by Keith Boyer, PE
In the age of high-thermal-performance building envelopes, there can be many surprises when the designed wall system does not meet the expected performance criteria. Sometimes these ‘misses’ are significant and can severely impact the energy use for a building. Transitions from one material type to another, such as opaque wall areas to windows and wall to roof, are often the cause.
The thermal performance of an entire wall assembly can be determined by knowing and understanding basic heat flow properties, such as U-values for materials and job-specific details. While there are very powerful 3-D finite element programs available to evaluate specific assemblies, these are complex, costly, and not used as often as they should be. There are simpler methods using the American Society of Heating, Refrigerating, and Air-conditioning Engineers’ (ASHRAE’s) U equation (found in its Fundamentals handbook), along with the THERM 7.3 program developed by Lawrence Berkeley National Laboratory (LBNL).
ASHRAE fundamentals provide a simple equation to determine the effective or average U-value of a wall system based on parallel heat flow paths and a weighted average based on area. U-values for various elements of a wall—like opaque areas and windows—are typically readily available from standard testing. For example, there is ASTM C1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot-box Apparatus, for opaque areas. National Fenestration Rating Council (NFRC) 100, Procedure for Determining Fenestration Product U-factors, and American Architectural Manufacturers Association (AAMA) 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, are available for window assemblies.
For the elements that do not have tested U-values, THERM 7.3 can be used to approximate a U-value. A good approximation for overall wall performance can be obtained through the use of the U equation and tested or modeled U-values. For this article’s purposes, the test assembly will be a wall that has only opaque and window areas. With this simplified assembly, the U equation has only two terms as shown below:
U= ((UOAAreaOA) + UwindowAreawindow))/Area
In this equation, AreaOA is the opaque wall area, whereas Area is the total wall area—the sum of AreaOA and Areawindow.
Case 1: Code requirements/whole-wall U-value
The International Energy Conservation Code (IECC) establishes the minimum thermal performance levels (i.e. maximum U-value) for the opaque areas and the windows. For Zone 5 of the 2012 IECC, the maximum U-value for opaque wall areas on metal-framed (studs) buildings is 0.363 w/(m2 K), (0.064 Btu/(hr sf F)), which converted to terms of thermal resistance is an R-15.6.
For the same zone, the fenestration maximum U-value is 2.158 (0.38), which is an R-2.6. Also established is the prescriptive maximum fenestration area, which is 30 percent. Given this input and the U equation, the IECC minimum code standard for a wall with opaque areas and windows is U= 0.90 (0.159) or R-6.3.2
Windows can severely degrade the whole wall thermal performance. The code assumes a seamless transition between window and opaque area.
Case 2: Window perimeter transition details
Frequently, the connection between a window and the surrounding opaque area focuses on weather-tightness and not thermal efficiency. Weather-tightness is needed and should not be compromised, but often better thermal details are desired. Details degrading the thermal performance include:
- metal panning or flashing that violates the window frame thermal breaks;
- the plane of the insulated glass unit (IGU) does not align with the insulation in the opaque area; and
- thermal breaks that are not properly aligned.
Through modeling these transitions using THERM 7.3, it was determined that, depending on the detail, the affected transition area can be approximately 305 mm (12 in.) wide around the window perimeter and can have a heat flow through this area much increased from the opaque material U-value to values approximately equal to the window.
To simplify this evaluation, and to demonstrate use of this evaluation process, it is assumed the affected area is 305 mm wide and the heat flow is equal to that of the window. This, in essence, is increasing the area of the window. For a 1.5-m (5-ft) square punched window with a 305-mm transition, the window area is essentially doubled and the opaque area is decreased by the same amount. Using this info in the U equation for the code minimum example above, the U value increases to 1.44 (0.254), or R-3.9 in IP units. This is 38 percent less than the expected code minimum of R-6.3.
The code is clear about minimum performance levels for both opaque areas and fenestration, but does not prescribe the transition details. Similarly, none of the standard test protocols establish material U-values. Unless designers are good stewards concerning the transitions, there can be a whole wall installation that, although it meets the letter of the code, fails to meet the spirit of the code. This can be achieved by avoiding thermal shorts via through-metal flashing and trim, aligning breaks in the adjacent materials, and, when possible, creating a thermal model using Therm 7.3.
ASHRAE-sponsored research to investigate the effects of transition details as was published in ASHRAE Research Project (RP) 1365, Thermal Performance of Building Envelope Details for Mid- and High-rise Buildings. However, some of the better transitions were not modeled.