Material Properties and Energy Impact

BIM for Architecture, Engineering, and Construction Curriculum


Thermal Properties of Building Materials

How is the R-value computed for a wall composed of many layers?

The R-values of each of the layers that compose the wall system can be simply added together to compute the R-value for the entire assembly.

If a layer of a wall assembly is made of two materials (for example, the core layer of many wood-framed walls includes wooden studs at 16 inches (0.40 m) on center and insulation between the studs), how is the R-value determined?

When a single layer contains more than one material, the R-value is computed as a weighted average of the R-values for each of the individual materials. Each material is weighted by the relative proportion of that material in a section of the wall. For the example cited, the wood material would receive a weighting of 1.5 inches(0.04 m) per 16 inches (0.40 m) of wall, and the insulation would receive a weighting of 14.5 inches (0.37 m) per 16 inches (0.40 m) of wall.

Does the location of the insulating materials affect the overall thermal performance of a wall or roof assembly?

The location of the insulating materials does not affect the thermal performance. Insulation is just as effective whether placed near the interior or exterior surface of a wall or roof.

Designers must be careful, though, about the placement of vapor barriers relative to the insulating layers. Vapor barriers should always be placed to prevent moisture in the air from condensing inside the wall or roof assembly.

How does the ratio of window area to wall area in the building envelope affect the energy consumed? As the percentage of window area increases, what typically happens to the energy consumed?

Windows typically have higher U-values (and lower R-values) than insulated wall assemblies. So as the percentage of window area increase, more energy is flowing through the windows. This effect may be offset, however, by the solar gains available from well-placed windows.

Several window manufacturers are now introducing ultra-high efficiency windows whose U-values are much closer to insulated walls, but the difference is still significant.

What is the relationship between changes in R-values (or U-values) and the energy consumption? Is it linear?

The relationship between R-values (and U-values) and the amount of energy flowing through a surface should be linear. But this may be difficult to see, because many other factors also affect the total energy consumption.

Can the results of analyzing a single classroom unit be extrapolated to estimate values for an entire school campus? Why or why not?

The results of analyzing a single classroom unit are a good starting point for making materials choices, but great care must be used when extrapolating these results to estimate the consumption for an entire campus. The single classroom unit is exposed to the exterior on all sides, so simple multiplication will produce an estimate that is too high. 

The configuration of how the individual classroom units will be joined to form buildings must be considered to determine the reduced area of the building envelope.

Thermal Comfort

How does the number of occupants, their activity level, and the time of use affect the heating and cooling required in a thermal zone?

All three factors work together to determine the amount of heat being generated inside the room by the users. More occupants, higher levels of activity, and longer periods of use all generate more heat.  

In colder months, this internal heat gain typically means that less energy is needed to maintain the zone at a comfortable temperature level. In summer months, however, the internal heat gain will add to energy required to cool the zone to a comfortable temperature level.

Why does changing the upper or lower limit of the thermal comfort range have such a big impact on energy consumption? What determines how much you can raise or lower the limits?

By widening or tightening the comfort band, we are allowing the zone to get hotter or colder before the HVAC system is used to heat or cool the space to bring the temperature within the comfort range. This has a very direct effect on the amount of energy consumed by heating and cooling.

The amount you can raise or lower the limits of the thermal comfort range ultimately depends upon the building's users and what temperatures they will find acceptable for a comfortable working environment. Comfort is highly subjective.

How can the Ecotect Analysis thermal analysis be used to help specify the type and size of a heating and cooling system?

Ecotect Analysis software's thermal analysis will predict the heating and cooling loads for various types of HVAC systems throughout the year based all factors considered in the energy model - including local climate data, materials choices, and building use patterns. Using this data, you can choose a heating and cooling system type that meets the project's requirements and size the system to handle the loads predicted.