Vapor barrier in walls

[CurtisS]

I am in the process of framing my garage. It is fairly simple construction with exterior plywood that will be used as the siding as well as providing sheer strength. My question is... we will painting the plywood but at some point we may want to insulate the exterior walls so would it be wise to install a vapor barrier (tyvek, felt, etc..) over the wall studs before putting on the sheathing? Hope I have the right forum for this questions.
Thanks!

[Rich]

In my opinion it's always good to put some type of exterior material like tyvek. One more layer of protection. Remember to follow the manufacturers recommendations when installing.

[mjpliv]

I don't want to sound like I am splitting hairs but Tyvec type products and felt are not vapour barriers. They are air barriers designed to control the passage of water vapour and slow down air migration. They are generally part of a rain screen which includes an airspace. Water must be able to bead and run down the surface of the air barrier. If you have standing water or water soaked materials against an air barrier, the barrier itself will start to "wick" the water into the wall cavity.

For the air barrier to be effective in this thread's application I believe you would have to apply the Tyvec to the studs then add vertical furring to the face of the stud and then attach your plywood sheathing. This will provide the cavity required to keep the walls dry. Then if and when you decide to insulate you can do so without the fear of the insulation getting soaked. You can then add a real vapour barrier (6 mil poly) on the inside on the warm side of the insulation.

[Rich]

That is very true mjpliv.. I need to learn to clarify posts on here :)
To really get down to what tyvek does - what mj said - yet is highly-permeable for vapor from the wall cavity. Actual water molecules are too big to pass through it.
The only part I may disagree with mj on is the airspace. Most installation of tyvek do not require an air gap at their face. Although in this instance I think it's a wise choice because there is nothing behind the tyvek.

[mjpliv]

Locally we are in the midst of adopting revisions to our building codes. As it stands now, since January 1, only vinyl siding will be permitted to be applied directly to the air barrier. The rational behind this is that vinyl siding is one sided profile so it creates its own air space. I tend to disagree with this for a couple of reasons -

1. All vinyl sidings leak like a sieve! No exceptions!

2. Most vinyl siding are a horizontal profile that creates horizontal channels on the back side that trap water.

3. All vinyl sidings leak like a sieve! No exceptions!

I believe reasons #1 and #3 are reason enough to install a complete rain screen. Here is a good source of information -

http://www.umass.edu/bmatwt/publications/articles/housewraps_feltpaper_weather_penetration_barriers. html

There is a new product out there that we are experimenting with for our Hardi-Plank installations that combines a housewrap material and a spun plastic matrix (a thinner version of the ridge venting material we are all familiar with now) that creates a uniform, unobstructed air space about 1/2" thick.

[CurtisS]

Thanks for the input guys. I think I will just forget the insulation idea. It's just a garage after all. :) It doesn't get that cold here (Seattle area).