hi... I have a outside room additiom 12x12 insulated walls and ceiling... 2x8 floor jouises no walls around the bottom... its like built on just the corners... so under the floor is all open to the air... if I insulate with 8 inches of fiberglass...
what should the under side of the floor be coverd with... I m guessing plywood to keep out the varnets... ect... plastic... then plywood or just the plywood or what? I live in md... so mainly I'm just trying to keep out the cold air from under the floor... from the winters...
thank you for your time and help... richard

You can use the fiberglass batting placed between the floor joists. Then you must do something about the perimeter of the bottom side of the room where it is open to the elements. You could use treated plywood to keep the cold and air out and varmits as well. If there are any vents, etc under the floor in this room that you need to get to I would suggest that you make some sort of "man door" in the plywood that you attach to the bottom of the floor outside. This way you have an access panel to enable you to get under the floor. Would also recommend that you place some sort of heavy plastic on the bare ground under the room as a moisture barrier as well. Otherwise a lot of moisture is going to get built up under the floor and cause rot problems. You also should look into an air vent for the side of the bottom enclosure. There are vents available that open and close automatically based on the outside temperature. This way the moisture build up in the summer will be dissipated and in the winter the room will be warmer with the vent automatically closed.

You don't have to use plywood under the floor to keep the fiberglass insulation in place. You can use a very heavy gauge plastic stapled onto the joists to keep the air out. Then the plywood around the perimeter of the room on the bottom.

Tableclocks. To properly insulate a crawl space the vapor barrier Has to be against the warm side.

In a crawl space the barrier has to be against the floor and the glass exposed to the cold side.
This exposes it to the critters. To stop that close in the sides to the ground but install vents for cross ventilation. You Don't want to staple plastic to the bottom of the floor joist even if there one against the floor. This will trap moisture and create mold and the insul will Loose its (R) value. Another way around this is spray foam and all is good. The critters don't really like hard sprat foam.

21 Boat - yes you are correct about the plastic under the fiberglass. I meant to say that the OP could use some sort of belly material such as they use under mobile home floors. This material is meant to be used to hold the fiberglass in place and to "seal" up the floor. Regular heavy weight plastic is not the same.

Thanks for the catch on this one!

Hi twink. No worries here. Thanks for the rating. I knew you knew and was laughing. Sometimes that's the problem. When its right off the head like breathing the fingers skip the brain and sequences gets skewed.

I had a couple post where I did the same thing and bal kiss and others covered my back. Group power I always say.