Did You Miss This Change To The 2015 International Energy Code (IEC)?

Did you know? B uildings designed and constructed under
the 2015 International Energy Code (IEC) can no longer use

Continuous Z-Furring to attach rain screen veneers to buildings.

Good Bye Continuous Z-Furring

The reason for this change is to eliminate the amount of thermal bridging that occurs through the insulation installed over the back-up wall. Previous versions of the IEC tried to accomplish this change by specifying continuous insulation as a means to meet the U-Factor requirements for exterior wall assemblies.

This led to a debate over the definition of continuous insulation despite the fact that the intent of the past and present codes were to require walls with a minimum U-Factor based on climate region.

“CONTINUOUS INSULATION (ci). Insulating material that is continuous across all structural members without thermal bridges other than fasteners and service openings. It is installed on the interior or exterior or is integral to any opaque surface of the building envelope.“

Factoring in the thermal bridging effect of Continuous Z-Furring in exterior wall assemblies shows that they do not comply with the intent of the code. Defining Continuous Insulation was a step in the right direction for compliance with the intent of the code.

Figure 1 below shows the effect of thermal bridging of Continuous Z-Furring on the thermal performance of an exterior wall assembly. The figure was taken from a thermal analysis using the THERM 6.3 software developed by Lawrence Berkeley Laboratories.

This assembly consists of

6” LGMF, 5/8” gypsum sheathing,
2 ¼” extruded polystyrene insulation with continuous Z-furring and
an ACM panel veneer.

Calculating the U-Factor of this assembly, using THERM 6.3 software, without factoring in the thermal bridging of the Continuous Z-Furring, the assembly U-Factor is 0.063. The equivalent R-Value is 1 divided by the U-Factor or R-15.87. Factoring in the thermal bridge created by the Continuous Z-Furring, using THERM 6.3, the U-Factor for the assembly increases to 0.117. The equivalent R-Factor is R-8.85 resulting in a reduction in the overall R-Factor of the assembly of R-7.2 or 44%. According to Table C402.1.4 of the 2015 IEC, this assembly will not meet the U-Factor requirement for any metal framed exterior wall assembly in any climate zone.

Table C402.1.4: Based on this change, a number of companies have offered attachment systems for rainscreen veneers to buildings. Not all of these deal with other code requirements and exterior wall assembly performance issues.

On the code requirements, the 2015 International Building Code has requirements for load-bearing elements in exterior walls such as these attachment systems.

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“704.10 Exterior structural members. Load bearing structural members located within exterior walls or on the outside of a building or structure shall be provided with the highest fire resistance rating as determined in accordance with the following:

As required by Table 601 for the type of building element based on construction of the building.
As required by Table 601 for exterior bearing walls based on the type of construction; and
As required by Table 602 for exterior walls based on the fire separation distance.“

In reviewing Tables 601 & 602, there are many construction types and fire separation distances that require the load bearing elements of the exterior wall to have a fire rating. Some of the attachments systems offered are made of or contain plastic or fiberglass materials or rely on plastic insulation boards to bear the loads transmitted from the veneer to the attachment system to the sheathing, to the LGMF and ultimately to the structural frame of the building. These types of products burn and therefore cannot be used in exterior wall assemblies require fire rated load bearing elements.

Table 601

Fire-Resistance Rating Requirements for Building Elements (hours)

Table 602

Fire-Resistance Rating Requirements for Exterior Walls Based on Fire Separation Distance

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Other considerations for the rainscreen veneer attachment systems available on the market are accommodating the installation of the continuous insulation and the fastening requirements for the insulation. Cutting and trimming the insulation to fit within or around the attachment system requires additional labor. Installing fasteners through the insulation results in holes in the air and water (and vapor barrier) membranes installed on the exterior sheathing.

While manufacturers of the membranes have claimed them to be self-sealing or self-healing, their claims are based on testing done under ASTM D1970. This standard contains a nail seal ability test whereby the membrane is applied to a piece of plywood, a nail is driven through the membrane and plywood, pulled halfway back out and then the water tightness of the penetration or the nail through the membrane is tested by creating a horizontal reservoir of water around the penetration for a period of time.

This in no way simulates the penetration through the membrane of a screw type fastener which then penetrates the gypsum sheathing, stalls in inward progress while self-tapping a hole through the LGMF member and kicks out the gypsum core of the gypsum sheathing due to the action of the threads of the screw type fastener. The industry is aware of this issue and ASTM is currently working on a standard test method for determining fastener seal ability through these types of membranes.

What are the properties needed and wanted for a rainscreen veneer attachment system that can be used for all construction types?

Does not burn
Does not rely on plastic insulation to bear any imposed loads
Installs easily with few parts to assemble
Provides required adjust-ability without having to shim
Reasonable cost
Does not require cutting and fitting of insulation
Provides for the mechanical fastening of the continuous insulation.
Results in minimal thermal bridging through the continuous insulation.
We searched for such a system but did not find it. So…….we invented it.

Introducing....

the Stand-Off MPV Bracket

An adjustable bracket for fastening metal panel veneers to a building. It eliminates thermal bridging and provides mechanical fastening of the cavity insulation.

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Reduce Thermal Bridging Using Stand-Off MPV Bracket

The reduction in thermal bridging when using the Stand-Off MPV Bracket versus Continuous Z-Furring, using the THERM 6.3 software, is obvious in looking at the graphic in Figure 2.

The model shows a reduction in the R-value of -2.02. The reduction would be half if the brackets can be spaced at 4’-0” O.C. with vertical attachment elements in lieu of the 2’-0” O.C. spacing of the Continuous Z-Furring. This also results in a substantial reduction in the potential for condensation with the Stand-Off MPV Bracket versus the Continuous Z-Furring.

Len Anastasi,

FABAA ,CSI, CDT
President

Do you have questions? C all 508-580-0901
or email Exo-Tec.

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