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Determining The R-Value You Need For A House

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.

Figure 47 and Figure 17 will help you to identify the type of insulation and its R-value as presently installed. Determine the kind of insulation you have from Figure 49, and circle it in Figure 47. 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 Figure 47 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 fiberglass, or about ½ inch if the old insulation is  Rockwool 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 Figure 49 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.

Figure 47: Evaluating the R-value of Insulation Previously Installed in Existing Homes (Includes Effects of Aging and Settling)

Insulation type

R-value per inch of thickness

Fiberglass blanket or batt

2.9 to 3.8 (use 3.2)

High performance fiberglass blanket or batt

3.7 to 4.3 (use 3.8)

Loose-fill fiberglass

2.3 to 2.7 (use 2.5)

Loose-fill Rockwool

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)

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

Real Estate Training Expert Home Inspector Appraiser Contractor Builder Handyman

x

Real Estate Training Expert Home Inspector Appraiser Contractor Builder Handyman

=

Real Estate Training Expert Home Inspector Appraiser Contractor Builder Handyman

Thickness (inches)

x

R-value per inch

=

Total R-value

Use this formula to determine how much insulation you need to add:

Real Estate Training Expert Home Inspector Appraiser Contractor Builder Handyman

-

Real Estate Training Expert Home Inspector Appraiser Contractor Builder Handyman

=

Real Estate Training Expert Home Inspector Appraiser Contractor Builder Handyman

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:

Real Estate Training Expert Home Inspector Appraiser Contractor Builder Handyman

/

Real Estate Training Expert Home Inspector Appraiser Contractor Builder Handyman

=

Real Estate Training Expert Home Inspector Appraiser Contractor Builder Handyman

R-value needed

/

R-value per inch

=

Approximate thickness needed

 However, remember to use the product information on the insulation packaging to determine the actual thickness for any new insulation.

 

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