Hi guys. My wife and I are in the late planning stages of building
our new home. I want it to be as energy efficient as possible. We
are having a slight disagreement.

I want foam insualtion in walls and attic. She is ok with that. What
we disagree about is how to do the attic.

Her ex-husband is a HVAC contractor. He says we should use the " foam
the underside of the roof, seal the attic and semi condition it"
method. Our builder agrees.

My problem with that is, why would I want to semi condition an area
the same size as my house if I am never going to use it for anything?
I want to foam the attic floor, and make sure the attic is well
venitaled. Why is allowing my warm and cold air (that I am paying to
heat and cool) to migrate up into my attic a good idea? Isn't it more
efficient to keep that expensive air in the house by well insulating
the attic floor?

Can anyone give me advice on why my thoughts are wrong?