Redblock Contractor Resource
Vapor Permeance of Shower Wall Materials
A source-linked reference for contractors comparing the water-vapor permeance of shower membranes, tile backer boards, and common materials used in exterior shower-wall assemblies.
Start here
Perm ratings are useful—but only when you know what was tested
A perm is a measure of water-vapor permeance: how readily water vapor passes through a specific material or product at a specific thickness under specified test conditions. Lower numbers mean greater resistance to vapor diffusion.
ASTM E96/E96M includes different test procedures and environmental conditions, and ASTM explicitly warns that a result obtained under one set of conditions cannot be used to predict the result under another. Thickness also matters. This resource therefore preserves the manufacturer-reported method, thickness, and threshold language wherever available.
For exterior shower walls, vapor control should be considered as part of the entire wall assembly—including climate, insulation, exterior sheathing, air control, drying direction, and the waterproofing layer immediately behind tile.
Reference classifications
How to read a perm rating
Class I
Vapor impermeable / vapor barrier in the common building-science classification.
Class II
Vapor semi-impermeable; substantially slows diffusion while retaining more drying potential than Class I.
Class III
Vapor semi-permeable. Many conventional building materials fall in this range.
Vapor permeable
Relatively open to water-vapor diffusion.
Classification thresholds follow Building Science Corporation's BSD-106 / Info-311 framework. Product suitability still depends on the complete assembly and application.
Master comparison
Manufacturer-published vapor permeance values for shower materials
This table prioritizes manufacturer technical data and retains the reported ASTM E96 method instead of normalizing unlike tests into a false ranking.
17 entries shown
Scroll horizontally to view all table columns.
| Product / material | Category | Thickness tested / reported | Reported permeance | ASTM E96 method / conditions | Classification* | Source / notes |
|---|---|---|---|---|---|---|
| Wedi Vapor 85 | Foam backer board | Complete assembly with specified seam / fastener treatment | 0.03 perms | ASTM E96, Procedure E | Class I |
Wedi 2026 Product Handbook Wedi 2026 technical literature reports 0.03 perm for the Vapor 85 assembly with the specified seam and fastener treatment. |
| Wedi Subliner Dry | Sheet membrane | Manufacturer system membrane | 0.05 perms | ASTM E96, Procedure E | Class I |
Wedi 2026 Product Handbook Corrected from the earlier 0.09 entry. Wedi technical literature reports an average permeance of approximately 0.051 perm. |
| ARDEX TLT™ Waterproofing Membrane | Sheet membrane | 5 mil | 0.051 perms | ASTM E96, Procedure E, 90% RH | Class I |
ARDEX Americas Manufacturer-published product value. |
| LATICRETE HYDRO BAN® Sheet Membrane | Sheet membrane | Not stated on product page | 0.06 perms | ASTM E96/E96M, Procedure E | Class I |
LATICRETE Manufacturer-published product value. |
| NobleSeal® TS | Sheet membrane | 0.030 in. | 0.15 perms | ASTM E96/96M, Procedure E, 90% RH | Class II |
Noble Company Manufacturer-published product value. |
| Schluter®-KERDI-DS | Sheet membrane | 20 mil | 0.19 perms | ASTM E96, desiccant / Procedure E, 90% RH | Class II |
Schluter Systems Schluter shower-system handbook reports 0.19 perms; some older pages report 0.18. |
| Mapelastic® AquaDefense | Liquid membrane | About 20 mil dry minimum | 0.35 perms | ASTM E96, Method E | Class II |
MAPEI Current MAPEI product data reports 0.35 perms. |
| RedGard® Waterproofing and Crack Prevention Membrane | Liquid membrane | 30 mil dry | 0.35 perms | ASTM E96, Method E | Class II |
Custom Building Products Value depends on cured film thickness; manufacturer reports 0.35 perms at 30 mil dry. |
| Schluter®-KERDI-BOARD | Foam backer board | 1/2 in. (12.5 mm) | 0.36 perms | ASTM E96, desiccant method, 100°F / 90% RH | Class II |
Schluter Systems Thickness-specific board value. |
| USG Durock™ Liquid Waterproofing and Crack Isolation Membrane | Liquid membrane | 20 mil dry | 0.38 perms | ASTM E96, Procedure E | Class II |
USG Manufacturer-published product value. |
| Noble AquaSeal® | Sheet membrane | Manufacturer sheet membrane | < 0.5 perms | ASTM E96/96M, Procedure E, 90% RH | Class II or better |
Noble Company Threshold value, not an exact permeance. |
| Schluter®-KERDI | Sheet membrane | 8 mil | 0.90 perms | ASTM E96, Procedure E, 90% RH | Class II |
Schluter Systems Manufacturer-published product value. |
| Johns Manville GoBoard® | Foam backer board | Current listed board thicknesses | < 1 perms | ASTM E96; procedure not stated in data-sheet table | Class II or better |
Johns Manville Threshold value across the listed GoBoard thicknesses; not directly comparable to Procedure E values unless conditions match. |
| LATICRETE HYDRO BAN® liquid membrane | Liquid membrane | 20–30 mil dry | 1.247 perms | ASTM E96-00e1, Procedure B | Class III |
LATICRETE product data sheet Different ASTM E96 procedure from most steam-shower Procedure E values; do not rank directly without noting test conditions. |
| Georgia-Pacific DensShield® Tile Backer | Faced backer board | Panel alone | < 1.5 perms | ASTM E96; conditions not stated in cited submittal excerpt | Exact class unresolved from <1.5 threshold |
Georgia-Pacific Gypsum Manufacturer submittal reports less than 1.5 perms for panel alone and 1.0 perm with dry-set mortar. |
| PermaBASE® Cement Board | Cement board | 1/2 in. and 5/8 in. | > 10 perms | ASTM E96 | Vapor permeable |
PermaBASE / National Gypsum product data Current product data reports greater than 10 perms for both listed thicknesses. |
| Wedi Building Panel | Foam backer board | Manufacturer board system | Not reported | Water-vapor transmission listed as N/A in Wedi technical literature | Not classified |
Wedi 2026 Product Handbook Wedi technical literature lists water-vapor transmission as N/A. Waterproofness tests are reported separately; no numerical perm value is assigned here. |
*Classification is assigned from the reported value using the reference thresholds above. Threshold values such as “<1” or “>10” are not exact measurements. The Class labels are for orientation, not a substitute for the manufacturer's application requirements.
Most steam-shower membrane values in the table use ASTM E96 Procedure E / desiccant-type conditions, but not all do. For example, LATICRETE's published 1.247-perm HYDRO BAN liquid value is Procedure B. That number should not be interpreted as a head-to-head ranking against Procedure E values without accounting for the different test conditions.
Visual reference
Representative Procedure E / desiccant-type values on a logarithmic scale
This visual includes exact manufacturer-published values that are reasonably similar in test approach. Threshold-only entries, the Procedure B HYDRO BAN liquid result, and Wedi Building Panel are intentionally omitted.
Visual position is logarithmic because permeance values span orders of magnitude. It is a communication aid, not a substitute for the full test-method table.
Beyond the waterproofing layer
Common exterior-wall materials and vapor-control context
An exterior shower wall is an assembly. These reference entries help place the shower membrane or backer board in the context of framing-cavity and exterior-side materials.
Scroll horizontally to view all table columns.
| Material | Category | Thickness / context | Vapor behavior | Basis | Classification | Source note |
|---|---|---|---|---|---|---|
| Solid sheet steel (material reference) | Solid metal | Any practical sheet thickness | Vapor-impermeable; steel permeability = 0 | Material physics / building-science reference | Vapor impermeable |
Building Science Corporation Building Science Corporation states that the permeability of steel is zero. This is not an ASTM E96 product test. |
| 6-mil polyethylene / sheet polyethylene | Vapor-control layer | Typical sheet | Class I; ≤ 0.1 perm by classification | Building-science classification | Class I |
Building Science Corporation BSC identifies polyethylene as a Class I vapor retarder. Exact product values vary. |
| Kraft-faced fiberglass batt | Insulation facing | Assembly-dependent | Class II; > 0.1 to ≤ 1 perm by classification | Building-science classification | Class II |
Building Science Corporation BSC identifies kraft-faced fiberglass as a Class II vapor retarder. |
| Latex-painted gypsum board | Interior finish layer | Assembly-dependent | Class III; > 1 to ≤ 10 perms by classification | Building-science classification | Class III |
Building Science Corporation BSC uses latex-painted gypsum board as a Class III example. |
| 1 in. extruded polystyrene (XPS) | Rigid insulation | 1 in. or thicker | ≤ 1 perm | Building-science reference | Class II or lower |
Building Science Corporation BSC notes that XPS at 1 inch or thicker has a perm value of 1.0 or less. |
| Plywood / OSB sheathing | Wood structural panel | Assembly-dependent | > 1 perm under wet-cup testing | ASTM E96 wet-cup context | Class III or more open |
Building Science Corporation BSC notes plywood and OSB can exceed 1 perm in wet-cup testing; humidity sensitivity matters. |
| Non-perforated foil- or polypropylene-faced rigid insulation | Faced rigid insulation | Product-dependent | < 0.1 perm | Building-science reference | Class I |
Building Science Corporation BSC identifies non-perforated foil and polypropylene-faced rigid insulations as below 0.1 perm. |
Contractor interpretation
How to use this information on an exterior shower wall
1Identify the vapor-control layer behind the tile
Start with the actual waterproofing membrane or waterproof backer board, not the tile. Tile and grout are finish materials; the dedicated waterproofing/vapor-control layer is what should carry the moisture-management design intent.
2Check the exact test method and thickness
A 0.35-perm value at a specified dry-film thickness is not interchangeable with a board value at another thickness or a result obtained under another ASTM E96 procedure.
3Consider the exterior side of the wall
Low-permeance exterior insulation, foil facers, membranes, or other vapor-closed layers reduce outward drying. The interior shower-side layer therefore cannot be evaluated in isolation.
4Preserve a safe drying direction
Building Science Corporation cautions against “double vapor barriers” because assemblies need a practical drying path. A very low-perm shower-side layer can be appropriate, but the complete enclosure should be reviewed rather than stacking vapor-closed layers by habit.
5Separate vapor diffusion from air leakage
Perm ratings describe diffusion through materials. They do not measure air leakage through seams, electrical boxes, framing gaps, or penetrations. Air control and vapor control are related but different design functions.
6Treat solid metal differently from membranes
Building-science literature describes steel permeability as zero. A solid sheet-metal enclosure therefore does not provide a vapor-diffusion path through its body, but perimeter joints and transitions still have to be integrated into the surrounding wall's waterproofing and vapor-control strategy.
Lower permeance is not automatically better in every climate or assembly. Climate zone, indoor humidity, exterior insulation, sheathing, cladding, air conditioning, and available drying direction all matter. For unusual assemblies or high-risk projects, use a building-envelope professional rather than selecting materials from perm values alone.
Methodology
Why two “ASTM E96” numbers may still not be directly comparable
ASTM E96/E96M-24a is a gravimetric water-vapor-transmission standard with multiple test conditions. ASTM states that a result obtained in one method under one set of conditions cannot be used to predict the result under different conditions or another method.
- Procedure matters. Dry-cup / desiccant conditions and wet-cup / water-method conditions can produce different results.
- Relative humidity matters. Some materials become more vapor-open as humidity rises.
- Temperature matters. Vapor pressure and material behavior change with test temperature.
- Thickness matters. Product permeance is tied to the tested thickness; do not reuse a board or coating value at another thickness without manufacturer support.
- Thresholds are not exact values. “<1 perm” may mean 0.95, 0.5, or far less. This resource never converts a threshold into a pretend exact measurement.
Scope and limitations
What this resource includes—and what it intentionally does not
Included: manufacturer-published permeance values for products commonly used in tiled showers, plus authoritative building-science references for common wall-assembly materials.
Not included as primary comparison values: isolated tile, grout, thinset, or sealant permeance. Those materials are highly installation-dependent and do not by themselves represent the continuity of the wall's intended waterproofing/vapor-control layer.
No value was inferred from marketing language. If a manufacturer says “low perm” but does not publish a numerical value in an accessible technical source, the product should not be assigned a number here until a source is located.
No endorsement is implied. A lower perm rating does not make one product universally superior; installation requirements, waterproofing performance, substrate compatibility, movement accommodation, service conditions, and system warranties remain separate considerations.
Why Redblock publishes this
A contractor resource first
Redblock Industries manufactures stainless-steel shower products, and publishes instructional articles on their website to help contractors and homeowners build better, safer showers. The purpose of this page is to make published vapor-permeance data easier to compare without stripping away the test conditions that give the numbers meaning.
The stainless steel material reference in the table above refers only to Redblock's own stainless shower niche, made from solid 1.2 mm 316L stainless-steel, a material considered to be vapor-impermeable according Building Science Corporation. This reference does not imply that sheet stainless steel products are used in any currently available shower wall waterproofing membrane system, as far as the publishers are aware.
Quick answers
Vapor permeance FAQ
Is a lower perm rating always better for a shower wall?
Yes, but the entire wall assembly and the exterior climate must be considered when deciding what materials should be used in the wall layers, and the order of these layers.
Is a waterproof shower membrane automatically a vapor barrier?
No. Waterproofing membranes are designed primarily to prevent liquid water migration, and vary drastically in their ability to manage water vapor transmission. Schluter notes that ANSI A118.10 is a liquid-water waterproofing standard and does not establish a universal vapor-permeance requirement.
Can I compare every ASTM E96 value directly?
No. ASTM E96 includes multiple methods and environmental conditions. Compare the procedure, RH, temperature, and tested thickness before treating two numbers as equivalent.
What perm rating is considered vapor-impermeable?
In the Building Science Corporation classification used on this page, 0.1 perm or less is vapor-impermeable / Class I.
Does cement board stop water vapor?
No. Even the most "waterproof" cement board available on the market is rated based on ANSI A118.10 which tests only liquid water penetration/ migration through the material. This standard does not involve any water vapor transmission testing.
Primary references
Core methodology and building-science sources
- ASTM E96/E96M-24a — Standard Test Methods for Gravimetric Determination of Water Vapor Transmission Rate of Materials
- Building Science Corporation — BSD-106: Understanding Vapor Barriers
- Building Science Corporation — Info-311: Vapor Open Assemblies
- Building Science Corporation — CP-1101: The Influence of Low-Permeance Vapor Barriers on Roof and Wall Performance
- Schluter Systems — Tiled Shower Waterproofing: note regarding water vapor
Every product row in the master table identifies its manufacturer source. Product data can change; Redblock should re-check manufacturer documents periodically and update the verification record only when that review has actually been completed.
Resource history
Verification and Revision Record
| Date | Verification / revision |
|---|---|
| September 9, 2026 | Initial publication with 15 product-specific entries and seven broader wall-assembly material references. |
| September 10, 2026 | Added Wedi Vapor 85 and Wedi Building Panel; corrected Wedi Subliner Dry permeance from the earlier 0.09 entry to 0.05 perm (average approximately 0.051). The full dataset was not reverified on this date. |