Rigid Insulation over Underlayment

[Guest]

I'm new to the forum, but would really like some help. Hopefully, someone has run into this before and can help me out.

I'm located in Central Texas and am building a house with my wife. The house was originally started by the previous owners and we are replacing and redoing quite a bit of it at the start. We are doing the majority of the work ourselves (taking forever) and we are getting close to working on the roofing. I've been reading a lot about Green Building and am trying to make the house as efficient as my budget allows. I found fiberfaced 2.5" Polyiso foam locally and had a whole bunch of it delivered to install on the roof. I plan to put one layer down over the roof and am wondering what the correct, or best, method is. The roof is sheathed with 7/16" OSB on 24" centers over 2x4 trusses. There is currently 15# tar paper on the sheathing but that will be coming off for synthetic roof underlayment. I plan to put the poly down over top of the underlayment and then another layer of underlayment (30# felt or synthetic, not sure) over top. I will then put 1x4 or 2x4 purlins at a 45 degree angle (to create venting) over the underlayment and poly and then I will have a standing seam metal roof installed.

So my plan is:
1. 7/16" OSB sheathing
2. synthetic underlayment
3. 2.5" Fiberfaced Polyiso
4. 30# roof felt
5. 1x4 or 2x4 (does the size matter?) purlins
6. standing seam metal roof


Does this sound like a good plan, or should I have another layer of sheathing or should I skip the roof felt, or is there any other suggestions you guys might have?

Thanks in advance.

I also attached a picture of the polyiso (111 sheets of 4x8) and a picture of the house/roof as of a week ago.

[Guest]

Since no one has responded yet, I'll ask another question with some design ideas that I found.

The picture attached seems like a good idea for getting the polyiso installed and still having the venting. My question is whether the extra layer of sheathing is necessary since I will be installing a standing seam metal roof and not shingles. Any ideas?

Jacob

[Guest]

Both shingles and steel panels must be connected securely,
OSB overlay is simple, & effective value.
Standing-seam roof-clip/fastener must align with flat 2x4 blocking, if OSB is omitted.

[Guest]

I'm now thinking of doing something similar to the picture below (taken from this website ). Again, it shows another layer of sheathing on top, but I'm hoping that I can do 2x4s perpendicular to the ridge, and then 1x4 purlins parallel to the ridge so I have a good base to mount the roof to. Would the 1x4 purlins be enough?