I got an email from someone who has recently bought an older home (2,000 square feet, one story). He looked in the attic and there is a layer of 40-year-old fiberglass insulation up there that may be four inches thick now. And he has a reasonable question – how much money could he really save if he beefed up the attic insulation?

That four inches of old insulation might be providing R-10 insulation value (See How Insulation Works for details on what that means). Adding insulation in the attic is usually easy and pretty inexpensive, especially if you can blow in a thick R-30 or R-45 layer. So let’s say you go from R-10 to R-45.

Say the average winter temperature is 45 degrees F (maps), and you like to set the thermostat at 70 degrees F. So the average temperature difference is 25 degrees. You have 2,000 square feet of ceiling in the house. So according to this page, the current house is losing this many BTUs through the ceiling:

2,000 square feet * 25 degree F difference / 10 = 5,000 BTUs

With a new layer of R-45 insulation it will be losing this many:

2,000 square feet * 25 degree F difference / 45 = 1,111 BTUs

Let’s call that a 4,000 BTU difference. Converting to watts that is 4,000 / 3.4 = 1,170 watts. With 24 hours in the day that works out to 28 kilowatt-hours per day. If you provide the heat for the house with electricity, and electricity is 10 cents per kilowatt-hour, it means that the new layer of insulation will save $2.80 per day. If winter is 100 days long (just to make the math easy), that is $280 per year saved on heating. And then some amount (maybe half that? It really depends on where you live) can be saved on air conditioning. You might be able to save something like $400 per year with that new layer of insulation.

So now you look at the cost of adding the insulation. Let’s say it’s $1,000. (you may be able to get tax breaks that lower the cost, or you might have a very easy attic, or you might do it yourself, but let’s just use that as a guesstimate). That means it takes 2.5 years to pay back the cost of the insulation, assuming energy prices don’t go up.

Since it is an older home, there may be several other ways to save money on heating and AC:

• Can you seal up gaps around the windows/doors with weatherstripping, foam and caulk? Older homes often have big air gaps, and sealing them is cheap. you get a big effect at low cost. Replacing the windows is another more expensive option.
• Is the floor insulated? Often there in no insulation in the floor in older homes.
• How old/inefficient is the furnace? Replacing the furnace with a new high-efficiency unit might cut your heating costs in half.

This article shows how successful a project can be, and what the costs are:

we spent a total of $7,525 for all the energy reduction projects, and we are saving $4,007 per year in energy costs!

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