Ribbed metal over OSB/stud wall with foam board insulation-do I need furring strips?

[IHDiesel73L]

Still kicking around ideas for an office/guest cabin in the back of my property and contacted an Amish builder about 3 hours away. They usually do LP Smartside but will also do metal over 7/16" OSB. I am considering ordering a building just skinned with OSB which would be a cost savings and then doing the following: Tyvek over the OSB (roof and walls), 1" or 1.5" rigid foam board over the tyvek, and ribbed metal over that. Inside I'll insulate the stud/rafter bays with fiberglass batts with the end result being R-20 walls and roof and a very airtight building with all Tyvek seams taped and all foam board insulation seams taped. The question is do I need to put furring strips (horizontally, like girts) on the walls, install the rigid foam in between them and screw the metal to the furring strips? Or can I just glue or mechanically fasten the foam to the wall and then use long self tapping screws to go through the metal and insulation directly into the OSB?

 

[jaxs]

Screws won't seal with foam cushion between head and osb. I'm not sure foamboard is worth the added effort. If your climate is exceptionally cold (member's location in avitar help's with questions ) compare added cost of material and labor for 6" studs to foam.

 

[KennyG]

I'm not sure the foam would be worth it either, but the attachment would depend on what type of foam you are using. The conventional poly sheets you see in the big box store isn't substantial enough to screw through - it will crush. However, if you used the high R value polyiso it would probably work fine. That stuff is really solid.

 

[s219]

You may not want to put the foam on the outside of the OSB -- could create terrible moisture problems. Normally you'd want foam right next to your batt insulation on the inside and you need to carefully calculate the location of the dewpoint temperature for your local weather conditions to make sure it falls within the foam layer in the interior part of the wall.

Actually, regardless, you need to carefully plan for and manage moisture and various layers of vapor barriers. I didn't detect any consideration of that in your plans and some of what you suggested doesn't make sense (Tyvek and then foam). Get that stuff worked out before going any further.

 

[jigs_n_fixtures]

Vapor barrier location depends on whether you are in a heating or cooling climate. latest from teh Green Building Advisor is tha tit goes on the outside in all but rare situations. but in all situations you need to make sure you don’t end up with a wall whihc lets the vapor in, and holds the water. those will always rot.

 

[s219]

If it's to be a conditioned space on the interior, then the real moisture problem is what comes from inside the building and how it's either blocked from getting into the walls or kept above the dewpoint temperature. That all needs to be carefully planned.

 

[mikester]

OSB - Tyvek - furring strip - metal cladding

Unless you fill the gap with spray foam bonded to the metal cladding, leave an air gap under the metal cladding. You need to remove condensation. Metal will sweat in cold climates.

 

[Diggin It]

If you really have need for something outside the OSB, consider Zip board and tape or some of the foil covered insulation panels and tape. I'm not sure the pink or blue board would accomplish much over OSB.

I used it 20 years ago on the outside of my 1940s concrete block house. Mounted 2x4s to the block, cut 1-1/2" foam panels to fit between the 2x4s, then hung vinyl siding. No problems since that I know of. Other than wasps trying to get into it.