Determining The R-Value You Need for an Existing Home

An insulated attic space
  • 1 hours
  • Beginner

The amount of insulation you need depends on the climate, type of heating (gas, oil, electricity) you use, and the section of the house you plan to insulate. The attic is the first area to consider because it is accessible and therefore less expensive to insulate.

A computer program is available to help you calculate the amount of insulation appropriate for your house. The program is called the ZIP-Code because it includes weather and cost information for local regions defined by the first three digits of each postal service zip code.

The program also allows you to define your own local costs and certain facts about your house to improve the accuracy of the recommendations. If you don't want to use this computer program (or other computer programs that exist), or just want to make a quick estimate, you can follow the steps outlined here.

Tables 1 and 2 will help you to identify the type of insulation and its R-value as presently installed. Determine the kind of insulation you have from Table 1, and circle it on Table 2. 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.

The next step is to compare the R-value of your insulation with the recommended R-values for your house and your type of space heating. Using these recommended R-values, subtract the R-value of the insulation already in your home. The result will be the R-value you should add.

You can use the information in 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 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 denser 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 fiberglass, or about ½ 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.