Something thats not mention much

[Kevin]

When doing post and pier foundations its a good idea to put a peice of plastic on the ground under the cabin. It will keep moisture down and help keep mold froming in the cabin. This is just a reminder.
Kevin

[Jimmy C.]

Would securing a layer of plastic to the base of your floor joists give you the same effect.
This brings up another question.. A little off topic.

What is the best way to insulate the floor for a post and beam foundation.  Plastic covering over insulation, or 1\4" plywood, insulation, with a plastic covering?

[Amanda_931]

Either plastic or keep the air moving!  

John had a post back in January about insulating floor joists.  In a topic called "insulating/under pinning sub floor"

(but the one I was thinking of involved movable pieces of perforated hardboard, and had pictures--search didn't turn it up)

author=John Raabe link=board=01;num=1104980387;start=0#1 date=01/05/05 at 23:19:14]The method is best described in the Little House plans but in general fill the floor joists with insulation and put up something on the bottom of the joists to hold it in place, keep air out of the insulation but not be a vapor barrier. You can use Tyvek with lath slats or black poly with holes poked into it to let out vapor and then held up with stapled chicken wire or pegboard.

There are about 30 other ways.... If the insulation is allowed to sag it can break the air barrier and lose its R-value.

Also skirt the foundation with something to keep racoons, dogs, and insects from setting up housekeeping.

[John Raabe]

There is a map somewhere if I can find it that shows the areas of the U.S. where a moisture barrier over the soil is recommended. It is largely in the moist coastal regions and northern parts of the country. In dry soil areas it is not needed.

If you do plastic flim under the floor insulation punch slits in it to allow a bit of airflow so you don't trap moisture. That's what we did when we built our version of the Little House and it has kept the insulation dry and the critters out (we have yet to skirt the cabin).

[Dave M.]

John,

Can you explain how putting slits in your plastic vapor barrier keeps critters out of your crawl space?

[John Raabe]

First of all I'm talking about the use of poly at the bottom of the joists - where you want a non-vapor barrier. Best would be to use Tyvek or another vapor permeable material. We used the same black poly that we put over the crawlspace soil but then cut 2" slits every 6' in each bay to vent any moisture trapped in the floor.

These are tiny slits. I suppose there may exist someplace where tiny ants would want to crawl into this narrow slit after climbing up the posts and then upside down across the slippery poly.

I don't live there however.

[JRR]

This (bottom of joists) might be a good application for "landscape fabric".