HI!

I have a screen porch on a cabin Northern Wisconsin that we will be converting into 2 bedrooms.

Right now, the porch is raised about 1.5 feet off the ground, with 2x8 joists, no current insulation.

We are curious as to how to insulate the floor, without spending too much time crawling underneath to porch.

Concerns: Rodents, warmth, sound, moisture, costs.

Ideas:

Since there is a cheap thin floor on the joints, we were thinking that we could just rip up the floor, and put insulation in from the top. We would do this by nailing furring strips on the joists, and then lay plywood (Which we have plenty of scraps of) on the furring strips, then lay down the insulation on top of that. We were thinking we may need to prime and paint the 'suspension' plywood for moisture and rot. Is this necessary?

Do you think this system will work? What type of insulation should we use? It will be seasonal use. Perhaps heated a few weekends in winter. But since there will be two bedrooms, we are looking on cutting down the noise too...

I have posted a picture of my idea: Please, give me your thoughts!

http://img250.imageshack.us/img250/7301/insulationideaforporchmj4.jpg

I think your plan is workable. I would offer a few ideas. First, I did something like this on a cabin I had in the woods. After laying the plywood bottom down, I dense packed the cavity with cellulose insulation by blowing through holed I drilled in the bandjoist at the end of each cavity. A year later I was under the cabin and found two spots where mice had been going in and out of two different cavities. In each case it was at the corner of a piece of plywood where there was a real small chip. The blown cellulose had quickly plugged the hole while I was blowing it but the mice found the weakpoint later. They have lot's of time kinda like I do now.(foot operation, on crutches) Detail the bottom of the space so it's tight.
I think that it is not necessary to use any coating on the underside of the floor space
If the floor above is sound, consider not tearing it up and bite the bullet and crawl under with an air nailer and fasten 3/8" plywood to the bottom of the floor joists. Then go topside and remove a narrow strip of flooring (5-6") at a right angle to the floor joists and then you can blow the floor cavities full. I'm an insulation contractor with a blowing machine and 500 bags in the barn so that is easy for me to say.
Actually, filling the floor joists is really overkill for a space that will rarely be heated. You might cinsider not insulating the floor although odds are this place will eventually become a year round residence.
Steve/Starbright Energy Services/Wellsboro

Just one thing I don't saee mentioned. Make sure your crawl space is vented.

Thanks guys! Good info.