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R-Value -: How Variable Are They?

A recent research study published by the Building Science Corporation (BSC), a respected engineering consulting firm, demonstrated that the R-values of the most common insulating materials vary based on the temperature. The good news for homeowners in climates with cold winters is that the R-value of the most common kinds increases as temperatures fall.

What Are R-values?

The "R" in R-value stands for resistance. Each digit in the R-value indicates that the product can resist temperature change for one hour. Nearly all solid surfaces have the ability to resist heat transfer to some degree, and when builders report the R-value of the insulation throughout your home, they not only include the R-value of the insulation, but also the materials that form your home's structure, like the exterior material, sheathing, framing lumber and the drywall.

The R-value of fiberglass and cellulose insulation is approximately 3 per inch of insulation. Rigid and expanding foam insulation have higher R-values per inch. A batt of fiberglass insulation that will expand to four inches has an R-value of 13, but four inches of rigid sheet of foam insulation will have a much higher value, depending on its type.

How Is an R-value Tested?

The American Society of Test Methods (ASTM) specifies the ways that manufacturers must test the ability of the insulating material to resist heat transfer. ASTM requires the laboratory to test the products by changing the temperature so that the outside exposure is 50 degrees different than the insulated space. Testing equipment measures how long it takes for the insulated space to gain or lose temperature.

Critics, however, state that these testing conditions don't reflect reality. The outside temperature used in laboratory testing is 100 degrees, and they measure the indoor temperature at 50 degrees. Even in hot climates where 100 degrees is the daily high, few spaces are kept at 50 degrees.

A more important aspect of the R-values for insulation is that the laboratories aren't required to test using cold temperatures, which are far more common in our climate than temperatures pushing 100 degrees.

BSC undertook a more thorough evaluation of various insulation types and tested them under hot and cold conditions. Their findings indicated that the R-value changed considerably as temperatures change.


The results of BSC's testing showed that different insulating products react to different temperatures with variable R-values across a temperature spectrum.

  • R-13 fiberglass batts - Fiberglass batts are the most common type of insulation used in homes throughout this region. The BSC's testing revealed its shifting R-value from -30 degrees to 150 degrees. Its R-value changed from 13 at 75 degrees to R-15 at -30 degrees and to a little more than R-11 as it approached 150 degrees. 
  • R-16 high density EPS - This type of insulation is expanded polystyrene. It's the same material that disposable styrofoam cups and those lightweight coolers are made from. Like fiberglass, its value rose as temperatures fell, going from R-17.25 at -30 degrees to R-14.75 as temperatures climbed to 150 degrees. 
  • R-20 XPS - This product—extruded polystyrene rigid insulation—was tested from -10 degrees to up to 140 degrees and performed fairly well at both extremes. Its R-value rose to 25 at -10 degrees and fell to R-21 at 135 degrees. Age matters with this product, and the BSC tested it using three age intervals. The highest performance came from the newest material, and its performance fell as it aged over the course of 44 months. 
  • One-inch polyisocyanurate- Known as polyiso, the BSC found that it's one of the few types of insulation that loses R-value as temperatures fall. It's used in homes and buildings because it has a high R-value per inch, typically R-6, and is easy to install, especially in tight wall cavities. Loose fiberglass and batts have an R-value of approximately 3 per inch, so it's easy to see why contractors turn to poliso when space is limited. 
  • Cellulose - The BSC did not test cellulose under varying test conditions, but the University of Massachusetts reports that cellulose has a slightly higher R-value than loose fiberglass. Cellulose also packs more densely, which reduces air infiltration, another factor in measuring the real-world performance of the insulation and structure in resisting temperature change. 

Why This Matters

The R-values of the insulation in your home are one of the most important components in the overall energy efficiency of your home. Knowing that the most common types of insulation used in Chicago and the surrounding region are more effective at colder temperatures means that you can count on what's in your walls, attic and floors to do its job, especially as the mercury falls.

However, the true energy efficiency of your home is more a sum of its parts than the insulation alone. Your heating and cooling bills rise and fall with the weather, and doing what you can to keep the weather out and your conditioned air in helps lower your energy usage and decreases the frequency of HVAC operation.

Recommended Insulation Levels

The U.S. Department of Energy (DOE) recommends that homes in our region have at least 13 inches of insulation in the attic, but notes that 16 inches is preferable. Insulation is an inexpensive product to purchase and install, and it starts working immediately to lower your energy bills. Inadequate or damaged wall insulation will increase conditioning costs, and professional contractors can increase the level using blown-in cellulose, fiberglass or foam.

The DOE also recommends that you have R-values of 25 to 30 in your floor. An insulated basement ceiling will provide thermal protection, as will subfloors and the flooring material itself. Padded and carpeted floors have a higher R-value than wood or tile, and thicker carpets provide more thermal protection.

Dealing With Air Leaks

Even if your home meets all the insulation standards established by the DOE, it could still suffer from thermal loses if it has air leaks in the building's envelope. You can discover them by inspecting the window and exterior door frames and closely examining the basement walls and the attic for breaches in the structure. Many can be sealed with caulk, expanding foam or metal flashing around chimneys and flues.

Another effective approach to sealing air leaks and dealing with insulation issues is to ask a licensed HVAC contractor to perform an energy audit of your home. A technician will use a blower door test that rids your home of air to test the amount and location of air leaks. This test is combined with thermographic equipment to identify air leak issues, as well as pinpointing insulation problems.

Dealing With Ductwork

Your energy audit may turn up problems with your ductwork, like compromised insulation or air leaks. Left untreated, these can drive up conditioning costs in proportion to their severity. It's a good idea not to put off having them repaired by an HVAC professional because leaky ducts can pull dust and loose insulation into your home's indoor air. If you use a combustion furnace or other ducted gas appliances, the leaks will also increase your risk of carbon monoxide exposure indoors.

Sealing Water Leaks

Regardless of insulation levels in your home, if it's wet, it won't insulate effectively. As you inspect your home, look for mold, mildew or damp spots on the insulation. Leaks in the roof or high humidity in the attic can promote mold growth and even cause ice dams during the winter—another problem associated with inadequate insulation levels, air infiltration or ventilation problems in the attic.

If you'd like more information about R-values, improving insulation and increasing your home's overall energy efficiency, contact We help homeowners throughout the greater Chicago area, northwestern Indiana and southwestern Michigan with top-notch HVAC services.

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