Need some feedback on IB idea for home theater build to get bass for free in garage

We are getting ready to start an addition on our home that has been in the works for over a year. To keep a long story short, I currently have Goldenear Triton 1 towers as the main speakers that have been in their boxes for over a year. I am running an anthem 200 wpc amp to them and they have their own subwoofer amp build in. Been considering an additional subwoofer to counter room modes and move some extra air on the first couple of octaves.

We are building the addition as a 3 car garage with an equal living space over it. Part of that space is a HT room about 16' x 24'.

I had been considering adding a pair of 18" subs on a manifold in the ceiling using the attic as the rear "enclosure". Today it occurred to me that instead of mounting the subs in the ceiling of the HT I could mount them from the garage in the floor and use the garage as the rear "enclosure".

Why? Because I could use one of the extra zones on my preamp to signal an amp in the garage, along with some bookshelf speakers, then use the backwave off the subwoofer manifold for bottom end in the garage.

What do you guys think? Am I missing something or would this be a good way to get bottom end in the garage basically for free? In my existing pole barn I have an old Velodyne 15" HT sub running bottom end and it works great. LMK your thoughts.

I had thought about doing that between my garage and shop, nothing really came out to me as it being a bad idea to be honest.

It will not in absolute polarity, but is that a concern in the garage?

maybe no issue, but I'd worry about a climate controlled situation, if the upstairs is cooled and the garage is not, maybe there is something easy I'm overlooking, but it seems like if the back of the subs are exposed to garage air and the front of the sub cones are in the living area, there will be some unpleasantness on cold or hot days, possibly ending in warped spiders or deteriorating surrounds.

I might not be understanding exactly, this scenario.

I know if the kind of IB dual use baffle were installed, and paper coned subs were selected, I'd have to worry some, even if they were "weatherized" due to the possible saturation as they would be the membrane between two thermal zones.

But it's a great idea, getting to use that dual zone option out of the receiver, maybe have a servo involved, lever up a panel against the cavity containing the baskets, where an air tight seal could be done, when the sub is not in use?

Otherwise, all those hours using the sub upstairs, keeps the cones in contact with the outside air...

but if the garage is going to get climate control year round, that would allow it?

Good points. I tried to get the garage included in the HVAC but was told that code would not allow vents into the garage. They said that I would need to have a separate HVAC unit for the garage. We'll have to wait and see if there is any money left in the budget.

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I would not worry about the temp delta... Many/all subs have a temp delta on that spider, Voice coils make a significant amount of heat.

I'd try it, what's the worse that could happen:(