Tag Archives: Wood framing

Getting Along With Stucco: Sometimes it just needs space


All images courtesy Alta Engineering Co.

by Brett Newkirk, PE
There are two maxims about stucco application over wood-framed structures: first, it will crack, and second, owners will not do much about it. Water intrusion through stucco claddings is so common in Florida, re-skinning buildings here after five or 15 years is commonplace, even though it is not always warranted.

In some cases, re-skinned buildings are needing to be re-skinned again due to moisture intrusion years later. Damage to building structures is frequently related to poor installation of the veneer system and associated flashings.

In some examples, the stucco systems have been installed in general compliance with the building code requirements and product manufacturer instructions, yet damage to the wood substrate still ensues (Figure 1). Why then, does a code-compliant system fail?

This article includes a study of code requirements for stucco veneer systems to help explain the situation for design/construction professionals.


Figure 1: Damage to an unpenetrated wall through a stucco veneer over Grade D paper and a polymeric water-resistive barrier (WRB).

Bond-break layer
A major change was introduced to the 2003 International Building Code (IBC) and the 2004 Florida Building Code (FBC), which required two layers of an approved water-resistive barrier (WRB), equivalent to two layers of Grade D paper, behind stucco veneers. The purpose of the second (outboard) layer is to create a ‘bond-break’ between the back plane of the stucco rendering and the WRB’s front face.

The bond-break layer (BB) is intended to provide a disruption to the potential capillary movement of moisture from the stucco across the WRB and into contact with the wood wall sheathing. To that end, the bond-break layer is supposed to create a small air space, allowing gravity to draw moisture to the base of the wall, where it presumably will drain out of the wall system before it absorbs across the cross-section of the WRB. While this code change was a huge stride and of sound substance, it was not quite enough.

The problem is the method by which stucco is secured to the walls and through the WRB and BB. Fasteners, typically staples, are installed with a pneumatic tool that draws the lath tight against the BB and WRB. The tightly bound sandwich of lath, BB, WRB, and wood wall sheathing at each connection does not offer sufficient separation to break the capillary path of moisture travel or allow for drying of moisture in the system, even when a BB is present. Thus, the BB’s purpose is negated, and capillary movement across the system is possible.

However, the greater concern is a hole (or two, thanks to a staple) is conveniently created by the fastener at the precise location where the weatherproofing sandwich is tightly squished together (Figure 2). Consequently, permeation through the WRB is not required. Instead, moisture accumulates within the tightly bound assembly, clings to the fastener shank, and travels through the fastener hole in the WRB into contact with the wall sheathing (Figure 3). Equally important is the inability of moisture within the assembly to dry due to the tightly bound, unventilated space between the veneer and the wood substrate. Instead, the chronically damp conditions are favorable for wood decay.

Figure 1

Figure 2: Typical stucco section at fastener—the two tiny holes are located right where the waterproofing is squished.


Figure 3: Damage to wood sheathing from state penetration.









By code, the metal lath is fastened to the wall sheathing with more than one fastener per square foot of wall area. Some rough math reveals there are on the order of 6000 holes through the WRB for a one-story 230-m2 (2500-sf) structure footprint.

Stucco accessories, such as casing beads, weep screeds, and control joints, are typically installed prior to lath. Accessories are also typically secured with pneumatic fasteners, which draw them tight to the BB, eliminating any meaningful drainage space behind the accessory. Consequently, water within the system tends to accumulate along the horizontal edges of the accessories, where it may find weaknesses in the WRB, prompting migration to the interior. Of course, fasteners securing the accessories also serve as conduits for water to migrate through the WRB.

Water-resistive barrier performance
Sheet-good WRBs are tested in a laboratory setting to ensure they perform under a battery of tests established by the International Code Council Evaluation Service (ICC-ES). The most commonly used WRBs are polymeric sheets called ‘house wraps’ or ‘building wraps.’

ICC-ES AC38, Acceptance Criteria For Water-resistive Barriers, evaluates the WRB material’s tensile strength, vapor transmission, air permeance, and resistance to water penetration, to name a few. The polymeric material’s adequacy in resisting water penetration is judged by its ability to prevent water passage for two hours when subjected to a 25 to 550-mm (1 to 24-in.) column of water on one side. Once proving performance, the WRB is considered code compliant. However, the battery of laboratory tests fails to include one little detail of real life: holes in the WRB from veneer fasteners.

A single fastener penetration would result in a ‘failure’ of the AC38 water penetration resistance test. Why should one expect field performance of a product that is installed in a different (and far inferior) manner than that under which it was tested and approved?

To this author, even more perplexing is AC38’s seeming double standard for the use of paper-based WRBs, which are not subjected to the same tests as those required of polymeric WRBs. Grade D paper is used as the backing for paper-backed lath, whose installation is the most common means of creating the BB layer. By definition, Grade D paper allows water to pass after only 10 minutes of exposure with no meaningful hydrostatic pressure, while polymeric WRBs are expected to perform for hours, at pressures of up to 5.36 kPa (112 psf). Why then are two layers of Grade D paper the code-mandated baseline for WRBs behind stucco veneers? Obviously, the very low tolerance of Grade D paper to resist moisture absorption can cause substantial moisture-related distress to wood-framed buildings where it is used.

Mockup testing
Full-scale testing was performed by this author to gage the performance of several ‘code-compliant’ stucco wall assemblies, with various polymeric-based and felt-based sheet-good WRBs. The tested wall assemblies consisted of 1.2 x 1.2-m (4 x 4-ft) specimens constructed with wood framing and oriented strandboard (OSB) sheathing, and then clad with various types of WRBs.

A three-coat stucco system was then applied over paper-backed metal lath secured with 25-mm (1-in.) crown staples, confined within 22-mm (7/8-in.) plastic casing bead accessories that were secured around the wall’s perimeter (Figure 4). A 0.6 x 0.6-m (2 x 2-ft) observation port was created in the center of the OSB substrate, where gypsum wallboard wrapped in kraft paper was substituted for the OSB (Figure 5). This way, the wallboard could be removed and the back side of the WRB could be observed after testing. The kraft paper also served as an indicator of water contact during the test.


Figure 4: Front side of a typical mockup wall test setup.


Figure 5: Back side of a typical mockup wall test setup.








Testing was performed using a variation of ASTM E2273, Standard Test Method for Determining the Drainage Efficiency of Exterior Insulation and Finish Systems-clad Wall Assemblies (AC38 drainage test), with the additional step of performing observations for moisture ingress during the test. (ASTM E2273 only calls for a comparison of the volume introduced to the specimen to that which escapes at its base. This measurement was not of interest to the author as it related to this evaluation.)

Water was introduced to the drainage cavity at the top of the wall at a rate of 3.38 L/min/m2 (5 gal/sf/hr) for 30 minutes. Perhaps unsurprisingly, the testing revealed water penetration occurred through the fastener penetrations in both polymeric and felt-based WRBs (Figure 6). Water migration also occurred directly through the field of some woven polymeric WRBs and Grade D paper. Intrusion through Grade D paper-based WRB mockups was substantially more severe than that of the polymeric or felt-based WRBs.


Figure 6A: Water penetration through fastener holes in a spun-bound polymeric WRB following testing. B: Water penetration through fastener holes in a woven polymeric WRB following testing. C: Water droplet below staple shank penetration through WRB during testing. D: Hyrion paper shows water ingress through fastener penetration.

Magnified observation of staple-penetrated WRBs reveals an oblong, torn annular opening is created around the shank (Figure 7). The torn annulus was more significant at woven than spun-bound WRBs. In some cases, an additional indentation and hole through the WRB was noted due to the actuation of the pneumatic tool which impacted its surface. Additionally, use of ‘slap’ or ‘hammer-tacker’ staples caused rupturing of the WRBs at the staple shank hole and the tool’s impact location, which could allow water passage.


Figure 7: The left image shows a 10x view of a staple shank shows an oblong torn hole in the WRB (and water migration through it). The right image depicts an enlarged, torn hole in the WRB at the staple penetration.

Where to go from here
It probably does not take the foregoing technical discussion to understand holes are paths for water entry or wood that stays wet will rot. So why do we build walls with thousands of holes in them, prevent them from drying out, and expect them to have watertight performance?

If repeating an activity and expecting a different result is the definition of insanity, then why do we tear stucco veneers off of water-damaged building structures and then replace it the same way? Accepting the realities of stucco cracking and inadequate owner maintenance, something else needs to change to give stucco-clad buildings
a longer life.

Based on the author’s testing and experience, there are a few installation practices that can help mitigate the compression of the veneer’s weatherproofing sandwich, which helps prompt drying and greatly reduces the likelihood of moisture migration through fastener penetrations in the WRB.

Separate stucco from WRB with furring or other drainage media
There are many randomly oriented polymeric filament products, typically 6 mm (1/4 in.) in thickness, that provide this function (Figure 8). Often called drainage media, drain screens, furring, or drainage mats, these products should be installed behind all accessories.


Figure 8: Test wall with drainage media installed in front of the WRB.

Figure 2

Figure 9: The recommended stucco section at a fastener.









The resulting air space creates a functional capillary break between the back side of the stucco rendering and the WRB, even at fastener locations (Figure 9). This gap provides an easy, unobstructed path for moisture to travel vertically down the face of the WRB (within the media). In combination with through-wall flashing vents and drainage weeps at the stucco base, the air gap also prompts ventilation within the cavity that prompts drying of moisture.

Control the installation force and depth of fasteners
This can be achieved by reducing the air pressure for pneumatic tools, but is most effectively completed by use of pan-head screws, which have installation depths that can be easily controlled and adjusted (Figure 10). The use of screws also inherently reduces the number of penetrations through the WRB by half, when compared to staples that have two shanks. Further, the cross-sectional diameter of the screw is greater than that of a staple shank, rendering it less vulnerable to corrosion over time. (The drainage system anticipates water to drain across the unprotected fastener shank.)


Figure 10: Pan-head screws are used to secure lath to the wood frame.

Use fluid-applied weather barriers.
Fluid-applied weather barriers cannot be torn; they create a ‘gasket’ effect around penetrating fasteners. However, these products’ detailing and reinforcing requirements demand a more skilled and conscientious craftsman to properly install. Designers should also recognize fluid-applied barriers are typically vapor-impermeable, which should be a consideration in the envelope design.

The recommendations in this article are an additional step to prevent water intrusion through stucco-veneered walls and to prompt drying beyond current code requirements. Of course, these suggestions are no guarantee against water penetration—after all, one still has thousands of un-gasketed holes through the sheet-good water-resistive barrier. However, with proper WRB installation, appropriate flashings and drains, incorporation of drainage media, and controlled depth fasteners, a stucco veneer has a much better chance at providing long-term performance.

Brett Newkirk, PE, is a practicing structural engineer with Alta Engineering Company in Jacksonville, Florida. He specializes in the diagnosis and repair of moisture-affected structures, and is a recognized author and leader in the building envelope repair industry in the southeastern United States. Newkirk is an associate member of the American Society of Civil Engineers (ASCE) and sits on ASTM committees for wood and gypsum. He can be reached at brett@altaengineeringco.com.

Improving Floor/Ceiling Sound Control in Multifamily Projects: Sound Testing Practices

by Josh Jonsson, CSI

The sound transmission class (STC) and impact insulation class (IIC) are ASTM-derived single number ratings that try to quantify how much sound a stopped by partition being tested.

Laboratory testing involves an ideal setting for the floor/ceiling assembly—it is isolated from the walls, and there are no penetrations for HVAC, plumbing lines, sprinklers, can lights, or electrical boxes. In the field (i.e. F-STC and F-IIC), the floor/ceiling assembly often sits on load-bearing walls, is connected to the structure, and contains many ceiling and floor penetrations for the items just mentioned. Consequently, the code allows for a lower rating for field scores over those in the lab.

The STC rating essentially tells how much noise is stopped from going through a wall. The test involves blasting loud noise at all the measured frequencies in a room. A Level 1 sound meter measures this exact noise in that room level at all frequencies, as well as the sound in the room on the other side of the partition. These two different levels are then essentially subtracted from each other, with some corrections made for background noise.

The IIC rating is not a comparative test like the STC. Rather, it uses an ASTM-specified tapping machine that sits directly on the floor—more specifically, directly atop the finished floorcovering. (Consequently, an IIC rating always lists the floorcovering with which it was tested.)

The machine has five steel hammers that spin on a cam shaft, falling onto the floor from the same height, no matter what or who is testing. These hammers put a consistent energy into the floor. The sound level meter is taken downstairs below the tapping machine and the sound level is measured at all the frequencies called out in the ASTM standard. These sound levels are plugged into the equations in the standard; a single number is generated summarizing how much sound was stopped.

To read the full article, click here.

Improving Floor/Ceiling Sound Control in Multifamily Projects


All images courtesy Maxxon Corporation

by Josh Jonsson, CSI

In recent years, demand has increased for better floor/ceiling acoustics in multifamily construction. This has been driven by consumer desires, new guidelines from code bodies, and stricter enforcement of existing codes. How do design professionals keep pace as the traditional approaches to multi-unit residential sound control evolve?

This article reviews important new guidelines that must be taken into account by architects and specifiers, and examines how construction manufacturers have created new products or enhanced existing ones in the pursuit of achieving higher acoustical performance.

Thanks to product technology improvements and more stringent regulations, the wood-frame multifamily industry is paying increasing attention to the acoustics of the floor/ceiling assembly.

Thanks to product technology improvements and more stringent regulations, the wood-frame multifamily industry is paying increasing attention to the acoustics of the floor/ceiling assembly.

Two of the principal measurement standards for acoustics in multifamily construction are:

  • sound transmission class (STC), which pertains to the amount of airborne sound contained by a given building element (i.e. walls, doors, windows, and floor/ceilings); and
  • impact insulation class (IIC), which deals with impact noise (i.e. footfall, chair scrapes, and dropped objects) transmitted through a floor/ceiling system.

Both these single-number ratings apply to the full assembly of building materials used to separate tenants, including floor/ceiling assemblies.

For more than 50 years, these measurements have helped architectural project design teams quantify the acoustic levels of floor/ceiling assemblies. In fact, the Department of Housing and Urban Development (HUD) wrote A Guide to Airborne, Impact, and Structure Borne Noise: Control in Multifamily Dwellings in 1967, helping reinforce the importance of sound control in multifamily construction. This document, along with the Uniform Building Code (UBC), helped project teams recognize an acoustical threshold was needed in multifamily construction. UBC required an STC and IIC rating of 50 (or 45 if field-tested as F-STC or F-IIC). The higher the rating, the better the performance. (See “Sound Testing Practices.”)

In 1997, UBC gave way to the International Building Code (IBC) as the widely accepted model code. This shift brought greater awareness of acoustical ratings and their deemed thresholds in unit-over-unit construction, however these code levels remained aligned with the UBC’s established minimum requirements of STC and IIC 50 (or 45 if field-tested).

As the multifamily industry became more competitive, developers began offering upgrades in flooring and lighting to tenants as an amenity, yet little to no attention was paid to acoustical performance. This is astounding when one considers acoustics continue to be one of the driving factors in maintaining low vacancy levels, as well as one of the most litigated issues in this type of construction. To compound the subject, many of the amenity upgrades offered, such as hard-surfaced finished floors and canister lighting, can adversely impact a floor/ceiling assembly’s performance.

CS_July_2014.inddUpdating acoustical recommendations
In response to the need for updated acoustical guidelines, the International Code Council (ICC), along with several respected acoustical experts, created ICC G2-2010, Guideline for Acoustics. The guideline recognizes:

the current level and approach of sound isolation requirements in the building code needs to be upgraded. The requirements are currently insufficient to meet occupant needs.

As shown in Figure 1, the guide provides two levels of acoustical performance: ‘acceptable’ and ‘preferred.’ Both exceed code minimums for airborne and structure-borne noise.

These new levels now give a clearer direction on what levels should be targeted for desired acoustical performance, depending on the building type. As the names suggest, when one wants a building that has an acceptable level of acoustical separation, ‘acceptable’ is targeted. When one is designing a building on the higher end of market rate or luxury level, or has tenants or owners sensitive to noise, the desire should be for a ‘preferred’ level of performance.

Components of acoustical design
How do these new recommendations apply to the current approach for multifamily construction? As Figure 2 shows, a commonly specified design for many multifamily projects, which is also recommended by acoustical consultants, includes:

  • hard-surfaced flooring;
  • 25 mm (1 in.) or more of gypsum concrete;
  • 6.4-mm (¼-in.) entangled mesh sound mat;
  • wood subfloor;
  • wood floor trusses or joists;
  • insulation;
  • resilient channel; and
  • one layer of gypsum board.

CS_July_2014.inddThe typical rating for the design would be an IIC 51 to 55 and STC 56 to 60, depending on the floorcovering (e.g. laminate, tile, floating engineered wood), the acoustical performance of which would be listed by the consultant and verified by test reports. These numbers exceed code minimum and exceed the STC requirement for ‘acceptable,’ but only marginally—at best—meet an ‘acceptable’ level for IIC and any nuisance impact noises from upstairs tenants (IIC).

It is important to keep in mind these listed ratings would be achieved by selecting and installing all the components based on proper acoustical design. For example, the resilient channel would need to be a product similar to a proprietary one using steel measuring 0.5 mm (0.021 in.) thick by 38 mm (1.5 in.) wide, as opposed to a similar but lesser design lacking proper acoustical performance. To increase the IIC rating, an upgrade to the sound mat and/or the resilient ceiling system must be made.

Improving IIC ratings with sound mats
Traditional sound control mats with entangled mesh enhance IIC performance through the mesh being attached to fabric, which is loose-laid over the subfloor and then encapsulated with a gypsum concrete topping. The entangled mesh acts as a spring and produces an air space with little surface contact (i.e. three to five percent). Until recently, IIC performance was upgraded by using a thicker sound mat and deeper gypsum concrete.

Sound mat manufacturers have added new technology that allows for higher ratings while continuing to meet industry expectations for the corresponding thickness of the gypsum concrete. Traditional entangled mesh mats are now being manufactured with an additional acoustical fabric—Figure 3 depicts a 6.4-mm (¼-in.) entangled mesh mat with this upgrade. The acoustical fabric is laminated to the underside of the mat, creating an additional vibration break and absorptive layer. This improved product requires the same thickness of gypsum concrete as its standard counterpart. The IIC performance of the system is improved by two to five points without adding any measurable thickness to the floor system.

Another option is to employ the original 6.4-mm entangled mesh sound mat and a secondary topical mat placed between the gypsum concrete and the finished floor. If this option is selected, this secondary mat should be high-quality and thoroughly tested for sound ratings. (The sound test showing this type of product’s performance must be specific to the assembly that is being used versus a sound test from an unrelated design—for example, using concrete test data for a 2×10 joist system.)

CS_July_2014.inddImproving IIC ratings with resilient clips and channels
Properly installed, high-quality resilient channel will improve IIC ratings, but the resilient channel’s effectiveness can be easily lessened through faulty installation. To install traditional resilient channel, proper-length screws are imperative so as not to penetrate the joist or remove the channel’s resiliency.

Penetrations from the drywall into the joist through the resilient channel create flanking paths that transfer sound through a floor/ceiling assembly, as does having a channel affixed tightly to the assembly. For these reasons, new resilient clips that are difficult to install improperly have been introduced to the market. These clips can deliver equivalent performance to properly installed, high-quality resilient channel.

Hanging systems that provide spring and reduce or eliminate resilient channel contact with the joist offer even better performance. Figure 4 shows two such products: the ceiling wave hanger and a spring isolator. Either of these products installed in conjunction with a 6.4-mm (¼-in.) entangled mesh sound mat on the floor above would help the system exceed the ‘acceptable’ level, and approach ‘preferred’ levels for the IIC rating with hard-surfaced floorcovering. See Figure 5 for various assemblies and their acoustics attributes.

How to design for desired acoustical performance
As a specifier or architect team leader, one must first determine the level of acoustic performance to which to design. This should not be a matter of just meeting code—rather, the entire conversation must be approached in a new light. The following questions should be asked:

  1. When considering the amenities offered to tenants, how important are the acoustics of the unit? In other words, how important is the quality of life related to acoustical privacy?
  2. Does the project team want to just meet code because complaints and vacancy rates are unimportant or not a factor? Do they want ‘acceptable’ performance, significantly reducing noise complaints and removing sound control from the vacancy equation? Or, do they want ‘preferred’ performance to meet client expectation and greatly reduce potential for noise complaints?
  3. Once the level is determined, which method makes the most sense for achieving that performance level? Does the sound mat get upgraded to a very high-performing mat (manufacturers offer many styles with differing performances)? Does the sound mat get upgraded while keeping the system as thin as possible? Does the sound mat stay the same and the ceiling hanger system get upgraded? Is a secondary sound mat added while upgrading the primary sound mat and/or ceiling system to reach optimal sound ratings? Or, do the mat and ceiling get upgraded to reach better ratings?

CS_July_2014.inddEven after the desired level of performance has been determined, there are other factors that should be considered, such as whether the project will always be apartments or if they could become condominiums. There is also the matter of whether carpet and pad areas will always have carpet and pad.

Projects that start as apartments and then plan on being converted into condominiums should be approached as if they were condominiums from the beginning. Future owners may tear out carpet and replace it with hard-surfaced flooring.

Sound mat manufacturers receive a high volume of phone calls every year where a condominium project put sound mat only in the hard-surfaced areas. The new owners want hard surfaced flooring throughout and are being told they need to provide levels of performance similar to the ‘preferred’ levels while only being able to add a thin amount to the profile of the floor. As they can only do work in their unit, they are left trying to use a thin, lower-performing sound mat to reach the requested, more stringent criterion.

Acoustic qualities of various fl ooring assemblies.

Acoustic qualities of various flooring assemblies.

Throughout the United States, the wood-frame multifamily industry is paying increasing attention to the acoustics of the floor/ceiling assembly. Innovative architectural acoustic products continue to see greater use in existing metropolitan areas as well as in new areas of the country. It is important specifiers continue to be educated on new products and adapt their specifications to ensure they meet defined levels of sound control that tie directly to the end user’s satisfaction with their living space.

Josh Jonsson, CSI, is an acoustical specialist and West regional manager at Maxxon Corporation. He has more than 15 years of experience in the architectural noise industry and has worked for acoustical and vibration consulting agencies. Jonsson is a member of CSI, Acoustical Society of America (ASA), and ASTM International committee E33 Building and Environmental Acoustics. He can be contacted via e-mail at josh@maxxon.com.