Thank you for your interesting question(s) concerning remodeling to incorporate vaulted ceilings in a hot and humid climate.
Moving the thermal barrier to the underside of the roof increases the volume of the conditioned space. The greater the volume of the conditioned space the greater the energy requirements to heat or cool the space. Efficiency is based on the performance of the HVAC equipment which should remain the same unless enlarging the space results in higher BTU requirements than the existing system can provide.
There are two schools of thought on insulating cathedral ceilings. I am a proponent of having a vented roof space.
- While insulation material will be very important in providing a proper air and thermal barrier, venting is always recommended (in my opinion) because you have to assume the system will fail eventually and venting will allow for removal of excess heat and moisture.
- If the roof leaks a vented roof will dry out while a sealed roof space may develop decay in the roof sheathing before you realize the roof is leaking.
If you are concerned with interior moisture (moisture generated by the occupant) you'll want to make certain that you have a moisture barrier in place to keep the moisture from passing through the insulation into the ceiling cavities. Most spray applied foams can qualify as both air and moisture barriers providing you have adequate space to install the depth of insulation required to be classified as a barrier.
If you have a radiant barrier material installed between the insulation and the underside of the roof you will also reduce the cooling load within the structure.
- If you have a sealed roof space you'll want to install roofing material with high solar reflective value for the same benefit.
- If you install a vented roof you'll need to install a radiant barrier material that creates a space between the underside of the roof and the insulation. Radiant barriers do not work unless there is an air gap to reflect the radiant heat. The roof vent will remove conducted heat.
Air flow throughout the house will be affected by the installation of vaulted ceilings unless you move your HVAC supply and return ducts so they have a positive impact on the new space created by the vaulted ceiling. Contact a qualified HVAC contractor prior to your remodeling project for a list of suggestions for ensuring occupant comfort.
For more information:
Read "How do you insulate a home with cathedral ceilings?" a Q&A answered by Kevin Holdridge.
Also, check David Willson's answer to the question "What's the best way to heat a large family room with 14 ft. ceilings?"