The rationale for non-permeable air barriers (fluid-applied or self-adhered sheet) is based on a number of factors. The most notable of these is the Code-driven mandate for exterior continuous insulation in many climate zones. However, building use and seemingly unlikely climatic regions may also employ non-permeable air barriers. The latter point could be brought about through the use of hygrothermal wall analyses.
For our climate zone (5), the IBC (Code) stipulates a minimum R-7.5 of exterior continuous insulation (c.i.) for steel-framed buildings. The most commonly-used c.i. is extruded polystyrene (XPS) which typically exhibit R-5.0 at one inch. To achieve the minimum 7.5 required by Code, designers naturally opt for 1-1/2-inch XPS, which naturally comes out to R-7.5.
At one inch, XPS is vapor-semi-permeable – meaning, it is vapor diffuse open, or “permeable”. At 1-1/2 inch and great thickness, however, XPS becomes vapor-retarding (i.e. a “vapor barrier”1). Polyisocyan- urate insulation (poly-iso) and closed-cell spray foam (SPF) at one inch and greater also is vapor-retarding. Both XPS and poly-iso are closed-cell types2,3. Both aforementioned insulation types at the noted specific thicknesses fall into the realm of a Class II vapor retarder, as defined in Chapter 2 of the Code.
Un-faced glass batt insulation used between framing members is highly permeable (est. 40-60 perms). As such, it easily allows the movement of water vapor – i.e. high drying potential. However, this drying capability only happens when it is allowed to “breathe” to either the interior or exterior. When vapor-retarding exterior c.i. is used, drying is generally directed towards the interior. The key to this configuration is to consider that interior conditioned environments are kept at a general constant temperature and humidity. Any residual moisture (humidity) at the interstitial space between framing members is “dehumidified” towards the interior by mechanical equipment.
As a parallel to the previously-mentioned information, a paraphrased reference of Part 1405.3.2 from the 2015 Code states when vapor-retarding c.i. is used, interior drywall and its finishes should be vapor-diffuse open. In essence, it is saying to avoid the ‘double vapor barrier’ condition by not using interior-side drywall plastic/poly vapor retarders (‘vapor barriers’) behind the interior drywall. Nor should oil or epoxy-based paint or non-permeable (vinyl) wallpaper get used. Permeable interior wall finishes such as latex (“breather”) paint and permeable wallpaper are advised when the exterior c.i. is vapor-retarding. This then allows the space between framing to become an extension of the interior environment – thus allow everything from the exterior sheathing-inward to dry.
The decision to apply all insulation outboard of the primary backup or to consider the “hybrid” wall of exterior c.i. + batt insulation between framing members does not affect Part 1405.3.2 of the Code – when exterior c.i. has a permeance less than 1.
Air Barrier – Permeable or Non-Permeable?
The cheeky response to the question of which type of air barrier to use (permeable vs. non-permeable) is: it depends. Air barrier permeability (choice) depends upon many factors including but limited to Code, geographic location, building occupancy use, interior conditions and most importantly – material composition. In some cases, ‘traditional’ fluid-applied or self-adhered air barriers may not be needed at all. Typical examples of the latter include stucco, EIFS and precast.
Part 1404.3.1 of the Code indicates Class I or II vapor retarders shall be placed on the interior side of framed walls in [climate] Zones 5 and up (and Marine 4). Class I and II vapor retarders are, for all intents, considered non-permeable as defined by Code. Where non-permeable c.i. (i.e. Class II vapor retarder) becomes the overall majority thermal layer, the primary backup wall is now situated above the dew point and on the interior of the structure. As such, the exterior c.i. essentially complies with Part 1404.3.1 of the Code as a Class II vapor retarder as its perm rating is less than 1.
Now, consider when that vapor-retarding c.i. is married to a vapor-retarding air barrier. Both combined materials create a composite consisting of all four control layers: air, water, water vapor and thermal. This ‘composite’ assembly now represents the complete environmental separation of interior and exterior environments.
A minimal layer of c.i. at or less than 1 inch –specifically XPS – is likely to be combined with batt insulation between framing members. At 1 inch or less, XPS is a Class III vapor retarder, qualifying it as a permeable material. In this instance, the air barrier of choice is would also be permeable. Permeable air barriers are typically applied on the cold-in-winter side of the primary thermal layer. For this wall assembly, interior-side poly vapor retarders will likely get used to comply with the Code.
For wall assemblies with approximately equal amount (values) of insulation outboard and inboard, it may be suggested to have a hygrothermal wall analysis performed. These programs, while very detailed, do not provide the type of air barrier to use (permeable vs non-permeable) nor are they evidence that the [wall] system works. They also do not account for building materials as they age. Material properties such as air and water vapor permeance can change over time.
From available data4, walls without exterior c.i. in mixed-humid, hot-humid, mixed dry and hot-dry climates commonly employ permeable air barriers. Everything else remaining the same, add a poly vapor retarder (‘vapor barrier’) beneath the interior drywall and this wall finds appropriate use in climate zones 5 and 6 (cold and very cold).
Air barriers are, and should always be considered the last line of defense against air, water and/or water vapor infiltration. As such, air barriers always installed on the structural back-up wall – whether it’s CMU or wood or gypsum sheathing over framing. Almost. In some instances, a structural wood panel (typically plywood or OSB) is designed in a bonded sheathing-and-exterior-c.i. ‘sandwich’ as a means for cladding attachment5. This design method obviates the need for sheathing behind (attached to the back-up wall). Manufacturers of these sheathing/insulation products naturally identify vapor-permeable air barriers for application to the exterior face of the sheathing. This, like the stucco and EIFS examples above, is also in keeping with placement of the ‘permeable air barrier on the cold-in-winter side of the insulation’.
Drying Potential of Moisture-Sensitive Materials
Water vapor drive (“vapor drive”) occurs in both directions within wall systems – exterior-towards-interior (typ. Summer); interior-towards exterior (typ. Winter). The greater the temperature and water vapor delta between interior conditioned environments and the exterior of buildings, the stronger the drive to the area of lower pressure.
As many structures employ metal framing and exterior gypsum sheathing, designers ought to heed gypsum’s relative fragility6 to long-term moisture exposure. The more exterior c.i. is used, the more exterior gypsum sheathing becomes relegated to the center-most portion (centerline in section) of a wall system. One building envelope consultant of the author’s experience suggests inward vapor drive through a vapor-permeable air barrier membrane facilitates moisture availability into the sheathing. Recognized via the use of hygrothermal wall analysis (“WUFI”), the drying potential of this now wetted and moisture-sensitive gypsum panel does not take place over days or weeks. His research discovered, in numerous examples in differing climate zones, that drying can take as much as several months or more. Climate zones for these studies varied anywhere from 3-6. This is one prime example of why gypsum panels – even water-resistant ones – should be avoided for use in natatoriums. They will ultimately succumb to degradation (disintegration) over time, given the above-referenced long drying time horizon. In these assemblies where the majority thermal layer occurs as exterior c.i., keeping the interior surface of the c.i. above the dew point is extremely important at maintaining the [wall] system’s drying potential.
The seminal article Understanding Vapor Barriers appears to parallel these findings. Figure 1, 5 and 7 in the article indicate the use of a vapor-retarding (non-permeable) air barrier in both all exterior c.i. and hybrid wall assemblies noted above. This article states walls with all their thermal insulation outboard have “applicability in all hygro-thermal regions”. The ‘hybrid wall’ with exterior c.i. + batt insulation states “applicability in all hygro-thermal regions except sub-arctic and arctic”. Vapor-retard- ing air barriers find use in Northern climates as well as mixed-humid and hot-humid regions. One of the key reasons is that the dew point collection occurs at, around or generally within the primary thermal layer. If the primary or most of the thermal layer is on the outside of the structure, the backup wall is kept above the dew point – i.e. largely avoiding condensation. For this wall, what little moisture does occur [within the vapor-retarding insulation and vapor-retarding air barrier] – is kept on the outside of the structure – which is where we want to keep all free moisture.
Can or should a vapor-permeable air barrier be used where a vapor-retarding thermal layer is applied overtop? Permeable air barriers have been used far longer than vapor-retarding air barriers. However, materials technology has continued to advance as have Code requirements (e.g. exterior c.i.). Many designers and contractors take what has been occasionally referred to as “the safe move” by specify- ing and installing vapor permeable air barriers. Given the information above, will a permeable air barrier cause a building to turn into a pumpkin at midnight at the fifth year of service? Not likely. However, it is important to keep in mind: it is not a question of if buildings get wet. Buildings, or more specifically, walls [and roofs] get wet through direct exposure to exterior elements as well as through vapor pressure deltas between interior and exterior environments. It is best to keep walls dry and their moisture-sensitive components through optimum design and material selection.
The information discussed herein is based on the writer’s research, reading of articles, and countless discussions with building envelope and forensic specialists (architects and consultants).
It is important to note, it is not the air barrier manufacturer’s position to recommend permeable or non-permeable membranes. The decision to use a permeable or vapor-retarding air barrier rests solely with the architect and/or design team. Information provided by manufacturers is a supportive aid used in conjunction with the architect’s research to determine the type of air barrier best suited to their projects.
- The term vapor barrier is an antiquated term that still holds sway with manufacturers, building science people and specifiers. It is technically incorrect as no material, given enough pressure and time, are true ‘barriers’. All materials will diffuse air and water vapor – hence the more [Code] correct term of ‘vapor retarder’.
- For simplicity, this article discusses only closed-cell c.i. insulation types. Both air barrier types discussed herein can and have also employ the use of mineral wool c.i.
- Poly-iso board stock insulation at 1 inch or greater is generally to fall into the Class II vapor retarder (e.g. a “vapor barrier”).
- Referenced from ‘Understanding Vapor Barriers’ – Building Science Corporation (www.building-science.com).
- These assemblies generally use poly-isocyanurate-type c.i.
- The water-resistant component of paraffin in gypsum sheathing provides intermittent water resistance. All gypsum sheathing manufacturers cite limitations to its exposure to moisture.
Fig 1.) KIMMU Extruded Polystyrene Foam (XPS)
Fig 2.) EcoHome Polyisocyanurate Foam Insulation Panel
Fig 3.) Nova Spray Foam
Fig 4.) Building Science Corporation
Fig 5.) Building Science Corporation
Fig 6.) USG Securock®
Fig 7.) National Gypsum Gold Bond® eXP® Sheathing
Fig 8.) Georgia-Pacific DensElement® Barrier System
Scott Wolff CSI, CDT | Architectural Specialist
W. R. MEADOWS, INC.