The tub should have installation instructions on line for you viewing pleasure. I bed all "plastic" tubs in a mortar bed. Foam is a shortcut that I think inferior, as it can still flex and compress over time. You'll need to make sure the height of the tub plus the thickness of the bed will allow the top of the tub to be at the correct height. If the subfloor is 3/4" ply, that's good enough. If it's something thinner, I'll screw down a piece of 3/4" CDX and place the mortar bed and the tub on that. The mortar bed not only support the base of the tub against flexing, but it allows the tub to be set dead-level on a not-so-level floor. Mongo

I gotcha, here's more background: the deck will be very close to the ground, about 18" so we probably won't do any skirting. The sod has been removed and replaced with landscaping cloth and stone pitched away from the house. My understanding is that i'm probably ok everywhere but the corner near the house, it seems like that wouldn't get as much airflow as the rest. There are 5, 6" x 16" crawlspace vents on that wall right below where the deck will be, some in the corner, a good amount of air flows through the crawlspace at any given time, might that cover me?

Oh, one other idea: instead of your last 2 boards being 5" and 3", you could rip down both of them to 4" (or whatever the exact measurement is when you take the spaces into account). That way it would be less noticeable.

I'm completely on fence. My deck is a main feature of house. After 10 years of mostly deferred maintenance, it's been pressure washed and sanded lightly and wow it is gorgeous. Muted grey, tan, blonde boards. I think is has an elegance. I have flipped a few board and tried sampling some stains - from Red Mahogany, to clear and even the clear really darkens it and it seem to lose the elegance. I would hate to get a mosaic of colors that aren't blending in the zone. Too bold would not fit with my 96 year old bungalow. Patina rules. Sure wish I could find a finish that would add mildew and uv protection without changing the patina color. Clear twp is just as dark as Penofin, ipe Brazilian oil stain or the rMesmers red mahogany. I don't get it. I think the scratch marks will be more visible if you have a stain because it will drag below the very surficial stain. Not much really penetrates deeply so you'd have lighter wood revealed in the scratch..