Replies

1. GBA Editor
   Martin Holladay | Dec 10, 2013 08:13am | #1

   Michael,
   You're probably right that you can get away with omitting the air gap behind the stucco in a dry climate -- assuming that your roof has wide overhangs -- but I still don't like it. I've seen too many stucco-clad homes with wet walls to take risks. But it's your house, and you get to make the final decision.

   (One possibility is to choose a different type of cladding -- for example, fiber-cement lap siding -- that is more tolerant of moisture entry than stucco. Stucco over wood is always risky, in my opinion.)

   The second issue concerns whether you can attach InsulWeb to rigid foam strips. There's always a way to do it, but you have to think about it. If you install horizontal 2x3 furring over the InsulWeb -- and I assume that you want the furring to be horizontal, to facilitate wiring -- then the insulWeb will only be pinned in a tic-tac-toe fashion, and I don't think that's enough. You'll end up fastening your InsulWeb with long cap nails, and that's fussy and a little expensive.

2. 
   Stu Turner | Dec 10, 2013 12:19pm | #2

   Hi Michael, I was in a similar position a few months ago, and hopefully these comments are helpful to you. I am building a home in a small mountain community (3000 population, 6500 ft elevation, 6000 HDD) a few hours outside Los Angeles. I have spent several years reading GBA but in the end I had to change my wall assembly around several times to fit the skills of the tradespeople operating in the town and "down the hill" in L.A.

   1. Before going with dense packed cellulose, I would contact several installers and make sure you've got someone with a lot of experience installing it. From what I understand, stapling the Insulweb properly requires a lot of practice and the right equipment. I was not able to locate an installer that I was comfortable with... many install loose fill in attics, but none who routinely installed dense pack. I really wanted dense pack cellulose, but only if I could find someone who did it often.
   2. Plywood is not a great vapor retarder, it is actually vapor semi-permeable at around 10 perms. Also it is tricky to tape properly, do a lot of reading first on what tape works best.
   3. You might want to consider a double stud wall rather than 2x6 + XPS strips. It is an easier way to eliminate thermal bridging. For example, you could have an exterior load bearing wall of 2x4s. Insulate the bays with Roxul. Then you could have a 2" gap. I would suggest insulating this with Roxul comfort boards, you can get them in the Southwest, they are 2" thick and more sturdy than the bats. Then you could have a 2x3 or 2x4 interior non load bearing wall, and again you could insulate it with bats. I think you will find this assembly has about the same R value, includes a thermal break, is 8" or 9" thick, and can be easily built by yourself or local insulation subs.
   3. its not expensive to find someone to do some energy modeling for you. Software can be used to predict your heating and cooling loads based on your R values, and that way you can have some data to guide your decisions.

   Best of luck on your project!

3. 
   piggyfinger | Dec 10, 2013 01:30pm | #3

   Martin,

   Thanks for your comments.

   I'll rethink the no air gap idea. In the past you've mentioned Delta-Dry as a WRB, perhaps that would work against my sheathing, with one layer of building paper, lath, stucco outside of that. Would that provide enough air for drying?

   I have considered using an alternative cladding, but literally every house in the neighborhood is stucco.

   I agree that horizontal furring would not secure the Insulweb properly. I was planning to install the furring vertically and dado a channel in the back of the 2x3 (near outlet height) to pull wire. Should only take a few minutes with a table saw. Maybe not ideal, but not much extra work.

4. GBA Editor
   Martin Holladay | Dec 10, 2013 01:49pm | #4

   Michael,
   According to tests and modeling studies performed by Achilles Karagiozis and John Straube, Delta-Dry performs well behind stucco -- so I think that Delta-Dry would make your wall much less risky.

5. 
   piggyfinger | Dec 10, 2013 02:48pm | #5

   Stu,

   Thanks for your comments.

   1. I will definitely check into which trades are available for my region. That's a great idea and (for DP cellulose) one that I hadn't really considered. I didn't know experienced DP cellulose installers were hard to come by.

   2. I'm not sure I agree about your perm rating for plywood. Maybe 10 perms when saturated, 1-3 under normal conditions. Here's a building material properties list from BSC.

   http://www.buildingscience.com/documents/information-sheets/building-materials-property-table/

   Many people around here seem to be taping plywood as their air barrier, but I will do more research on it.

   3. I do like double stud walls. I've had some difficulties thinking my way though some of the design and buildability details of a double stud wall. Also, I wonder if a whole other wall is really necessary in my climate.
   I like Roxul too, just not the price.

   3. I will look into this. I've done some back-of-the-envelope calcs. Unfortunately there is very little (free) energy modeling software out there for Macs.

6. 
   Stu Turner | Dec 10, 2013 04:46pm | #6

   Hi Michael, just to clarify, in some areas of the country DP cellulose is a pretty standard practice, but elsewhere - Southern California is one of them - it is hardly used at all. I can't speak for Arizona. And you've got me on the BSC link, I need to go back and do some investigating. I had referred to this page, but I don't know enough to reconcile the difference between the two.

   http://www.buildingscience.com/documents/information-sheets/info-312-vapor-permeance-some-materials