Analysis of thermal performance of different wall construction materials

Example wall section detail showing construction layers and insulation locations

BEM is an excellent tool for evaluating the energy and HVAC sizing impact of different wall constructions. The schematic design phase is a time when many fundamental decisions are made regarding building structural systems, and the information provided by a BEM study can help the design team capture cost effective savings opportunities.

For a discussion of using BEM during conceptual design, see Constructions and thermal mass options.

Impact of wall constructions

Obvious impacts of the thermal performance of wall construction are heating and cooling loads and the associated HVAC energy consumption and peak demand. BEM can be used to evaluate these impacts.

Other environmental impacts include the embodied carbon represented by wall materials and the global warming potential of blowing agents used in some foam insulation products.

Alternatives

Exterior wall example. Exterior glass fiber reinforced concrete (GFRC) panels with interior metal furring and spray foam insulation

During schematic design, the alternatives to be evaluated with BEM might be a specific set of wall constructions identified by the design team, or it may be up to the modeler to identify a set of alternatives.

Baseline wall construction for analysis:

As designed. If the design team has identified a likely construction type and insulation level, then that wall construction may be an appropriate starting point. See Detailed design input data for an example of wall constructions shown on architectural plans.
Energy code minimum insulation. If a construction type, e.g. metal frame, wood frame or concrete, has been selected but the insulation level is not identified, then a good starting point is the minimum insulation level required by energy code based on the construction type.
Performance compliance baseline. If neither wall type nor insulation level is yet to be determined, then the baseline wall construction defined in the energy code’s performance compliance approach is a good choice. See an example here: Define opaque envelope constructions.

Some of the most common alternatives evaluated using BEM:

Framed walls: frame material (metal or wood), frame depth and spacing, cavity insulation R-value, continuous insulation (typically foam board or mineral wool) R-value, thermal mass of sheathing and cladding material, exterior solar reflectance.
Mass walls: exterior or interior continuous insulation R-value, exterior solar reflectance

Other considerations:

Thermal bridging. Alternatives that minimize thermal bridging caused by framing materials, slab edges and other construction details should be considered.
Spandrel glass. In designs where portions of the façade consist of spandrel glass, pay attention to insulation options and thermal bridging from window framing.

See also the conceptual design page on Constructions and thermal mass options.

Guidance on modeling approach

As with most studies performed during schematic design, the modeler will need to make a number of assumptions regarding systems that are yet to be designed and also make reasonable simplifications in order to provide quick and timely design feedback.

Base model

The model to use as a starting point will typically be based on the latest version of the floorplan and elevations, with an understanding that they may be evolving. A simplified approach to thermal zoning is typically appropriate. Input for Internal heat gains and HVAC systems should be reasonable for the building type. Some judgment will be required, but it is important to remember that quicker analysis with more assumptions may be more helpful than later analysis with more detail.

Some inputs warrant extra attention:

Schedules for occupancy and HVAC operation. The impact of wall construction may be different for buildings that are occupied only during the day and those that are occupied 24 hours per day.
Window area and performance. Reasonable inputs that reflect the likely façade design will help ensure that the results for wall constructions will be appropriate. Due to solar heat gain and high thermal transmittance of windows, their presence will affect the amount of time that a perimeter zone is in heating mode or cooling mode and will indirectly affect the impact of wall constructions.

Wall construction modeling approach

A material layer modeling approach is recommended in order to represent wall heat capacity as well as thermal transmittance. See Define opaque envelope constructions for more information.

Guidance on finding material thermal properties is provided here Sources for material thermal property data and here Material thermal property data.

Surface properties are discussed here Solar reflectance, absorptance, and emittance.

Attention to thermal bridging is very important. See Accounting for thermal bridging and Thermal bridging - modeling approach.

Check inputs

Check that the intended wall construction alternatives are accurately reflected by model inputs. As discussed in Review and analysis to verify model quality, simulation output reports often include a summary of opaque constructions that can be reviewed as a quick check.

Additional guidance

See these pages for more guidance

Guidance on presenting results

There are several potential messages to convey when presenting results.

Annual energy and peak demand impact and the associated operational carbon emissions.
Impact on HVAC system sizing.
The contribution of opaque constructions to annual and peak heating and cooling loads in the context of all sources including other envelope components and internal heat gains, to help the design team understand priorities for load reduction.
The diminishing returns of increasing insulation R-value.
The potential of high insulation levels to minimize heating loads to a point that separate perimeter heating systems may not be required.

Information about the embodied carbon for wall construction alternatives can be very helpful if it is available.

General BEM results that are typically found useful are described in this page: Analyzing Model Outputs. Other important considerations:

Example charts showing wall insulation impact on EUI and chiller sizing

Example chart showing contributors to peak cooling load, including exterior walls

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