There are several simple questions when it comes to attic insulation: Where should it go? On the floor of the attic, or on the underside of the roof? And of course, what material should you use? Foam, cellulose, fiberglass, or something else?
The answer goes something like this:
- Decide what is inside or outside your building. Is the attic space inside the building envelope or is it outside?
- It would be outside the building if there is nothing in the attic (including ductwork and HVAC equipment) and no one spends any time up there. If you attic is used for anything, it is considered inside the building.
- 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.
- 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.
Check out this whiteboard video for a one minute explanation!
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.