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October 28, 2010

3-D thermal models keep pace with complex wall assemblies

3-D computer modeling that determines actual or real thermal values of a building envelope is expected to prove its worth in years to come as complex wall assemblies are increasingly specified to meet higher energy performance standards.

The advanced modelling software is an important step forward from 2-D analysis to calculate accurate heat flow paths through wall assemblies, says Patrick Roppel, building science engineer, Morrison Hershfield Limited (MH).

Roppel says the analysis would be used to help designers and architects design assemblies to meet specific energy standards. “There is a drive to doing more whole building energy analysis, especially with programs like LEED and P3 projects where the same people doing the building will be operating the building. That translates to a drive to get accurate results.”

Not much research has been done on the thermal properties of wall assemblies using 3-D software but MH has been retained to conduct 3-D analysis on 40 typical building envelope details for mid- and highrise construction for the American Society of Heating Refrigerating and Air Conditioning Engineers (ASHRAE).

The study, which includes details on 17 steel-stud assemblies, covers all the climate zones in North America.

The research looks at simulations relevant to existing and future building stock and captures both retrofit and new construction details, he says.

The study assesses typical interior finishing and cladding systems and attachment methods. Heat loss around windows, parapets, floor slabs and other details in wall assemblies are being analyzed in mid and highrise construction covered by ASHRAE’s 90.1 standard. The standard is referenced by a number of building codes in Canada.

A key priority is to analyze details where thermal bridging is an issue, Roppel says.

The two-year study, which wraps up in January, will provide ASHRAE with design procedures to calculate the effect of thermal bridging on the thermal performance of steel and concrete framed buildings, explains Roppel. Thermal bridging is a process where heat is lost through highly conductive materials such as steel studs or concrete.

A big concern is that any cladding with exterior insulation (an increasingly common application today) typically incorporates steel elements to attach it to the structure and that steel can be a major point of heat loss through thermal bridging, he points out. Other options, he notes, are available that MH is looking into.

While calculations for common wall assemblies have been reasonably accurate, calculating the thermal performance for more complex wall systems has been difficult to estimate using 2-D models, he says. “We’re refining that now with our 3-D model to include actual real life 3-D heat flows and other influences such as contact resistance (steel studs against drywall, for example).”

Roppel says that the nominal R-value of an insulation batt in a steel stud wall is not an accurate measurement of the R-value of many steel -stud wall assemblies. In fact, the overall heat loss through the wall assembly indicates the real R value of the wall is only 40 to 60 percent of the nominal R value of the insulation batts.

That’s not news, says Roppel, because extensive testing and research has been done since the 1990s. However, until now thermal analysis software was primarily 2-D, not 3-D.

“We’re starting to put more insulation outboard, use different methods to attach cladding, choose more assemblies and different construction methods. This is where there is a lack of information that designers can use with confidence.”

The 3-D software used by Morrison Hershfield to assess thermal values can also be used for other 3-D analysis such as structural and stress problems, says Roppel.

MH’s research isn’t dealing with bulk air movement from air leakage or convection. It is simply addressing thermal performance due to heat flow by conduction (heat flow through solid materials) and radiation (heat flow through the air from surfaces in plain view of each other).

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