Define opaque envelope constructions
The input process to define opaque envelope constructions consists of two steps:
- Defining the thermal properties of the envelope constructions of the building including walls, roofs, floors, doors, and any other surfaces of the building exposed to exterior conditions. Important performance characteristics include overall thermal transmittance (U-factor), heat capacity and surface properties (solar reflectance and thermal emittance).
- Assigning the constructions to the geometrical surfaces of the building model.
Defining thermal properties of envelope constructions
Simple input approach
Some simulation tools allow an opaque envelope construction or material to be defined with only a U-factor or R-value and with no heat capacity. These are sometimes called “quick” or “no mass” constructions or materials. The drawback to using this method is that the impact of thermal mass on dampening and delaying heat transfer is not represented, and there may be a fairly significant impact on heating and cooling load calculations.
Material layer input approach (recommended)
The recommended more accurate approach is to define opaque envelope constructions with materials that have heat capacity. These are sometimes called “delayed” constructions. They are typically defined as layers of materials -- such as gypsum board, concrete or insulation -- and those layers have conductivity, density, specific heat and thickness properties defined. With that information, most simulation tools use methods to estimate the time delay of heat transfer.
Layer-by-layer representation of construction assembly. (Source: IBPSA-USA BEM Workshop)
Thermal properties of opaque construction materials. (Source: IBPSA-USA BEM Workshop)
Accounting for thermal bridging
A common limitation of whole building energy simulation programs is that they do not explicitly model the impact of thermal bridging, which is a significant performance issue in many opaque constructions such as steel-framed walls. Constructions are defined in these tools as layers of homogeneous materials, but typical actual framing layers are not homogenous. A common construction layer includes steel studs with insulation in the cavity between studs, but in most simulation tools its performance must be represented by a single material. Many tools include libraries of materials that include materials that approximate the framing/insulation layer performance, and these materials may be a reasonable choice for representing the impact of thermal bridging. Otherwise, there are reference tables available in Appendix A of ASHRAE Standard 90.1 and other sources that provide effective R-values for insulation in metal framing. It is important to verify how thermal bridging is being addressed in your analysis.
There are several calculation approaches to account for thermal bridging, depending on the type of construction materials (e.g. wood vs. metal frame). See the page: Thermal bridging - modeling approach for additional guidance.
Slab-on-grade and below-grade surfaces
The method for modeling slab-on-grade and below-grade surfaces can be important, especially for low-rise buildings where they represent a significant portion of surface area. It is important to understand how the heat transfer through these surfaces is calculated in your simulation tool and to pay attention that inputs and results are reasonable.
Solar reflectance, absorptance, and emittance
Surface property inputs for solar reflectance (or solar absorptance) and thermal emittance are also important, especially for roofs. The impact of these surface properties may be a good subject for study with a simple box model.
Solar reflectance is a value between zero and one that represents the fraction of total solar irradiance that is reflected by a surface, and the solar absorption is simply equal to one minus the reflectance. Common roofing materials such as composite shingles and mineral cap sheets have solar reflectance values in the range of 0.10 to 0.30. White roof membranes are available with solar reflectance greater than 80% and are commonly referred to as “cool roof” membranes. For the purpose of an energy model, it is recommended to enter somewhat lower values to account for dirt and degradation, typically in the range of 0.55 to 0.65.
Thermal emittance is also a value between 0 and 1 that represents the ability of a surface to freely emit radiant energy. For exterior surfaces, a high value is usually desired, so that the surface re-radiates absorbed heat. Fortunately, for most building materials the thermal emittance is in the range of 0.8 to 0.9. One exception is metallic materials, which have lower thermal emittance. In the extreme case, materials like low-e coatings on windows and radiant barriers can have thermal emittance below 0.05.
Assigning constructions to the model surfaces
After the thermal properties of the constructions have been defined, you must assign them to the model. Many software tools can automate this process by automatically assigning an exterior wall construction to all of the model's exterior walls, or a selected subset. If different constructions are used on different orientations of the building, then a user may select all the zones on that orientation and assign a chosen set of constructions, then repeat this process for other orientations.
If there are unique constructions used only in certain places, then the assignment process may involve clicking an individual surface on the model and manually assigning the construction. Some software tools allow you to select a surface (or surfaces) from a list - in this case, it is strongly recommended that, while creating the geometry, you give any special surfaces a unique, easily-identifiable name such that you'll be able to find it on a long drop-down list.
Simple box model approach
The impact of opaque envelope component performance is often a subject for simple box modeling analysis, so some attention to how those constructions are defined is warranted. Important performance characteristics include overall thermal transmittance (U-factor), heat capacity and surface properties (solar reflectance and thermal emittance).
At the time of a simple box modeling analysis, the type of construction and the amount of insulation are often not yet decided. If information about a likely choice is not available from the design team, then a common approach is to choose a construction for your base case model based on energy code requirements. There are, however, often several classes of construction defined by energy codes, such as wood-framed walls, metal-framed walls, and mass walls, and they all have different insulation requirements. Therefore, some judgment is necessary to choose an appropriate construction type for the simple box model. A common choice is light-weight construction types for the walls and roof: steel framed walls with cavity insulation and metal deck roof with insulation on top; these are the construction types used for the baseline building in ASHRAE Standard 90.1’s performance rating method calculation, and the table below lists the required insulation levels.
| Climate | Roof, insulation above deck | Steel framed wall | ||
| Zone | Insulation | U-factor | Insulation | U-factor |
| CZ0 | R-25 | 0.039 | R-13 | 0.124 |
| CZ1 | R-20 | 0.048 | R-13 | 0.124 |
| CZ2 | R-25 | 0.039 | R-13 + R-3.8 | 0.084 |
| CZ3 | R-25 | 0.039 | R-13 + R-5 | 0.077 |
| CZ4 | R-30 | 0.032 | R-13 + R-7.5 | 0.064 |
| CZ5 | R-30 | 0.032 | R-13 + R-10 | 0.055 |
| CZ6 | R-30 | 0.032 | R-13 + R-12.5 | 0.049 |
| CZ7 | R-35 | 0.028 | R-13 + R-12.5 | 0.049 |
| CZ8 | R-35 | 0.028 | R-13 + R-18.8 | 0.049 |
Detailed design input data
Detailed design model envelope properties should represent the project's constructions as specified in later phases of a project such as the design development and construction documents phases. Compliance models are generally representative of the final design.
Envelope thermal properties should be based on architectural drawings and specifications, but some information may not be explicitly shown on the drawings and will need to be gathered separately in order to define the model inputs appropriately.
Architectural drawings
Architectural drawings will generally tell you what type of constructions are proposed. For example, they will indicate whether the walls are steel framed construction or concrete. The elevation drawings may include notes that indicate wall and window types. But, this doesn’t tell us everything we need for the energy model.
Section drawings provide an extra level of detail by showing the layer-by-layer materials that make up a particular construction. These drawings will generally call out the material layers by name (e.g. gypsum board, metal frame with batt insulation, air gaps, concrete panels, etc.), however they generally do not include detailed thermal properties of the materials.
Example of an architectural elevation drawing with notes to indicate the surface types. (Source: IBPSA-USA BEM Workshop)
Example of wall section drawing showing layers of a wall construction. (Source: IBPSA-USA BEM Workshop)
Material thermal property data
For detailed models where construction assemblies are defined with layer-by-layer materials, the thermal properties of each layer must be input to appropriately account for heat transfer.
Most BEM software these days includes libraries of typical construction materials and their thermal properties, so they can be selected from a list and easily defined.
In some cases, the material may not exist in the software library, so you may need to research the data for a particular material.
A good reference for this information is the ASHRAE Handbook Fundamentals.
As noted above, material layers that consist of framing with insulation must calculate an overall R-value for the composite layer to properly account for thermal bridging effects—a steel framed wall with R-30 insulation will have a significantly lower overall R-value than 30!
Comparing to pre-calculated values
A good reference for overall assembly U-factors for different types of roofs, walls and floors is Appendix A of ASHRAE Standard 90.1. The U-factors vary depending on frame depth and spacing as well as cavity insulation R-value and continuous insulation R-value. This is a good source of information to see if the assemblies you created for your model are reasonable. If you are performing modeling for the purpose of compliance with Standard 90.1, then you are required to model assemblies that match these values.
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