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Measuring Thermal Performance

By Robert Naini

What do you think of when you hear the words thermal performance? Do you think of insulation, R-Value, energy efficiency?

That’s a good start, because it’s all of them; it’s how something performs with respect to temperature. But how do you measure thermal performance? Let’s consider a simple example that many of us are familiar with, a coffee cup.

When you are at home you probably use a ceramic mug for your coffee. But when you pick up a cup of joe to go from the convenience store, like me, you probably get it in a foam cup. Why?

Well, the obvious reason is the EPS foam cup is considerably more cost-effective for the store to give away, but moving beyond that, consider the thermal performance of each.

After you fill it with piping hot coffee, what is the temperature of the outside of the ceramic mug? What about the temperature of the foam cup?

You will feel the heat directly through the outside of the mug, that’s why they have handles, but you pick up the foam cup without hesitation.

From a conceptual standpoint, you understand this, because you are familiar with coffee, served both in a ceramic mug and in a foam coffee cup, but how does this relate to measuring thermal performance?

R-VALUE AND COFFEE MUGS

Well, when it comes to a building, most often, the first consideration of thermal performance is R-Value.

R-Value is the amount of heat that does NOT pass through a thickness of material per unit area in one hour, if the temperature difference between the hot and cold side of the material is one degree. This is thermal resistance, so the higher the R-Value, the better the thermal performance.

So, considering our coffee cup example, the R-Value of ¼-inch ceramic is R-0.02, compared to the R-Value of 1/2-inch EPS foam is about R-0.58.

coffee-cup-1797280Consider this for a moment: The foam cup is almost 30 times more effective at resisting the heat than the ceramic mug.

When you are at home or out to eat, you are in a controlled environment and the expectation is that you will be drinking the coffee at that moment, so there is little risk that the coffee will get cold before you finish it. But when you get that cup to go, the foam cup helps minimize temperature loss, keeping the coffee warmer, longer, especially on those cold winter mornings.

AIR INFILTRATION: PUT A LID ON IT

After addressing thermal insulation, the second consideration of thermal performance is air infiltration.

Once again, we can return to the foam coffee cup, because after you fill it up, and add cream and sugar, you put a lid on it. Obviously, this helps prevents spills, but it also creates a barrier between the hot coffee and the cooler air around it. When the lid is off the cup, the coffee loses heat to the air around it, through convection, and cools down faster.

This also happens in buildings.

During the winter, a building with 10 air changes per hour will lose more heat than a building with five air changes per hour.

You can think of lowering the air leakage of the building the same way you think of adding a lid to a coffee cup.

Taking these ideas into account, you will see that both R-Value and air-tightness of a building have a considerable effect on the thermal performance of the structure. This is why spray foam insulation has gained such momentum over the past 10 years, because it can offer both insulation and air barrier properties in a single application.

Spray foam insulation regularly offers some of the highest insulation values in the industry, even for applications with limited space.

And, by providing an air barrier, spray foam can reduce a building’s heat load related to air infiltration, a thermal benefit that is not measured by R-Value.

Additionally, spray foam insulation can also have an impact on radiant heat transfer and the associated radiant loads. Have you wondered why a traditional, vented attic temperature could easily be over 120°F when it is only 90°F outside? This is primarily due to heat radiating off the superheated roof deck to the inside of the attic.

Under the hot sun, a black shingle roof could easily exceed 150°F surface temperature and as the heat conducts through the roof deck, the interior roof deck surface might be close to 140°F. This heated surface acts like the heating element in your toaster and radiates heat into the attic heating the air up to 120°F. By installing spray foam insulation to the underside of the roof deck, just like turning off your toaster, you change the temperature of the interior radiant surface. The new radiant surface is the exposed surface of the spray foam, and rather than being 140°F, like the exposed roof deck, it will be closer to the interior conditioned temperature, around 80 to 90°F. Thus, having a tremendous effect on the radiant heat load.

Overall spray polyurethane foam can address all three forms of heat transfer:

  • Conduction, of course, with its R-Value
  • Convection because of its air barrier qualities
  • And, radiation because the temperature of the exposed radiating surface is significantly lowered.

Therefore, spray foam outperforms other insulation materials, because spray foam can fundamentally, scientifically, have an impact on all three forms of heat transfer. And with all of this in mind, and a few key tools, the overall thermal performance of a building can be measured and projected.

I am talking about energy modeling software, such as:

  • REScheck™
  • COMcheck™
  • EnergyPro
  • Elite RHVAC
  • Wrightsoft Right-J8
  • Florida Solar Energy Center’s EnergyGauge.

These programs capture thermal efficiency data for a building and are regularly used to analyze the thermal performance of residential and commercial buildings.

Obviously, these programs require input related to the building envelope, including R-Value of various materials and performance ratings of the windows and doors, but there are additional factors that go beyond R-Value to help document how the building works. For example, the air handler and ductwork can be moved into the building envelope, and once these are within conditioned space, duct leakage can be reduced to a minimum.

Furthermore, these programs generally have an input for the building leakage rate, or natural air changes per hour, and the building gets an energy performance benefit for having a tightly sealed envelope. These software programs can also provide a compliance report for building code approval purposes under the performance method of the energy code.

Sometimes, knowing that insulation, R-Value, and tighter envelopes means better thermal performance is not enough, and you need to measure and project a building’s energy efficiency, it can be done.

The answer is energy modeling software and there are professionals that can help you. •

Robert Naini has a Bachelor’s of Science in Mechanical Engineering and an MBA from the University of Texas in Arlington. With more than a decade of experience on the cutting edge of spray foam insulation, he has helped hundreds of owners and managers grow their business with a unique knowledge base including spray foam sales & marketing, employee & applicator training, building science awareness and building code expertise. Spray Foam Advisor offers web-based training and education, with videos, articles, blogs and more, to help solve problems for spray foam professionals and the construction industry.