Home Innovation Insights

Bob Hill, PE
November 8, 2012

When You're Hot, You're Hot

The cost of energy to heat and cool homes is always a hot topic of discussion for product manufacturers, builders, designers, and homebuyers. The stringent requirements in the 2012 IECC for building envelope thermal performance have ratcheted up not only the code requirements, but also the focus on the topic. Testing and labeling of insulation materials, as required by the FTC, provides an easy guide for those who need to identify the appropriate R-value with those products, but insulation is not the only element in a home’s thermal envelope that contributes to its overall energy efficiency.

As with most elements of building science, there are formulas and analytical techniques to calculate the overall R-value (or U-value) of an individual wall, ceiling, or other building component. But testing the thermal performance of an entire exterior building system – including siding, house wrap, sheathing, framing, insulation, and drywall – is the only way to tell its total value in terms of energy efficiency. To this end, ASTM developed its C1363 “Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus,” one of the many test methods for which Home Innovation Research Labs is accredited. The Home Innovation’s lab is accredited by both IAS and NVLAP to run ASTM C1363.

What is a Hot Box?

When Home Innovation Labs built its state-of-the-art product testing laboratory in 2007, we expanded our capabilities to include a hot box. The apparatus consists of two chambers – one that’s heated and one that’s cooled – and both have fans to circulate the hot or cold air. The system being tested is sandwiched between those two chambers and exposed to the hot and cold temperatures. The chambers and the surface of the test assembly are instrumented with several thermocouples to measure temperature. Once we establish the temperature difference between the hot and cold side and the amount of energy flowing through the test specimen, we can then calculate the R-value or U-factor for the system. The temperature measurement is very easy to collect from the thermocouple output. Determining the energy transmitted requires considerably more care.

The Home Innovation uses DC current for both the heating element and the circulating fans in the hot side of the box because DC current (and thus energy into the box) can be more accurately measured than AC current. We also take steps to ensure that all the energy going into the box actually goes through the test specimen – the hot side of the box is heavily insulated to R-60; and the entire hot box apparatus is enclosed in a room that is heated to the same temperature as the hot side of the box. This results in essentially no temperature difference between the interior and exterior of the hot chamber. Without any discernible temperature difference, there is no heat flow into or out of the hot chamber from its surroundings, so the heat must go through the test specimen. There are some slight “flanking losses” in this process, wherein a small amount of heat does leak through the mounting frame. To account for these losses, the hot box is calibrated with a known R-value material. Buried within the walls of the hot box is a heat flow meter. Using a combination of the output from the heat flow meter and the known R-value of the calibration wall, we can adjust for any flanking losses.

Not to Brag, But...

The Home Innovation has one of the largest hot boxes in the country for conducting ASTM C1363 – we can test specimens as large as 9’ x 16’. The larger the test specimen, the smaller the flanking losses as a percentage of the overall energy usage. The Home Innovation’s hot box has been used to evaluate conventional wood-frame wall and attic systems, as well as metal building systems, reflective insulation systems, masonry systems, and other special materials. Because orientation can also affect the hot box results, we’ve designed ours so it can be rotated allowing for walls to be tested vertically, floors and ceilings to be tested horizontally, and cathedral ceilings to be tested at the appropriate “natural” angle.

Do you have an innovative insulation system that could benefit from testing in our hot box? I'd love to hear from you and see how our unique testing capabilities can help best position and market your product.

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