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BPI Phrase of the Month: R-value
posted at 2017-01-16
"R-value" is one of the building science terms you spot in everyday life. You see it on products in the home improvement stores, hear home inspectors talk about it, remodelers, energy auditors, and even your super cool neighbor who just had work done on his house. So let's talk about what R-value is, why it matters, and why you are probably missing out on its importance.
What is R-value?
R-value itself is a measurement, a rating, which corresponds to a material's thermal resistance (the ability to resist heat transfer through conduction). The higher the R-value, the greater resistance to heat transfer.
Have you ever grabbed a handle on a pot of boiling water and burnt your hand? That is conduction - molecule to molecule heat transfer. Heat moves from areas of higher heat to areas of lower heat. In this case, heat moved from the bottom of the pan to the handle. That law, the First Law of Thermodynamics, is true everywhere.
The higher the R-value, the greater the ability to resist conductive heat transfer. Think of pulling something out of the oven after baking at 400 degrees Fahrenheit. You wouldn't take it out barehanded (not twice at least). Instead, you would use something that resists conductive heat transfer to your hand: an oven mitt. The longer you hold the object, the more you realize that it is a time-based resistance - your hand gets increasingly warmer. The rate at which your hand warms depends on the material of the oven mitt (also known as its R-value).
Why does R-value matter?
The R-value of a material, such as insulation, is only useful when there is a temperature difference on opposing sides. Insulation doesn't care if it is resisting heat from leaving or resisting it from entering a home. But we, as homeowners and home occupants, do. We care about both depending upon the season. Heat loss, or gain, because of inadequate or missing insulation can have an enormous effect on your comfort and energy usage.
The greater the difference in temperature between inside and outside, the greater the impact. More insulation is better, but only up to a point. There comes a point where the cost (and amount of space available) to install the insulation outweighs the benefit of additional R-value.
Real life example of insulation R-values
While different types of insulation have different R-values per inch, the most important piece of this R-value equation does not lie in the type of insulation, but rather how and where it is installed. For example, fiber batt insulation typically has a rating of anywhere from R-19 to R-38 depending on how thick it is. Compressing the insulation to fit in the cavity reduces the overall R-value. Compressing an R-38 batt (12 inches thick) into a 2x8 inch cavity reduces the R-value to R-27. A 64-square foot batt of R-38 can cost $77 whereas a 64-square foot batt of R-25 is closer to $47.
Without proper air sealing prior to the installation, wind-washing protection, and allowing gaps, voids or compression, the R-value of the material drops remarkably faster. Doing it wrong wastes money in more ways than one.
Not all insulation types are susceptible to these problems and they all have their pros and cons. But, no matter which insulation you choose, if it is not installed correctly, you may as well throw your money out the window, and you could cause more harm than good.
A BPI Certified Professional or BPI GoldStar Contractor can help you understand what level of insulation is best for your area, and the benefits of adding the correct amount. Find a knowledgeable contractor in your area by using our locator tool.
Categories: General
