Replies

1. GBA Editor
   Martin Holladay | Feb 05, 2011 05:49am | #1

   Christian,
   Q. "Will high-density spray foam on the floor of a ventilated attic cause moisture problems in a hot-humid climate?"

   A. No -- not if the foam is thick enough. There is no reason for the top side of the insulation to be cold enough to allow condensation, unless the foam installer made a mistake and installed it very thin.

   If there is evidence of condensation on the top of your insulation, something is wrong. You should call back your insulation contractor to have the situation remedied (by installing enough insulation to prevent the problem).

   You are in Climate Zone 2. According to the 2006 IRC, the minimum R-value for ceiling insulation in your climate zone is R-30, so you need at least 4.5 inches of closed-cell foam. If your insulation contractor installed less than that, your insulation doesn't meet minimum code requirements.

2. 
   christianwagley74 | Feb 05, 2011 03:15pm | #2

   Thanks for the answer, Martin. There are currently only two inches of closed-cell foam installed, as the idea is to go back this spring and install several inches of cellulose on top of the foam. The foam was installed to stop air leakage through the tongue and groove ceiling of the home (air was passing through many of the grooves--especially in winter when the wood shrinks), which a blower door test revealed. Would the cellulose or other insulation on top of the foam remedy the potential for moisture?

3. GBA Editor
   Martin Holladay | Feb 06, 2011 05:04am | #3

   Christian,
   If you really have 2 inches of closed-cell foam, you shouldn't have a problem. Can you see any moisture on the attic side? Or are you just worried?

4. 
   christianwagley74 | Feb 08, 2011 06:18pm | #4

   I haven't noticed any moisure yet--the insulation has only been in for about two months, so I was just worried for what might happen in the warmer months. It sounds like adding any additional insulation to get to minimum R-30 would eliminate any possibility of trouble from moisture. Does this sound right?

5. GBA Editor
   Martin Holladay | Feb 08, 2011 06:37pm | #5

   Christian,
   No, that's not exactly right. During the summer, the closed-cell foam needs to be thick enough to separate your cold interior from the hot, humid attic -- thick enough to keep the top of the foam layer at a different temperature from the bottom of the foam layer.

   In other words, you want the foam layer to be thick enough for the top of the foam to be hot during the summer, not cold. You don't want a cold surface in your attic. Two inches of closed-cell foam should do the trick -- as long as it is really 2 inches.

   Adding cellulose insulation on top of the foam is good, because it helps save you energy. But note:
   - The cellulose doesn't stop the attic moisture from reaching the top of the foam.
   - The cellulose tends to insulate the top of the foam, making it colder in summer and therefore MORE subject to condensation, not less.

   However, the top of the foam will never be cold enough for condensation to form, even if you install R-30 cellulose -- assuming, as always, that you really have 2 inches of foam and not less.

   So, in conclusion:
   - Verify the depth of your foam by measuring it with a stiff wire.
   - If it is a uniform 2 inches, stop worrying and install the cellulose.

6. 
   christianwagley74 | Feb 08, 2011 08:56pm | #6

   Thanks for the thorough explanation, Martin. No more worries!