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Is There a Gap in Your Air Barrier Wall Design?
--Overlooking joint sealing where it really matters--in the structural backup wall.

All day long, our Specification Development Engineering Team opens and reviews architectural details.  We look at hundreds of cross sections per week.  One of the most often seen reflects the industry’s move towards exterior wall design based on air barrier principles. 

Driven by the desire for more energy efficient buildings, air barrier codes have were developed and adopted in Canada years ago and have been adopted in Massachusetts, Michigan, and Wisconsin with other states close behind.  In many other states, the principles of air barrier design are being widely adopted because they make sense--building science, waterproofing, and energy-use sense.

What do we see?
One of the details we see repeatedly shows a plan-section through the wall at a structural expansion joint (see Figure 1).  This can be at the mid-span or elevation change of the building or, as commonly, at additions between new and existing structures.  The detail shows the structural backup wall, sometimes block, sometimes steel-stud and exterior gypsum.  The structural wall is wrapped or coated with an air barrier membrane and extruded, polystyrene insulation board is detailed over the air barrier.  The cavity-wall gap comes next and then the exterior façade material—often brick, but also precast concrete, metal panels, etc.

At the structural expansion joint in the façade we very often see COLORSEAL—a structural expansion joint hybrid sealant that fills these larger joints, insulates them, makes them watertight, coordinates through color with the aesthetic of the building, and installs without the use of invasive anchors.

Figure 1: Typical detail shows 2" expansion joint gap in precast facade, wall cavity, extruded polystyrene insulation, air barrier membrane, exterior gypsum over steel stud backup wall.  Note air barrier membrane looped into joint and lack disruption of insulation at the structural joint in the backup wall.

Where’s the problem?
In the structural joint in the backup wall we see a sliver of air-barrier membrane or other “tape” across or looped into the joint.  The structural joint gap itself is often empty.  When it is filled with something it is usually fiberglass batt insulation shown.  All three of the above are the problem.

Firstly, this gap is a dynamic, moving, opening and closing structural expansion joint.  It will cycle continuously throughout its life.  Air barrier membranes and joint bridging “tapes” if installed without any slack will tear.  Even if installed as drawn with a loop into the joint, these products will fatigue and tear where they wrap around the joint corners.  These materials are not designed to handle joint cycling and are particularly prone to failure when they become brittle at low temperatures.

Secondly, membranes and tapes provide no insulation. The detailing of extruded insulation board in the cavity is intended to insulate the wall from the outside of the structural wall.  This means that at the structural joint gap, instead of a thick piece of insulation board all you have is a few mils of modified rubber sheet between the cold wall cavity and the inside of the building.  The structural joint gap is a direct path to the interior of the building.  In this detail the joint-gap constitutes a major thermal break and could be setting the R-value for the  entire exterior wall system. 

As was pointed out in “Exterior Wall Systems, R-Value, and Revenue” by Tom Kuckhahn published in the September 2003 Construction Specifier, heat “seeks the path of least resistance, so the R-value of an actual wall is closer to the R-value of the least insulating portion of that wall.”

Furthermore, in details where the structural joint gap is left unfilled, condensation can form on the interior side of the membrane.

The use of fiberglass batt insulation to fill this cavity invites two problems.  Fiberglass insulation cannot handle joint cycling.  It takes on compression set and when set it loses much of its insulation value.  Once set and no longer properly insulating, it is still porous and will absorb moisture that condenses behind the uninsulated membrane “bridge” across the gap.

What to do?
An elegant solution to this gap in air-barrier wall design is to detail a preformed, impregnated, cellular foam sealant in the structural joint in the structural wall. EMSEAL’s 25V is uniquely suited to this application (see Figure 2).

Figure 2:  "25V" by EMSEAL installed in the backup wall behind the air barrier membrane ensures watertightness through joint cycling AND maintains continuity of thermal insulation.

25V is produced by infusing a high-quality, polyurethane, cellular foam with water-based, acrylic modified asphalt.  The asphalt is compatible with the air barrier membrane and can be installed against the edges of the membrane wrapped into the joint a small distance, or beneath the membrane run continuously over the 25V.  The product uses the stored strain energy of compression to lodge itself firmly in joint and to cycle, fatigue-free, with joint movement. 

25V by virtue of its depth, density, and infused foam composition has an R-Value of approximately 3.28 per inch of depth which matches that of ¾” (20mm) extruded polystyrene (3.75 per inch of depth). 

This means that on the typical detail with a 2” (50mm) structural expansion joint, 25V by EMSEAL for this joint would come with a standard depth of  2 ½” (60mm).  The R-Value of the 25V sealant would be: 2.5 x 3.28 = 8.2.  This would equal the R-Value of 1 3/4” (45mm) thick extruded insulation installed in the cavity on the face of the structural wall. 

If you want the joint seal R-Value to be higher, simply specify a greater depth of seal.  For example, if you are designing 2” (50mm) insulation board with a R-Value of 10, then specify 2” x 3” (50 x 75mm) EMSEAL 25V for an R-Value of 9.84.

Coming Soon: 
Window Perimeter Seals—Filling the Small Gaps in Your Air-Barrier Design.

Expansion joints and precompressed joint sealants by EMSEAL

1-800-526-8365 -- 508-836-0280 --  techinfo@emseal.com -- Fax: 508-836-0281

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