would like to insulate under a deck 2x8 joists the deck has been converted to a sun room / living space vapour barrier or no ? Batt insulation ? Talked to a guy about spray foam but he wants big $$$ lots of room to get under it thanks
bob

Install batt insulation supported with wire rods for support, moisture barrier to warm side (floor). Install plywood sheathing to bottom edge of joist.

I have crawled in crawlspaces you couldn't breath heavy in to insulate under places.. You can do anything you want.

I used solid insulation for most of my personal projects including this,the house and shop both have upwards of 4 inches of solid polyisocyanurate insulation(R-7 per inch)

Being a solid board and that you have space to work with,this would be the type I would suggest.

Installing the boards then applying strips of wood to each edge of the floor joists left exposed would keep them tight to the floor.A vapor barrier would be a good idea as well.

Hope this helps you.

Whether you need a vapour retarder and where it should be located depends upon where you live.

For colder climates, the vapour retarder needs to go on the warm-in-winter side.

For southern warm and moist climates, no vapour retarder should be installed or it should be installed on the outermost side of the floor assembly.