Replies

1. 
   user-2310254 | Jun 11, 2017 09:00am | #1

   Where are you located?

2. 
   user-2310254 | Jun 11, 2017 02:24pm | #2

   Alan,

   Some spray foam contractors oversell the performance characteristics of their products. This helps them to "compete" with lower cost materials that deliver more R-value for the dollar. Many areas of the country require R-49 or R-38 for the ceiling. You have several options for safely achieving this level insulation. Is this a custom or spec house? Are you working with a builder or managing the project yourself?

3. 
   user-5253450 | Jun 11, 2017 09:47pm | #3

   I'm located in West Virginia and I'm building it myself. It started out as a hunting cabin, but is turning into a weekend get away home. I just want to make sure I don't have moisture problems and mold.

4. 
   user-5253450 | Jun 11, 2017 10:01pm | #4

   It looks like from the map I'm in zone 4A.

5. GBA Editor
   Martin Holladay | Jun 12, 2017 06:19am | #5

   Alan,
   In Climate Zone 4, most building codes require a minimum of R-49 ceiling insulation. Check with your local code enforcement authority to find out what your local code requires.

   It sounds like you want to insulate your cathedral ceilings using the so-called "flash-and-batt" approach. You can do that if you want. Here are links to two articles that provide more information:

   How to Build an Insulated Cathedral Ceiling

   Flash-and-Batt Insulation

   Fortunately, your spray foam layer is thick enough (in your climate zone) for you to proceed with the flash-and-batt approach without any worries about moisture problems.

   Your questions about insulating your ceiling contain a few confusing sentences, related to the question of "over" and "under." A lot of homeowners get these terms backwards when talking about ceilings. With the flash-and-batt approach to a cathedral ceiling, the batt is under the spray foam, not over the spray foam. In other words, the spray foam is on the exterior side, and the batt is on the interior side.

   Concerning the question of whether to install R-21 insulation or R-49 insulation, your spray foam contractor is wrong. You definitely want R-49 insulation rather than R-21 insulation. Every time you double the R-value of your insulation, you cut the rate of heat flow in half. For more information on this topic, see It’s OK to Skimp On Insulation, Icynene Says.

   P.S. Three inches of closed-cell spray foam is probably closer to R-18 or R-19.5 than R-21. Don't worry, though; the spray foam is still thick enough to keep you out of trouble when it comes to moisture worries. If you decide to add R-19 batts to the existing spray foam, you'll end up with a total assembly of about R-37 or R-38.5 -- not bad, even though it's not R-49 (and way better than the R-19.5 that your spray foam contractor left you with). Thicker fiberglass would be even better.

6. 
   user-5253450 | Jun 12, 2017 11:16am | #6

   Martin,

   Thanks for the detailed answer and sorry for the confusing sentences.

   I meant exactly what you were thinking, the spray foam was applied facing the exterior side of the sheathing and the batt insulation would be installed on the interior side. The roof and walls are sheathed with Huber Zip panels which are taped and sealed completely.

   I actually used the article you referenced as my insulation plan while doing research for insulating cathedral ceilings. I originally intended to use exterior foam as my first choice, but due to the time constraints and access (12/12 roof) I needed to get it under roof and out of the weather. I am building this cabin myself and work full time, so bad weather was an issue causing me to get it sealed in quickly. This made me go with option #2 the flash and batt. I upsized my rafters size to allow for the room for 3 inches of foam and R19 batts.

   The product used was SES Nexseal 2.0 which advertises a R7 per inch. It sounds like I will be in good shape if I end up with an R38-40, after installing R19 batts.

   Are there any other steps I need to take?

   I just noticed there is a high density R21 batt that is only 5.5 inches thick. Is it worth the extra cost?

7. GBA Editor
   Martin Holladay | Jun 12, 2017 11:42am | #7

   Alan,
   Q. "I just noticed there is a high density R-21 batt that is only 5.5 inches thick. Is it worth the extra cost?"

   A. The answer obviously depends on (a) the cost, and (b) your budget, but of course R-21 is better than R-19.

8. Expert Member
   Dana Dorsett | Jun 12, 2017 11:46am | #8

   R19 batts are about the crummiest fiber insulation product sold today due to their extremely high air permeability. They're more of an air filter than a convection blocking air retarder. An R19 is a "fluffed up" R13- exactly the same amount of fiberglass, but manufactured with more space between the fiber.

   What are the rafter dimensions? That is, how much space is available to be filled between the surface of the closed cell foam? From "I upsized my rafters size to allow for the room for 3 inches of foam and R19 batts." can we surmise they are 2x10" (9.25" nominal depth)"

   If yes, an R19 doesn't have sufficient manufactured loft to reliably fill the space. The manufactured loft of an R19 is a nominal 6.25", and your nominal foam depth is 3". While that adds up to 9.25", in order to guarantee a decent fit it has to be either a precision rather than nominal thickness to both the foam and the batt, or it has to be a compression fit. Since precision isn't possible here, R19s are guaranteed to not fit perfectly, and will under perform.

   Consider adding some edge strips of 1" polyiso + 1x lumber (or 1.5" wide strips of 3/4" plywood or OSB), to increase the remaining cavity depth to nominal 8.0", in which would be deep enough for a high-density R30 "cathedral ceiling" batt (8.25" nominal thickness) with a small amount of compression, or a mid-density R30 (9.5" nominal thickness) to fit with even better compression. Compressed to 8" a cathedral ceiling R30 would deliver ~R28-R29, a standard R30 would deliver ~R27-R28. Either one would get you to code-min on a U-factor basis due to the R6 thermal break on the rafters, even if it ended up R1-R3 shy of the full R49 at center cavity. See:

   http://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge

   https://www.greenbuildingadvisor.com/sites/default/files/Compressing%20fiberglass_0.JPG

9. 
   user-5253450 | Jun 12, 2017 04:49pm | #9

   Dana,

   Thanks for the info on the R19 batts, that's interesting to know.

   You are correct, I used 2x10's for my rafters, so yes, around 6.25" space left for batts.

   I will check out those articles.