Is it worthwhile putting spray foam between joists in my attic ceiling?

Asked by Jim
Doylestown, OH

I have 18 inches of blown-in insulation on the attic floor.


David Willson

Answered by David Willson

Sebastopol, CA

Advanced Home Performance

August 3, 2011


The short answer is probably not. The long answer is, well, longer, and contains some "ifs," both in favor of foam and against it.

Applying 2 to 6 inches of foam to the underside of your attic sheathing will do a couple of things.

  • It will drop the air temperature in your attic, although how much depends on the outside temperature, the angle and slope of your roof, and the type and color of your roofing.
  • Without knowing those variables, I can't even make a guess how much the temperature would drop in your attic, though I've seen drops of 10 to 40 degrees.
  • If your furnace, AC coil, and duct system are exposed in the attic, dropping the attic temperature even 10 degrees would be a significant help to your AC system.
  • On the other hand, blowing more insulation over the ducts in the attic would probably give you as much energy savings for far less money.

Make sure you have a new roof

If you add foam insulation under your roof decking and you get a leak in your roof, it can be difficult to find and can promote rot.

  • The foam traps water and holds it against the roof decking, where hot temperatures can cause fungus rot relatively quickly.
  • If you DO install spray foam, make sure you have a new roof.

Also, if you have foam sprayed under your roof decking, the person spraying it, along with the hose he uses, will cause significant damage to the loft of your existing loose fill insulation.

Consider loose fill fiberglass instead

Lastly, there's cost.

  • Here in California, spraying foam in the attic runs from $2.50 to $5/square foot, depending on the type and thickness of foam.
  • Contrast that with less than $1/foot for 6" (R-19) of blown-in loose fill fiberglass.

You may choose to halfway fill your attic with another foot or so of loose fill fiberglass and get more effective insulation for less cost than spray foam.

However I love spray foam for some applications 

Now, I have to say, in the right place and for the right reasons, I love spray foam and recommend it.

  • Spray foam is a good choice when air sealing is also important, or in the crawlspace when batt insulation won't work well.
  • For your application, however, consider blowing in more loose fill insulation instead.
  • But even before that, contact a local energy efficiency guy or a Home Performance Contractor and have him do an analysis on your home to target where your real energy losses are.
  • The money is well spent.

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