I am replacing 6 rows of clapboard on a back wall and finding no vapor barrier between the clapboards and the sheathing (22 yr old house). The plywood looks in good shape, but there is residue of what I assume are carpenter ants behind the clapboards. Does it make any sense to add Tyvek to just the part of the sheathing I am exposing?

I personally like utilizing tyvek. Unfortunately the material, along with the tape is very expensive and you would have to buy a whole roll to do one little area. It would be like waxing one small part of your car and not waxing the rest. Unless you were doing the whole wall, I'd say forget about it.

some help

Hello
Tyvek is a great product but will off no help here. Before you cover up the plywood find out where the 'ants' are coming from and take care of the problem there, all around and under the house they will not go away and can make problems for you later. you should take a sample of the ants to a pest control office at the county or city they can help you with the best product to use by name and type for your area.