+---
+title: Vapor Retarders
+category: Building Envelope
+toc_depth: 3
+description: >
+ When to use: Below-slab vapor barriers under concrete slabs-on-grade and above-grade wall and ceiling vapor retarders for moisture and water-vapor control in commercial, institutional, and residential buildings. Covers the two distinct applications addressed by this standard: (1) the plastic sheet water-vapor retarder installed in contact with soil or granular fill beneath a concrete slab-on-grade, classified to ASTM E1745 (Class A, B, or C) and installed to ASTM E1643, with its thickness, permeance, lapped and taped seams, sealed penetrations and pipe boots, and the placement debate of ACI 302.1R / 302.2R over whether the retarder sits directly under the slab or below a granular cushion; and (2) the above-grade vapor retarder in framed wall and ceiling assemblies, classified by permeance as IBC/IRC Class I, II, or III, placed on the correct (warm-in-winter) side of the assembly governed by climate zone, including kraft-faced batt facings, polyethylene sheet, and variable-permeance (smart) membranes. Addresses class selection, permeance verification per ASTM E96, climate-zone placement logic, seam and penetration sealing, and field inspection before concealment.
+ Not intended for: Air barriers, whose function is to stop bulk air movement rather than vapor diffusion (see [[sync/air-barriers]]); below-grade dampproofing and waterproofing of foundation walls and slabs that retain earth and resist liquid water and hydrostatic pressure (see [[sync/below-grade-waterproofing]]); roofing vapor control layers furnished as part of a low-slope or steep-slope roof assembly (see [[sync/membrane-roofing]]); the thermal insulation itself, even where its facing also serves as a vapor retarder (see [[sync/building-thermal-insulation]]); and the concrete slab mix, finishing, and curing (see [[sync/cast-in-place-concrete]]).
+---
+
+# Scope {toc}
+
+## This standard covers the materials, classification, placement, and installation of vapor retarders in two distinct applications: the below-slab vapor barrier installed in contact with soil or granular fill under a concrete slab-on-grade, and the above-grade vapor retarder installed within framed wall and ceiling assemblies. {note}
+## Although both applications control the diffusion of water vapor, they are governed by different standards, classified on different scales, and placed by different logic, so the two are specified separately within this standard and shall not be confused with one another. {note}
+
+## A vapor retarder is a material that limits the rate at which water vapor diffuses through a building assembly, rated by its water-vapor permeance in perms. {note}
+## The term "vapor barrier" is used loosely in the field for the most impermeable retarders (Class I, at or below 0.1 perm), but every product in this standard is more precisely a vapor retarder of a defined class; this standard uses "below-slab vapor barrier" for the under-slab sheet by industry convention and "vapor retarder" for the above-grade material. {note}
+
+## The single most consequential decision in this standard is which side of the assembly the vapor retarder is placed on, because a retarder placed on the wrong side traps moisture against a cold surface and condenses it inside the assembly, causing exactly the damage the retarder was meant to prevent. {note}
+## For below-slab work, the governing question is instead whether the retarder is placed directly beneath the slab or below a granular cushion, a placement choice with real consequences for slab curling, bleed water, and floor-covering moisture that this standard addresses explicitly. {note}
+
+## A vapor retarder is not an air barrier, and the two shall not be substituted for one another: the vapor retarder controls diffusion of vapor through a material, while the air barrier controls bulk air leakage through gaps and laps, which carries far more moisture than diffusion ever does. {note}
+## The air barrier scope is covered by [[sync/air-barriers]] and is referenced, not duplicated, here. {note}
+
+## The below-slab vapor barrier shall not be confused with below-grade waterproofing: the under-slab sheet resists upward water-vapor diffusion from the ground, not liquid water under hydrostatic pressure, which is the scope of [[sync/below-grade-waterproofing]]. {note}
+
+## All work shall comply with the adopted edition of the International Building Code (IBC) and International Residential Code (IRC), the referenced ASTM standards and ACI guides, and the vapor retarder manufacturer's published installation instructions.
+
+# Referenced Standards {toc}
+
+## Materials, classification, testing, and installation shall comply with the latest adopted editions of the standards and codes listed below.
+## Where the contract documents, the adopted building code, the manufacturer's instructions, or a referenced standard conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
+
+| Standard | Title |
+|----------|-------|
+| IBC | International Building Code — Chapter 14 (Exterior Walls), vapor retarder provisions |
+| IRC Section R702.7 | International Residential Code — Vapor Retarders (Class I/II/III by climate zone) |
+| ASTM E1745 | Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs (Class A, B, C) |
+| ASTM E1643 | Standard Practice for Selection, Design, Installation, and Inspection of Water Vapor Retarders Used in Contact with Earth or Granular Fill under Concrete Slabs |
+| ASTM E1993 | Standard Specification for Bituminous Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs |
+| ASTM E96 / E96M | Standard Test Methods for Gravimetric Determination of Water Vapor Transmission Rate of Materials (perm rating; wet-cup and dry-cup) |
+| ASTM E154 / E154M | Standard Test Methods for Water Vapor Retarders Used in Contact with Earth under Concrete Slabs, on Walls, or as Ground Cover (conditioning) |
+| ASTM D882 | Standard Test Method for Tensile Properties of Thin Plastic Sheeting |
+| ASTM D1709 | Standard Test Methods for Impact Resistance of Plastic Film by the Free-Falling Dart Method (puncture resistance) |
+| ASTM F1249 | Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor |
+| ACI 302.1R | Guide to Concrete Floor and Slab Construction (vapor retarder placement) |
+| ACI 302.2R | Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials |
+| ASHRAE 160 | Criteria for Moisture-Control Design Analysis in Buildings |
+
+## Materials, classification, testing, and installation shall comply with the latest adopted editions of the referenced standards and codes.
+## Where the contract documents, the code, the manufacturer's instructions, or a referenced standard conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
+
+# Submittals {toc}
+
+## Action Submittals {toc}
+
+### The vapor retarder, its seam tape, and its penetration accessories must be submitted together as a coordinated set, because the seam and penetration seals are where below-slab and above-grade retarders most often fail. {note}
+### The Contractor shall submit the items listed below for the Engineer's review and return before procurement and installation.
+
+- Product data for each vapor retarder, stating the water-vapor permeance per ASTM E96 (and the resulting IBC/IRC vapor retarder class for above-grade materials, or the ASTM E1745 / E1993 class for below-slab materials), the nominal thickness in mils, and the tensile strength and puncture resistance for below-slab sheet
+- For below-slab vapor barriers: certification of the ASTM E1745 class (A, B, or C), including the permeance measured both before and after the ASTM E154 conditioning regimen
+- Product data for the matched seam tape, pipe boots, and pre-formed accessories supplied or approved by the vapor retarder manufacturer
+- For above-grade variable-permeance (smart) membranes: the dry-cup and wet-cup permeance values demonstrating the responsive permeance range
+- A sample of each vapor retarder and of the seam tape
+- A placement plan for below-slab work identifying whether the vapor barrier is placed directly beneath the slab or below a granular cushion, with the basis for that decision coordinated with the structural engineer and ACI 302.1R / 302.2R
+- The manufacturer's installation instructions for each product
+
+```datasheet
+label: Action Submittals Required
+type: checkbox
+options:
+ - "Vapor retarder product data (permeance per E96, class, thickness)"
+ - "Below-slab ASTM E1745 class certification (permeance before and after E154 conditioning)"
+ - "Matched seam tape, pipe boots, and accessory product data"
+ - "Variable-permeance (smart) membrane dry-cup and wet-cup permeance data"
+ - "Samples of each vapor retarder and seam tape"
+ - "Below-slab placement plan (direct under slab vs over granular cushion)"
+ - "Manufacturer installation instructions for each product"
+default: "Vapor retarder product data (permeance per E96, class, thickness)"
+```
+
+### The Contractor shall submit the action submittal items listed above for the Engineer's review and return before procurement and installation.
+### Installation shall not proceed until the corresponding submittals have been reviewed and returned without rejection.
+
+## Closeout Submittals {toc}
+
+### At substantial completion, the Contractor must provide the closeout records listed below before the vapor retarder work is accepted.
+
+- Daily installation and inspection records, including the ambient and substrate conditions at the time of installation
+- The pre-placement inspection record for the below-slab vapor barrier, documenting that laps, seams, penetrations, and any repairs were inspected and accepted before concrete placement
+- A record of any damage found and repaired, identifying the location and repair method
+- Manufacturer's warranty documentation for products carrying a manufacturer warranty
+
+```datasheet
+label: Closeout Submittals Required
+type: checkbox
+options:
+ - "Daily installation and inspection records (ambient and substrate conditions)"
+ - "Pre-placement inspection record for below-slab vapor barrier (before concrete)"
+ - "Record of damage found and repaired (location and method)"
+ - "Manufacturer warranty documentation where applicable"
+default: "Pre-placement inspection record for below-slab vapor barrier (before concrete)"
+```
+
+### The Contractor shall provide the closeout submittals listed above at substantial completion before the vapor retarder work is accepted.
+
+# Quality Assurance {toc}
+
+## Installer Qualifications {toc}
+
+### Vapor retarder work is simple to install but unforgiving of inattention, because a single unsealed lap or an unsealed pipe penetration in a below-slab barrier is permanently concealed once the slab is poured and cannot be reached for repair. {note}
+
+```datasheet
+label: Installer Qualification Basis
+type: radio
+options:
+ - "Experienced installer following ASTM E1643 and manufacturer instructions (standard)"
+ - "Manufacturer-trained installer for the specific below-slab system"
+default: "Experienced installer following ASTM E1643 and manufacturer instructions (standard)"
+```
+
+### The vapor retarder installer shall be experienced in the installation of the type of vapor retarder being installed and shall install it in accordance with ASTM E1643 (below-slab) or the manufacturer's instructions (above-grade).
+
+## Pre-Placement Inspection {toc}
+
+### The below-slab vapor barrier shall be inspected and accepted before any concrete is placed, because once the slab is poured no defect in the barrier can be located or corrected without demolition. {note}
+
+```datasheet
+label: Below-Slab Pre-Placement Inspection
+type: radio
+options:
+ - "Required; laps, seams, and penetrations inspected and accepted before concrete placement (standard)"
+ - "Not required (vapor barrier not specified for this slab)"
+default: "Required; laps, seams, and penetrations inspected and accepted before concrete placement (standard)"
+```
+
+### The Contractor shall not place concrete over the below-slab vapor barrier until the installed barrier, its laps and seams, and all penetration seals have been inspected and accepted.
+### Any damage to the below-slab vapor barrier discovered before concrete placement shall be repaired and re-inspected.
+
+## Moisture-Sensitive Flooring Coordination {toc}
+
+### Where the slab will receive a moisture-sensitive floor covering or coating, the vapor barrier is not optional, and its placement directly beneath the slab is the current recommendation of ACI Committee 302 precisely to protect that flooring from slab moisture. {note}
+
+### Where the slab-on-grade will receive a moisture-sensitive floor covering, adhesive, or coating, a below-slab vapor barrier conforming to ASTM E1745 shall be provided.
+### The vapor barrier placement and the slab moisture acceptance criteria shall be coordinated with [[sync/cast-in-place-concrete]] and with the flooring manufacturer's maximum allowable moisture-vapor emission rate and internal relative humidity.
+
+# Environmental and Service Conditions {toc}
+
+## The correct vapor retarder strategy depends entirely on climate, because the direction of the dominant vapor drive — and therefore the side of the assembly that must be protected — reverses between cold heating-dominated climates and hot-humid cooling-dominated climates. {note}
+## In a cold climate the vapor drive in winter is from the warm, humid interior outward, so the retarder is placed toward the interior; in a hot-humid climate the dominant drive is inward, and an interior retarder is counterproductive because it traps inward-driven moisture. {note}
+## The climate zone is the single input that governs above-grade vapor retarder class and placement, and it shall be established before any above-grade retarder is selected. {note}
+
+## Climate Zone {toc}
+
+### The climate zone governs the above-grade vapor retarder requirements throughout this standard and shall be obtained from the energy code report or the contract documents. {note}
+
+```datasheet
+label: Project Climate Zone (IECC/IRC)
+type: select
+drawing_ref: "energy code compliance report"
+options:
+ - "Zone 1-2 (hot / hot-humid)"
+ - "Zone 3 (warm)"
+ - "Zone 4 (mixed, non-marine)"
+ - "Marine 4"
+ - "Zone 5 (cool)"
+ - "Zone 6 (cold)"
+ - "Zone 7-8 (very cold / subarctic)"
+default: "Zone 4 (mixed, non-marine)"
+```
+
+### The Contractor shall obtain the project climate zone from the energy code compliance report; the climate zone governs the above-grade vapor retarder class and placement.
+
+## Moisture Design Analysis {toc}
+
+### Where assemblies are unconventional, where interior humidity is elevated, or where the dew-point location is uncertain, a hygrothermal moisture-control analysis removes the guesswork from vapor retarder selection. {note}
+
+### Where the assembly is unconventional or the building has elevated interior humidity (such as a natatorium, food-processing space, or commercial kitchen), the vapor retarder strategy shall be evaluated by a moisture-control design analysis in accordance with ASHRAE 160.
+
+# Vapor Retarder Classes and Selection {toc}
+
+## The IBC and IRC classify above-grade vapor retarders into three classes by their water-vapor permeance measured per ASTM E96, and this classification is the basis for every above-grade selection in this standard. {note}
+## Below-slab vapor barriers are classified on an entirely separate scale (ASTM E1745 Class A, B, C) based on strength and puncture resistance, not permeance, and the two classification systems shall not be cross-applied. {note}
+
+## Above-Grade Vapor Retarder Class {toc}
+
+### The vapor class is the controlling material decision for an above-grade retarder, because permeance that is too low for the climate traps moisture while permeance that is too high fails to control it. {note}
+### Class I (at or below 0.1 perm) includes polyethylene sheet and foil; Class II (greater than 0.1 to 1.0 perm) includes kraft-faced batt facing and coated membranes; Class III (greater than 1.0 to 10 perm) includes latex paint on gypsum board. {note}
+
+```datasheet
+label: Above-Grade Vapor Retarder Class
+type: radio
+options:
+ - "Class I (≤0.1 perm) — polyethylene sheet or foil facer"
+ - "Class II (>0.1 to 1.0 perm) — kraft-faced batt facer or coated membrane"
+ - "Class III (>1.0 to 10 perm) — latex/vapor-retarder paint on gypsum board"
+ - "Variable-permeance (smart) membrane — Class I/II by dry cup, >1 perm by wet cup"
+ - "None required (Climate Zone 1-3)"
+default: "Class II (>0.1 to 1.0 perm) — kraft-faced batt facer or coated membrane"
+```
+
+### Above-grade vapor retarders shall be classified by water-vapor permeance determined per ASTM E96, with Class I at or below 0.1 perm, Class II greater than 0.1 and at or below 1.0 perm, and Class III greater than 1.0 and at or below 10 perms.
+### Class I retarders shall be used only where specified, because their very low permeance can trap moisture in assemblies that experience a seasonal reversal of vapor drive.
+
+## Variable-Permeance (Smart) Membranes {toc}
+
+### A variable-permeance membrane is low-perm when the surrounding air is dry (winter, controlling outward diffusion) and opens up to a higher perm when the air is humid (summer, allowing the assembly to dry inward), which is why the IBC and IRC permit a responsive Class I or II membrane on the interior side in any climate zone. {note}
+
+### Where a variable-permeance (smart) membrane is specified, it shall be a Class I or Class II retarder by dry-cup permeance that also exhibits a water-vapor permeance greater than 1 perm by wet-cup permeance, qualifying it for interior use in any climate zone.
+
+# Below-Slab Vapor Barriers {toc}
+
+## The below-slab vapor barrier is a plastic sheet placed beneath a slab-on-grade to stop water vapor from diffusing up out of the ground, through the slab, and into the building or its floor covering. {note}
+## It is classified to ASTM E1745 by tensile strength and puncture resistance into Class A, B, and C, all three of which share the same maximum permeance of 0.1 perm; the class is therefore a measure of physical durability during installation and concrete placement, not of vapor performance. {note}
+## A thicker, higher-class sheet survives the abuse of rebar chairs, foot traffic, and concrete placement with fewer punctures, which is why higher classes and greater thicknesses are specified where the barrier must endure heavy traffic before the pour. {note}
+
+## ASTM E1745 Class {toc}
+
+### Class A is the most robust (minimum 45 lb/in tensile, 2200 g puncture), Class B is intermediate (30 lb/in, 1700 g), and Class C is the least robust (13.6 lb/in, 475 g); the permeance limit of 0.1 perm is identical for all three. {note}
+### Class A is the common specification for commercial work because the barrier must withstand significant construction traffic and reinforcement loading before the slab is placed. {note}
+
+```datasheet
+label: Below-Slab Vapor Barrier — ASTM E1745 Class
+type: radio
+options:
+ - "Class A — minimum 45 lb/in tensile, 2200 g puncture (commercial; heavy traffic before pour)"
+ - "Class B — minimum 30 lb/in tensile, 1700 g puncture (moderate-duty)"
+ - "Class C — minimum 13.6 lb/in tensile, 475 g puncture (light residential)"
+default: "Class A — minimum 45 lb/in tensile, 2200 g puncture (commercial; heavy traffic before pour)"
+```
+
+### The below-slab vapor barrier shall conform to ASTM E1745 and shall be the class specified above for the project.
+### The barrier shall have a water-vapor permeance not greater than 0.1 perm, measured per ASTM E96, both before and after the conditioning regimen of ASTM E154.
+
+## Thickness {toc}
+
+### The sheet thickness is specified in mils, and although ASTM E1745 classifies by performance rather than thickness, the market correlates the two: a 15-mil sheet is the common commercial Class A choice, while heavy-duty applications use 20 mil and light residential work uses 10 mil. {note}
+
+```datasheet
+label: Below-Slab Vapor Barrier — Nominal Thickness
+type: range
+unit: mil
+options:
+ min: 10
+ max: 20
+ setpoints: [10, 15, 20]
+default: 15
+```
+
+### The below-slab vapor barrier shall be not less than the specified nominal thickness, and shall meet the specified ASTM E1745 class regardless of thickness.
+
+## Material Type {toc}
+
+### Most below-slab barriers are multi-layer polyolefin or high-density polyethylene plastic sheet (ASTM E1745); a reinforced bituminous membrane (ASTM E1993) is an alternate where its higher puncture resistance is wanted. {note}
+
+```datasheet
+label: Below-Slab Vapor Barrier — Material Type
+type: radio
+options:
+ - "Plastic sheet (polyolefin / HDPE) per ASTM E1745 (standard)"
+ - "Reinforced bituminous membrane per ASTM E1993"
+default: "Plastic sheet (polyolefin / HDPE) per ASTM E1745 (standard)"
+```
+
+### The below-slab vapor barrier shall be a plastic sheet conforming to ASTM E1745, or a bituminous membrane conforming to ASTM E1993 where specified.
+
+## Placement Relative to Slab and Cushion {toc}
+
+### Whether the vapor barrier is placed directly beneath the slab or below a granular cushion is a genuine engineering debate, and the right answer depends on the floor covering and on the contractor's ability to manage bleed water. {note}
+### Placing the barrier directly under the slab gives the greatest protection to a moisture-sensitive floor covering and prevents rain, saw-cut slurry, curing water, or cleaning water from being trapped in a fill course beneath it; this is the current ACI 302 recommendation where the slab receives a vapor-sensitive covering. {note}
+### The drawback of direct contact is that the impermeable sheet prevents downward bleed water from escaping, so all bleed water rises to the surface; if it cannot evaporate before finishing, the trapped water can cause surface delaminations, blisters, and increased curling at slab edges. {note}
+### Placing the barrier below a granular cushion (the "blotter" layer) lets the slab bleed and drain downward, reducing curling, but the cushion itself can become a reservoir that traps construction water against the slab underside and re-emits it for months, which is why ACI 302 advises against a blotter beneath slabs that will receive moisture-sensitive flooring. {note}
+
+```datasheet
+label: Below-Slab Vapor Barrier — Placement
+type: radio
+options:
+ - "Directly beneath the slab (in contact with slab underside) — ACI 302 recommendation where slab receives moisture-sensitive flooring (standard)"
+ - "Below a granular cushion / blotter layer — only where no moisture-sensitive flooring and curling control governs"
+default: "Directly beneath the slab (in contact with slab underside) — ACI 302 recommendation where slab receives moisture-sensitive flooring (standard)"
+```
+
+### Where the slab will receive a moisture-sensitive floor covering, adhesive, or coating, the vapor barrier shall be placed in direct contact with the underside of the slab, with no granular cushion between the barrier and the slab.
+### Where the vapor barrier is placed in direct contact with the slab, the Contractor shall manage bleed water and shall not begin finishing operations until surface bleed water has dissipated, to prevent delaminations and blisters.
+### A granular cushion shall not be placed between the vapor barrier and a slab that will receive a moisture-sensitive floor covering.
+
+# Wall and Ceiling Vapor Retarders {toc}
+
+## The above-grade vapor retarder is placed within a framed wall or ceiling assembly to limit vapor diffusion through the assembly, and its class and its side of placement are both governed by the climate zone. {note}
+## The governing rule is that the retarder goes on the warm-in-winter side of the insulation in heating-dominated climates, so that interior humidity cannot diffuse outward and condense on the cold sheathing behind the insulation. {note}
+## The IRC and IBC set the class required by climate zone, and this standard records that requirement; the design professional remains responsible for confirming the assembly does not trap moisture given its specific insulation and cladding. {note}
+
+## Requirement by Climate Zone {toc}
+
+### Climate Zones 5, 6, 7, 8, and Marine 4 require a Class I or Class II vapor retarder on the interior side of frame walls, because their long, cold heating seasons drive interior moisture outward where it will condense without a retarder. {note}
+### Climate Zones 1, 2, and 3 do not require an interior vapor retarder, and one is generally counterproductive there because the dominant vapor drive is inward; Zone 4 (non-marine) permits Class I, II, or III. {note}
+
+```datasheet
+label: Above-Grade Vapor Retarder — Requirement by Climate Zone
+type: radio
+options:
+ - "Class I or II required on interior side (Zones 5, 6, 7, 8, and Marine 4)"
+ - "Class I, II, or III permitted on interior side (Zone 4 non-marine)"
+ - "Not required (Zones 1-3); Class III permitted with no special conditions"
+drawing_ref: "energy code compliance report"
+default: "Class I or II required on interior side (Zones 5, 6, 7, 8, and Marine 4)"
+```
+
+### In Climate Zones 5, 6, 7, 8, and Marine 4, a Class I or Class II vapor retarder shall be provided on the interior side of frame walls, except where a variable-permeance membrane or a continuous-insulation ratio permits otherwise per the IRC.
+### In Climate Zones 1, 2, and 3, an interior vapor retarder shall not be provided on frame walls except where specifically required by the design professional, because it traps inward-driven moisture in cooling-dominated climates.
+
+## Placement Side {toc}
+
+### Placing the retarder on the wrong side of the insulation is the most damaging error in above-grade vapor control, because it converts a moisture-control measure into a condensation trap. {note}
+
+```datasheet
+label: Above-Grade Vapor Retarder — Placement Side
+type: radio
+options:
+ - "Interior side of insulation (warm-in-winter face) — heating-dominated climates (standard zones 4-8)"
+ - "Exterior continuous insulation keeps sheathing above dew point; interior Class III permitted"
+ - "Variable-permeance membrane on interior side (responsive; any climate zone)"
+default: "Interior side of insulation (warm-in-winter face) — heating-dominated climates (standard zones 4-8)"
+```
+
+### The above-grade vapor retarder shall be placed on the warm-in-winter (interior) side of the insulation in heating-dominated climates, unless sufficient exterior continuous insulation keeps the sheathing above the dew point or a variable-permeance membrane is used.
+### A vapor retarder shall not be installed on the exterior side of the insulation in a heating-dominated climate where it would trap interior moisture against a cold plane.
+
+## Material {toc}
+
+### The retarder may be the integral facing of the cavity batt, a separate polyethylene sheet, a variable-permeance membrane, or a vapor-retarder paint, each appropriate to a different assembly. {note}
+
+```datasheet
+label: Above-Grade Vapor Retarder — Material
+type: radio
+options:
+ - "Kraft-faced batt facing (Class II, integral to cavity insulation)"
+ - "Polyethylene sheet (Class I, 6-mil, sealed to framing)"
+ - "Variable-permeance (smart) membrane (responsive Class I/II)"
+ - "Vapor-retarder (Class III) latex paint on gypsum board"
+default: "Kraft-faced batt facing (Class II, integral to cavity insulation)"
+```
+
+### The above-grade vapor retarder material shall be of the specified class and shall be installed continuously on the specified side of the assembly.
+### Where the vapor retarder is the integral facing of cavity insulation, the facing shall face the conditioned space side of the assembly in heating-dominated climates, coordinated with [[sync/building-thermal-insulation]].
+
+## Avoiding Double Vapor Barriers {toc}
+
+### Two low-perm layers on opposite faces of an assembly trap any moisture that gets between them with no path to dry in either direction, which is why an assembly shall never be built with a vapor retarder on both sides. {note}
+
+### The assembly shall not contain a Class I or Class II vapor retarder on both the interior and exterior faces, because a double vapor barrier traps moisture between the two layers with no drying path.
+### Where the exterior face already includes a low-permeance layer (such as foil-faced sheathing or closed-cell spray foam), the interior vapor retarder class shall be reduced or omitted so the assembly can dry inward.
+
+# Seams, Laps, and Penetration Sealing {toc}
+
+## The field membrane rarely fails; the failures concentrate at laps, at seams, and at penetrations, exactly the locations that are smallest in area and most often rushed. {note}
+## For the below-slab barrier these seals are permanently concealed by the slab, so they must be made correctly the first time; for the above-grade retarder they must be made before the finishes close the wall. {note}
+
+## Below-Slab Laps and Seams {toc}
+
+### A taped lap of adequate width is what makes the separate sheets of a below-slab barrier behave as one continuous membrane; an untaped or narrow lap is an open seam through which ground vapor bypasses the entire barrier. {note}
+
+```datasheet
+label: Below-Slab Vapor Barrier — Minimum Lap Width
+type: select
+unit: in.
+options:
+ - "6 in. minimum lap, taped (ASTM E1643)"
+ - "Per manufacturer's published lap, where greater than 6 in."
+default: "6 in. minimum lap, taped (ASTM E1643)"
+```
+
+### Below-slab vapor barrier seams shall be lapped not less than 6 in. and sealed continuously with the manufacturer's seam tape or an approved equivalent method, in accordance with ASTM E1643.
+### Seam tape shall be applied to clean, dry sheet so that the seal is continuous and free of voids.
+
+## Below-Slab Penetration Sealing {toc}
+
+### Every pipe, conduit, reinforcing bar, and footing that pushes up through the below-slab barrier is a hole in the membrane, and each one must be sealed tight to the penetrating element or the barrier leaks at that point. {note}
+
+```datasheet
+label: Below-Slab Penetration Sealing Method
+type: radio
+options:
+ - "Pre-formed pipe boot sealed and taped to barrier at pipe and round penetrations (standard)"
+ - "Membrane collar cut, fitted, and taped to penetration per ASTM E1643"
+default: "Pre-formed pipe boot sealed and taped to barrier at pipe and round penetrations (standard)"
+```
+
+### Every penetration of the below-slab vapor barrier — pipes, conduits, ducts, reinforcing, and footings — shall be sealed continuously to the barrier with a pre-formed boot or a fitted membrane collar sealed with the manufacturer's tape, in accordance with ASTM E1643.
+### The barrier shall be sealed to the foundation wall, grade beam, or footing at its perimeter so the vapor control plane is continuous from the slab to the foundation.
+
+## Above-Grade Seams and Penetrations {toc}
+
+### The above-grade retarder is only as continuous as its seams and the seals around boxes and pipes, and a torn or unsealed retarder is no retarder at all over that area. {note}
+
+### Above-grade vapor retarder sheet seams shall be lapped a minimum of 6 in. over a framing member or supported backing and sealed with a compatible tape or sealant.
+### Penetrations of the above-grade vapor retarder for electrical boxes, pipes, and mechanical connections shall be sealed with compatible materials per the manufacturer's instructions.
+### Vapor retarder material that is cut, torn, or punctured during installation shall be repaired before the assembly is closed, lapping the repair at least 6 in. onto sound material.
+
+# Installation {toc}
+
+## Below-Slab Installation {toc}
+
+### A below-slab barrier installed over a sharp or rutted subgrade punctures under the first foot traffic, so the surface it is laid on matters as much as the sheet itself. {note}
+
+### The subgrade or granular fill receiving the below-slab vapor barrier shall be level, compacted, and free of sharp projections, debris, and standing water before the barrier is placed.
+### The below-slab vapor barrier shall be laid out so that sheets are lapped, taped, and sealed into one continuous membrane, with the barrier turned up and sealed at all perimeter foundations.
+### The Contractor shall keep traffic off the installed barrier to the extent practicable and shall repair any puncture or tear before concrete placement.
+### Concrete shall be placed on the barrier in a manner that does not displace, tear, or puncture it.
+
+## Above-Grade Installation {toc}
+
+### The above-grade retarder must be installed continuously and sealed at every edge, because gaps at the top plate, bottom plate, and openings are diffusion bypasses just as seams are. {note}
+
+### The above-grade vapor retarder shall be installed continuously on the specified side of the assembly, lapped and sealed at all seams, and sealed at the top plate, bottom plate, and around all openings.
+### Where a kraft or foil facing serves as the vapor retarder, adjacent batts shall be installed with the facing flanges lapped and stapled to provide a continuous retarder plane.
+### The above-grade vapor retarder shall not be installed over wet framing or wet sheathing.
+
+# Inspection and Testing {toc}
+
+## Below-Slab Pre-Pour Inspection {toc}
+
+### This inspection is the only opportunity to verify the below-slab barrier, because every defect becomes permanently inaccessible the moment concrete is placed; it is a hold point, not a punch-list item. {note}
+
+### The installed below-slab vapor barrier shall be inspected for continuity, taped laps, sealed penetrations, sealed perimeter, and the absence of unrepaired punctures before concrete is placed.
+### The Engineer or the designated inspector shall accept the below-slab barrier as a hold point before the pour proceeds.
+
+## Above-Grade Inspection {toc}
+
+### The above-grade retarder is verified by visual inspection before the wall is closed, confirming the right class is on the right side and that seams and penetrations are sealed. {note}
+
+### The above-grade vapor retarder shall be inspected for correct class, correct side of placement, continuous seams, and sealed penetrations before the assembly is concealed by gypsum board, cladding, or finishes.
+### Any deficiency found shall be repaired before the assembly is closed.
+
+# Delivery, Storage, and Handling {toc}
+
+## Vapor retarder rolls are easily damaged by sunlight, abrasion, and crushing, and a roll torn or degraded in storage carries those defects into the assembly. {note}
+
+## Vapor retarder materials shall be delivered in the manufacturer's original, labeled packaging and rolls.
+## Vapor retarder materials shall be stored off the ground, under cover, and protected from prolonged sunlight, mechanical damage, and abrasion until installation.
+## Seam tape and accessories shall be stored within the temperature range specified by the manufacturer so that the adhesive performs as intended.
+## Any roll that is torn, punctured, or UV-degraded in storage shall be repaired or discarded, not installed.
+
+# Warranty {toc}
+
+## Vapor retarders are typically warranted for material defects, and the Contractor shall provide the manufacturer's standard warranty along with the installation warranty for the work. {note}
+
+```datasheet
+label: Vapor Retarder Manufacturer Warranty
+type: select
+unit: years
+options:
+ - "1 year"
+ - "5 years"
+ - "Per manufacturer's standard product warranty"
+default: "Per manufacturer's standard product warranty"
+```
+
+## The Contractor shall provide the vapor retarder manufacturer's standard warranty for the products furnished.
+## The Contractor shall warrant the vapor retarder installation against defective seams, laps, and penetration seals for a period of not less than one year from substantial completion, or for the period stated in the contract documents if longer.
+## Any moisture damage to an assembly attributable to a defective vapor retarder seam, lap, or penetration seal, or to a vapor retarder installed on the wrong side of the assembly, shall be corrected at the Contractor's expense.