Insulation: The Facts Part 3

The Old House Web

Confused about R-values, rolled or blown-ininsulation, vapor barriers and other insulation terms? This series of stories,adapted from the U.S. Department of Energy "InsulationFact Sheet" explains the ins and outs of home insulation -- and may helpyou save significantly on your heating and cooling costs.

Topicscovered in Part 3: Evaluating the R-Value of insulation inexisting 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 toinsulate. The attic is the first area to consider because it is accessible andtherefore less expensive to insulate.

Take the information from Table1, 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 fromTable1, and circle it on Table2 (below). Then, multiply the thickness of your insulation by the "R-value perinch". 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)
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 recommendedR-values. Below is a quick table of recommended R-values by geographicalarea:

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

heating map

* 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 ofinsulation appropriate for your house. The program also allowsyou to define your own local costs and certain facts about your house to improvethe accuracy of the recommendations. Using recommended R-values,subtract the R-value of the insulation already in your home. The result will bethe R-value you should add.

Use this formula to determinethe R-value of your existing insulation:

  • Thickness (inches) x R-value per inch = Total R-value

Use this formula to determinehow much insulation you need to add:

  • Recommended R-value - existing insulation R-value = R-value needed

Do you want to know if you havethe space available to add the insulation you need? Then use this formula todetermine 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 requiredfrom different materials to achieve this added R-value. This approximatethickness may help you choose your insulation material, especially if you areworking within a confined space. However, when purchasing or installing newinsulation, always consult the product label for accurate thickness information.Many special products have been developed to give higher R-values in a smallerthickness. On the other hand, some materials require a greater initial thicknessto offset eventual settling or to assure that you get the rated R-value under arange of temperature conditions.

When you stack new insulation on top of existing attic insulation, theexisting insulation is compressed a small amount. This will slightly decreasethe total R-value of the insulation. This effect is most important if the newinsulation is more dense than the old insulation. You can compensate for thisstacking effect and achieve the desired total R-value by adding about one extrainch of insulation if the old insulation is fiber glass, or about 1/2 inch if theold insulation is rock wool or cellulose.

For example, consider an existing house in St. Paul, Minnesota (zip code55103) with a gas furnace. The recommended R-value for attic floor insulationfor this house is R-38. If the existing attic floor insulation has an R-11insulation value, then an additional R-27 would be needed to bring the atticfloor insulation up to the level recommended for that house. The homeowner couldthen check Table 1 to find several choices. Remember to buy the new insulationbased on this R-value, and to check the product label to determine the properthickness 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 willpay for itself in reduced energy consumption, particularly where the amountalready installed is substantially less than recommended. If fuel and electricalpower costs rise, it will make even more sense to invest in insulation. If youare financing a new home, or a major home improvement, you may wish to check tosee if banks in your area allow larger loan amounts for energy efficienthousing.

The insulation levels recommended by the Department of Energy were chosen based on a life-cycle costanalysis. This analysis included many assumptions about your house, heating andcooling system efficiencies, and what rate of return you would like to earn onyour investment.

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