R-Value Ratings for Insulation

R-Value Ratings

As energy efficiency has become an increasing concern among builders and homeowners, the attributes and performance of building materials and components are being scrutinized more closely. In order to maximize levels of energy efficiency by examining the details of how each individual component of a house performs on its own and as part of a dynamic system, very specific properties are measured and taken into account. This can be especially helpful when trying to select the best building materials for a given application.

R-value is the measurement used when quantifying a specific material’s level of thermal resistance, which is the inverse of U-value, which measures thermal conductance. R-value is often the standard consideration when discussing the effectiveness of insulation. It’s good to be familiar with the specifics of R-value ratings, especially when wanting to understand insulation, energy efficiency, heat transfer and energy audits.

How Does R-Value Relate to Insulation?

Heating and cooling costs account for 50% to 70% of energy used in an average U.S. home. Inadequate insulation can account for a lot of wasted energy, so it is important to be sure that insulation installed is doing its job properly and increase energy efficiency.

The function of insulation is to provide resistance to the flow of heat, and R-value is the measure of exactly this attribute for a given material. A higher R-value equates to higher resistance to heat flow and greater effectiveness in insulating. An insulation material’s R-value, in conjunction with how and where it is installed, will determine its overall thermal resistance and energy efficiency effectiveness. Adding the R-values of each layer of material contained in one building component, such as a wall or ceiling with multiple layers of insulation, will help determine the thermal resistance of the whole component. The way the insulation is installed, as well as other factors, will also affect its thermal resistance, heat transfer and energy efficiency.

Important Factors to Consider When Measuring R-Value Thermal Resistance

When considering R-value as a means to determine the thermal resistance of a building component, there are other factors that must also be taken into account. While R-values are an excellent guide for comparing the attributes of different insulation products, they apply only when the insulation is properly installed. For example, if two layers of insulation are smashed into the thickness intended for one layer, the R value does not double. Likewise, if a single layer of insulation is compressed during installation, it will not be as effective. Stuffing batt insulation sized for 5 inches into a 4-inch wall cavity will actually lower its R-value. Ensuring that insulation is correctly installed will help allow the product’s full benefits to be realized.

Also important to consider is the fact that even when installed correctly, insulation affects heat transfer through the insulation itself but not through other materials, such as glass windows and studs. If there are structural gaps in any building penetrations, even insulation with a high R-value that’s installed properly cannot mitigate heat loss from air leaks. Studs and windows provide a parallel heat transfer conduction path, and insulation between studs in a wall does not restrict heat flow through the studs. This heat flow is called thermal bridging, and the overall R-value of the wall will be different from the R-value of the insulation itself.

Calculating and Converting R-Value

The equation used to calculate R-value may be of interest to some inspectors because if the R-value is known, the equation can also be used to help calculate heat loss. The equation for determining R-value is as follows:

R-value = temperature difference x area x time ÷ heat loss

The temperature difference is expressed in degrees Fahrenheit, the area in square feet, the time in hours, and heat loss in BTUs. Since European R-value uses different units of measure (Celsius, Kelvin, meters, etc.), it may be helpful to know how to convert a European R-value into a U.S. R-value. This is done by multiplying the European value by 0.176 and dividing 1 by the result.

The FTC and DOE on R Value

In the 1970s, the Federal Trade Commission (FTC) created a rule requiring insulation manufacturers to disclose R-values at the point of sale and in some ads. This is intended to protect purchasers from false claims made by manufacturers and to create a standard of comparison for products.

insulation r value1 R Value Ratings For Insulation

The U.S. Department of Energy (DOE) has issued recommendations for insulation R-values in new and existing homes. The recommendations are based on a comparison of the cost for installing insulation versus potential future energy savings. Their recommendations for attics, cathedral ceilings, walls and floors are generally greater due to more heat flow than what is actually required by most current building codes.

R-value ratings are a useful tool, especially when comparing the effectiveness of insulation products for energy efficiency, heat transfer and controlling heat loss. Understanding a bit about how other factors such as heat flow affect a building component’s thermal resistance is important if insulation is to be used to its full benefit.

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U-Factor Window Rating

U-Factor Ratings for Windows

When quantifying the energy efficiency of a window assembly, the rate of loss of non-solar heat can be expressed as its U-factor (or U-value). Understanding the U-factors of windows is helpful for inspectors performing energy audits, as well as for consumers planning a new build or updating a house with energy-efficient windows.

U-Factor or R-Value?

While windows are rated using both U-factors and R-values, the U-factor is used to express the insulative value specifically of windows, while the R-value is used primarily to rate the energy efficiency of insulation installed in other areas of the building envelope, such as beneath the roof, in the attic, behind the walls, and beneath the floors. In order to translate a window’s U-factor into its R-value, divide 1 by the U-factor. For example, a window with a U-factor of 0.25 is calculated as 1 ÷ 0.25 = 4, so the same window has an R-value of 4.

What is the U-Factor?

The U-factor rating system was devised by the National Fenestration Rating

Council (NFRC). The NFRC is a non-profit group that administers a uniform, independent rating and labeling system for the energy efficiency of building components, including windows, doors, skylights and attachment products. The U.S. Department of Energy and the Environmental Protection Agency’s Energy Star Program take the U-factor into account when evaluating the energy efficiency of windows for product certifications, and federal incentive and rebate programs.

Windows that have the best resistance to heat flow and, thus, the best insulating qualities, have a low U-factor. Less efficient windows with poor insulating ability have a high U-factor. The combination of a window’s U-factor, air leakage, sunlight transmittance, and solar heat-gain coefficient add up to determine its level of energy efficiency.

The temperature difference between the interior and exterior of a building creates the non-solar heat flow that results in windows losing heat to the outside during the winter, and gaining heat from outside during the summer. Compensating for this by cranking the thermostat or turning up the AC results in added energy needs and higher bills. Greater energy efficiency calls for a closer examination of the individual building components to see how they can work individually and in relation to each other in more effective ways. U-factor ratings can help in formulating standardized comparisons and objective evaluations.

Determining the U-Factor

The U-factor generally refers to the energy efficiency of the complete window assembly, which includes the glazing, window frame and spacer. The spacer is the component of a window frame that separates the glazing panels, and often reduces the U-factor at the glazing edges. The performance rating of the glazing alone, independent of the frame, is known as the center-of-glass U-factor, but use of this rating is less common. For most energy-efficient windows, the U-factor for the entire window assembly is higher than the U-factor at the center of the glass.

The best, high-performance, double-pane windows may have a U-factor of 0.30 or lower, indicating that they are very energy-efficient. Some triple-pane windows may have a U-factor as low as 0.15. Manufacturers have started to incorporate low-emittance coatings and gas fills between panes in attempts to further decrease U-factors and provide an even more energy-efficient product.

U-Factors in Different Climates

While beneficial in cooling-dominated climates, a low U-factor is most important for windows in heating-dominated climates. The following are recommendations for the most effective window U-factors based on the major climate zones in the United States.

In colder climates in the North that are heating-dominated, the U-factor should be less than or equal to 0.30 for windows, and less than or equal to 0.55 for skylights. In areas where air-conditioning needs are minimal, windows that allow for solar heat gain during the day (a solar heat-gain coefficient of 0.40 or higher) can be considered energy-efficient with a U-factor as high as 0.32. Low U-factor windows are most important and will be most effective in this colder climate area where minimizing heat loss is critical to energy efficiency.
In mixed climates in the North and Midwest regions that use both heating and cooling, the U-factor should be less than or equal to 0.32 for windows, and less than or equal to 0.55 for skylights. Heating bills can help determine the importance of U-factors in this climate. Higher bills indicate the importance windows with a lower U-factor for added energy efficiency.
In mixed climates in the South and central regions that use both heating and cooling, the U-factor should be less than or equal to 0.35 for windows, and less than or equal to 0.57 for skylights. In these climates, again, heating costs can determine if a lower U-factor could be beneficial and more energy-efficient. If costs are high and a list of factors for heat loss is being addressed, window U-factor can be taken into consideration. A low U-factor for windows can also be helpful during hotter seasons when it is important to keep heat out, though a low solar heat-gain coefficient is more important in such situations.
In hot climates in the South that are cooling-dominated, the U-factor can be less than or equal to 0.60 for windows, and less than or equal to 0.70 for skylights. A lower U-factor is still useful during any cold times of the year when heating is needed in this climate. Such low ratings can ensure that heat is kept out on hot days when combined with a low solar heat-gain coefficient, which is the most important consideration in this climate.

Understanding the function and rating criteria for U-factors is a helpful tool for inspectors who perform energy audits. They can then pass this information along to their clients who may have questions about their windows and their home’s overall energy efficiency.

U-Factor Rating Information

Styrofoam Homes FYI

Styrofoam Homes

While Styrofoam may be most commonly associated with meat trays and disposable coffee cups, the material is considered by many to be a residential building material for Styrofoam homes.

Communities composed entirely of Styrofoam™ are already a reality in progressively minded Japan. These structures have been approved by Japan’s Land and

Transport Ministry, and, with the proper permit, they can be erected anywhere in that country, where they are known as “dome homes.”

Prefab home manufacturers use the material to construct easy-to-assemble, igloo-shaped modular kit homes. The Aso Farm Land resort village in Kyushu is one notable instance, where 480 Styrofoam™ domes serve as lodging, recreational facilities and retail shops. They are built from 7-inch thick, snap-together wall sections made from a type of Styrofoam™ that is considerably stronger than ordinary packing foam. Dome house kits start at around $30,000, and this price does not include the cost of transport or assembly.

Manufacturers of dome homes boast the following advantages over homes built with wood or metal:

They do not rust, decay or attract termites. It’s still wise to have an InterNACHI inspector search for such defects.
They are earthquake-resistant. In fact, models tested by the Federation of American Scientists (FAS) did not merely meet earthquake standards; they remained fully intact after being shaken harder than the strongest earthquakes ever recorded. Styrofoam™ houses can withstand typhoons, too.
The walls are treated with a flame retardant that emits no toxic fumes during a fire.
Construction is quick and easy. The prefabricated pieces, which each weigh about 175 pounds, can be carried by two or three people and assembled in a few hours.
They have excellent thermal-insulation properties, resulting in higher energy efficiency and lower heating and cooling costs. Tests performed by the FAS found that Styrofoam™ can cut heating and cooling costs by 50% to 60%, in some cases.

Even in the United States, where dome homes are still the stuff of hobbits and smurfs, Styrofoam™ homes have passed uniform building code standards, and they represent a growing niche market. In Hawaii, for instance, entire conventional multi-story homes – not Japanese-style dome homes – are being constructed from Styrofoam™. The ceiling, stairs, walls, support beams and roof of such a home are constructed with cement-coated Styrofoam™ and lack straps, tie-downs, nuts and bolts. Even the gutters are cut into the foam, and everything conforms to the building code. According to the manufacturer, these homes are cooler than homes made from conventional building materials, and they cost 10% less to construct.

Another growing market in the United States is homes that are built from a concrete-foam hybrid. The result is a synergy of the insulative capability of Styrofoam™ and the strength of a standard building material. The foam provides continuous insulation, as opposed to traditional insulation that has a break at every wood stud. The technique is about 3% to 5% more expensive for new-home construction.

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