Article #1492 - COMcheck Web-Based Training Questions - Building Envelope

This article contains questions asked during the August 18, 2005 live broadcast of the COMcheck Web-Based Training session. The original language of the questions and answers has been modified to make them as generic as possible while retaining the applicable specific information.

For the continuous insulation value, do I add the R-values of construction materials?

No. Construction materials are included in the drop-down list of assemblies. For assembly types that do not fit into any of the assemblies shown in the drop-down list, you can choose "Other" as the assembly. If you use "Other" as the assembly type, you must enter an overall U-factor for the entire assembly, including air films. Be prepared to provide the building department with manufacturers' literature or documentation of U-factor calculations.

I am adding new rooftop unit onto an existing, unaltered 1970s structure. The structure does not pass in COMcheck. Now what?

What you are describing is an alteration to a building. In this case, the major national codes do not require that the entire building meet energy code; only the equipment or building component that is being changed or added must meet the energy codes.

I have had a situation where a city requires compliance with current code when adding a single door in the wall of an existing 40,000 sq. ft. building. The building cannot be brought into compliance without extensive additional insulation. What do you recommend?

The major national energy codes typically only require that the added or changed component(s) comply with the code. Please verify this with your local officials.

I am renovating an existing building. The demolition process reduces the building down to only 8-inch block walls and a built-up roof. Would it be acceptable to treat the building as new construction?

This sounds like a reasonable approach and would likely be acceptable to the local building official.

If I have a covered breezeway, are the walls considered interior or exterior?

If airflow exists next to the wall, it would act thermally like, and therefore be considered, an exterior wall.

How do I handle a clerestory?

The glazing and wall or roof areas of the clerestory are handled in COMcheck the same way as other glazing and wall/roof components.

How are demised partitions treated between adjacent retail spaces in a strip retail center? Are they treated as exterior walls or not included at all in the wall area?

Typically, envelope compliance is done on the entire building at the time the shell is designed and/or constructed, in which case the demising walls are not included. If you are doing a tenant space after construction of the shell, the demising walls would still be ignored because the original design assumes these will always be between conditioned spaces.

What is "budget" U-factor? Can I go beyond this value?

The term "budget" refers to the values that are allowed, as in the amount you have to use. To comply, you may not go beyond this value.

Do thermal mass credits exist?

COMcheck accounts for thermal mass in walls using the same algorithms and parameters used in ENVSTD: heat capacity and insulation position. In ENVSTD values for these inputs are entered directly by the user; in COMcheck default heat capacity and insulation position parameters are provided based on your selection of wall type. The exception to this is the Other wall category, for which you must specify the heat capacity.

All COMcheck compliance calculations assume that any insulation in an above-grade exterior wall (or above-grade portion of a below-grade wall assembly) is integral with the thermal mass of the wall, as opposed to assuming the insulation is either on the exterior or on the interior of the wall. This assumption was made because the insulation position has little impact, producing a maximum of about a 1% change in cumulative space-condition loads in a high-mass wall. In addition, for many wall assemblies, it is difficult to determine which of the three options -- interior, integral, or exterior -- is the most appropriate.

This assumption also appears to be appropriate for the most common wall assemblies; e.g., metal stud walls. Coincidentally, the integral insulation position appears to result in the most favorable impact from high heat-capacity walls. Assuming integral insulation is the best case for advocates of masonry construction and effectively eliminates any grounds for criticism of simplifications in COMcheck in this area.

How do I account for louver square footage within the building envelope?

In ASHRAE 90.1, up to 1% of the opaque area of the assembly is exempted from the envelope requirement if the entire assembly is insulated. Otherwise, you need to use an area averaged U-factor in the calculation.

How do I figure the insulation value in steel buildings where the insulation is compressed?

COMcheck has the following assembly options for metal buildings (roof and wall assemblies) where the insulation is compressed.

Metal Roof without Thermal Blocks and Metal Roof with Thermal Blocks. The base assembly consists of a roof where the insulation is draped over metal purlins and compressed where the metal structural members are attached to the metal purlins. R-values for additional continuous insulation may be added to the base assembly. Two cases of screw-down metal building roofs are considered in COMcheck. One case involves the use of a 1 in. x 3 in. foam thermal block (other than compressed insulation) between the purlin and metal roof members. The other case is identical but without the thermal block material at the purlins. The base assembly R-value for uninsulated roofs is 0.78, representing the interior and exterior air film coefficients. Balance of assembly U-factors and framing factors are used as coefficients of a linear regression equation developed to represent the assembly U-factors of standard insulation R-values for metal building roof assemblies, as listed in the table.

Metal Building Roof (MBR) Assembly U-Factors for Standard Insulation Thicknesses

Other. COMcheck allows you to define a roof assembly by specifying its overall effective U-factor. This option permits you to accurately describe the performance of any roof assembly not adequately covered by the predefined roof types.

Metal Wall Without Thermal Blocks.The base assembly consists of wall insulation that is compressed between metal wall panels and the metal structure. The heat capacity of the wall is assumed to be 1.0. For un-insulated metal walls, the overall R-value for the assembly is 0.85 assuming only indoor and outdoor air film coefficient R-values of 0.17 and 0.68. The balance of assembly R-value and framing factor for insulated metal building walls is used as coefficients in a linear regression equation developed to represent assembly U-factors for various standard insulation R-values, as provided in the table for metal building wall systems with 7-foot girt spacing.

Metal Building Wall (MBW) Assembly U-factors for Standard Insulation Thicknesses

Other. COMcheck allows you to define a wall assembly by specifying its overall effective U-factor. This option permits you to accurately describe the performance of any wall assembly not adequately covered by the predefined wall types.