Constructions and thermal mass options
Use BEM to compare performance of constructions and thermal mass options during conceptual design. A BEM analysis can compare different construction types, such as steel, wood or concrete. The analysis might also evaluate the impact of insulation levels, thermal bridging, or the thermal properties of the exterior surfaces. These analyses are especially valuable during conceptual design because some decisions about opaque envelope constructions are locked in at an early design phase, such as decisions about the fundamental structure type, and they can have a significant impact on performance.
Impact of Construction and Thermal Mass Decisions
The choice of constructions affects heating and cooling loads in two fundamental ways: 1) heat transfer through the exterior envelope, which can be either heat loss or heat gain depending on outdoor conditions and 2) heat storage in constructions exposed to indoor air (thermal flywheel effect), which affects the rate of change of indoor air temperature.
Two important properties of constructions that affect the heat transfer and heat storage are 1) thermal transmittance, typically represented by the U-factor of the construction, and 2) heat capacity, which is an indication of the construction’s ability to store heat. Constructions with high heat capacity, such as concrete walls, are also said to have high thermal mass.
Due to the impact on heating and cooling loads, the choice of construction can have large impacts on energy consumption and energy demand for comfort conditioning as well as sizing of HVAC equipment associated with peak heating and cooling loads.
Construction and Thermal Mass Alternatives
A useful BEM analysis during conceptual design includes alternatives that cover the range of possible thermal transmittance and heat capacity. Those alternatives might include cases with no insulation, moderate insulation and very high insulation. Similarly, the constructions might have low, medium and high heat capacity. With results that represent some extremes, the designers gain an understanding of the magnitude of the potential impact of their choice of constructions.
One source for information on moderate insulation values is the relevant energy code. See the page define opaque envelope constructions for some examples.
Solar reflectance is another property to consider for evaluation during conceptual design, especially in warm sunny climates with high cooling load. See solar reflectance, absorptance and emittance.
For ideas on additional options, see the list of energy conservation measures.
Guidance on Modeling Approach
During conceptual design, a simple box model approach is often appropriate for comparing options for envelope constructions. To get accurate results, pay attention to the issues described in the page define opaque envelope constructions. It is especially important to use a modeling approach that includes the effect of thermal mass, especially for heavy constructions, and to account for thermal bridging, especially for constructions with metal framing.
A second approach to comparing envelope constructions during early design is to start with a prototype model that is a reasonable match for the expected building design and modify the prototype model to create versions with different constructions.
See also the general guidance on other model inputs, such as internal gains and HVAC systems here: Guidance on Modeling Approach.
Guidance on Presenting Results
Presentations are most effective if they focus on results that are important to the project team, which might include EUI, energy cost, peak cooling load, or other metrics. Ideally, the presentation helps the team to prioritize strategies by showing the relative impact of walls, roof and floors.
The following types of presentation can be helpful for illustrating the impact of the choice of constructions.
- X-Y line charts showing how results vary with insulation level or mass, for example a chart of peak cooling load as a function of insulation R-value.
- Pie chart or bar chart showing the relative contribution of walls, roofs and floors to peak loads along with sources of heat gain and loss.
- Histograms for operating hours in each load range, which could, for example, show how many hours each year that cooling load exceeds 100 tons for several different constructions.
- Heat map of hourly results to show annual patterns of load, temperature or consumption, perhaps to show the impact on indoor temperature of constructions with different levels of thermal mass.
BEM results that are typically found useful are described in this page: Analyzing Model Outputs. Other important considerations:
For more inspiration in developing visual presentation of massing study results, see examples on the website Project Stasio such as the following:
Other Considerations
The choice of constructions has impacts beyond loads and energy, and it may be useful to consider these other factors when evaluating design decisions.
- Embodied carbon of building materials
- Occupant thermal comfort
- Construction cost
- Resilience, such as the comfort benefit of thermal mass if HVAC systems cannot operate
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