Replies

1. GBA Editor
   Martin Holladay | Apr 18, 2014 05:26am | #1

   Stacey,
   There are many ways to create a tight air barrier. Material selection is only one factor; the more important factor is attention to detail by the construction crew.

   The key to creating a tight air barrier is to have a plan to seal seams and penetrations. It's also essential that the designer and the builder know where your air barrier is located.

   In Option #1, your primary air barrier will be at the drywall level. This is a tried-and-true method that is decades old. How well it works depends entirely on the conscientiousness of your crew.

   In Option #2, I have no idea which layer is intended as the primary air barrier. In theory, your plywood wall sheathing could be your primary air barrier; if you choose that layer, you would need to seal the plywood seams with a high quality tape. It's also possible to use your smart vapor retarder as an air barrier.

   To repeat: You won't achieve good results unless everyone on the job site knows where the air barrier is, and unless the entire crew pays attention to air sealing details.

   For more information on this topic, see Questions and Answers About Air Barriers.

2. 
   wjrobinson | Apr 18, 2014 10:23am | #2

   I would not use MgO as I have not seen it yet to be an established product. If you have please post such. Have a thousand homes used it yet?

   Rainscreen, I do not feel 3/4" rainscreens should be used. If you use plywood or another product the space can be much smaller and do the job.

   As to air barrier, you can have two and then you will want to keep the perm high which your chosen products do.

   Be careful not to mix and match a million ideas especially from web site posting like here.

3. 
   yellowbrickroad | Apr 18, 2014 12:33pm | #3

   Thanks for your response, Martin. Judging by the 4:13am response to my Air Krete post, you sure are an early riser :).

   In my question, I was trying to determine if one method of creating an air barrier was superior to the other. In the second option, I was indeed looking at the smart vapor retarder as being the air barrier. Do you see benefit in having a service cavity along with a smart vapor retarder? If you were choosing where to put your very conscientiously attended to air barrier, would you choose option 1 or option 2 for a mixed humid climate?

4. 
   yellowbrickroad | Apr 18, 2014 01:02pm | #4

   Thanks for weighing in, AJ. I too am a little uneasy about the newness of MgO, however, in not being a fan of gypsum, I don't know of a better alternative (cannot afford lath and plaster!).

   As for the rainscreen gap, the 3/4" came from the following by Martin: https://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-rainscreens

   "Finally, if your furring strips are being installed over thick rigid foam, you don’t really have a choice on furring strip thickness. You’ll need 1x3s or 1x4s that are 3/4-inch thick in order to have something to attach the siding to."

   Since we are planning to use 2" of Roxul ComfortBoard IS, I thought this applied. Please correct me if I am wrong!

   Which of the two options for air barrier would you select? Do you see benefit in having a service cavity along with a smart vapor retarder?

   Finally, I'm not sure what you mean by:
   "Be careful not to mix and match a million ideas especially from web site posting like here."
   Trying to build a modest home that is friendly to both occupant and planetary health is proving to be a challenge indeed! I am coming to all of you, with much greater experience and knowledge, to learn and make the best choices I can for my family.

5. 
   wjrobinson | Apr 18, 2014 08:06pm | #5

   Roxul is a rainscreen, but 3/4" spaces are critter condominiums IMO. If you could inset the strapping with two layers of roxul, then the space could be less or gone.

   As to my other concern , dreaming up a high tech home is fun, but then finding someone who understands your dream and has built such successfully for a decade is asking the impossible. Which then means you are the architect, the drafts-person, the GC,and lastly the crew methods trainer and fully responsible for you success or failure. Don't dare blame one person doing this work if they are being guided by you and not by their past success doing something they have proved to them self and their clients to be a good building assembly period.

   Most of what you post will work fine. Now you have to figure out how and who will build what. That has been a life long passion of mine and I learn more everyday.

   Building a home with all the new ideas discussed here at GBA isn't rocket science but it is a bit more than learning to change a tire.

6. 
   KeithH | Apr 19, 2014 01:13am | #6

   Unless you find someone who is drinking the cool aid on the service cavity, expect resistance from your contractor. That said, I think a big part of the point of Roxul's incredible fit is compromised if you run a bunch of wire through it. Every time I've had an electrician pull new wire in a remodel situation, I have been disappointed with the state of the insulation afterwards. DIYer, not a pro.

7. 
   wjrobinson | Apr 19, 2014 06:00pm | #7

   Keith, reread. No one is pulling wire through roxul. There is to be a cavity.