A team of software developers is offering a free online calculator you can use to compare the thermal performance and cost of a variety of roof, floor, and wall assemblies.

Ekotrope is part of a larger application of the same name that the company sells to residential designers. Nick Sisler, one of Ekotrope’s founders and a product development engineer at the company, said by e-mail that the full version of the software is accredited as a HERS rating tool.

The company would clearly like to sell you their software. But if you’re not interested in buying into the full application, the calculators provide a quick way of comparing different combinations of materials.

Change the type of insulation, from high-density polyurethane foam, let’s say, to cellulose and the calculator instantly reflects changes in R-value and cost per square foot. A series of drop down menus lets users adjust a variety of values, such as stud depth and spacing, and the type of sheathing and cladding.

Sisler says the calculator determines the effective R-values for conduction and convection but does not take into account bulk air movement. As a result, the effects of air leaks in a wall or roof assembly can’t be factored in.

“Though somewhat limited, it is still useful in understanding the effects of thermal bridging and also for quickly comparing different materials from an insulation and cost basis,” Sisler’s e-mail said.

Data come from a variety of sources

Sisler says the algorithm used in the calculator is based on the same method described by John Krigger in Residential Energy, supplemented with material from both ASHRAE Fundamentals and RESNET standards.

As to the estimated costs of various assemblies, he says information comes from Ekotrope’s internal research and from projects the company has worked on. Users can edit the values if local costs are different. The cost values are updated periodically but not on a regular basis.

The Oak Ridge National Laboratory’s Building Envelopes Program also offers several online calculators, including one for estimating whole-wall R-values. They’re also available online for free.