| Slab-on-Grade Perimeter | R-10 (24" vertical/horizontal depth) | R-10 (48" vertical/horizontal depth) |
3. Steep-Slope Roofing Assemblies, Underlayment & Flashing Details
Steep-slope roofs shed water via gravity using overlapping shingles, tiles, or standing seam metal panels.
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| STEEP-SLOPE ROOF SLOPE CATEGORIES |
| |
| SLOPE RATIO DECIMAL / DEGREE CODE STATUS FOR ASPHALT SHINGLES |
| ----------------------------------------------------------------------- |
| < 2:12 < 9.5° PROHIBITED for asphalt shingles. |
| (Flat / Membrane) Must use BUR, Mod-Bit, or Single-Ply|
| |
| 2:12 to < 4:12 9.5° to < 18.4° PERMITTED ONLY with mandatory |
| (Low-Slope Shingle) DOUBLE UNDERLAYMENT (19" overlap) |
| |
| ≥ 4:12 ≥ 18.4° STANDARD STEEP SLOPE. |
| (Standard Steep) Single underlayment (2" lap) |
+-----------------------------------------------------------------------------+
+-----------------------------------------------------------------------------+
| ICE & WATER BARRIER EAVE DETAIL (IRC R905.1.2) |
| |
| / |
| / Self-Adhering |
| / Polymer-Modified |
| / Bitumen Membrane |
| / (Ice Barrier) |
| / |
| +-------------------------------+ |
| | ≥ 24" INSIDE WARM WALL LINE | |
| +-------------------------------+ |
| / |
| / |
| / |
| /| |
| / | [Interior Conditioned Space] |
| / | |
| [Drip Edge] / | |
| + / | |
| | / | [Exterior Wall] |
| +----+---------------+ | |
| | Fascia Board / Gutter | |
+-----------------------------------------------------------------------------+
Critical Roofing Detailing Mandates:
- Ice Barrier Requirement (IRC R905.1.2): In regions with historic ice damming (all of West Virginia), a self-adhering polymer-modified bitumen membrane must extend from the eave edge up the roof slope to a point at least 24 inches (610 mm) inside the exterior warm wall line.
- Drip Edge Installation:
- At Eaves (bottom edge): Drip edge is installed directly onto the roof sheathing underneath the underlayment/ice barrier.
- At Rakes (gable edges): Drip edge is installed over the underlayment to protect against wind-driven rain.
- Valley Flashing (IRC R905.2.8.2):
- Open Valleys: Lined with minimum 24-inch wide corrosion-resistant sheet metal formed with a central $W$-shaped water-diverting rib.
- Closed-Cut / Woven Valleys: Shingles from one or both roof planes weave continuously across the valley center line over a 36-inch wide ice-and-water underlayment membrane.
- Step Flashing & Counterflashing: Required wherever a sloped roof meets a vertical wall or chimney. Individual L-shaped metal pieces ($4" \times 4" \times 8"$) are layered with each course of shingles with a 2-inch overlap. At masonry chimneys, counterflashing is cut into mortar joints (reglets) and overlaps step flashing by at least 3 inches.
Roof Attic Ventilation (IRC R806 / IBC 1202.2):
Attic ventilation prevents winter ice dams and lowers summer cooling loads.
- Base Rule (1:150 Ratio): Minimum Net Free Ventilating Area (NFVA) of 1 square foot per 150 square feet of attic floor space.
- Exception (1:300 Balanced Ratio): NFVA is reduced to 1:300 if:
- A Class I or II vapor retarder is installed on the ceiling plane in Climate Zones 4/5, OR
- Balanced Ventilation: Between 40% and 50% of the required vent area is located in the upper portion of the roof (ridge vents or high louvers located $\le 3$ feet below the ridge), and the remaining 50% to 60% is provided by continuous eave/soffit intake vents.
4. Low-Slope Membrane Roofing Systems (Slope < 2:12)
Roofing assemblies on flat or low-slope decks (minimum slope 1/4-inch per foot for positive drainage) utilize continuous monolithic, waterproof membranes.
+-----------------------------------------------------------------------------+
| LOW-SLOPE MEMBRANE ROOFING SYSTEMS |
| |
| [BUR] Built-Up Roofing -> Alternating plies of asphalt-saturated |
| organic/fiberglass felts mopped with hot |
| bitumen pitch + gravel aggregate flood. |
| |
| [MOD-BIT] Modified Bitumen -> Factory-manufactured polymer asphalt: |
| - APP (Plasticized, torch applied) |
| - SBS (Rubberized, hot-mop/self-adhered) |
| |
| [EPDM] Synthetic Rubber -> Thermoset single-ply rubber membrane |
| (45-90 mil). Seams glued with tape. |
| |
| [TPO] Thermoplastic Polyolefin-> Thermoplastic single-ply (white/reflect).|
| Seams hot-air welded at 800°-1,000°F. |
| |
| [PVC] Polyvinyl Chloride -> Thermoplastic single-ply. Resistant to |
| fats, oils, grease, & chemicals. |
+-----------------------------------------------------------------------------+
5. Fenestration: NFRC Ratings & Shingle-Lap Flashing Integration
Windows and glazed exterior doors must comply with National Fenestration Rating Council (NFRC) thermal ratings and watertight flashing integration.
+-----------------------------------------------------------------------------+
| NFRC PERFORMANCE METRICS |
| |
| METRIC MEASUREMENT CODE MANDATE (ZONE 4/5) |
| ----------------------------------------------------------------------- |
| U-Factor Conductive heat loss U ≤ 0.27 to 0.30 |
| (Lower is better) (Btu/h·ft²·°F) |
| |
| SHGC (Solar Heat Solar radiation admitted SHGC ≤ 0.40 |
| Gain Coefficient) (Scale 0.0 to 1.0) (Reduces cooling loads) |
| |
| Visible Transmittance Natural light admitted VT 0.30 to 0.70 |
| (VT) (Scale 0.0 to 1.0) (Higher is brighter) |
| |
| Air Leakage (AL) Air infiltration rate AL ≤ 0.30 cfm/sq ft |
+-----------------------------------------------------------------------------+
+-----------------------------------------------------------------------------+
| SHINGLE-LAP WINDOW FLASHING SEQUENCE (AAMA 100) |
| |
| STEP 1: Install Sloped Sill Pan Flashing with Back-Dam and End-Dams. |
| | |
| v |
| STEP 2: Apply 3-Side Bead of Sealant (Top & Sides of Sheathing; leave |
| bottom open for sill drainage) and Set Window Level & Plumb. |
| | |
| v |
| STEP 3: Apply Self-Adhering Membrane Flashing over Left & Right Jambs |
| (Overlapping Sill Pan Flashing). |
| | |
| v |
| STEP 4: Install Rigid Metal Drip Cap / Head Flashing over Window Top. |
| | |
| v |
| STEP 5: Apply Self-Adhering Membrane over Head Flashing Upturn Leg. |
| | |
| v |
| STEP 6: Fold Upper Water-Resistive Barrier (WRB) Down over Head Tape |
| and Seal with Weatherization Tape (Completes Gravity Drainage). |
+-----------------------------------------------------------------------------+
6. Interior Finishes: Gypsum Board & GA-214 Finish Standards
Gypsum wallboard provides fire containment, acoustic isolation, and finished architectural surfaces.
Gypsum Board Classifications:
- Standard Gypsum (1/2"): Standard residential walls and ceilings with 16" o.c. framing.
- Type X Gypsum (5/8"): Specially formulated non-combustible core containing glass fibers and unexpanded vermiculite. Provides a certified 1-hour fire resistance rating when installed on standard wood studs. Mandatory for garage-to-house separation walls (IRC R302.6), boiler rooms, and multi-family party walls.
- Water-Resistant Board ("Green Board"): Treated moisture-resistant paper face. Permitted in humid areas (laundry rooms, bathrooms), but prohibited as a backing board in direct wet areas (showers, tub surrounds).
- Cement Backer Board (ASTM C1325): Non-decaying Portland cement and fiber mesh board (Durock, WonderBoard). Mandatory substrate behind ceramic tile in high-moisture shower and tub enclosures.
Gypsum Association Drywall Finish Levels (GA-214 / ASTM C840):
+-----------------------------------------------------------------------------+
| GA-214 DRYWALL FINISH LEVELS |
| |
| LEVEL 0: No taping, finishing, or corner beads. Temporary dust partitions.|
| |
| LEVEL 1: Tape embedded in compound at joints and interior angles. Tool |
| marks acceptable. Concealed ceiling plenums, attics (fire tape). |
| |
| LEVEL 2: Tape embedded with thin compound wipe; fastener heads covered |
| once. Substrate for tile backer board; garages, storage spaces. |
| |
| LEVEL 3: Two separate coats of compound over joints; fastener heads |
| covered twice. Prepared for heavy texture coats or heavy vinyl. |
| |
| LEVEL 4: Three coats of compound over flat joints; two over angles; three |
| over fasteners. Standard commercial finish for flat paints. |
| |
| LEVEL 5: High-grade Level 4 finish PLUS a full continuous SKIM COAT of |
| joint compound or drywall primer across the entire surface. |
| Mandatory under gloss/semi-gloss enamel paints & raking light. |
+-----------------------------------------------------------------------------+
Test Your Knowledge
Under the International Residential Code (IRC) and International Building Code (IBC), what is the definition and placement rule for a Class I vapor retarder in West Virginia's climate?
A
B
C
D
Test Your Knowledge
When installing an ice barrier membrane on an asphalt shingle roof in regions subject to freezing and ice dam formation, how far up the roof slope must the self-adhering polymer-modified bitumen membrane extend?
A
B
C
D
Test Your Knowledge
According to Gypsum Association standard GA-214, which drywall finish level requires a continuous thin skim coat of joint compound applied across the entire surface of the drywall boards?
A
B
C
D
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