I am in the process of designing a 1.5 story U-shape house with 3,600 ft^2 on the ground floor. The main section of the house is a rectangle, it will have an 8/12 TJI framed roof with load bearing ridge beam having and has a center section containing a large open room with vaulted ceiling for the living room, kitchen, and dining area with a ~500f^2 loft covering part of it. One end has the bedrooms and the other a garage, these ends will have insulated attic space above them that I will likely leave unfinished which can be later finished for use as a bonus room or storage.

The house will be built in Dewey, AZ. I have designed a double 2x4 exterior walls 24” oc 10” thick with blown in cellulose should yield ~ R-30 and 14” TJI rafters and 11-7/8” TJI floor will give enough space for R-40 or more fiberglass batts. I plan on sealing up the house as airtight as practical. The house is designed to get part of its heating load from its passive solar design https://www.youtube.com/watch?v=fNt-D_1CvAw. I have pretty well decided I want to go with hydronic system with an air-water heat pump of some sort for heating and cooling because I will be building the house and will not have to deal with refrigerant and I would like to be able to shift the cooling load to off peak electric rates with heat storage tanks. I am undecided about using air handlers or radiant panels for distributing heating/cooling loads in the house. My biggest beef with traditional HVAC is running poorly insulated duct work in unconditioned space. I do have space in the attic (which is in insulated space) for running central air ducts if needed. That being said, independent ductless air handlers or a radiant system would be preferred for its zoning capability.

I have checked out some options of radiant ceilings with pex tubing behind the drywall. I have an engineering background and am not intimidated by control systems but I am nervous about a system failure in monitoring the dew point and getting moisture behind drywall due to condensation or a leak. My last idea I have is to do a dropped ceiling with 2x2 ceiling tiles but use radiant panels like this http://www.sigmaproducts.com/radiant-panels.php for heating/cooling. I think done with the right kind of trim on the grid it could look nice and not have the industrial office building look.

Some questions.
1. Is blown in cellulose in a double wall going to be an issue in my climate and should I consider other insulation materials?
2. Can I seal up the airspace in my rafters or does it need to be vented?
3. If I use purely radiant heating/cooling with no air handlers will an HRV ducted to one central location in the house be sufficient or will I need to provide ventilation to several rooms in the house?
4. Is it feasible to do radiant only cooling without an air handler or any means of controlling humidity for my climate?
5. Does anyone know of any US suppliers of radiant ceiling panels for dropped ceilings (or anything where the panel is not behind drywall? Closest I found is in Canada.

Any other feedback welcome.
Thanks,
-Adam

[Edited 5/8/17 added line breaks. Had to use "< br >" for some reason else would not show up.]

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1: Cellulose insulation is just fine in your climate zone. The cellulose itself is somewhat protective, since it wicks and distributes moisture away from the structural wood, but it's buffering capacity isn't infinite. Yavapai county is officially US climate zone 4B, but at higher elevations it can be more similar to zone 5B, where the details start to matter a bit more. The type of exterior sheathing & siding may determine whether an interior side vapor retarder is needed, but in most cases it'll be fine/best to use just standard interior latex on wallboard as the interior side vapor retarder. Back ventilated siding allows the sheathing to dry to the exterior much more quickly than say EIFS.

2: Soffit to ridge roof/attic venting has a lot of advantages if it can be done correctly & everywhere. The details at the vaulted ceiling area have to be worked out in advance to ensure it doesn't get screwed up.

3: With ducted ventilation in a tight house it's good to provide individual room supply ducts in rooms that matter (such as bedrooms) and exhaust ducts from areas that are sources of indoor air pollution (such as kitchen & bath areas), but for most rooms jump-ducts can be used for either the supply or return paths, as long as there is at least a dedicated supply OR return register in every room.

4: Radiant cooling is definitely possible in your area, but you may need to control humidity separately from time to time in July/August when outdoor dew points hit north of 60F.

5: Can't help you on US suppliers of radiant cooling panels for suspended ceilings.

Thanks for the advice Dana.