Zip walls and Perm rating
I have noticed over the last few months that there seems to be some disagreement about the permeability of the Zip wall system. There are some that say the Zip sheathing is not a vapor permeable product. I have frequently read that the zip system will not allow drying to the exterior of a wall assembly. I have even read on some forums and blogs that the Zip system should not be used with double wall construction and cellulose. However, I have contacted Huber several times and have been told that the OSB component in the Zip system is 2-3 perms and the WRB facing is 12-16 perms. I understand that some folks stand firmly behind their hatred for OSB, but is there any real science to back up the arguments that the Zip system will not allow drying to the exterior of a wall assembly?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Joseph,
have you seen John Straube's post #31 at this blog?
https://www.greenbuildingadvisor.com/blogs/dept/musings/airtight-wall-and-roof-sheathing
Yes, I have seen that comment. I have no dog in the fight, but to me paragraph # 2 contributes to the ambiguity.
I would like to ask Mr. Straube what sheathing he would recommend on the exterior of a double wall or if he is just not in favor of a double wall without exterior sheathing. If a double wall assembly is going to need rigid insulation on the exterior then why should one go to the trouble of a double wall instead of simply building a 2x6 wall with more insulation on the outside?
I have customer who wants to potentially use double wall construction. We are looking at a double wall about 9 inches in thickness filled with cellulose. I have been doing a lot of research on double wall construction. It seems like a lot of folks are using the Zip walls on the exterior for double walls...I am also aware that this is a relatively new probuct. For those who caution against the Zip walls with the double wall construction what sheathing is recommendable?
I do not want to use foam on top of the exterior sheathing or anywhere in the assembly. The climate is cold with 7300 HDD and no A/C will be installed. The location gets about 60" of annual precip. Attenion would be given to the interior air barrier. There would be two layers of #15 felt and a 1/2" rainscreen beneath the exterior cladding. HRV for mechanical ventilation will be utilizied as well.
Joseph,
From recent threads, here's one recommended strategy: place the structural sheathing at the inner face of the inner frame, then add horizontal strapping (cross-hatching) to create a service core. The service core avoids penetrations of the AB. Enclose the single insulated cavity (the double frame) at the exterior with a high-perm material such as solid wood sheathing or exterior gypsum board.
Or, more like the Sunset Roost home, place the structural sheathing at the outer face of the inner frame, and you'll have two cavities to insulate. If the inner frame is 2x4 and the outer frame is spaced far enough out, then just insulate the outer cavity (Thorsten Chlupp recommends this for less extreme climates). But with the sheathing in the middle like this, it may be less straightforward to attach the two frames together.