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Foam then vapor barrier or v.b. then foam?

I am in Minnesota and need to re-side my home and add some insulation. Home was built in 1980 and (from the inside out) the walls are: 1/2" sheetrock, 4" paper faced baths between 16" o.c. studs, 25/32" 'buffalo board' and 1/4" rough face plywood siding and 16" o.c. battens. The home is lacking in insulation and weather tightness, both significant factors in Minnesota. We like the look of the 'board and batten' appearance the home has, and plan to refinish with the same, but I want to improve the envelope when I do it.
My plan is to remove the exist. siding and install tyvek and 1/2" of foam insulation, then again apply a rough face plywood, or a similar engineered product (LP Smartsiding or cement board panels or similar). I have a couple of questions:
1. Should I strip the walls with 3/4" board's, install the 1/2" foam between them to allow an air space behind the siding for drainage?
2. Vapor barrier, moisture barrier, air barrier... which do I need here?
3. Foam first, then the barrier or the other way around?

I am working out all the flashing and trimming scenerio's as we speak, but the order of operations has me stumped. Any other options or recommendations? Thanks in advance. -Mike

Re: Foam then vapor barrier or v.b. then foam?

"Buffalo board"? I don't know what you are calling buffalo board but buffalo board plywood is a trademark of a European brand plywood with a wire mesh and slip resistant surface used as decking for boats, and it is way too expensive to use as sheathing on a house.

But all that aside, only one vapor barrier and you already have that on your fiberglass batts. Be careful when removing your current siding. It is providing important structural support to keep the walls from racking. If you remove all the siding off one side of the house at once, you could cause structural damage that will be very expensive to fix.

1/2" foam is not going to provide you with much difference in your insulation level, but is will help break the thermal short circuits from your sheetrock, through the studs and then to the siding to be radiated. 1" foam would be significantly better, but you will need longer screws or nails for the siding. Hardened and ceramic coated screws would be the best to use, and use plenty. You can mix some screws with 16d nails but with 1" of foam between the siding and the studs, you will need a nail or screw every 6" on every stud and along the headers and footers to resist racking forces.

BTW, push the fiberglass as far towards the interior of the cavity as you can and let it expand back. The old method of installing fiberglass batts with an air gap between the batts and the sheetrock was wrong and reduces the effectiveness of the fiberglass. The gap, if any should be on the outer side of the insulation.

Re: Foam then vapor barrier or v.b. then foam?

Keith, thanks for the reply... definitely a different buffalo board! Here in MN that is what it is called, but I guess it is generically called fiberboard (google buffalo board sheathing and you'll see the stuff I mean).

Re: Foam then vapor barrier or v.b. then foam?

If you are going to replace the sheathing anyway why not try an all in one product like radiant barrier/OSB and associated tape, it gives some insulating qualities and eliminates the need for a seperate vapor barrier/housewrap (yes,it's more expensive but you save on labor)??

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