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Adding the right insulation to a building is critical to maintaining indoor comfort with energy efficiency. There is an optimum level of insulation that will provide the maximum comfort for the minimum cost. This amount is specified by the R-value of the insulation per area in the building based on climate conditions. The US Department of Energy recommends insulation R-values as a guide to achieving this optimum insulation level. |
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| What is R-Value? |
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Building materials vary widely in their ability to resist the transfer of heat. Metal, for example, is a very good conductor of heat. Materials that do not conduct heat well are used for insulation. Air is an excellent insulator. Houses are built with air cavities in the walls and the attic to resist the flow of heat. Traditional insulation materials use air spaces between fibers or in plastic foam bubbles to trap expanding warm air and prevent it from escaping the building.
Insulation is rated by its ability to resist both conductive and convective heat flow in units called R-value. R-value gives the insulation resistance per inch of material. Construction materials with higher R-value ratings are more effective insulators than materials with lower ratings for the same thickness. For example, heat will flow through an R-12 insulated wall only half as fast as through an R-6 wall.
R-value is a function of material type, thickness and density. The R-value of an insulation system (including wood, drywall, siding, and air spaces) is calculated by adding the R-values of the individual components together to achieve the recommended insulation protection based on climate. As increasing resistance to heat flow saves a lot of money in heating and air conditioning costs, the US Department of Energy has raised their R-value recommendations in recent years for both metal frame and wood frame new construction buildings. R-values are used for all types of insulation except foil radiant barriers.
Insulation system for a cathedral ceiling includes rigid and reflective insulation, two air spaces, drywall, and the roof decking
From top layer to bottom:
- Roof
- Rafters (Ventilated Air Space)
- Rigid Insulation
- Wood Strapping
- Reflective Insulation
- Wood Strapping (second air space)
- Drywall
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R-value is helpful in comparing different types of bulk insulation as well as different brands of the same type of insulation. For example, rigid foam insulation has R-values that are almost double the R-value per inch of fiberglass or cellulose insulation. R-values for fiberglass batts average 3.2 per inch while R-values for rigid foam range from 3.6 - 8 per inch for foil faced polyisocyanurate rigid foam board.
State and federal agencies recommend insulation R-values for different areas inside of a building based on local climate conditions with the attic requiring the most insulation. Divide the recommended R-value by the R-value per inch of the type of insulation you want to use to determine the necessary insulation thickness. You can determine the correct R-values for the different areas in your building based on type of construction, the type of fuel you use and your zip code using the Department of Energy`s R-Value Recommendation online form found at
http://www.ornl.gov/sci/roofs+walls/insulation/ins_16.html.
If you use reflective insulation and bulk insulation in combination, you can add in up to an additional 14.5 R (Details) depending on whether the reflective insulation has foam, plastic bubbles or fiberglass for its central layer. Foam core reflective insulation (like foam board insulation) has the highest R-value. To be effective, reflective insulation must face an air space. ASHRAE assigns a 1" air space R-2.77 for upward heat flow and R-4.5 for downward heat flow. The Masonry Advisory Council adds an additional R-2.89 to rigid foam insulation for a foil facing if it faces an air space. If you use either reflective foil or foil faced insulation facing an air space, you can add an additional R-value of 2.8 for upward heat flow without increasing the insulation thickness. The foil will also act as a radiant barrier preventing up to 97% of radiant heat from entering through the roof or walls.
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| Table 2. R-value of Insulation Previously Installed in Existing Homes |
| Insulation type |
R-value per inch of thickness |
| Fiber glass blanket or batt | 2.9 to 3.8 (avg. 3.2) |
| High performance fiber glass blanket or batt | 3.7 to 4.3 (avg. 3.8) |
| Loose-fill fiber glass | 2.3 to 2.7 (avg. 2.5) |
| Loose-fill rock wool | 2.7 to 3.0 (avg. 2.8) |
| Loose-fill cellulose | 3.4 to 3.7 (avg. 3.5) |
| Perlite or vermiculite | 2.4 to 3.7 (avg. 2.7) |
| Expanded polystyrene board | 3.6 to 4 (avg. 3.8) |
| Extruded polystyrene board | 4.5 to 5 (avg. 4.8) |
| Polyisocyanurate board, unfaced | 5.6 to 6.3 (avg. 5.8) |
| Polyisocyanurate board, foil-faced | 7 |
| Spray polyurethane foam | 5.6 to 6.3 (avg. 5.9) |
| Reflective foil with foam core (Non-DOE source) | 14.5 (Details) |
| Source: US DOE Insulation Fact Sheet
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Reflective Foil Insulation Facing the Attic Air Space
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Reflective insulation is most effective at resisting the downward flow of radiant heat. That is why it is so popular in attics where it reflects the heat of the sun back into the atmosphere. It is also useful in reflecting indoor radiant heat back inside the house. According to the Reflective Insulation Manufacturers Association, laboratory experiments and computer modeling suggest that floor radiant barrier systems may exhibit R-values as high as R-7.5 to R-8.0 for reducing heat loss to basements and crawl spaces. If you use a reflective insulation instead of just a radiant barrier, R-values will increase.
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| R-value test for Mass Insulation |
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R-value measurement for the purposes of labeling an insulation material is defined and regulated by the Federal Trade Commission (FTC) in Rule 460. An excerpt appears in the Appendix below. Another good source for R-value calculations for reflective insulation and radiant barriers is the Reflective Insulation Manufacturers Association (RIMA) Handbook found at http://rima.net/handbook/content.htm. The appendix has formulas and tables for estimating the R-value of airspaces for either downward or sideways movement of heat.
FTC regulations specify using American Society of Testing and Materials (ASTM) procedures for measuring R-value. R-value is determined by placing a prepared insulation sample between a cool and a warm plate in a laboratory apparatus and measuring heat flow through the sample. This provides a relative number that is useful in comparing different types of insulation products.
The major drawback of this test is that it does not have a component measuring the effect of air movement on the ability of the insulation to resist the flow of heat. In all real world uses there is some air flow, either naturally, or by design for adequate building ventilation. This makes installed R-value always lower than the measured R-value. Many other factors degrade actual insulation performance including air infiltration at the intersections of walls, doors and windows, moisture, compaction, freeze-thaw cycles, convection and presence of insects.
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| R-Value Test for Reflective Insulation |
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While other types of insulation are made to resist or impede the flow of warm air, reflective aluminum foil insulation reflects back radiant (infra-red) energy from the sun so it does not penetrate the building. It can also reflect back radiant heat inside the house so it does not escape. R-value is not used to rate a material`s ability to resist radiant heat. It is only a measure of convective and conductive heat transfer while radiation is the dominant method of heat transfer in a building. As such, R-value cannot be used exclusively to compare the indoor comfort level or energy savings potential of reflective versus bulk insulation.
Aluminum foil systems with more than one sheet and a core layer are tested with the same ASTM methods as bulk insulation. To get the actual R-value for either a reflective insulation or a single sheet of foil, the foil must also be tested with a specific size air space and either downward or sideways heat flow. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers` (ASHRAE) Handbook provides tables to use for estimating these values.
A note about foil:
Although an aluminum foil radiant barrier reduces heat loss and gain through the building envelope because it is installed in vented cavities (like attics), it is not an insulation material per se and has no inherent R-value. A radiant barrier system is defined as a reflective material facing an air space. When the radiant barrier faces an enclosed air space it becomes a reflective insulation with a measurable R-value.
Reflective Aluminum Foil Radiant Barrier
Reflective foil insulation products incorporate trapped air spaces as part of the system by facing an air space and will thus carry an R-value. Reflective insulation traps air with layers of aluminum, foam, or plastic bubbles. The foil then faces an air space such as an attic or wall cavity creating a system with two air spaces for maximum R-value.
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| Difficulties Using R-value to Compare Building Materials and Make Choices |
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R-value is a relative number and cannot be used exclusively to compare different types of insulation as each has different properties relative to sealing the building against air infiltration and resistance to moisture which can seriously diminish insulation effectiveness. R-value alone cannot be used to compare bulk insulation with reflective insulation either. The combination of reflecting radiant heat and trapping convective heat makes reflective insulation consistently out-perform bulk insulation materials.
The R-value for a specific type of insulation will also vary between manufacturers based on the type of materials used, density, and type of facing (if any). Foil facings will generally have higher R-values than other facings or un-faced insulation.
As a general rule of thumb, avoid using fiberglass insulation if you can. It has the lowest R-value per inch, is hazardous to install, and is prone to degradation from moisture, air infiltration and compaction. Instead, use materials that can be installed over framing members and not just in between them to avoid thermal bridging (gaining or losing heat through either wood or metal frame studs) and air infiltration. Rigid foam, spray foam, and reflective insulation will all work well. Rigid and reflective insulation are both thin with high R-values and work well in wall cavities, roofing and places where there are space limitations.
Experts recommend loose-fill cellulose over fiberglass batts for wall cavities and the attic floor based on studies by the University of Colorado School of Architecture and the Oakridge National Laboratory (ORNL). While the laboratory R-value of cellulose is only slightly higher than fiberglass, tests after installation showed major differences. For the Colorado study, researchers built two houses and insulated each to the same initial R-value - one with cellulose and one with fiberglass. The cellulose insulated building was seven degrees warmer and used 26.4% less energy than the fiberglass building after 3 weeks of monitoring. The study concluded that blown-in cellulose outperforms fiberglass batts by up to 38% in severe climates as the blown-in insulation tightened the structure providing better protection against air infiltration. The ORNL test found similar results using an attic simulator.
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| Installation |
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Proper installation is critical to achieving the anticipated R-value of the insulation system. Gaps allow air infiltration. Compressions of batts or loose-fill, and dents or chips in rigid boards all reduce insulation effectiveness. While framing members make up as much as 15 percent of total wall area, thermal bridging through un-insulated studs can reduce the energy efficiency of a wall by up to 50 percent (this number is somewhat higher for metal studs than wood).
Installation tips to maintain R-value:
- Insulate over framing members
- Insulate the attic door
- Insulate to the recommended thickness
- Measure boards and batts precisely to achieve the tightest fit
- Do not compress loose-fill or blanket insulation to make it fit
- Install reflective surfaces to face an air space of at least 1"
- Seal all cracks with tape, foam or caulk
- Tape seams with aluminum tape
- Tape edges to electrical boxes
- Repair tears or punctures in the vapor barrier with tape
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| REFERENCES |
US Department of Energy. "Insulation fact sheet: determining the R-value you need for an existing house." http://www.ornl.gov/sci/roofs+walls/insulation/ins_05.html.
Virginia Department of Mines, Minerals and Energy. "The Virginia Energy Savers Handbook, Insulation materials selection and installation" 2006, http://www.dmme.virginia.gov/De/hbchap3.html.
Houle Insulation Inc, "R-Value". http://www.houleinsulation.com/r-value.html
Reflective Insulation Manufacturers Association, "Reflective Insulation for Residential and Commercial Applications" www.rima.net
Reflective Insulation Manufacturers Association, "Understanding and Using Reflective Insulation, Radiant Barriers and Radiant Control Coatings", 1999
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| Appendix |
LABELING AND ADVERTISING OF HOME INSULATION 16CFR460
"Sec. 460.5 R-value tests.
R-value measures resistance to heat flow. R-values given in labels, fact sheets, ads, or other promotional materials must be based on tests done under the methods listed below. They were designed by the American Society of Testing and Materials (ASTM). The test methods are:
- All types of insulation except aluminum foil must be tested with ASTM C 177-85 (Reapproved 1993), "Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus;" ASTM C 236-89 (Reapproved 1993), "Standard Test Method for Steady-State Thermal Performance of Building Assemblies by Means of a Guarded Hot Box;" ASTM C 518-91, "Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus;" ASTM C 976-90, "Standard Test Method for Thermal Performance of Building Assemblies by Means of a Calibrated Hot Box;" or ASTM C 1114-95, "Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Thin-Heater Apparatus." The tests must be done at a mean temperature of 75 deg. Fahrenheit. The tests must be done on the insulation material alone (excluding any airspace). R-values ("thermal resistance") based upon heat flux measurements according to ASTM C 177-85 (Reapproved 1993) or ASTM C 518-91 must be reported only in accordance with the requirements and restrictions of ASTM C 1045-90, "Standard Practice for Calculating Thermal Transmission Properties from Steady-State Heat Flux Measurements." These incorporations by reference were approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies of the test procedures and standard practice may be obtained from the American Society of Testing and Materials, 1916 Race Street, Philadelphia, PA 19103. Copies may be inspected at the Federal Trade Commission, Consumer Response Center, Room 130, 600 Pennsylvania Avenue, NW, Washington, DC 20580, or at the Office of the Federal Register, 800 North Capitol Street, NW, Suite 700, Washington, DC.
- For polyurethane, polyisocyanurate, and extruded polystyrene, the tests must be done on samples that fully reflect the effect of aging on the product`s R-value. To age the sample, follow the procedure in paragraph 4.6.4 of GSA Specification HH-I-530A, or another reliable procedure.
- For loose-fill cellulose, the tests must be done at the settled density determined under paragraph 8 of ASTM C 739-91, "Standard Specification for Cellulosic Fiber (Wood-Base) Loose-Fill Thermal Insulation." This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies of the test procedure may be obtained from the American Society of Testing and Materials, 1916 Race Street, Philadelphia, PA 19103. Copies may be inspected at the Federal Trade Commission, Consumer Response Center, Room 130, 600 Pennsylvania Avenue, NW, Washington, DC 20580, or at the Office of the Federal Register, 800 North Capitol Street, NW, Suite 700, Washington, DC.
- For loose-fill mineral wool, the tests must be done on samples that fully reflect the effect of settling on the product`s R-value. When a settled density procedure becomes part of a final GSA Specification for loose-fill mineral wool, the tests must be done at the settled density determined under the GSA Specification.
- Aluminum foil systems with more than one sheet must be tested with ASTM C 236-89 (Reapproved 1993) or ASTM C 976-90, which are incorporated by reference in paragraph (a) of this section. The tests must be done at a mean temperature of 75 deg. Fahrenheit, with a temperature differential of 30 deg. Fahrenheit.
- Single sheet systems of aluminum foil must be tested with ASTM E408 or another test method that provides comparable results. This tests the emissivity of the foil--its power to radiate heat. To get the R-value for a specific emissivity level, air space, and direction of heat flow, uses the tables in the most recent edition of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers` (ASHRAE) Handbook. You must use the R-value shown for 50 deg. Fahrenheit, with a temperature differential of 30 deg. Fahrenheit.
- For insulation materials with foil facings, you must test the R-value of the material alone (excluding any air spaces) under the methods listed in paragraph (a) of this section. You can also determine the R-value of the material in conjunction with an air space. You can use one of two methods to do this:
- You can test the system, with its air space, under ASTM C 236-89 (Reapproved 1993) or ASTM C 976-90, which are incorporated by reference in paragraph (a) of this section. If you do this, you must follow the rules in paragraph (a) of this section on temperature, aging and settled density.
- You can add up the tested R-value of the material and the R-value of the air space. To get the R-value for the air space, you must follow the rules in paragraph (c) of this section.
[44 FR 50242, Aug. 27, 1979, as amended at 55 FR 10055, Mar. 19, 1990; 55 FR 12110, Mar. 30, 1990; 61 FR 13665, Mar. 28, 1996; 63 FR 71587, Dec. 28, 1998] "
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