I have a 16'x17' screened in porch with shed roof and want to convert it into a fully heated room off my kitchen. All inspections have passed. I used 6x6's to support the deck floor and the floor is traditional nailed treated deck boards.

I will more than likely run 240 electric to this room and add a heater/AC unit on separate thermostat.

Regarding the floor...should I brick in a complete foundation and create a crawl space like my house or is it OK to simply nail treated plywood to the underside of the deck...insulate it...and then put in a traditional floor...Trying to do it right but also want to save money

Can anyone help me out there??

Creating a 'crawlspace' would be a waste of time and money, although it could help to create a warmer, cooler room.

But the cost to benefit ratio just would not be there.

Insulate the floor to the maximum R value you can get and required for your region...

The underside can be covered with rigid foam if necessary for added insulation and covered with any type plywood or soffiting material if desired.

Inexpensive vented vinyl soffit material is a good choice for the underside...it will allow ventilation of the floor insulation, and inhibit entrance of insects and other vermin.