Am framing exterior gable wall with 2x6 dimension lumber. Will custom cut eps to fill cavities. Drywall on interior, osb on exterior. Vapor barrier interior,  felt exterior?

Felt is OK. Tyvek (or eq.) might be a better choice, depending on your circumstances. (climate, exterior finish, etc.)
Also, the INTERIOR vapor barrier is very climate specific. It should not be used in hot, humid climates, etc.

Where are you? Only in very cold climates do you need a vapotr barrier. A vapor retarder, such as certianteed's Membrain or kraft facing or latex paint is ok, but plastic in a wall anywhere south of Duluth is asking for trouble. search www.buildingscience.com for vapor barriers fort more then you'd ever want to read on the topic.

Dan

Am  in the Kansas City area; summers here can be extremely hot and humid. Sounds like a vapor barrier is not a good idea.

Definitely skip the vapor barrier in the Kansas City area.

@ Wes Shelby

I understand the logic behind vapour barrier HOWEVER people often refer to the use of it in very cold climates. I am 1 hr north of Buffalo NY.

In this modern day and age, a barrier on the inside so moisture in my warm interior does not condense in the insulation zone seems far less intuitive than a barrier on the outside so the warm and FAR MORE HUMID summer air doesn't condense when meeting my central air conditioned house.

Summer humidity is MUCH higher than the humidity indoors in the winter. Seems to me there is more of an issue with vapour penetration/condensation from the outside in the summer than from the inside in the winter.

I have seen the odd comment from media source advisors (meg ruffman, mike holmes, steve maxwell, ) (all tv celebrities past present) suggesting a vapour barrier on the outside-- IF DESIRED, but never a proper discussion of the issue.

The names I mentioned are for example but it was not necessarily from any of these celebrities shows that the acknowledgement of summer condesation came from.

Anyway, if you could shed knowledge on why in this air conditioned age it is not common practice to vapour line the outside of a house I would appreciate it.

drewski,
I'm not the expert in this area. I've never used a vapor barrier in a project, and not that we work with SIPS and ICF's, they are not necessary.
Check out Joe Lstiburek's writings on the subject at www.buildingscience.com