Determining the R-Value You Need for an Existing 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.
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.
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.
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 .5 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. 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.
