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Getting A Handle On R-Value

March 12, 2015 | Posted by Mitt Jones
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iStock_hotpot_crophigh.jpgAs much we try to avoid jargon, in the home energy world we talk a lot about R-value. Every now and then a customer asks a very good question: “What is R-value, anyway?” I usually explain that R-value is a measure of the effectiveness of insulation, the higher the better. That isn’t bad, but the question deserves a better answer.

Thermal Resistance

R-value provides an indication of the thermal resistance of a material, meaning its ability to resist heat transfer. A substance with a very low R-value, such as metal, lets heat pass through very quickly. Imagine grabbling the metal handle of a pot of very hot soup.

A material with a high R-value doesn’t let much heat through. Think about the potholder you’re smart enough to wrap around that metal handle.

To keep our homes comfortable and energy efficient, we want the pot holder, not the metal handle. The higher the R-value in our attics, walls, and crawlspaces, the easier it is to keep the heat in during winter and the heat out during summer.

R-Values of Common Materials

Here are rough, representative R-values for a few common building materials. The R-values are given per inch of thickness. With an R-value of 3 per inch for fiberglass batts, for instance, a 3.5-inch thick batt would have an R-value of 10.5, or about 11. (High-density 3.5-inch fiberglass batts offer an R-value of 15.)

Concrete 0.3 per inch
Brick 0.8 per inch
Wood 1 per inch
Drywall 1 per inch
Loose-fill fiberglass 2.5 per inch
Fiberglass batts 3 per inch
Loose-fill cellulose 3.7 per inch
Extruded foam board 5 per inch
Foil-faced foam board 6.5 per inch

 






 

 

The best way to determine the R-value of any given insulation is to look for the rated R-value on the packaging or on the product itself. Rated R-value is usually determined based on measurements at a constant temperature, though real-world R-value can vary with temperature.  

Doing the Math

If formulas bring back bad memories, consider yourself warned: From here on we’re talking math and physics.

The opposite of thermal resistance is thermal conductivity, or u factor.

A simplified but commonly used heat transfer equation is Q = uΔT, where Q is the amount of heat that flows through the material and delta T is the difference in temperature from one side of the material to the other. The higher the u factor, the greater the heat transfer.

R-value is the reciprocal of u factor, as in: R-value = 1/u

That makes our simple heat transfer equation Q= ΔT/R.

Visualizing R-Value

Heat_transfer.gifI borrowed the graph here from our blog post “Why More Insulation Isn’t Always the Answer.” It’s one of my favorite graphs. The three curves plot heat loss as a function of R-value for temperature differences of 10, 30, and 50 degrees.

Notice how heat transfer plummets from R-0 to R-10 or so. Also notice how the shapes of the curves differ for the examle temperature differences of 10, 30, and 50 degrees Fahrenheit.

Summer attic temperatures in Portland can easily lead to a temperature difference of 50 degrees between your living space and the attic, particularly if you use air conditioning.  

The graph does a pretty good job of illustrating what you may already know intuitively: The bigger the temperature difference you need to maintain across your walls, floor, and/or ceiling, the more important it becomes to improve your insulation.

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