Mini split sizing in Houston – GreenBuildingAdvisor

I’m hoping on getting some thoughts on sizing Mitsubishi mini splits. I’m in the Houston area (zip 77536)

We’re gutting the entire second story and adding many big dormers to actually make the slanted ceiling bedrooms usable for our growing kids. 

Walls will be 2×4 framing, fully filled with open cell spray foam and 1.5” of exterior rigid foam, then Hardie siding. 

Roof will be 2×6 construction with open cell spray foam, 3” exterior foam and shingles. Bedroom 2 and west side of bedroom 3 actually have 2×10 roof rafters due to the size of the shed dormer. 

The entire roof will be a fully closed, unvented design. 

The bedrooms and bath will have high cathedral ceilings. So ceiling height will vary between 8’ at the walls and 11 feet at the ridge.

Window sizes will be about 30”x48”, low E with Argon.

Bedroom 1: 10’ dog house dormer with 2 windows on east side. 2 windows on north side. 11’ Shed dormer with 2 windows on west side.

Bedroom 2: French door on west side. 2 velux skylights 30×46” U factor 0.49 in roof

Bedroom 3: single glass door on west side.  2 velux skylights 30×46” U factor 0.49 in roof. 10’ dormer on east side with 2 windows.

Bedroom 4: two 6’ doghouse dormer on East side with one window each. one window on south side. 22’ shed dormer with 3 windows on west side. Half of that room is on top of my garage but I plan to put spray foam in the floor.

Bathroom has one window. I may take some of the bathroom square footage to make a closet for bedroom 1.

So my original HVAC plan was for 4 separate 6000 BTU Mitsubishi units for each of the bedrooms. 

I’ve been playing around with coolcalc and coming in around 3500 and 4500 BTU cooling load for the different bedrooms. I’m really not sure if I’m doing the calculations correctly though. 

My other option would be for a ducted Mitsubishi air handler in the tiny space that I have available over the hallway. From there I could reach all room, including the bathroom pretty easily. No idea about the necessary heat pump size though. 

Is one option more efficient than the other? After reading through many posts, I was worried about short cycling with 4 individual wall units, but depending on how accurate my load calculations actually are, that might not even be a problem? Especially since the 6K Mitsubishi units modulate down pretty far? 

I am somewhat concerned about not getting enough air flow into the bathroom, since the kids like to keep their doors closed. 

With a ducted air handler option, would I still be able to regulate temperature for each room? What if I want to shut off one room completely? Like if one of the kids goes off to college…

I would appreciate any kind of guidance you could give me. My HVAC guy proved to be pretty useless. Trying to sell me a giant multi split pump with wall units between 9 and 12 K.