Insulation levels in a home heated by natural gas.  From the top down:  R38 in the peak attic, R13 in the 2×4 kneewalls, R38 in the kneewall flats, R-13 in the the 2×4 walls, R30 in the 2×10 floor.  Sketched by Kyle, a great employee and artist.

The ideal amount of insulation in a home can depend on numerous things, such as climate, exposure, and comfort.  But from a cost efficient perspective (for every dollar you spent how much do you save), expensive fuels get more insulation, cheaper fuels get less.

If your fuel source was free, there would be no financial benefit to adding insulation to your house.  For every dollar you spent on insulation, you would have zero dollars of savings on your fuel.  There would be benefits of comfort, durability, and human health, but not cost savings.

If, on the other hand, your fuel source was outrageously expensive, it would make sense, from a financial standpoint, to insulate the heck out of the home.  For every dollar you spent on insulation, you would have a dramatic savings on your fuel costs, tapering up to a certain point.

Where is the point at which it makes sense to stop insulating a home?  Puget Sound Energy, the provider of our most expensive fuel source (electricity) here in Bellingham, says an R49 for attics, R13 for walls, and R30 or fill the floor joists.  Cascade Natural Gas, the provider of one of our cheapest fuel sources (natural gas) says R38 for attics, R11 for walls, and R30 or fill the floor joists.  Both provide rebates to add insulation up to those levels if no insulation is present.

But what if you already have some insulation in the home?  Does it make sense to go up to an R49 if you already have R40?

PSE and CNG answered this question, confusingly, by ending their rebates at similar R values.  Neither will provide rebates if you have more than R18 in the attic (PSE says 6 inches, CNG says R18, both are roughly equivalent).  PSE will only rebate walls that have no insulation, CNG will rebate walls with less than or equal to R4.  PSE will rebate floors that have 0 to 4 inches of insulation, CNG will rebate floors that have up to R11 (about 3.5 inches of insulation).