The thick and thin of fluid-applied air barriers

August 26, 2016

All images courtesy Henry Company

by Scott Wolff, CSI, CDT, and Todd C. Skopic, CSI, CDT, LEED AP
The fluid-applied air barrier (FAAB) concept was originally a Canadian-developed technology from more than 40 years ago. Back in the early 1970s, the genesis was an adhesive combining air- and vapor-retarding characteristics—eventually, this material would find its way into building codes.

The earliest FAABs were heavy-bodied products with predictably thick wet film/dry film applications. As it turns out, thicker membranes would have many advantages. Industry organizations such as ARCOM’s MasterSpec created the “fluid-applied membrane air barriers’ specification for these membranes with “not less than 40 mils dry film thickness (DFT),” that is,
1 mm (0.04 in.).

It is important to acknowledge all other ‘sides’ of the building enclosure ‘box’ similarly employ thicker membranes in the prevention of water and air infiltration. Foundations are commonly covered with 60-mil (i.e. 1.5-mm [0.06-in.]) styrene butadiene styrene (SBS) self-adhered sheet waterproofing (e.g. rubberized asphalt) or single or two-ply fluid-applied waterproofing. These are commonly in the 60 to 120 mils (i.e. 3-mm [0.12-in.]) DFT range. Roofing membranes, such as single-ply membranes, are also applied in larger thicknesses—typically in the 60 to 80-mil (i.e. 2-mm [0.08-in.]) DFT range or higher.

Enter the ‘thin-film’ era of air barriers. This phenomenon began approximately 15 years ago and was born out of the exterior insulation and finish systems (EIFS) industry. EIFS manufacturers provide single-sourced water-resistive barrier (WRB), insulation, and finished exterior wall assemblies. All may be well and good, but within the past 10 years, MasterSpec eventually addressed this new class of FAAB with a new specification titled “Air Barrier Coatings.”

In this specification, DFT values are listed at 14 and 17 mils (i.e. 0.35 and 0.43 mm [0.014 and 0.017 in.]) for vapor-retarding and vapor-permeable ‘coatings,’ respectively. Absent in the air barrier coatings specification language for DFT are the words “not less than.” Essentially, these ‘coatings’ can be even less than 14 mils.

AB06 off lift
Thick-film FAAB installation over plywood sheathing. As an alternative to the sheet membrane detail strip over the plywood sheathing joints, an approved sealant can be also be used.

In 2015, MasterSpec decided to combine the two specifications into one. Gone is the “Air Barrier Coatings” title while the “Fluid-applied Membrane Air Barriers” remains. This new version contains three sections in Part 2−Products and Part 3−Execution for different thicknesses:

MasterSpec has provided a demarcation line for a minimum of 35 mils (i.e. 0.9 mm [0.035 in.]) DFT for the high-build FAAB. Conversely, low-build FAABs are held to not less than 6 mils (i.e. 0.15 mm [0.006 in.) DFT.

Differences in thickness
The commonalities of all FAAB types include meeting respective industry requirements for maximum air water vapor permeance based on:

This is, however, where much of the similarities end. Pointing to the obvious fact buildings move brings this discussion to performance and long-term membrane durability. The disparity begins with the singular performance aspect MasterSpec does state—ultimate elongation (ASTM D412, Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers−Tension). Only the high-build vapor-retarding FAABs are currently held to a minimum of 500 percent ultimate elongation. Both medium- and low-build FAABs are held to a minimum of 350 percent ultimate elongation—essentially, this shows thin-film FAAB are held to a lower performance standard.

The discussion points that must be considered by the specifier for what is lacking in thin-film membranes are summed up in the paragraphs that follow.

AB 31 McD
Installation of thick-film fluid-applied air barrier.

Long-term durability expectations
To provide an analogy, use of thick-film air barrier membranes is akin to using coal-tar pitch roofing. By sheer virtue of ‘having more membrane,’ it takes longer for the membrane to ‘wear.’ Additionally, as this article discusses, thin-film membranes’ application are inherently more difficult to control (to achieve correct wet/dry film thickness). Thick-film air barriers are high-bodied membranes typically needing only one application instead of multiple ones.

How long the company has been engaged in FAAB manufacturing
This question is not meant to favor a longstanding air barrier manufacturer over a newer one; rather, it merely establishes a ‘company regularly engaged in the manufacture of’ these types of membranes with many successful years. Of course, in any area of building science, newer technologies are not to be ignored, and should be heralded. It should be noted, however, it is not unheard of some companies rushing a new product to market before proper and thorough field testing.

Crack-bridging performance properties
A standard commonly used by thick-film manufacturers is ASTM C836, Standard Specification for High-solids-content, Cold-liquid-applied Elastomeric Waterproofing Membrane for Use with Separate Wearing Course. Some thin-film manufacturers may state “crack-bridging,” but neglect to reference the specific standard to back up the claim. (Thicker membranes have more body and mass to span the cracks.)

Tensile and elongation properties
These performance properties may be lower in thinner-film FAABs. Design professionals should ensure the numbers are being reported.

Single-source opportunities
Are all accessories provided by the primary air barrier manufacturer? Some thin-film providers do not manufacture and provide a complete accessory compliment such as through-wall flashings, transition membranes, termination sealants, and primers. Compatibility of accessories by other manufacturers then becomes another question.

AB 31
Thick-film FAAB applied around complex geometries and masonry ties. This project would have been very difficult with a sheet-applied air barrier.

Air barrier warranties apply to the primary membrane and accessory items. Warranties are
solely for defective material (e.g. material-only warranty). If a contractor cherry-picks the membrane and various accessories from multiple companies, each company will provide warranties for only those products each has provided. Single-source helps ensure compatibility.

In projects where exterior insulation and finish systems (EIFS) are employed, to acquire the ‘system’ warranty, the EIFS manufacturer often requires its thin-film air barrier be installed beneath.

Marked coverage rate disparity based on substrates
Thin-film coverage rates can be as much as five to six times that of thick-film air barrier per gallon of installed product. Costs become an overriding deciding factor where the bottom line is the only thing that matters. However, the first cost of an FAAB is not as important as the lifecycle cost for the building’s entirety.

Back-rolling adds another layer of installation cost for thin-film products. Roller-only formulations
require more time and labor to install.

‘Additional material’ required
For thin-film products, added material is commonly required, based on substrate porosity, as mentioned in the manufacturer’s literature. [Some thick-film manufacturers recommend initial higher-build applications of their membranes when applied to “rough” surfaces. The phrase “additional material required” is language specific to thin-film manufacturer’s literature. To clarify, “additional material” is required in the event the initial application has failed to achieve the specified wet film thickness (or achieve ‘hide,’ as yet other thin-film manufacturer’s literature would dictate)]. Who is policing the installer when higher-porosity substrates (such as exterior gypsum sheathing, precast concrete, concrete masonry units [CMUs]) mandate more material? General contractors might be enticed at the prospect of a lower-cost air barrier, but are they really getting what was specified? Are all the transitional conditions getting proper detailing with transition membranes and through-wall flashing?

It is an established fact the thinner a membrane is, the more difficult it is to achieve a complete and uninterrupted film during application. This is based on the marked variations in substrate porosity of exterior gypsum sheathing, concrete, concrete masonry units, etc.

Testing specifics
Thin-film FAAB performance standards are tested at X mils. However, the specifications may require Y thickness, which is often two to three times that ‘X.’ This leads to the question of what those performance characteristics are at Y thickness.

Figure 1
Figure 1: For fluid-applied air barriers (FAABs) some thin-film manufacturers’ data states gypsum sheathing products require additional material to achieve hide and ensure pinhole-free coating to meet the spec. When the sheathing manufacturer’s logo can easily be seen, desired mil thickness was not achieved.

Who’s minding the store?
In many cases, contractors are solely left to decide if enough material is applied to the correct wet film/dry film thickness. The language in some thin-film manufacturer’s data states many gypsum sheathing products require additional material to achieve hide and ensure pinhole-free coating to meet the spec. When the sheathing manufacturer’s logo can easily be seen (Figure 1), it is a telling sign desired mil thickness was not achieved.

Not being discussed in great detail is the increased difficulty (and needed vigilance) of achieving a uniform, unbroken membrane over substrates—especially in the case of CMU backup walls exhibiting higher surface porosity. Thin-film coatings typically require several coats over CMU or what is called out as ‘block filler.’ Each coat is required to be back-rolled and must be fully cured before the second coat is applied. This equates to additional material, labor, time, and cost to finish the installation (presuming the specification is being followed correctly).

Vapor-permeable FAAB specification
Some FAAB specifications contain both thick-and thin-film membranes. In a number of these, thickness is required to be “not less than 40 mils dry film thickness.” Where a thin-film product has been tested at a DFT of 15 mils (i.e. 0.4 mm [0.015 in.]) and a vapor permeance of 10 perms, installing that same membrane at the project specification (requirement) of not less than 40 mils will yield wildly different permeance results. In the event this thin-film membrane were to comply with the 40-mil minimum DFT, it would result in a more than doubling the thickness and a reduction of its water vapor permeance. As a result, the drying potential
of the wall assembly may come into question.

The minimum performance and physical properties discussed herein are based on available industry-leading specifications such as MasterSpec. As noted, these are minimum standards by which all FAAB should be based. Physical performance criterion such as tensile strength and crack bridging—not listed in MasterSpec—hold even more consideration for long-term envelope performance and durability.

Air barrier technical data and specifications should reference these performance capabilities as well as include their accompanying standard, such as:

Case study
On a recent project in New England, a thick-film FAAB was specified, submitted, and approved. Shortly thereafter, the general contractor was persuaded to accept a so-called ‘value-engineered’ alternative thin-film FAAB reportedly to “save $0.45 per square foot.” In the mockup in Figure 2, the thin-film FAAB application to CMU (at left) is shown to bear discontinuities in the form of breaches in the membrane exposing the substrate underneath. The thick-film FAAB is shown at right on the  same mockup.

The back-up wall is CMU. The thin-film manufacturer was going to require a substrate smooth enough so the product could be installed at its recommended thickness without breaches. To get this required smooth surface on the CMU, the mason required an additional $1.45/sf in surface prep. The presumed cost savings would not only be lost by this additional cost, but to use the thin-film it would also cost the owner an additional $1/sf. This project was approximately 3700 m2 (400,000 sf), so to use the thin-film material would have actually cost the owner an additional $400,000.

Figure 2
Figure 2: In the mockup at left, the thin-film FAAB application to concrete masonry units (CMUs) is shown to bear discontinuities in the form of breaches in the membrane exposing the substrate underneath. The thick-film FAAB at right is quite different.

Thick-bodied thick-film air barriers are typically applied in a single application over substrates with varying porosity without the need for an additional layer. One argument of thin-film air barrier manufacturers is that substrate panel edges and transitional areas are the only areas in need of reinforcement (via transition membranes, etc.).

However, one only need to think about racking forces acting in the x and y axis of an exterior wall. Z-axis movement is also noted due to positive/negative wind and air pressure forces. Gypsum panels have been known to fracture within the field areas, not always at their edges. It is for these and other reasons thicker membranes, such as those used on foundations and roofs, should be used to maintain the same robustness on above-grade wall assemblies.

Durability and exposure
Some building envelope consultants have been concerned with the influx of newcomers in the FAAB market, requiring them to comply with various ASTM test procedures. In some cases, they have tested fluid-applied air barriers to ASTM D471, Standard Test Method for Rubber Property Effect of Liquids. This test is intended for rubber ([EPDM]) pond liners, not FAAB. The industry has been working on more applicable durability standards. One of these initiatives falls under ASTM Committee E06−Performance of Buildings.

The committee’s Work Group WK50742 is writing a new standard, Standard Practice for Assessing the Durability of Fluid-applied Air and Water-resistive Barriers. Katherine Wissink of engineering group Simpson Gumpertz Heger (SGH) presently chairs this work group. This proposed standard aims to develop test methods for FAAB that do not have an established track record. This proposed standard practice provides test methods and accelerated aging techniques used to assess the durability of these products.

“In most cases, fluid-applied air-barrier membranes serve as the primary air barrier and water-resistive barrier for the exterior wall system and, in some cases, serve as the vapor retarder depending on the vapor permeance of the membrane,” explained Wissink. “The integrity and continuity of the membrane is essential given that this single material may perform up to three important functions and is critical to the performance of the wall system in terms of moisture migration and waterproofing.”

Given the ever-expanding FAAB industry, it is critical all peripheral data is read and fully understood. Only then, can the designer be fully informed on what they are truly delivering to an owner. When it comes to deciding on a fluid-applied air barrier, it is evident physical properties and material and system performance are not equal. Just because a certain FAAB has passed certain requisite ASTM tests does not make it equivalent in regard to long-term durability, crack-bridging, and elongation.

Scott Wolff, CSI, CDT, is the Midwest building science manager at Henry Company, and has been in his current position for a decade. He has worked in an architectural capacity for 15 years within the industry and is active with the Chicago Chapter of the Building Enclosure Council (BEC). Wolff can be reached at[1].

Todd C. Skopic, CSI, CDT, LEED AP, is a building science manager at Henry. He has been in the air barrier industry for 16 years, working with different manufacturers. Skopic is active in BEC, RCI International, and ASTM, and serves on the Terminations and Flashing Committee for the Air Barrier Association of America (ABAA). He can be contacted via e-mail at[2].

 To read a letter to the editor regarding this article, and the authors’ response, click here[3].
  3. here:

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