Evaluating the Impact of Opaque Envelope Components: A Comparative Analysis

Published by firstgreen on

A building’s envelope – composed of its roof, walls, and windows – plays a crucial role in determining its overall energy efficiency. These components, characterized by properties such as conductivity and thickness, affect the amount of heat transfer between the building’s interior and the external environment. In this blog post, we will evaluate the impact of different envelope assemblies, particularly opaque components, on a building’s energy consumption.

To perform this analysis, we considered four cases: three design variations and a base case as per ASHRAE standards. We used data and simulated the building energy model using a building energy simulation software Design Builder.

Table: Building Envelope Assemblies and Corresponding U-Values

CaseComponentConductivity (W/mK)Thickness (m)LayersU-Value (W/m²K) EPI
ASHRAE Base CaseWindow0.80.0432.50 434.06
Case 1Window0.60.0431.88 429.14
Case 2Window1.00.0433.13 437.07
Case 3Window0.70.0432.19 430.93

                     ASHRAE BASE CASE (wall/roof)                                                                      CASE 1(wall/roof)

                               CASE 2 (wall/roof)                                                                                       CASE 3(wall/roof)

(Note: U-value refers to the heat transfer coefficient, lower the U-value, the better the material insulates.)


Our results reveal significant variations in the output energy across different cases due to differences in U-values, which are dictated by the thermal conductivity, thickness, and number of layers of the materials used.

ASHRAE Base Case: The total EPI for the base case was 434.06. This case uses standard materials as defined by ASHRAE and serves as our comparison benchmark.

Case 1: By using materials with lower thermal conductivity for the window, wall, and roof assemblies, we significantly improved insulation, reducing total EPI to 429.14, a  reduction from the base case.

Case 2: This case, in contrast, used materials with higher thermal conductivity. This resulted in higher U-values and consequently, more heat loss. The total EPI increased to 437.07, an increase from the base case.

Case 3: This case used materials with slightly improved thermal conductivity than the base case. As a result, there was a modest reduction in EPI to 430.93, a reduction from the base case.


This analysis underscores the crucial role that a building’s envelope, particularly its opaque components, plays in determining its energy efficiency. By selecting materials with low thermal conductivity and suitable thickness, we can significantly reduce a building’s energy consumption, resulting in cost savings and a reduced carbon footprint.

Choosing the right envelope materials can lead to substantial energy savings. Hence, investing time and resources in optimizing the building envelope design and assembly can yield long-term benefits.

Remember, the perfect envelope assembly will depend on a variety of factors, including climatic conditions, building use, and local building codes. It is, therefore, essential to use building energy simulation tools in the design phase to optimize these selections.

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