Confused about R-values, rolled or blown-in insulation, vapor barriers and other insulation terms? This series of stories, adapted from the U.S. Department of Energy "Insulation Fact Sheet" explains the ins and outs of home insulation -- and may help you save significantly on your heating and cooling costs.
Topics covered in Part 3: Evaluating the R-Value of insulation in existing homes * Cost *
The amount of insulation you need depends on the climate, type of heating (gas, oil, electricity) you use, and the section of the house that you plan to insulate. The attic is the first area to consider because it is accessible and therefore less expensive to insulate.
Take the information from Table 1, in Part 2 of this series. With information from Table 2 in this story, you can figure out the total R-value of your existing insulation.
Determine the kind of insulation you have from Table 1, and circle it on Table 2 (below). Then, multiply the thickness of your insulation by the "R-value per inch". This will give you the total R-value of your existing insulation.
| TABLE 2: Evaluating the R-value of insulation previously installed in existing homes (includes effect of age and settling) | |
| INSULATION TYPE | R-VALUE PER INCH OF THICKNESS |
| Fiber glass blanket or batt | 2.9 to 3.8 (use 3.2) |
| High performance fiber glass blanket or batt | 3.7 to 4.3 (use 3.8) |
| Loose-fill fiber glass | 2.3 to 2.7 (use 2.5) |
| Loose-fill rock wool | 2.7 to 3.0 (use 2.8) |
| Loose-fill cellulose | 3.4 to 3.7 (use 3.5) |
| Perlite or vermiculite | 2.4 to 3.7 (use 2.7) |
| Expanded polystyrene board | 3.6 to 4 (use 3.8) |
| Extruded polystyrene board | 4.5 to 5 (use 4.8) |
| Polyisocyanurate board, unfaced | 5.6 to 6.3 (use 5.8) |
| Polyisocyanurate board, foil-faced | 7 |
| Spray polyurethane foam | 5.6 to 6.3 (use 5.9) |
The next step is to compare the R-value of your insulation with the recommended R-values. Below is a quick table of recommended R-values by geographical area:
| CEILING INSULATION R-VALUES * | |
| HDD Zone | Ceiling R-value |
| 1 (0-500) | R-19 |
| 2 (501-3,000) | R-30 |
| 3 (3,001-5,000) | R-38 |
| 4 (5,001-6,000) | R-38 |
| 5 (6,001-10,000) | R-49 |
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| * Source: The 1995 Model Energy Code (MEC) and DOE Insulation Fact Sheet | |
The DOE Web site has a computer program to help you calculate the amount of insulation appropriate for your house. The program also allows you to define your own local costs and certain facts about your house to improve the accuracy of the recommendations. Using recommended R-values, subtract the R-value of the insulation already in your home. The result will be the R-value you should add.
Use this formula to determine the R-value of your existing insulation:
- Thickness (inches) x R-value per inch = Total R-value
Use this formula to determine how much insulation you need to add:
- Recommended R-value - existing insulation R-value = R-value needed
Do you want to know if you have the space available to add the insulation you need? Then use this formula to determine the approximate thickness you need to add:
- R-value needed / R-value per inch = Approximate thickness needed
You can use the information on Table 2 to estimate the thickness required from different materials to achieve this added R-value. This approximate thickness may help you choose your insulation material, especially if you are working within a confined space. However, when purchasing or installing new insulation, always consult the product label for accurate thickness information. Many special products have been developed to give higher R-values in a smaller thickness. On the other hand, some materials require a greater initial thickness to offset eventual settling or to assure that you get the rated R-value under a range of temperature conditions.
When you stack new insulation on top of existing attic insulation, the existing insulation is compressed a small amount. This will slightly decrease the total R-value of the insulation. This effect is most important if the new insulation is more dense than the old insulation. You can compensate for this stacking effect and achieve the desired total R-value by adding about one extra inch of insulation if the old insulation is fiber glass, or about 1/2 inch if the old insulation is rock wool or cellulose.
For example, consider an existing house in St. Paul, Minnesota (zip code 55103) with a gas furnace. The recommended R-value for attic floor insulation for this house is R-38. If the existing attic floor insulation has an R-11 insulation value, then an additional R-27 would be needed to bring the attic floor insulation up to the level recommended for that house. The homeowner could then check Table 1 to find several choices. Remember to buy the new insulation based on this R-value, and to check the product label to determine the proper thickness of the new insulation. Choosing a slightly higher level of insulation, such as R-30, would serve to offset the stacking effect discussed above.
| A word about cost |
The initial investment in insulation will pay for itself in reduced energy consumption, particularly where the amount already installed is substantially less than recommended. If fuel and electrical power costs rise, it will make even more sense to invest in insulation. If you are financing a new home, or a major home improvement, you may wish to check to see if banks in your area allow larger loan amounts for energy efficient housing.
The insulation levels recommended by the Department of Energy were chosen based on a life-cycle cost analysis. This analysis included many assumptions about your house, heating and cooling system efficiencies, and what rate of return you would like to earn on your investment.
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