Attic Fan or Insulation?


Image source: Zieak

Dear Pablo, is it cheaper to install a roof fan or add extra insulation? When the sun shines on your roof, the dark shingles (assuming you have a shingle roof) collect the solar energy and direct it to your attic. The temperature in your attic can easily exceed the outside temperature by 30 ° F. The energy absorbed by your roof is transferred to your attic through both convective and radiant heat transfer.

There are several ways to prevent the roof from absorbing heat in the first place, including installing a “cool roof” or shading your roof with a tree or solar panels. Once the energy has been absorbed, you can run your air conditioner, install a roof fan, or add more insulation to your attic. Of these two options, the latter two are the least expensive, but which one is better?

Geek Alert: The insulation’s ability to withstand the flow of heat from one side (your attic) to the other (your living space) can be measured by its R-value. The R-value is defined as ft2 x ° F xh / BTU or the square area multiplied by the temperature difference between the two sides, multiplied by the length of time, divided by the energy transferred from one side to the other. You probably know the square area of ​​your attic, and we can assume the temperature difference is around 30 ° F and we are looking at a 1 hour period. A contractor can tell you the R-value of your attic insulation (mine was estimated at 13.7) with the heat loss (in BTU) remaining the only unknown.

13.7 = 1.886 ft² x 30 ° F x 1 hour / δ BTU
Heat transfer 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 BTUs from my house every hour. Of course, this is not the whole story as it is only responsible for convective heat transfer. Heat is also radiated from the roof in the form of infrared, which heats exposed rays and the insulation itself and transfers heat through conductive heat transfer. My home services contractor, Sustainable Spaces, not only recommended an R-value of 42 for my home, which would reduce convective heat transfer into my home to 1,347 BTU / hour, but it would also install a radiation barrier that actually reflects back the radiated heat the roof. Adding the extra insulation and radiation barrier would cost me over $ 3,000.

The alternative would be to install a roof fan that draws cooler outside air into the attic while the hot air is expelled. Assuming the outside air is 10 ° F warmer than the desired indoor temperature, a roof fan could lower the attic temperature by 20 degrees. This reduction would have almost the same effect on convective heat transfer as adding insulation, but it would not change radiant heat transfer. In addition, the attic fan creates a negative pressure in your attic, which draws the cool, conditioned air out of your living space through leaks (which practically all houses have). After all, the attic fan costs more than $ 1000 and consumes electricity (solar-powered models are available). But even in hot climates like Florida, a solar powered roof fan can have a payback period of 20 years.

In comparison, both options are an improvement on the status quo. However, when combined with a radiation barrier, insulation is a better choice, especially if you find that the additional insulation also helps lower your heating bills in winter. For my home the choice is clear, but your home can be different. I recommend contacting your local health care provider with your decision.

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