Replies

1. Expert Member
   ARMANDO COBO | Jul 07, 2012 01:08pm | #1

   See Martin's blog on Persist, plus there are several threads here in the GBA on Persist wall systems:
   https://www.greenbuildingadvisor.com/blogs/dept/musings/getting-insulation-out-your-walls-and-ceilings or https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/15208/explain-correct-usage-rigid-insulation-exteri

2. 
   Jaahk | Jul 07, 2012 04:35pm | #2

   Thanks Armando,

   I've read everything I can find on this site and buildingscience.com, including the links you posted. I am still confused. Is a wall built with Zipwall and taped seams, covered rigid foam, essentially the same as a PERSIST wall?

3. 
   wjrobinson | Jul 07, 2012 05:10pm | #3

   Cold or warm sheathing seems to survive. In between in cold climates is recommended to be avoided. The sheathing rots and or wets.

4. Expert Member
   ARMANDO COBO | Jul 07, 2012 06:04pm | #4

   John, the way I see it is, if the rigid insulation is installed, taped and sealed properly, there should not be air and/or moisture movement thru the wall. In addition, if you have the proper ventilation and humidity control built with your HVAC system, then you should not have to worry about condensation on the sheathing, nor should you worry about the wall system drying to the outside.
   Any wall system improperly built with air or moisture leaks, is the one we need to worry about, no matter if it is Persist, Remote, Larsen, Double-stud or any other kind. That’s why all the latest building science point to air sealing and moisture management as the number one issue to take care for high performing homes.
   I would assume you could use zip panels (taped and sealed) and then rigid foam on top, if I follow Martin's description of a Persist wall. What I really do not like od that system is the lack of everhangs on a house; I believe they help protect the house in may ways.

5. 
   Jaahk | Jul 08, 2012 08:03am | #5

   Thanks again, So assuming water is prevented from entering from the exterior, you are saying that air sealing is the key to preventing water/moisture damage to the wall due to water vapor and condensation? The reason I find all of this confusing is that the recommendations for a PERSIST wall specify no more than 1/3 of the insulation r-value in the stud bays to keep the sheathing temperature above the dew point. But I find construction detail all over both GBA and Buildingscience.com that show 1" or 2" of foam over a 5 1/2" wall filled with insulation.

   I also read John Abrams article High Performance Homes on a Budget, from JLC Jan 2011 http://www.southmountain.com/smclibrary/articles/2011_01_JLC.pdf, he's detailing a wall that uses tons of cavity insulation with nothing insulating the exterior of the sheathing. Your advice seems to be the only thing that reconciles these inconsistencies in design.

   If I build with tape sealed sheathing and achieve a tight shell I'll need to allow the walls to dry to the inside and use a HRV? Can I just use Class III vapor retarder to control vapor? Any thoughts on roof design? I need to build cathedral ceilings in my 1&1/2 story building in zone 5 and like everyone I'm on a budget.

6. Expert Member
   ARMANDO COBO | Jul 09, 2012 10:49am | #6

   John,
   I don't follow rules of thumb when it comes to specifying outsulation; I perform a dew-point analysis on all wall and roof assemblies. The amount of outsulation depends on what type of insulation you are using, weather at your location, indoor humidity, indoor and outdoor temperatures, how air-tight your building is, etc, and in CZ5, you could be using anywhere from 2”-4” of outsulation.
   Mechanical ventilation can be achieved with HRVs, ERVs or regular HVAC systems. The issue is, they need to be properly designed, installed and commissioned in order to perform as they should.
   It all starts with a holistic design, integrating all components from the beginning.

7. 
   Jaahk | Jul 09, 2012 04:53pm | #7

   Armando, There's a lot to designing a building. I've been working on this design for close to two years and it's time to stop thinking and pull the trigger. Thanks for the advice it's helped me make up my mind.