Zip Wall water control layer with rigid foam - providing a drainage gap?

[NathanS]

Been doing lots of research. I like the Zip Wall system.

My plans:

1) 2x6 framing with cavity insulation of unfaced R-21 fiberglass, or R-23 rockwool
2) Sheath house with zip wall, install windows on this plane. These are called "Innie" windows. Zip has diagrams on how to install everything using their products. This will be the water control barrier, and the air control barrier.
3) 4 inches of EPS insulation ~R-20. Already found a supplier that will ship to my door for 20% less than the same amount of XPS from Lowes.
4) 1x4 furring strips screwed into the 2x6 frame. This acts as a rain screen.
5) Wood siding

Here is a basic drawing I made:



This is a vapor open wall. I think the weak link would be that water can get stuck between the EPS and the Zip-wall. When this happens, it can slowly dry via diffusion inward and outward.

I guess my concern is one of scale, is some amount of 'stuck' water too much?


Zip-wall specifically says that as long as there is no vapor barrier on the inside of the wall assembly, no drainage gap is necessary. The green stuff on their wall is 12-16 perms, but the OSB itself is only like 3-5 perms, which is technically vapor open. They are saying that the inward vapor diffusion is enough to dry the wall out. This would allow of poly-iso or XPS, which is vapor closed.

4 inches of EPS is vapor open, I'm not sure the exact permeability though. 'My' wall, should have drying potential in both directions.
Here is the zip-wall diagram I'm talking about-
http://www.huberwood.com/assets/user/library/F-10-ZS-Exterior_Foam_ISO_-_Comb.pdf


Joe Lstiburek (buildingscience.com) wrote that there needs to be a way for the water to drain between OSB and the foam. Problem is, I'm not so sure he wrote this with zip-wall in mind. Basically, the only water that touches the zip wall should be on the green side, and when the water defuses through the wall, it will not condense on the unprotected side of the OSB. I think he says a 1/8 inch gap between the OSB and foam will only reduce thermal performance by about 5%.
http://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh


Cutting these grooves seems smart. I don't have a lot of hope that the supplier could do this, and it seems like an impossible amount of labor to do it myself. I am wondering if doing this might help protect against screw ups.



I'm not sure if this something I need to be worried about. Basically just dumping my mind out right now. I'm trying to build this house so when I screw something up, nothing rots. If I'm going to tape.. uh, some ungodly length of zip-wall seams, I'm probably going to have a hole some where. If water is leaking through that, I don't want the OSB to rot.

Anyone have any thoughts?

[JRR]

I'm not familiar with this Zip system.  I owe it some reading.  In my scheming up 'til now, I have always seen "standard" permeable plywood as the sheathing and then wanted to make sure the R value of the exterior rigid insulation was much higher than the R value of the sheathing plus all inward.  This, to my thinking, would keep the wet-dry interface some where inside the rigid exterior insulation.  I'm no expert and need to restudy the whole subject more before I go further with any design.

[NathanS]

I'm not familiar with this Zip system.  I owe it some reading.  In my scheming up 'til now, I have always seen "standard" permeable plywood as the sheathing and then wanted to make sure the R value of the exterior rigid insulation was much higher than the R value of the sheathing plus all inward.  This, to my thinking, would keep the wet-dry interface some where inside the rigid exterior insulation.  I'm no expert and need to restudy the whole subject more before I go further with any design.

Thanks for the response. Plywood has a much better track record than OSB. Plywood's permeability increases as it gets wet, so if you have a weak spot in your wall (which I think we should plan this way..) plywood would handle a 'wetting event' way better than OSB. Zip is made out of OSB.

I've mainly been using Joe Lstiburek's books and his building science website to learn all of this stuff. I'm a little confused because it seems like he contradicts himself on water control and rain screens (but maybe I am just out of the loop).

In my first post, the picture with the grooves cut in the back of the EPS is from his 'mind the gap' article. Which says that if your water control barrier is underneath the rigid foam, you should have some amount of a 'gap' (drainage space) between the water control surface and the surface of the rigid foam.

In my 'cold climate building' book by Joe, he never mentions 'minding the gap' anywhere, but often puts the water control layer inside of the rigid foam in his recommendations.

More recently, in this article (http://buildingscience.com/documents/insights/bsi-085-windows-can-be-a-pain)

he says "I think the winning technologies are to make the structural sheathing itself the water and air control layer—and to install continuous insulation over the structural sheathing (Figure 1). Back in the day we called this the "perfect wall" (see BSI-001: The Perfect Wall)."

With this graphic:

He isn't allowed to say zip-wall, but it's definitely what he's talking about. He doesn't say anything about creating a drainage space between the foam and the zip-wall.


Of course, all of this is climate sensitive. On the coasts, where you have tons of wind driven rain, I can see having the rain screen + a gap between the zip and the foam.

Where I live in NY, we still get like 35-40 inches of rain a year, but it almost always falls vertically, and I think that a good sized roof overhang will really do the trick.

A whole different tangent, buildingscience.com has research showing that water problems and walls are very highly correlated with the size of roof overhang.

[Don_P]

I've used Zip system, I'm not in love with it. On a conventional wall it makes the stickum of the tape the flashing. When time proves that their brand of highly overpriced duct tape will last a century I guess I'll be convinced. Do price out the entire installation, the tape costs almost as much as the osb. On a roof there is a slight time advantage but it is slight, although a roll of Titanium is up to about $150 now. I'd want a drainage plane, one of the crinkly housewraps at the least, there are also some synthetic drainage grid type products, sort of like plastic hardware cloth.

[NathanS]

I've used Zip system, I'm not in love with it. On a conventional wall it makes the stickum of the tape the flashing. When time proves that their brand of highly overpriced duct tape will last a century I guess I'll be convinced. Do price out the entire installation, the tape costs almost as much as the osb. On a roof there is a slight time advantage but it is slight, although a roll of Titanium is up to about $150 now. I'd want a drainage plane, one of the crinkly housewraps at the least, there are also some synthetic drainage grid type products, sort of like plastic hardware cloth.

Haha, thanks Don. I have only ever put up tar paper. I actually forgot to include the price the tape. I like the idea of the zip wall partly because it can also act as an air barrier. I'm not married to it though.

I did price out one of those plastic mats once.. I think it was as a rain screen for cedar shingles... that was not cheap.

[JRR]

I tend to agree with making the (plywood) sheathing the main air barrier, along with minimum insulation in the framing and maximum insulation outside the sheathing.  To my mind, this best fits all climates ... just let the framing breathe interior air.  I also agree with the importance of generous overhangs ... as much as wind loading will stand.  Windows and doors? ... there is no solution.  I'm thinking minimize the number and area dedicated to each ... and have two planes for each, an "inner" and an "outer", with restricted (but not sealed) air space between.