Ok, it’s not too late to save big bucks, it’s all about the basics
Infiltration - this is the air that leaks into the house, cheap to address while it’s under construction, the goal is to seal every penetration that goes from the inside to the outside, some of this may be done by the insulation installer, check behind them
o Foam the electrical and plumbing holes
o Foam around windows and doors after they are installed
o Tyvek type exterior house wrap, tape all seams and wall penetrations (windows and doors)
o Use a foam seal or caulk under the sole plate (the bottom 2x4 of the exterior walls), if the wall is already up then remove all sawdust and put a caulk bead where the 2x4 hits the sub floor
- The goal is to make the house air tight but that’s just impossible.
No matter how hard you try you’ll still have plenty of leaks. If not you can add them later.
Ventilation - continuous ridge vent and soffit vents, need plenty of attic ventilation
Insulation - R-19 in floors, R-13 in walls, R-38 in flat ceilings
- you said the 3rd floor (attic) will be finished, big problem
- it probably has vaulted ceilings framed with 2x6s, that doesn’t give enough room for insulation, add a 2x2 nailer to get more space
- must use a baffle to allow upward air flow then R-19 is all that will fit
- the walls are probably exposed to “attic” space (knee walls), use R-13 but put an insulation board on the back side to add more insulation
- if the space is used for storage the insulation won’t be exposed, those walls need as much insulation as possible.
HVAC is the most tricky. This is definitely not a place to cut corners. The unit needs to be properly sized. Too large and the power bill will be high. The goal is that on a 95 degree day it should run and never shut off. When it shuts off the humidity is not being controlled. Air conditioning is not about just cold air it’s about humidity control. 78 degrees and 50% humidity feels the same as 75 degrees and 65% humidity. That 3 degree temp difference will cost you 3