Been reading these forums for a long time, and learned a ton.

I have started building my new house, and I am going to be at the point in a week that requires a final insulation decision.

First of all over view:
Dallas, TX
3600sqft living, 1500sqft attached garage (roughly 1000sqft of house over the garage).
House has a low slope roof, 2.5:12 along one side.
2 rooms have fully vaulted ceilings(2x10 stick above them), roughly 15x15 each. Rest of house has flat ceilings.
AC systems (5 and 3 ton, 2 stage, 16 seer) in the attic
2X6 walls
I have a fairly decent amount of plywood down on the attic floor to access AC units (300sqft probably)
mostly Stucco exterior

My original plan was to go 100% open cell foam with an unvented attic. Was quoted roughly $8000. However, I found out that my AC guy installed a 80% furnace. My GC didnt tell him that the attic was going to be unvented. Oops.

I do not want to spend the money to remove the furnace and replace, considering I am in TX and I will probably never recoup the costs associated with a higher efficiency unit, let alone the install cost.

I had 2 foam folks come by:
1) wanted to foam the walls with open cell, foam the cold floor over garage, install fiberglass batts over vaulted ceiling areas and blow in fiberglass in the attic for a vented attic. Install batts under plywood in attic. 5.5" in the walls
2) Wanted to foam everything with open cell, attic floor included. Then blow in fiberglass to get the R value needed. 3.5" in the walls.

Both these guys wanted ~$7K

I like the idea of #2, except there is no way he can spray the corners of the attic floor at the edges of the 2.5:12 roof. its really narrow. Guy #1 said exactly that, hence no foam in the attic. I would also probably go ahead and fill the wall up to 5.5.

I am thinking about doing a hybrid. Using open cell foam on the vaulted areas attics, effectively making those areas completely sealed.
Foam under the plywood for AC access in the attic, and entire accessible attic floor to seal. Glass batt on top of open cell under attic plywood, and blow in fiberglass everywhere else.

Basically, two small areas (above the two 15X15 rooms, which both happen to be against the outside corner of the house on opposite ends) of the attic would be sealed, with the rest open giving the furnace plenty of combustion volume. No can lights in any vaulted areas.

Thoughts? I am going to bridge the furnace issue with my HVAC guy, but either way it is going to cost me thousands ($3k+)more to go with a sealed attic, and in Texas I am not sure the payback would be their.

I have even thought about getting day labor to seal everything up with calk, and using cellulose in the walls and everywhere except cold floor and vaulted areas. Friend did that and his power bills are exceptional, but I feel over time the cellulose will collapse.

Open to any suggestions/thoughts especially on my hybrid arrangement.

Tons of pics in this thread:
http://www.garagejournal.com/forum/showthread.php?t=218086

Ill leave them there so I dont bog this one down.

The house is built with traditional framing techniques, etc. I had a very tough time finding an available framer in a reasonable price range, and of them that would consider more advanced methods wanted 2X as much. Sucks when you are building only one house! :) I think if I was one of you Northerners, I would consider passive building techniques.. that is very interesting for an engineer like myself.

Thanks!

When you say flat ceilings, does that mean trusses? And how much ceiling insulation can you get in there?

What is the orientation? Does the garage face south or the pool?

Been wiring all day, snapped a few pics of the areas I was talking about. One major thing I left out above (durrr) is that the entire hallway is vaulted, just not "fully", it is about 2ft from the roof deck.

To answer your question, the garage is perfectly due west... so front door is due south. That is why I have the huge 9ft overhangs on most of the big windows that face west/south.

I meant that all the other rooms were flat, no trusses.

The "full vaulted" rooms I can get roughly 9.5". (2x10). The other vaulted area int he hallway has about a 2ft gap.

I think it really should be full encapsulated, but even if that saves me $50/m in energy bills it would take a LONG time to recoup the probable 4-5 K extra it is going to cost me My insulation guy insists that doing the attic in a combination of blown in and bats, with spray foam walls will get me 95% of energy efficiency as fully foamed, and doesnt believe it is even $50/m difference.

Here are the floor plans and also a few pictures I snapped today of the small attic areas: Clicky