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The recently commissioned a research team to examine how multiple common building methods affect a building envelope’s ability to keep warm or cool air either in or out.

The team spent two years looking at different construction 鈥渁ssemblies鈥� 鈥� steel-stud construction, for example 鈥� and the way heat can travel through them. The research was accepted by the ASHRAE research board, and researcher and building science specialist St茅phane P. Hoffman recently presented the results to a group of engineers, architects and officials at the Center for Architecture in Portland.

Hoffman’s team believes that its data, gathered from laboratory experiments and three-dimensional models, will help create more accurate energy consumption predictions. It also expects that the information will eventually be incorporated into building codes.

Hoffman recently discussed the research with the 91视频.

91视频: The overall effect of your research will be creation of energy models with greater accuracy. Why is that important?

St茅phane P. Hoffman: Our research provides an understanding of the actual energy consumption of the building, and the knowledge of how to selectively make interventions to reduce energy consumption.

This is of particular interest if you are designing to the Living Building Challenge standards or a net-zero energy building. You need to understand how much energy will be lost or consumed through the building envelope, because that drives the sizing of your mechanical systems, which drives the sizing of your photovoltaic (solar panels), or your geothermal or other site-generated energy.

If you’re looking to minimize how many solar panels you have to put on the building, you need to really understand the real loads they need to support.

91视频: For projects trying to meet the Living Building Challenge, there are concerns about three things that are difficult to model: how much energy a building will need, how much energy these new solar or other technologies will generate, and how people will behave in the building. How does your research fit into that challenge?

Hoffman: The information in this study helps you address one of those components, which is the actual energy consumption of the building enclosure. What people do in the building is probably a much more sophisticated model. Our research provides a tool on which to make better-informed decisions about the design of an enclosure system.

91视频: To start with, introduce us to the idea of building envelope performance. A lot of this is measured with an 鈥淩鈥� value. What is an R value and how does it work?

Hoffman: The R value is a measurement of the thermal resistance of an assembly to heat flow 鈥� how good a material is at minimizing heat loss. It’s a property of the material, but also of how it’s used in the construction of the building and interfacing with various assemblies.

Some materials 鈥� for example, board insulation 鈥� are very good and have a high R value. Some materials like steel studs are great conductors 鈥� so they’re the opposite, and have a very low R value; this is called a 鈥渢hermal bridge.鈥�

When you mix these materials you have to come up with an effective R value for that assembly, and that is the problem.

If you have, for example, a batt (fiberglass) insulation in a steel stud assembly, the batt insulation has an R of 19 鈥� but the bridging that occurs through the steel stud reduces the overall R value by more than half.

91视频: I understand that the 鈥渢hermal bridge鈥� problem allows heat to travel through the conductive materials, like steel, to circumvent the insulating values of insulation. How do people deal with this problem currently?

Hoffman: Over recent years, the expected R values of building have been decreasing, but they don’t reflect actual performance.

There’s a concept out there that if a little insulation is good, more must be better, and a lot is best.聽 At a certain point, though, you can add more and more insulation but you won’t overcome the thermal bridge. With our research, you can find out the optimal level of insulation.

The code had just been saying 鈥渁dd more insulation,鈥� but I think what we’ll go to at some point is more effective insulation. This is the current drive in a lot of the industry in terms of modeling: a change to more performance-based modeling instead of prescriptive modeling.

91视频: What is the problem for people designing buildings facing this 鈥渢hermal bridging鈥� issue?

Hoffman: Right now, there’s very little accounting for additional heat loss. It’s traditionally been made up for with additional safety factors that the mechanical consultant applies when sizing his mechanical equipment.

However, the lack of an ability to accurately model this means that you can’t give the mechanical consultant a good sense of what the actual energy use of the building will be 鈥� and they might use more money to make a bigger system than is needed as a result. How can you size a mechanical system if you don’t know the actual energy use of the building?

As technology has improved, we can more and more accurately model the actual performance and help the mechanical consultant understand what the actual loads will be on the mechanical system.

91视频: Why hasn’t there been another study like yours to find the actual impact of thermal bridges?

Hoffman: Up until now, all thermal modeling of assemblies has been done only in two dimensions, which works well when you’ve got something that’s infinitely long. If you’re doing a warehouse building, and it’s infinitely long, you have the ability to take a cross-section and model it very consistently, because heat loss at the periphery is minimal.

Unfortunately, we don’t build in two dimensions. That’s where the impact of our use of 3-D modeling comes in, is being able to determine how these interfaces impact the value of the walls 鈥� especially in areas where the plane of construction changes, like in window transitions, corners and slab edges.

91视频: How can someone use your findings, and how might this change how a building is designed?

Hoffman: The report provides a series of graphs and tables, which allow practitioners to determine heat loss without having to do these 3-D models themselves. From this they could find out that is the actual heat loss, and what is the impact of additional insulation. You can also determine the risk of condensation, by determining surface temperature at critical points.

One of the ways this information can be used is to figure out where a majority of the heat loss is 鈥� Is it in support for cladding, or in the slab edge? It helps you focus your design efforts. It also helps you understand the elements in that assembly that really influence the thermal bridge, and how those could be modified to reduce the impact.

91视频: Who will and won’t be affected by this research, now and in the future?

Hoffman: For the average practitioner building to code, there isn’t an incentive to use this right now, because it shows you a lot of energy use that is not taken into account in current code. If you really want to know how much energy is being used, it’s a good tool. If you’re just looking to meet the baseline code, you don’t need that additional information.

In the interim, it provides better information for those individuals who are pursuing low-energy buildings 鈥� things that are already going beyond the code and need more sophisticated tools to assess the energy consumption of the building.

Eventually, that will come into adoption. I think that at some point, there will be a realignment of what’s expected from a thermal performance perspective.

It takes time for things to be adopted through the code process, but down the road we expect this to form the basis of how energy consumption is modeled and required under the code.