Insulating the floor

Valerie asks:

Hey Jake,

I am adding on a new 3-seasons porch. I will add a gas line for a fireplace as well and since the furnace room is so close, may run ductwork out there in the future. 

The 11'x20' porch will have a gable roof tied into the house w/single hung windows and 2 doors all around. It is being built over 1/2 existing concrete patio and 1/2 lawn. There will be about 18" for joists underneath. 

I live in ND with extreme temps and last year tons of snow melt. To prevent moisture/mold will plastic with rocks over the grassed area suffice? How do you seal the perimeter from snow melt/rain? Green treated lumber dug into the ground a bit? I have a contractor but he wasn't going to put any plastic down and said it would be fine. The inspector suggested it. What would be the best way to insulate the floor? I was thinking plywood under the joists, seal all with that expanding foam sealer and foamboard insulation? Thanks in advance for your expertise. I truly appreciate it. Valerie

Jake's answer:

Valerie,

You might want to consider using treated joists.  At 18” and less the code requires it.  By using treated lumber you minimize the chances of rot and insect problems. 

Definitely put a 6 mil or thicker plastic over the soil and patio, with a minimum of 2” of gravel.  This will help to reduce moister from permeating the floor system.

You should use some sort of treated lumber to seal around the perimeter of the room.  The code does require that these crawl spaces be ventilated year round.  For the size of room you are constructing, you will need 2 vents at least 1 square foot each.

The 2006 International Residential Code calls for an R-30 floor insulation.  Anytime you use batt insulation you should also provide for ventilation.  This reduces the chances of mold forming. 

I hope you are using a professional designer to take into account these issues.  In the long run, you will have a very comfortable room to enjoy those North Dakota winters in.