Synthetic vs felt over wall sheathing

[Arky217]

I used Grace Tri-flex for my roof underlayment and really liked it.
Is there a downside of using it for wall sheathing covering before hanging vinyl siding. It's only about $2/sq more than 30# felt in my area.
Just not sure if it would have a disadvantage vs felt on the walls.

Arky

[MountainDon]

I believe the only potential downside to the synthetics is the possible troubles that can and have resulted in cases where liquid water gets trapped between the wrap and the sheathing under it.

The nature of the synthetics is that they are vapor permeable, meaning water can evaporate through it. They are impervious to liquid water, a good thing when the water is something like rain from the outside that gets past the cosmetic outer layer (vinyl or other siding, whatever).

But if water gets through from the outside because of some faulty installation issue the liquid under the wrap becomes trapped. Perhaps the OSB will absorb it. In another scenario it might be possible for humidity from the inside to pass through the wall as water vapor. Then if the wrap is cold enough to cause that vapor to condense on the inside face the now liquid water stops right there; potential trouble.

Building felt on the other hand can absorb water from one side and then as conditions permit the water will wick to the warmer side and evaporate. When the felt absorbs water the fibers swell and become more densely packed and repels liquid water quite well.

Wrap is used a lot. Pro builders like it because I believe it can be installed in less time than felt. Some folks pick it, I believe, because it is a newer idea and newer ideas must be better, right?

Anyhow that's my take on the question.

Oh, for walls you'd use #15 felt'#30 would be difficult to work with. And #15 rolls cost the same as #30, but cover twice as much.  FYI, for stucco two layes of #15 is the preferred weather resistant barrier.

[Jeff922]

This is always a contentious issue.    So here is my 2 cents.  I live in a very cold climate and I believe a continuous air-barrier (this is different than a vapor barrier/retarder) is VERY important.  A twelve foot roll of house wrap can go a long way without any seams and the few seams there are will be taped.  I believe this is a significant advantage in cold climates as most of a home's heat loss is through air infiltration.  Also, with a little care and a proper flashing and siding job, water is at little risk of getting behind the houses wrap.  Never tape anything countinuously horizontally directly to the sheathing, "skip tape" instead.  That way if water gets behind the housewrap, it can run downward.  Also, pay close attention to your housewrap choice, the perm ratings vary widely and some stand up to UV degradation better than others.  Some like the idea of using a poly interior vapor/air barrier in cold climates or relying on an "airtight" drywall job to provide air barrier protection, but it seems to me that air infiltration should be kept out of the wall assembly to begin with.  P.S. I'm no expert, but this is what made the most sense to me.

[glenn kangiser]

Keep in mind that only vapor can get back out of house wrap.  Water cannot.

Water penetration through felt will be drained or wicked out.  Through a synthetic it will become a plastic bag full of water.

Entire housing projects have had problems from synthetics.  I heard of a project in Oregon that had to be torn down when new due to synthetic wrap problems.

[Erin]

I've torn down a lot of old buildings.  Houses, barns, chicken coops... Whatever.  Tarpaper is always behind the first layer of interior siding.  The exterior siding is usually in rough shape. The interior is always newish looking, golden wood colored, with stain from the tarpaper. 
OTOH, I've heard some really mixed reviews about house wrap.  Some folks swear by it.  Some don't. 

But personally, until a structure is old enough to tear down and see the results of a few decades of use, I'm going to go with what I know works in my area. 

[Arky217]

Keep in mind that only vapor can get back out of house wrap.  Water cannot.

Water penetration through felt will be drained or wicked out.  Through a synthetic it will become a plastic bag full of water.

Entire housing projects have had problems from synthetics.  I heard of a project in Oregon that had to be torn down when new due to synthetic wrap problems.

Ok, I'm seeing where felt would be a better choice over the wall sheathing due to its ability to absorb and pass through water as well as vapor.

Actually, I was planning at first on not using anything since the sheathing is Zip panels with the joints taped with Zip tape. The makers of the Zip panels say felt or house wrap is not necessary, but I thought a second barrier would be just extra protection. Maybe I should just put the vinyl siding directly on the Zip panels after all.

I do know that the Zip roof panels held up pretty good through the 6 months of weather before I got the metal roofing on, but I think maybe the membrane on the roofing panels is different than on the wall panels. (roof panels say for roof only, wall panels say for walls only).

Before putting the metal roofing over the roof Zip panels, I did apply synthetic underlayment even though the makers of Zip panels said it was not necessary. The advice I got from roofers was to use an underlayment anyway. So I went ahead and applied the synthetic thinking that a backup layer of protection would be a good idea in case the Zip tape did not hold up over the years.

But I hope I didn't mess up using the synthetic instead of felt on the roof. However, it seems the roof would be a different situation than the walls (not dealing with a wall structure and permability issues).

So, the questions now are:
     Should I go ahead and put felt over the Zip wall sheathing (maybe at least one layer of 15# felt) or should I just install the vinyl siding directly over the Zip panels ?

    Am I ok on the roofing underlayment, or should I have used felt instead of synthetic ?