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Dear Pablo: Is it more cost effective to install an attic fan or add additional insulation?When the sun shines on your roof, the dark shingles (assuming that you have a shingled roof) collect the sun's energy and pass it into your attic. The temperature in your attic can easily exceed the outside temperature by 30 ° F. The energy absorbed by your roof is transmitted into your attic by both convective and radiative heat transfer.
There are several options for preventing the roof from absorbing heat in the first place including installing a "cool roof," or shading your roof with a tree or solar panels. But once the energy has been absorbed the options become running your air conditioner, installing an attic fan, or adding more insulation to your attic. Of these options, the latter two are the most cost effective, but which one is better?
Geek Alert: The ability of insulation to resist the flow of heat from one side (your attic) to the other (your living space) can be measured in terms of R-value. R value is defined as ft2 x ° F x h / BTU or the square area, times the temperature difference between both sides, times the period of time, divided by the energy transferred from one side to the other. You presumably know the square area of your attic, and we can assume that the temperature difference is about 30 ° F and that we are looking at a 1 hour period. A home performance contractor can tell you what the R-value of your attic insulation is (mine was estimated to be 13.7), leaving the heat loss (in BTU) as the only unknown.
13.7 = 1,886 ft2 x 30 ° F x 1 hour / ? BTU
Heat transferred into the house = 4,130 BTU/hour
This means that, if I don't do anything, my air conditioner will have to remove 4,130 BTU from my house each hour. Of course this is not the whole story since it only accounts for convective heat transfer. Heat is also radiated from the roof in the form of infrared, which heats exposed beams and the insulation itself, transferring heat by conductive heat transfer. My home performance contractor, Sustainable Spaces, recommended not only an R-value of 42 for my house, which would reduce the convective heat transfer into my house to 1,347 BTU/hour but also installing a radiant barrier that actually reflects the radiated heat back to the roof. Adding the additional insulation and radiative barrier would cost me over $3,000.
The alternative would be to install an attic fan that would draw cooler outside air into the attic while expelling the hot air. If we assume that the outside air is 10 ° F warmer than the desired indoor temperature then an attic fan could reduce the attic temperature by 20 degrees. This reduction would have almost the same effect on convective heat transfer as the added insulation but would not alter the radiative heat transfer. Additionally, the attic fan will create negative pressure in your attic that will draw the cool air conditioned air from your living space through leaks (which virtually all houses have). Finally, the attic fan will cost you upwards of $1000 installed and will use electricity (there are solar-powered models available). But even in hot climates like Florida, a solar-powered attic fan can have a payback period of 20 years.
So in comparison, both options represent an improvement over the status quo but, when combined with a radiant barrier, the insulation is the better choice especially when you realize that the added insulation will also help reduce your winter heating costs. For my home the choice is clear but your home may be different. I recommend contacting your local home performance contractor with help in making your decision.
Additional Resources on Improving Your Home's Performance:
Attic Fans-Just Blowing Hot Air?
How to get a Roadmap for Greening your Home
Should I Switch to Dimmable CFL Bulbs?
Does Turning Down My Water Heater Really Make a Difference?