I'm going to put Hardieboard siding on my SIP house (7/16" osb skin).  I am going to screw 1/2" plywood strips over the tyvek at 12" OC.  I will then blind screw the hardie to  the strips and osb.  My question, do I need to use treated plywood or can I use regular?  Regular plywood would save me a ton of money, but I don't want to use it if treated would be required (not by code, but practicallty).
Thx

You could probably just use OSB strips. Whatever the material, if you want the extra protection, you could just staple 30# felt to the face of it. At the end of each board(which should be over the furring), I would place another piece of felt that runs from the top of that board to beyond the top of the bottom board(but hidden by the top board). If any moisture gets through the joint, it will simply drain to the lower board. Overkill? Yes, but it doesn't hurt. And, of course, seal all joints with a high quality caulk prior to placing the next horizontal board.

Good Luck!

Gary BK: Actually your method above with an exterior drainage plain will be the future of sheathed structures. It is a proven fact that felt holds moisture and causes rot on vertical substrates that are covered with impervious materials like cement board siding. The wood construction industry is ripe for the same problems that where created in the EIF's industry years ago. The results are actually shown that the products with the least moisture protection like vinyl siding are the best without an exterior drainage plain due to there ability to dry-out because of air leakage.

Dave

So you think strips of 7/16" OSB onto the 7/16 OSB SIPS and about 2"??? screws through the hardiboard ought to do it? The osb is $10 a sheet cheaper than 1/2" plywood. That would really help the pocketbook. I was thinking of making the strips 3" wide 12" oc, is 3" about right?
Thx again.

I would use 1-1/4" - 1-1/2" screws, and 2" OSB strips would leave a larger area ventilated.

Good Luck!

I wouldn't buy them period. Many local builders including your self will have plenty left over as scrap from there job that they would be willing to give away to save dumpster fees.

Dave

What would you think of using the metal strips that have a z shape and are used for sound isolation to attach sheet rock?
You wouldn't have to worry about protecting it since it is galvanized or plated I believe.
Bruce

Ouch, that would cost a fortune on a 3000ftsq house with walkout and steep pitched a frame roof....