Hello, I know this topic has been beaten to death already, and I have done a lot of reading and talked to some other local builders, but am still uncertain of the best way to proceed. Here’s my situation…

I am building a 12×20 cabin on piers. I am in northern NY state. The cabin will be heated, and used intermittently throughout the year. I am aware of local building code and not trying to meet R-value requirements. The floor is framed. I will be using Advantech on top of the 2×8 joists. I have between about 18″ and 4′ of clearance under the cabin, enough to work, though I’d like to do the majority of the insulating form the top. I want to insulate the floor to a certain extent, but being that it will never be heated all winter long and is only used intermittently I am not concerned with getting the highest possible R-value. I am more concerned with managing moisture, rodents, and doing so economically. For now, it will remain open underneath with plenty of airflow. I may eventually add skirting but not likely any time soon. 

I have some 2″ felt faced polyiso and can acquire more cheaply. Initially, this is what I was going to use, but I am concerned with having to take additional steps to rodent proof it. I’m also a little concerned about moisture from underneath, as the fabric facing isn’t really a vapor barrier. I recently talked to a friend who has a cellulose blower and can acquire cellulose at a good price, and I am leaning toward this. With this option, rodents and moisture are my main concerns. 

Here is my question… 
If I were to use some sort of sheathing attached to the underside of the joists, say 1/2″ plywood, or maybe 7/16″ OSB, or, possibly treated Plywood as one corner is about 18″ or less off the ground, fill the joist bays with blown in cellulose, and put down Advantech subfloor, will I encounter moisture issues due to condensation, and where would that condensation most likely form?  And if I do this, should I take steps to make sure this floor box is air tight? i.e. tape/caulk seams of the sheathing under the joists etc. I had thought of using a 1″ layer of foil faced polyiso under the joists, but it adds cost and I’m not too concerned about the amount of thermal bridging I will have or the gain in R-value. It would also add a vapor barrier on the cold side, and I’m not sure if that’s a good thing. I will likely use some sort of hardwood flooring on the Advantech.

I have been very impressed with the wealth of knowledge on this form. Any opinions or suggestions would be appreciated! 

Ben