Kohler Proflex AirTub Possible - Subfloor/mortar prep?
I didn't ask...just assumed, your joist are they full wood or "I" type? If they are full wood, I wouldn't think it would be a problem mainly because with the mortar bed, use full 2x4s glued and screwed. With either full wood or "I" joist , you could always cut the sub-floor out to the center of the joist and put a new piece in with or with out the sisters. I would do the same in my house. The weight is being spread out to the footprint of the mortar bed. I believe the mortar bed is optional (at least it was on my air tub) so if that were the case the entire weight sits on top of 5 4x4 blocks glassed to the bottom of the tub.
As for the coldness...insulate the heck out of things and seal any air hole going between the garage and house.
when it comes to breakfast, the chicken is dedicated, the pig is committed.