Hi folks. I am in the planning stages of building a small home. In the interests of money, simplicity, and time (I will be most of the labor), I've stuck with a simple design with a shed roof (sloped on both inside/outside). From the back to front it goes from a ridge height of around 10' to 15' at the front (over a span of 25ft). The entire front area is combined kitchen/living space. I may frame in 8' ceilings in the bedrooms as they are in the back. Part of the impetus for the shed roof aside from easy framing is allowance for a kids loft in the front to maximize the small footprint.

Moving along.. is there a consensus on insulation and ventilation in roofs like this, with reasonable energy efficiency considerations in mind?

I've seen several articles with some suggesting keeping it airtight is the best way to go. That said I'm not sure if local code comes into play here either. I am in borderline zone 2/3 territory.. Southeast.. hot and humid. I know when I used to be in the industry we would use continuous baffles from the soffit up, with cathedral ceilings.

Insights appreciated

What roofing material? Standard plywood or OSB decking with felt and asphalt/fiberglass shingles? If using standard plywood or OSB for decking, I would strongly consider the use of radiant barrier decking with you being in a hot humid climate. The cost of the radiant barrier decking over standard decking is minimal and it significantly reduces summer radiant heat gain below the decking.

Here are examples:

https://www.homedepot.com/p/7-16-in-x-48-in-x-8-ft-OSB-TechShield-Radiant-Barrier-22493/202106242

https://www.lowes.com/pd/Solarbord-Sheathing-7-16-CAT-PS2-10-Radiant-Barrier-OSB-Application-as-4-x-8/50432994

Will the roof be framed with dimensional lumber or trusses? Blown or sprayed cellulose is generally your best bang for the buck insulation; however, that can be hard to implement in a cathedral style roof. If you did put air chutes the entire length of the roof line, then you could blow in cellulose and fill the cavity between the air chutes and the ceiling board. If using dimensional lumber for rafters, you might also consider cross strapping under the rafters to get a level surface for the ceiling board and to create a thermal break between the rafters and the ceiling board to limit thermal conduction through the wood members. 1x4 strapping is pretty common; however, I prefer ripping 2x4s down to 2x2s (1.5" thick x 1.75" wide) and use those instead because it gives you a 1.5" thermal break, and assuming you are using blown cellulose, 1.5" of insulation between the ceiling board and rafters. Also, SPF 2x4s are typically about the same cost as 1x4 per linear foot, so if you rip a 2x4, you get almost twice as much strapping for your money with a little extra time involved to rip the lumber (this is assuming you have a table saw available).

Depending on the roof load, you may be able to achieve an all in one solution with a steel SIP , a 12" (R-51) has been used before to clear span 25 ft and no additional roof cover is needed with seam tape.
one product and your rough frame/ insulation /finished roof is all done in one step
and yes air tight (conditioned space) is the best in hot-humid climates

Im doing a shed roof on my house too.

32ft span, 0.5/12 pitch. short side of trusses 24 inches, long side 40"

my goal is to have 2" of closed cell spray foam below the deck, and then get the rest of the insulation through fiberglass batts up at the roof. I should be able to get at least r49 without too much cost this way.

the roofing contractor is also suggesting one inch of foam board above the deck.

I believe the way to do it in a humid area is vapor barrier on the exterior side and then an ERV if your envelope is tight and possibly dehumidification.

Extreme cold goes opposite, vapor barrier inside.

In hot and dry areas, it seems like vapor retarder only works.

As for the roof, if you have a cathedral ceiling with the insulation directly under the roof deck, you have to make sure you control moisture in the roof by ventilating the insulation area or some other means.

What you don't want is to trap moisture between to vapor barriers.

For me, I think I'm going with ZipR sheathing on my roof to make a vapor barrier between the sheathing and the roofing. It will be well taped and liquid flashed on top. I will have blown in blanket style dense pack insulation below that and hopefully wood tongue and groove paneling beneath that to make a vapor ~permeable layer so the ceiling can dry to the interior should any moisture get up there (hopefully it will just be seasonal humidity variation).

On top of the ZipR, I'm going to have a radiant barrier underlayment, possibly a ventilated roof mat and then the standing seam metal roof so I might have 4 moisture barriers in total on the roof. Fingers crossed, it won't leak...

Using energy efficiently can reduce the cost of heating, ventilating, and air-conditioning, which account for a significant part of the overall cost of housing. Energy costs recur month-to-month and are hard to reduce after a home has been designed and built. The development of an energy-efficient home or building must be thought through using a systems approach. Planning for energy efficiency involves considering where the air is coming from, how it is treated, and where it is desired in the home. Improper use or installation of sealing and insulating materials may lead to moisture saturation or retention, encouraging the growth of mold, bacteria, and viruses. In addition, toxic chemicals may be created or contained within the living environment. These building errors may result in major health hazards. The major issues that must be balanced in using a systems approach to energy efficiency are energy cost and availability, long-term affordability and sustainability, comfort and efficiency, and health and safety.