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How much insulation do I need?

When it comes to adding insulation, the old adage of, “if some is good, more is better,” is not necessarily true. The law of diminishing returns has a huge impact when it comes to thermal insulation. The calculations can get pretty convoluted so bear with me as I try to give it to you in layman’s terms, which by the way, is how I understand it. So, how much of an R value is right for your home?

Before I get ahead of myself, let me give a definition of R value. It is a measure of a material’s resistance to heat flow. In other words, how good it is in keeping heat on the side where it originates. This can be heating from your furnace inside your home during the winter or heat from the sun outside in summer. In both cases, the objective is to keep the heat where it started, reducing heat loss in the winter and heat gain in the summer.

When you have an uninsulated space, the base assumption is that is has an R value of 1 by virtue of the construction materials. Now, add a 6” batt of fiberglass insulation. Fiberglass has an R value that ranges from 3.1 to 4.3 R per inch. For convenience, most folks use R19 as the value. The impact of adding the batt is significant…it stops 95% of the heat transfer. Adding another R30 only stops an additional 3% of heat transfer. So you see, more is better, but not as much as the first R19.

So why are there different recommended levels of insulation for different parts of the country? Again, in my layman’s parlance, it has to do with the size of the temperature difference. The second law of thermodynamics says that heat will naturally flow towards cold. Let’s look at heating as an example. If it is 0o outside and 68o inside, you are trying to insulate against a 68o temperature differential. You are going to need more insulation to provide the same resistance to heat flow because the insulation will eventually get cold (thermodynamics again – trying to reach a uniform temperature) and start the heat moving on its way outside. Scientists, check here for a description you’ll enjoy: Thermodynamics

So, for folks here in Virginia, R38 – R60 is the recommended value for flat ceilings while those living in parts of Alaska are going to need R49 – R60. The US Department of Energy has a handy table you can use to see how much insulation is recommended. If you have an existing home with 6” of fiberglass batting in the ceiling (R20) and 4” in the walls (R12), you should look at other options before getting into the expense of adding insulation.

• Be sure to seal and weather-strip extensively – find and seal every leak you can. This applies to windows, doors, any pipes that penetrate the outside of your home and ductwork.
• Replace you heating system (when it is ready) with a more efficient model.
• Be diligent in cleaning filters.
• Install a smart or programmable thermostat.
• Adjust your temperature settings.

You may just find these measures are going to improve your comfort and lower your bills without going to extra insulation. If you want to add another 6” later to hit R39, there are calculators online that can help Green Building Advisor  seems the easiest to use but I have a little more comfort with this one,Chuck-Wright Calculators just because it seems to give me more control over the inputs. You can find your own by searching on this: calculate roi of extra insulation.