Hello,
I was wondering if someone could give me some advice about flooring over an unheated garage. The rest of the house uses HVAC, and we live in a cold climate.
the sub-floor is 2x8 diagonal 3/4" decking 12 O.C. I know the floor is strong, but I plan on adding an extra 3/8" to the rest of the house, and I figure I should do it here too.

I am considering using engineered hardwood (vintage) because of humidity differential between the inside and unheated garage below.

Under the floor are joists of course, and I planned on installing dense pack cellulose insulation during the install (i will pick up a few of the floor boards and just stick the hose in) When I live, a poly 6 mil vapor barrier is required, which i will install between the sub floors. the vapour retarder's purpose this barrier will prevent moisture flow from the inside from reaching the outside (heating mode).
the question
I know hardwood requires a vapour barrier that has a higher value perm rating to still allow for slight moisture movement. I want to know if I am creating some sort of issue with the engineered hardwood by using a vapour barrier that is not very permeable.

thanks

Engineered wood doesn't present near as many problems when exposed to conditions that are less than ideal. You are correct in stating that wood needs air flow. Engineered will be the best application if you think you'll greatly reduce the permeability.