Crawl space
 

I have a regular, about 24" tall crawl space under my house with no insulation.
As the joists are on 16' centers I was planning to use regular r19 batts.
My question is can I use plastic sheeting on the outside, not only to hold the batts in place but as a leak seal?
As my part of New Mexico has almost no rain and the average humidity is normally in single digits I don't think condensation is a problem.

Any input please.:thumbup:

That's a good question!
Here in Ontario, Canada, the building code accepts the combined qualities of your plywood sheathing, subfloor and finish floor as a "vapour barrier", as it occurs with an insulated floor over an unheated garage, or a cantilevered bay window. Given the fact that we have generous amounts of rain, snow and humidity and there is no specific requirement to add poly to the floor I would think that you would also not likely need one. I might suggest the installation of a housewrap like Typar or Tyvek. They form an air barrier to keep air from wandering into your insulation on a windy day, but allow moisture to dissipate, a handy consideration if you should ever have the misfortune of a toilet, tub or sink overflow, dishwasher leak or broken pipe.

I' use reinforced poly and unfaced batts if you can't go for the poly and cellulose. And r 19 fiberglass is for 6" and your floor joists are at least 8".

I wanted to leave a gap for a possible underfloor heating setup later.