Replies

1. Expert Member
   Dana Dorsett | Mar 23, 2016 05:05pm | #1

   Are you talking about an unvented attic, or something else?

   A radiant barrier cuts peak loads under low-slope roofs, but is otherwise pretty useless unless the attic is otherwise completely uninsulated, or with uninsulated ducts and air handlers above attic floor insulation. Using a "cool roof" material on the exterior is more effective, but still not sufficient.

   R20 open cell foam is fine under roof decks, but would not meet the R30 code minimums unless you also had R10 of rigid foam above the roof deck. Contractors tend to stop at R20 since it's the thickest layer than can be installed safely and well in a single pass. There has to be a cooling/curing period between passes to be fire-safe during the curing period of the second lift.

   Open cell foam can be great for air-sealing an attic, which is important. But as a retrofit there will often still be leaks at the soffits unless verified/rectified by blower door testing before they break down the equipment and leave.

   If it's either/or, definitely take the R20 open cell foam, and seal the attic tight. That makes the house less susceptible to hurricane damage too, since vented roofs get stripped far more readily than unvented roofs. When it's time to re-roof, install a 2.5" polyiso nailbase panels above the structural roof deck to bring it up to code, and use only CRRC rated cool roof materials (http://coolroofs.org/ ) on top.

2. GBA Editor
   Martin Holladay | Mar 23, 2016 05:52pm | #2

   Clinton,
   Dana gave you good advice.

   R-20 insulation beats a radiant barrier any day, but R-20 is still less that the minimum code requirement for roof insulation.

   So you should ideally do better -- and at least meet the minimum code requirement of R-30.

3. GBA Editor
   Martin Holladay | Mar 24, 2016 04:51am | #3

   Clinton,
   I forgot to include a link to an article with more information. Here it is: Radiant Barriers: A Solution in Search of a Problem.