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Q&A: Continuous Insulation and Insulated Metal Panels

CENTRIA’s Keith Boyer, director of architectural wall technology, tackles some of the most common questions and misconceptions regarding continuous insulation and insulated metal panels (IMPs).

Q. What is continuous insulation?

A.  Continuous insulation is defined by ASHRAE 90.1 as “insulation 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.”

Q. Do IMPs meet this definition?

A. There are not any IMPs on the market today that will meet this definition. As soon as there is a formed side joint that will encroach into the foam thickness, this definition does not apply.

Q. If IMPs are not continuous, how do they meet energy code requirements?

A. The term “continuous insulation” is often confused or misunderstood as a requirement for the thermal barrier in buildings. Actually, there are two options to determine code compliance for opaque wall areas noted in both the ASHRAE 90.1 2010 and the 2012 International Energy Conservation Code (IECC) documents. The intent of both of these options is to establish the same functional amount of wall insulation for a given climate zone regardless of the option used. Of note, the ASHRAE 90.1 2010 standard is used to write the 2012 IECC code documents.

The two options for compliance of opaque wall areas are stated in ASHRAE 90.1-2010
Section 5.5.3.
1. "Minimum Rated R-values" method; and
2. "Maximum U-factor Assembly" method.

Insulated Metal Panels use the Maximum U-factor Assembly method.

Q. What is the difference between the two options for determining code compliance of the thermal barrier for opaque wall areas?

A. To demonstrate, look at the Minimum Rated R-values method for Zone 5 prescriptive Table 5.5-5 in ASHRAE 90.1 for opaque areas supported on steel frames (studs). It indicates:
R-13 + R-7.5 c.i. (continuous insulation)
In this case, “R-13” is the rating of the insulation in the stud cavity and “7.5 c.i.” represents continuous insulation typically located outboard of the support frame. This same requirement is presented in the 2012 IECC code, Table C402.2. Note that this code compliance option cannot be used for metal-faced insulated composite foam panels because these panels do not meet the ASHRAE definition of continuous insulation.

The R-13 rated cavity insulation in a steel support cavity is not fully effective due to the thermal interruptions of the studs. A stud spacing of 16” o.c. results in an effective R-value of R-6, and a stud spacing of 24” o.c. results in an effective insulation of R-7.2.

The "R-7.5 c.i." is the rating of the fully effective continuous insulation typicallylocated outboard of the stud frame. Adding the thermal benefits of the interiorgypsum and the air films results in the following opaque wall area R-value:

Interior gypsum

0.45

Cavity insulation

6.0 to 7.2

Exterior sheathing

0.45

Continuous insulation

7.5

Air films

0.85

Total R-value

15.3 to 16.5 (Using the prescribed minimum R-values, based on stud spacing and depth of studs)


CENTRIA's Formawall Dimension Series 3"-T

In the Maximum U-factor Assembly method used by IMPs, the ASHRAE Assembly Maximum U-factors Table 5.5-5 indicates that for steel framed walls, the maximum U-factor is 0.064. This same value is shown in the 2012 IECC Table C402.1.2.

Converting the U factor to an R-value: R = 1/U = 1/0.064 = 15.6

Hence, both code-compliance options yield similar requirements as they should. Note that the Maximum U-factor Assembly option does not require continuous insulation as was required in the Minimum Rated R-value option.

 

Q. So, how do IMPs show compliance with the Maximum U-factor Assembly R-value of 15.6?

A. ASHRAE Appendix section A9.3.2 for Assembly U-Factors states that the U-factors can be determined by test using ASTM C1363.

CENTRIA’s standard Formawall® Dimension Series® panels with 1/2" reveals have been tested by witnessed ASTM C1663 tests for 2", 2.5", and 3" thicknesses. The results are noted below and include air films.

Formawall Dimension Series
R-Values Determined via ASTM C1363

Panel Thickness

R-Value with Air Films

R-Value with Air Space and Interior Gypsum

2"

13.9

15.3

2.5"

16.6

17.0

3"-T

21.6

23.0

In order to meet the R-15.6 for Zone 5—per this example—a 2.5" or 3"-T Formawall Dimension Series panel would suffice.  

Summary

  • The IBC gives two prescriptive options for demonstrating compliance for opaque wall areas.
  • Metal-Faced Insulated Composite Panels do not meet the ASHRAE definition of continuous insulation.
  • The Minimum R-value option includes cavity insulation, which is ineffective in metal stud framed walls, along with a continuous insulation requirement.
  • IMPs use the Maximum U-Factor Assembly option and are tested per ASTM C1363 to prove compliance.

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