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Attic Insulation Quickie

Posted on Tue, Apr 17, 2012

One of the most frequent questions we get on attic insulation is: where should it go?  On the floor of the attic or on the underside of the roof (celiling) of the attic?  The next most frequent question we are asked is: what material should you use to insulate your attic?  Foam, cellulose, fiberglass, something else?
In two of our whiteboard sessions, I have tried to answer these questions.
The answer goes something like this:

  1. Decide what is inside or outside your building.  Is the attic space inside the building envelope or is it outside?
    1. It would be outside the building if there is nothing up in the attic and no one spends any time up there.
    2. It would be inside the building envelope if there is something up in the attic, like living space, storage or more likely heating and cooling equipment, like ductwork, heat pumps, air handlers or other HVAC equipment.
  2. If the attic is outside the building area then the insulation should go on the floor of the attic.  In this case you would want to use a product like blown cellulose or fiberglass or batts of cellulose or fiberglass.
  3. If the attic is inside the building area then the insulation should go on the underside of the roof (ceiling) of the attic.  In which case you would want to use foam insulation.
Once you understand where your building envelope begins and ends things are pretty straightforward.

For those with a more academic approach to the world, here is a great paper from the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Journal (Vol. 48, April 2006), entitled Understanding Attic Ventilation by Joseph Lstiburek, Ph.D., P.Eng., Fellow ASHRAE.  

Check out the whiteboard videos for one minute explanations!

Harold Simansky