3.5.2 Construction Assemblies

Assembly Name
Applicability	All projects
Definition	The name of a construction assembly that describes a roof, wall, or floor assembly. The name generally needs to be unique so it can be referenced precisely by surfaces.
Units	Text, unique
Input Restrictions	Construction name is a required input.
Baseline Rules	Not applicable

Specification Method
Applicability	All projects
Definition	The method of describing a construction assembly. The more simple method is to describe the U-factor of the construction assembly which can account for thermal bridging and other factors. However with this method, the time delay of heat transfer through the construction assembly is not accounted for. Generally, with the U-factor method, heat transfer is assumed to occur instantly. The more complex method is to describe the construction assembly as a series of layers, each layer representing a material. With this method, heat transfer is delayed in accord with the thermal mass and other properties of the assembly.
Units	List: choices are U-factor or Layers
Input Restrictions	The layers method shall be used for all constructions except for metal building or similar constructions with negligible thermal mass.
Baseline Rules	For each construction, the proposed design specification method shall be used.

U-factor
Applicability	All construction assemblies that are specified by a U-factor
Definition	The steady state rate of heat transfer through a construction assembly
Units	Btu/h-ft²-°F
Input Restrictions	U-factors should be consistent with values in Appendix A of ASHRAE Standard 90.1-2013.
Baseline Rules	Not applicable

Layers
Applicability	All construction assemblies that use the layers method of specification
Definition	A structured list of pairs of material names that describe a construction assembly, beginning with exterior finish and progressing through to the interior finish. Material names must be from the standard list (Appendix D) or defined (see above).
Units	Data structure: construction assembly
Input Restrictions	The user is required to describe all layers in the actual roof assembly and the proposed design will be modeled as input by the user.
Baseline Rules	See building descriptors for roofs, walls, and floors.

3.5.2 Materials

Energy simulation programs commonly define construction assemblies by listing a sequence of material layers that make up the construction assembly. Typical construction assemblies and their respective material layers are defined in Normative Appendix A of Standard 90.1-2019.

Material Name
Applicability	When construction assemblies reference materials that are not standard
Definition	The name of a construction material used in the exterior envelope of the building
Units	Text, unique
Input Restrictions	Material name is a required input for materials not available from the standard list. The user may not modify entries for predefined materials.
Baseline Building	Not applicable

Density
Applicability	All non-standard materials
Definition	The density (or mass per unit of volume) of the construction material
Units	lb/ft³
Input Restrictions	Density is a required input when non-standard materials are specified as documented in an ASHRAE handbook, a comparably reliable reference, or manufacturers’ literature
Baseline Building	Not applicable

Specific Heat
Applicability	All non-standard materials
Definition	The specific heat capacity of a material is numerically equal to the quantity of heat that must be supplied to a unit mass of the material to increase its temperature by 1°F
Units	Btu/lb·°F
Input Restrictions	Specific heat is a required input when non-standard materials are specified. The specific heat capacity of the construction material as documented in an ASHRAE handbook, a comparably reliable reference, or manufacturers’ literature.
Baseline Building	Not applicable

Thermal Conductivity
Applicability	All non-standard materials
Definition	The thermal conductivity of a material of unit thickness is numerically equal to the quantity of heat that will flow through a unit area of the material when the temperature difference through the material is 1 °F.
Units	Btu/h·ft·°F
Input Restrictions	Thermal conductivity is a required input for non-standard materials as documented in an ASHRAE handbook, a comparably reliable reference, or manufacturers’ literature.
Baseline Building	Not applicable

Thickness
Applicability	All non-standard materials
Definition	The thickness of a material
Units	ft or in.
Input Restrictions	Thickness is a required input for non-standard materials
Baseline Building	Not applicable

Construction assemblies for the proposed design shall be created by selecting from a library of building construction layers in Standard 90.1-2022, Appendix A. The software shall specify all composite layers that consist of both framing and insulation and shall use established methods defined in the Standard 90.1- 2022 Appendix A or the ASHRAE Handbook of Fundamentals for calculating effective R-values of composite layers.

In accordance with ASHRAE 90.1-2022, it is mandatory for projects to identify each linear and point thermal bridge associated with the proposed design, as required in Section 5.5.5. The impact of these thermal bridges must be captured in the proposed energy model using one of the following methods:

Inclusion of a Separate Model: A separate model of the assembly can be incorporated within the energy simulation model.
Adjustment of Clear-Field U-Factors: Alternatively, clear-field U-factors can be adjusted in accordance with Section A10.2.

The inclusion of thermal bridging requirements in 90.1-2022 stems from the findings of ASHRAE Research Project (RP) 1365 “Thermal Performance of Building Envelope Details for Mid- and High-Rise Buildings.” This research highlighted that thermal bridges can significantly increase annual energy consumption when compared to buildings without such bridges.

Alterations other than additions are exempt from thermal bridging requirements according to Standard 90.1-2022 Section 5.5.5 exception #6.

Example of a project that is adjusting the clear-field U-factor in accordance with Section A10.2

A 33,000 square foot, 4-story multifamily building includes thermal bridging due to brick shelf angle cladding support, a parapet that follows the entire perimeter of the roof, and wall to vertical fenestration intersections. Below is a high-level graphical depiction showing the location of these thermal bridges. The length of the parapet would equal the perimeter of the building since it runs along the edge of the roof, the brick shelf angles run continuously along the perimeter once per floor so the total linear length would be the perimeter of the building multiplied by 4-stories, and the length of the wall to vertical fenestration would be the sum of the perimeter of each window as depicted with the yellow lines around the perimeter of the window shown in the graphic below.

Figure 5. Location of Thermal Bridging

The design team needs to quantify the length of each linear thermal bridge as measured on the outside surface of the building envelope (Li) and the number of occurrences of each type of point thermal bridge (ni). Below is an example of the minimum information needed to determine the proposed U-factor to model (Utot) for example assembly EW-1. This process would need to be completed for all above grade wall assemblies. This example only includes linear thermal bridges. Project teams can use the built-in functionality in COMcheck for 90.1 2022 to complete and document these calculations.

Table 21. Calculation of Linear Thermal Bridging

Assembly ID	Class of Construction	Uo Proposed Design		Atotal
EW-1	Steel framed and metal buildings	0.055		13,294
	Thermal Bridge Type	Li	ni	Meets 90.1 5.5.5 Prescriptive Requirements?
	Cladding support	1,162.0	0	Yes
	Parapet	415.0	0	No
	Wall to vertical fenestration intersection	1,387.2	0	Yes

The proposed Utot is calculated using equation 90.1 2022 A10.2 which is shown directly below.

Utot = {[(∑ψi × Li) + (∑χj × nj)]/Atotal) + Uo}

(7)

Where:

Utot = overall thermal transmittance, including the effect of linear thermal bridges and point thermal bridges not included in the construction assembly Uo-factor, Btu/(h·ft²·°F)

Uo = clear-field thermal transmittance of the construction assembly as determined in accordance with Section 5, Btu/(h·ft²·°F)

Atotal = total opaque projected surface area of the construction assembly, ft²

ψi = psi-factor, thermal transmittance for each type of linear thermal bridge from 90.1 2022 Table A10.1. The default column shall be used where the thermal bridge meets 90.1 Section 5.5.5 prescriptive requirements. The unmitigated column shall be used where the thermal bridge does not meet the prescriptive requirements., Btu/(h·ft·°F)

Li = length of a particular linear thermal bridge as measured on the outside surface of the building envelope, ft

χi = chi-factor, thermal transmittance for each detail type of point thermal bridge from 90.1 2022 Table A10.1. The default column shall be used where the thermal bridge meets 90.1 Section 5.5.5 prescriptive requirements. The unmitigated column shall be used where the thermal bridge does not meet the prescriptive requirements., Btu/(h·°F)

ni = number of occurrences a particular type of point thermal bridge

The “Proposed Design Psi-Factor, Btu/(h·ft·°F) per Table A10.1” column in the graphic below shows the Psi-factor used in the calculation of Utot for the proposed design for this example assembly. This is determined based on whether the specified thermal bridging mitigation techniques (if any are specified) meet the prescriptive requirements in 90.1 Section 5.5.5. If the specified thermal bridging mitigation techniques do not meet prescriptive requirements, then the unmitigated column in Table A10.1 must be used. For reference, the “Unmitigated/Default Psi-Factor, Btu/(h·ft·°F) per Table A10.1” column below shows both the Unmitigated and Default columns from 90.1 2022 Table A10.1 (left value is unmitigated and right is default). The chi-factors are not shown because they are not applicable to the thermal bridging types shown in the table.

Table 22. Psi-Factor Calculations for Linear Thermal Bridge

Assembly ID	Class of Construction	Uo Proposed Design		Atotal
EW-1	Steel framed and metal buildings	0.055		13,294
	Thermal Bridge Type	Li	ni	Meets 90.1 5.5.5 Prescriptive Requirements?	Proposed Design Psi‑Factor, Btu/(h·ft·°F) per Table A10.1	Unmitigated/ Default Psi‑Factor, Btu/(h·ft·°F) per Table A10.1
	Cladding support	1,162.0	0	Yes	0.217	0.314/0.217
	Parapet	415.0	0	No	0.289	0.289/0.151
	Wall to vertical fenestration intersection	1,387.2	0	Yes	0.112	0.262/0.112

Using equation A10.2 the proposed Utot required to be modeled in the proposed design model for this example assembly is calculated as follows:

Utot proposed for EW-1 = 0.055 + [(1,162 ft * 0.217 Btu/(h·ft·°F)) + (415 ft* 0.289 Btu/(h·ft·°F)) + (1,387.2 ft * 0.112 Btu/(h·ft·°F))]/13,294 ft² = 0.055 + 0.0353 = 0.095 Btu/(h·ft²·°F)

Below is an example format for complete documentation/supporting calculations. These, or similar, calculations would need to be submitted for each above grade wall assembly. The chi-factors are not shown because they are not applicable to the thermal bridging types shown in the table. Project teams can use the built in functionality in COMcheck for 90.1 2022 to complete and document these calculations.

Table 23. Utot Calculations for Linear Thermal Bridge

Assembly ID	Class of Construction	Uo Proposed Design		Atotal	90.1 2022 A10.2 Utot Proposed Using Actual Default or Unmitigated Psi and Chi Factors
EW-1	Steel framed and metal buildings	0.055		13,294	0.095
	Thermal Bridge Type	Li	ni	Meets 90.1 5.5.5 Prescriptive Requirements?	Proposed Design Psi‑Factor, Btu/(h·ft·°F) per Table A10.1	Unmitigated/ Default Psi‑Factor, Btu/(h·ft·°F) per Table A10.1
	Cladding support	1,162.0	0	Yes	0.217	0.314/0.217
	Parapet	415.0	0	No	0.289	0.289/0.151
	Wall to vertical fenestration intersection	1,387.2	0	Yes	0.112	0.262/0.112

Assembly Name
Applicability	All projects
Definition	The name of a construction assembly that describes a roof, wall, or floor assembly. The name generally needs to be unique so it can be referenced precisely by surfaces.
Units	Text: unique
Input Restrictions	Construction name is a required input
Baseline Building	Not applicable

Specification Method
Applicability	All projects
Definition	The method of describing a construction assembly. The simpler method is to describe the U-factor of the construction assembly, which can account for thermal bridging and other factors. However, this method does not account for the time delay of heat transfer through the construction assembly. Generally, with the U-factor method, heat transfer is assumed to occur instantly. The more complex method is to describe the construction assembly as a series of layers, each layer representing a material. With this method, heat transfer is delayed in accord with the thermal mass and other properties of the assembly. For below-grade constructions, a C-factor can be specified; for slab-on-grade constructions, an F-factor is specified.
Units	List: layers, U-factor, C-factor, F-factor, R-value
Input Restrictions	The layers method shall be used for all constructions except for metal building or similar constructions with negligible thermal mass may use the U-factor method
Baseline Building	Construction assembly for the baseline building will be defined in layers

Layers
Applicability	All construction assemblies that use the layers method of specification
Definition	A structured list of material names that describe a construction assembly, beginning with the exterior finish and progressing through the assembly to the interior finish. Materials are described in Section 3.5.1
Units	List: layers of construction assembly
Input Restrictions	The user is required to describe all layers in the actual assembly and model the proposed design based on the layer descriptions
Baseline Building	See building descriptors for roofs, exterior walls, exterior floors, doors, fenestration, and below-grade walls