This is regards to the floor insulation BTW!!…

Batt-type insulation is a poor choice for insulating under floors because the batts will be compressed at installation and you'll get less than the stated R-value. Then, over time, they'll sag and it will be worse. You need to insulate with a material that air seals well and has a high R-value. I suggest you install spray foam insulation. It's what we use when we convert a screen porch to conditioned living space in our area.

Unfortunately the Froth pak DIY spray foam is only meant to be applied at 2” thickness. This only gives you an R value of R-14 (approx R-7 per inch), so I wouldn’t get a high enough R-value. Plus the cost of the DIY kits is high…I can only imagine a contractor would be MUCH higher.

I believe the 2" limitation is the recommended maximum application thickness to be applied in a single pass. You can achieve more thickness with multiple passes.

I don't recommend installation of spray foam as a DIY task because of the safety risks.

Don't forget the vapor barrier.

Urethane spray foam insulation installed to the code-required R-value doesn't require a vapor barrier.

I called DuPont…max thickness on diy froth paks is 2”!! I can afford $1500 to spray foam 140 sq ft.

Spray foam insulation gets special treatment in most building codes. In the 2018 International Residential Code, the requirements are described under section 2603 (foam plastic insulation) of the code. Froth-Pak is permitted to be installed up to 2" in thickness without installation of a thermal barrier. The manufacturer may be limiting it to that in order to ensure DIYer installations meet that code requirement or it may be to limit the heat generated during the reaction of the foam to reduce fire risk. I recommend a thermal barrier for your application if only to keep insects and rodents out of the foam.

IRC Section R316 covers Foam Plastic (also R302.8). The applied thickness does not govern a thermal barrier for this use. There are other characteristics that determine that, like where it is used, what it is exposed to, and if there is other cover like the batt insulation. If you choose to enclose the under deck space to a crawl space, or whatever your AHJ will define it as, will determine a thermal barrier. However, there is functionality of enclosing and air sealing the bottom of the deck. Many DIYers choose to use foam board under the joists as the air seal plane, with batts fully between the joists, then covering with a soffit material.