I would think about insulating it from below and thin cement board on top of a vapor barrier.... and then maybe some LVP flooring on top of that?

You might find this article helpful and given it will be used all year, suggest insulating as well. What heat source will you be using?

https://www.clairoliviawayman.com/best-type-of-flooring-for-an-unheated-sunroom/

Another idea if you will have heat and be insulating is perhaps painting and adding an area rug. See pros/cons in article.

Info:https://www.bhg.com/decorating/paint/how-tos/should-you-paint-your-floors/

To me this is like going from a deck to an addition if you really plan to use this space all yrar round sune rooms are part of thr house and need to be treated as such .You will need heating for sure. Yes yo could do this your self if you had the right knowledge but please wait to do this right .

Agree that you need to think about HVAC, insulation etc - but if you’re committed to DIY then Youtube will be your friend https://www.youtube.com/watch?v=mHHtXHygjrM

Thanks all. Will probably be heating cooling with mini split unit. And it is more like a covered deck honestly. Tired of the incredible amount of pollen that accumulates from April to June and then very hot temps.

What do you want to use the space for? It looks like it's currently being used as outdoor living space. If you want to make it true extension space, hit up your local ordanances to see what is in code. Some places have some serious code about what you can do or can't rig up. Some are more lax or don't have much codes about it.

For example, some places are ok with converting a deck into a living space like a sunroom. Other places might say no and you need to tear that out and put in a proper foundation/footing first. Or that you can just wall in your current framing- or you might need to put up proper wall framing for siding or other finishing requirements.

But if you can just go ahead with what you have... I would try sealing up the floor from the underside first. And also you would want to remove the lattice and enclose that underspace to help reduce the wind whistling around down there. The better you insulate and close up the underside, the better it keeps out the weather you don't want and keep in the temps you do want.

Before considering topside flooring, figure out your walls. How are you planning on enclosing your walls? Can your current decking build support the additional weight of enclosure? If you happen to have a lot of glass handy, great. But if you don't, your glass order and wall build is going to be just as serious of an issue as sealing up the floor is.

We would use it to watch TV, watching sporting events, have people over in a semi-outdoor place. Have a large screen TV in it already. Would likely put in windows that you can easily open for screens when the weather is nice. In east TN so winters don't really factor in that much, Summers can be pretty hot though but with two large fans and a split unit, I don't think that would be an issue. Re walls, plywood covered with vinyl siding like the rest of the house. Because of the way the porch is situated between the house and the garage, almost no wind. That actually contributes to the discomfort out there because there is literally no breeze most of the time.

Good that you probably don't need to heat the space and keep out the cold much. Your battle will be keeping the heat out and also keeping the cold in. If you are planning on having an AC in there, you will want to have that room sealed up. You might not need heavy insulation, but you will need some under the deck. Still close up the underside of the space.

DIY heating and cooling with a Costco lifetime warranty.

And I forgot to add, it already has walls (vinyl siding) around half the porch already.

Are you saying you have already started enclosing the screen porch? As in those are old pics and no longer accurate to the current state of the porch? In the pics we can see the house has siding, and the porch was built off that wall. What else is half the porch sided?

The porch is between the house and the garage. On the house side, vinyl siding is prett much the length of the wall and then part of another. Here are some pics. Ignore the clutter, cleaning out the garage and doing recycling ...