Vapor Barrier & Zip Panels

Started by redside, January 17, 2018, 06:12:32 PM

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redside

Just putting the final touches on my materials list and was very tempted to go with Huber's Zip Panels for the sheathing.  My concern came when I started thinking about using this system in arctic with a vapor barrier.  With a vapor barrier (6 mil plastic) you essentially keep the inside humid air from condensing inside the wall cavity where the dew point is.  You do as good of a job as you can, but there will inevitably be places where moisture get in.  If it gets in, you should probably allow a place for it to get out.  That is why I am leaning toward using plywood with 15# felt.  The beauty of these products is that the permeability increases as they get wet and allow faster drying through the exterior wall.  The osb in the zip panel has a very low permeability which could cause mold/damage inside the wall cavity.  Am I off base with my thinking?  Your thoughts.  Would be great to hear from those that have used a vapor barrier (not retarder) with the zip panels.

MountainDon

My experiences with cold climate Manitoba was that in heating season the air inside the building is so dry that if water vapor makes it into the wall cavity it can easily dry to the inside.  In that case you would not use a plastic vapor barrier on the inside.

Google "building science cold weather wall".  They have shown that foam towards the exterior with batts in the cavities is the way to go for severe cold climates. Using enough foam towards the exterior prevents the interior wall space from getting cold enough to have condensation. The colder the climate the thicker the foam.
Just because something has been done and has not failed, doesn't mean it is good design.


NathanS

Foam on the outside would be best. I would agree that plywood + felt is more vapor open than zip, and maybe (?) a little less susceptible to rot. Going that route, boards at a 45 degree angle + felt would be the most vapor open and resistant to rot.

We did foam on the outside w/ zip, it was a lot of extra labor, but heating the house effortlessly on sub zero Fahrenheit days forever was worth it. Also the foam is a greater protection against rot than any type of sheathing.

Note that an air tight house will not have dry air. It is 8F and I had a window cracked with bathroom fan running to drop the humidity after making dinner.

I have found Huber, company that makes Zip, to make awesome products.

MountainDon

Quote from: NathanS on January 17, 2018, 07:43:10 PM


Note that an air tight house will not have dry air. It is 8F and I had a window cracked with bathroom fan running to drop the humidity after making dinner.


Glad you mentioned that. It reminded me that friends in CO installed an air to air heat exchanger to deal with that automatically.
Just because something has been done and has not failed, doesn't mean it is good design.

Rys

Isn't there a line of Zip Panels that come with a foam backing made on them? Has anyone used them or have an opinion about them?


NathanS

They do, overall I think it would be a good system. I did skim the installation manual, and like almost all manuals and guides they tell you to refer elsewhere for the difficult details (windows). Overall seems like a pretty good installation manual though.

It definitely is not as straightforward as just installing sheathing, but maybe a little easier than separately using foam. Also for the 2.5" thick you would need a minimum of 4" long nail. If you're using a nail gun that cost could really add up fast. I also think that it would be necessary to put on a rain screen with structural screws back to the studs, otherwise you're fastening siding into 1/2" osb which will probably not hold up in the long run.

SouthernTier

If you use outside foam over regular OSB, do you need to add a vapor barrier?  Does the foam serve that purpose (say, with taped seams), or do you still need the vapor barrier between the OSB and the foam?

MountainDon

Installed correctly foam is the vapor barrier.  The foam must be thick enough, have a high enough R-value, to keep the inside wall face at a temperature above the dew point. This required R-value changes with climate zones. We used two layers with staggered seams. The outermost layer was foil faced. The panels were secured with foam friendly adhesive and long screws into the OSB sheathing. The long screws used large nylon washers under the heads, designed for the purpose. Once that was all seam taped we installed vertical furring 1x's with structural rated screws securing those furring strips to the wall studs. Then horizontal lap siding was applied over that. Windows had boxes built to secure the windows to.
Just because something has been done and has not failed, doesn't mean it is good design.

SouthernTier

Thanks.  I decided to go with a one-inch foam layer on the outside after running Rescheck and found I needed it.  I will still have R-19 or R-21 in the wall cavities.  I have read through that Joe Lstibruck/Building Science book about the building in cold climates and predicting the due points, and it seems like this arrangement would be appropriate for me (northeast, cold climate).  So if I am adding the foam (will stick with one layer of one-inch), and tape it properly, I was thinking that a separate vapor barrier wouldn't be necessary. 

I don't want to go too much thicker than one-inch.  I'd like to stay away from the furring strips because with a constraint of staying with 16-foot rafters, my overhang is already less than I would like, and I don't want to be pushing the wall out any more than I have to.  I figure I can fasten the lap siding (amm thinking of going with LP smartside) through one-inch foam into the studs behind.


NathanS

I think 1" in upstate NY is cutting it close. Some of the examples in that book will make assumptions of 20% interior RH to drop the dew point sufficiently.

Also I didn't realize until looking at the code that people install siding into 7/16" OSB. That just seems like a bad idea to me.

Don_P

That is how SIPs work, not a fan but it works I suppose.
For overhangs I've scabbed 4'-6' pieces of 2x4 or 2x6 to the rafters to create the tail without going to custom lengths. At least half the length inboard and secured very well. You have to do a little value math to see which option is better.

SouthernTier

Is there a good way to model how thick I should go (other than just plain thicker is better)?

It would defeat the purpose of a continuous barrier, but I could suppose put the furring strips over the 1" and put half-inch in-between the strips.  Doesn't get you anywhere with Rescheck, but maybe gets me somewhere with condensation.  I would have a similar climate as Nathan.

NathanS

#12
I guess after missing studs when nailing up the sheathing I just really didn't like how loose the nails were, and on the inside you could see how the OSB would kind of fragment. Well lots of siding is held on that way just fine.. I wonder what would happen if ya jiggled those boards a little though...

I don't think you have to model anything out. Somewhere in the code book there is a chart that says how much exterior insulation you need to not have an interior vapor barrier. In my zone (guessing same for you) a 2x6 wall is supposed to have like R-11.25 or thereabouts. That would be 2" of polyiso.

What Lstiburek says is the right way to do it, just to be realistic about how dry the air in your house is going to be.

When I did it my process was a conservative 3 month cold average, 70 degrees inside, and then it would be checking how dry the air in the house would need to be to stay above that dewpoint.