Question

Should attic insulation extend into the eave soffits?

Asked by Ralph Fumante Jr., Oyster Bay Cove, NY

I want to add more insulation to my attic. I have a ranch house with eaves that extend about 24 inches beyond the exterior walls. The present insulation is between the floor joists in the attic and it continues over the soffits of the eaves. Should the soffits be insulated like this, or should insulation stop at the top of the exterior walls? In either case I plan to add rafter vents.

Answer

Jesse Terzi

Answered by Jesse Terzi

Inwood, NY

New Eco, Inc.

April 23, 2009

To answer simply: no, your soffits should not be insulated, and there is a chance that your insulation has been damaged by this.

On a positive note, congratulations on having 24-inch eaves. Overhangs that extend this far, especially on the south side, will significantly reduce the internal solar gain of your home during the summer, when the sun is higher in the sky.

Wind washing

The problem with insulation extending into the soffits is that air getting sucked in through the soffit vents runs over the insulation.

  • This phenomenon, known as wind washing, dramatically reduces the insulation's R-value and could permanently damage insulation material as well.
  • It is crucial that correct attic baffling with a wind-wash barrier be installed. Installing baffles will let the insulation extend only enough to cover the top plate, no further.
  • This directs air from the outside upwards, along the baffle, instead of across the insulation. I recommend the AccuVent soffit insulation baffle by Berger. It's a great system, made of 100-percent recycled PVC.

Air sealing rather than adding insulation

As for your desire to add more insulation, I suggest you have a building analyst come over and inspect the situation. It could be that you only need to air-seal rather than adding insulation.

  • You might also need to remove and replace the current insulation if it has become damaged over time from the exposure to outside air.
  • Air-sealing is always a good idea while you're making changes to the attic anyway; it will increase the lifetime of your insulation and minimize damage done by airflow.

Having a Home Performance with Energy Star contractor perform the work makes you more likely to receive financial incentives or rebates and usually assures a quality job. Our company performs this work in Long Island, and you can go to the Long Island Power Authority (LIPA) website to search for other participating contractors near you.

 

For more information:

Read Danny Kelly's Q&A "I recently had blown-in cellulose put in my attic. Can I install batt insulation over it to increase the R-value?"

If you want to learn more, BuildingScience.com has a lot of good information on topics like this. And if you really want to get serious, I recommend checking out the Energy & Environmental Building Association's online bookstore. They have some great books and builder's guides on home insulation and energy efficiency.

Tagged In: attic insulation, energy audit, attic ventilation

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