Unless you provide heat to the room or leave the door to the house open, it will stay cold in winter. And no matter what you do to the walls, there will be a lot of cold coming from the floor, which looks like concrete directly on the soil.

Short of tearing it down and starting over, remove all the mouldy messy fiberglass, caulk and polyurethane foam the gaps in what you have, fill with tightly fitted polystyrene (expanded or extruded) and cover with plywood or drywall.

With such a small area, I'd go with spray foaming the stud bays--that will fill the gaps and provide better R-value than polystyrene sheets.

Most important, you have to insulate the floor--if it is concrete, add the polystyrene sheets here, then cover with plywood and flooring. If it is a wood floor, try to spray foam from below or use unfaced batts or poly sheets.

Thanks!

We can actually access underneath the porch floor from our basement, so that's good.

The plan was to remove the door between the house and the porch and run a little bit of baseboard in there. It's a little intimidating now that it's all exposed!

Spray foam will give you good sealing against air flow and the best R value in the bays (remember to caulk the sill plates while you have access). Even better is to add an additional inch of rigid foam over the entire surface to give you a thermal break where the studs are. It shouldn't be that expensive for such a small space. If you have access underneath you can just have them spray foam over the entire structure and you'll have happy warm feet.

Professionally sprayed high density foam will do a great job. But the costs for such a small area will not be modest. For the applicators, it's the same setup and travel time as for a large job.

This would a good size for a diy foam spray kit if you're up to it.

Thanks, all!

I now have a spray foam person coming tomorrow for an estimate.

Between that and the carpenter needed to fix the gaps in the wood, this is turning out to be a lot less DIY and a lot more $$$ of a project! But I try to keep reminding myself that this is cheaper than an addition!

If your house is vinyl sided on the outside, usually they went over the original siding...so the gaps aren't going through to the outside, just to the next layer which could be rosin paper or the backs of original siding. You don't need to fill the gaps--the foam will do that for your. The carpenter isn't needed for that job, just to do the extra heating.