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Free Online Calculators for R-Values and Costs

A Massachusetts software company offers part of its application for free, allowing builders to compare different roof, wall, and floor assemblies

Posted on Feb 18 2014 by Scott Gibson

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 HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. 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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. 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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. and claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. .

Sisler says the calculator determines the effective R-values for conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow. 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 bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. 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 ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. 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.

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Tue, 02/18/2014 - 10:37

by Dwight Harris

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BeOpt works well for this purpose and runs multiple simulations to plot costs. I'm interested in it as a HERS rating tool if it's cheap. I won't do the HERS rating myself, but I'd like to mess around with an inexpensive calculator to predict final scores myself before building/rating.

Tue, 02/18/2014 - 12:06

BEopt: Response to Dwight Harris
by Nick Sisler

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Hi Dwight,

Thanks for your feedback! Yes, BEopt is a great program. Martin Holladay wrote a nice post on BEopt a couple years ago:

As you mentioned, Ekotrope is accredited by RESNET for HERS and IECC Performance ratings, which is an important part of what Ekotrope does. Ekotrope is also a web application and is being continually improved as we work with our clients. The calculator is a great way to see what the User Interface of Ekotrope is like.


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