It would help to know the climate, the R values you want to achieve, whether or not the attic will be conditioned and what kind of air barrier system will be used. Also what kind of spray foam is your contractor recommending?

In general, your contractor is correct about the importance of the roof vs wall insulation even if he has exaggerated the difference. I doubt he's correct about ventilation if the walls and windows are well designed.

The Project location and construction techniques have a major effect on these issues so that's where you need to start.

Thank you JDS

We are deep south texas (think almost in mexico) Very hot and humid. I believe he'll be using open cell foam. I'll be getting more info from him soon.

Your concern will be heat gain from the sun rather than heat loss. The heat gain from the roof will be much greater than through the walls and if the insulation is in the floor of the attic heat will be radiated to the attic floor. Even if the foam insulation is in the rafter bays, a radiant barrier will be helpful but it must be installed correctly.

Thank you for the great information Springtime Builders. We are actually planning to put the a/c unit inside of our living space in a closet. The air ducts will only be in the attic.

That's good, most HVAC leakage occurs around the air handler (inside unit) but ducts and their connections to boots or registers are important too and a good reason for bringing attic within the conditioned space. Another nice feature of the certification route is that ducts are also diagnostically tested (like blower door for building envelope) to verify minimum duct leakage.

Is the roof framed with trusses or rafters?

There is increasing evidence that Open Cell Spray foam under roof sheathing poses rot risks not just in hot humid climates, as in east Texas, but in all climates.

I have never used open cell foam. When I use foam it is to maximize the R value for the thickness.

A better use of foam in general is as board sheathing completely outside of the framing or structural sheathing. For walls, Zip+R is a great product combining the two and has an integrated air and water barrier. For roofs it's rare for builders to use foam sheathing on top of the roof sheathing but it's a better way to go from a thermal standpoint, closer to Dr. Joe's perfect wall (roof and slab).

Does open cell foam really rot roofs is a good counterpoint to Mr. Holladay's article. Judging from the vast number of open cell roof decks being built in the south (without dedicated conditioning) I feel pretty good about it. Certainly adding a means of conditioning these attic spaces seems to be a best practice that makes most building researchers happy. Cold climates have greater risks for sure.

Codes in colder climates are beginning to require exterior insulative sheathing for walls. In the OP's case, even 1/2" of exterior foam would represent a huge addition in performance, especially if choosing polyiso with a foil facing. Use a rainscreen to get full performance out of the radiant barrier and you have an extremely cost-effective use of foam.

There can be a difference between expected and actual performance with wall and ceiling assemblies. I've used open cell foam as insulation to create conditioned attics in a number of homes in our mixed-humid (coastal Virginia) climate including my own home. No rot issues so far with 16 years of experience. Duct sweating is minimized, HVAC performs better, and the homes are more comfortable.

a good counterpoint

All Bailes says is:

1) lots of homes with oc foam in conditioned attics have worked well, though he has no explanation of why that may be so.

2) as per Lstiburek, you can add returns and supplies with the rest of the house, even though there's no easy, economical way to do it.

I've used open cell foam as insulation to create conditioned attics in a
number of homes in our mixed-humid (coastal Virginia) climate
including my own home.

Allan Edwards, a Texas custom home builder, reports the same results in the GBA linked piece above.

Do you add returns/supplies with the conditioned space below the attic as he reports doing?

*****

Every time I think about doing a conditioned attic in our cold climate--all HVAC in the basement--I hesitate and go unconditioned.

When we starting constructing conditioned crawls and attics in 1999, duct leakage alone could be counted to provide adequate conditioning of the spaces. Now with duct leakage < 5%, we install a continuously operated fan to ventilate conditioned crawl spaces as per prescriptive code It is indeed better to introduce conditioned air from the living area versus the HVAC supply. We do not advocate returning air from the crawl or attic to the living space (although allowed by code) in order to minimize the potential for soil gasses, mold spores, mildew, moisture, etc. to migrate between the spaces.

We don't have experience with conditioned attics in cold climates, so we'd have to defer to others regarding what works under those conditions.

"Codes in colder climates are beginning to require exterior insulative sheathing for walls."

These codes are beginning to allow insulative sheathing but do not require it.

Considering insulative sheathing has made it's way into prescriptive minimums, I think it's more of a requirement than allowment. The "+" value in wood framed wall R value below denotes insulative sheathing prescriptions.

Typical framing results in walls where 20-30% of a wall's total area, has no cavity insulation, which drags down the entire wall area's performance. For owners or designers after the most "R per inch" insulative sheathing is more effective than cavity insulation as it better encapsulates framing and structural components.

This potentially increases airtightness, reduces thermal bridging and keeps structural components warmer, reducing chances of condensation and rot. The last feature may be why codes are beginning to adopt insulative sheathing as best (minimum) practice and calculating the minimum insulative sheathing is important for wood framed walls in cold climates. In the warmer climate zones of 2 and 3, a 2x4 wall with 1/2" of insulative sheathing outperforms a 2x6 wall and is thinner.

As for Open Cell foam in roofs, I agree it's a relatively new practice posing risk and the same is true for Closed Cell. That's why many experts recommend insulative sheathing on top of roof structural sheathing and why the traditional unconditioned attic is appealing, as long as it has an airtight ceiling plane (tough part) and a thick layer of loose fill insulation. Roof leaks are typically more common and damaging than condensation and I think closed cell complicates discovery and repair more than open cell, but in cold climates I would prefer closed cell.

Martin's article sounding the alarm about modeling studies is useful, but I think the onus mainly falls on field results and the vast majority of open cell roof decks seem to be working fine, other than the unfortunate reality that most spray foam installations are below code minimum performance.

A good friend of mine is building in TN and we just went through all this. He was able to get good pricing for a flash coat of closed cell against the bottom of the roof deck, followed by filling the rest of the rafter cavities with open cell.