Home Inspection FYI

Providing Help, Knowledge and Other Useful Information To Home Owners Everywhere

Home Inspection FYI - Providing Help, Knowledge and Other Useful Information To Home Owners Everywhere

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.insulation r value R Value Ratings For Insulation

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.

 R Value Ratings For Insulation

U-Factor Window Rating

U-Factor Ratings for Windows

 u factor windows U Factor Window Rating
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 Ratingnfrclabel U Factor Window 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.  winter U Factor Window Rating
  • 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

Window Gas Fills

Window Gas Fills: What Consumers Should Know

Improvements in window design are an important step toward reducing overall energy costs, as heat gain and loss through windows account for up to half of a home’s heating and cooling needs, according to the U.S. Department of Energy.

Gas fills, as well as other efficient or green window features, such as low-E coatings and Heat Mirror® insulating glass, help plug “thermal holes” in the building envelope.

When used in conjunction with low-E coatings, gas-filled window units can yield incredibly high R-values, so they make a good choice for retrofitting a home to enhance its overall energy efficiency.  Consumers can educate themselves on the benefits of multi-paned gas-filled windows, and home inspectors who perform energy audits and/or thermal imaging inspections can help their clients determine whether a retrofit with this type of window is a cost-effective upgrade.

What are Window Gas Fills?

Gas fills are special gases pumped inside insulated glass units (double- and triple-paned window assemblies also known as IGUs) to improve their thermal efficiency. These viscous, slow-moving gases (typically, argon and krypton) allow for less convection than would ordinary air, minimizing convective currents within the window and reducing the transfer of heat across the window. Argon Window Gas Fills Homeowners may be able to tell if a building’s windows are gas-filled by checking the specifications on the original work order or the window tag, which is typically found along the bottom inside track of the window. Two small holes may also be observed along the spacer — one where the gas entered the unit and the other for air to exit.

Argon and krypton are the gas fills used most often by window manufacturers to displace the air between the panels in windows. Argon, which comprises slightly less than 1% of the Earth’s atmosphere, is non-toxic, inert, clear and odorless. Its thermal conductivity is roughly 67% that of air and it’s inexpensive, making it an attractive gas fill.

Krypton shares many qualities with its fellow noble gas argon, except that it’s an even better insulator, albeit more expensive to produce. When cost and functionality are considered, argon is a more efficient thermal barrier per dollar spent, especially in the larger ½-inch (11mm to 13mm) gaps between double-paned windows. Krypton is more commonly used in the tighter ¼-inch to 3/8-inch (6mm to 9mm) gaps within triple-paned windows. A mixture of krypton, argon and oxygen gases is sometimes used to reconcile performance and price, and occasionally xenon and nitrogen are used, as well.


Window Gas Leaks


Gas-filled windows will leak over time — as much as 1% per year, according to some estimates — depending on the quality of the window and its installation, the building’s climate, including its exposure to the sun and the altitude, and other factors.  However, these windows will usually perform adequately even after many years of gradual depressurization.  According to the National Glass Association, if 80% of the gas remains in spite of gradual leakage over time, a window can be expected to maintain its properties and effectiveness.  That means that, “even if 1% was leaking out per year, the window would still be effective in 20 years.”

If the breach is significant, however, the window will no longer be an effective thermal barrier and may require replacement. Condensation or fog inside the window unit indicates that the gas fill has escaped and been replaced with moisture-laden air. Homeowners should clean the window’s exterior to be sure that the observed moisture is, in fact, within the window rather than on its surface. The manufacturer or installer should be contacted if the window fails. Leaks can be detected only with special gas-detection equipment.  But homeowners should rest assured that leaked argon or krypton poses no health hazards to a home’s occupants.

One rare yet shocking consequence of argon leakage is the sudden implosion of the window. Popped argon window Window Gas FillsDue to molecular differences between argon and the principal components of air, nitrogen and oxygen, argon under pressure to escape a window unit may exit the seal faster than it can be replaced with air. Under this circumstance, the glass will bend inward to accommodate the gradual reduction in pressure within the window. If conditions are right, according to USGlass Magazine, “units have been reported to shatter with a bang, sometimes described as loud as a gunshot. While the broken glass usually stays within the unit, on at least one occasion, glass has shattered with enough force to send glass shards flying outward.” Elevation differences between the location where the IGU was first pressurized and its installation location may also account for the negative pressure. This phenomenon is quite rare, however, especially in newer windows with superior seals.

Double- and triple-paned windows are often filled with the gases argon or krypton to reduce convection within the window units to improve the building’s overall energy efficiency.  IGUs can be huge energy-savers for homeowners, and InterNACHI inspectors can help their clients determine whether these windows are the right choice.