Does anyone have any ideas on how I can "seal" my crawl space in order to condition the space?  I am just outside of Atlanta (so we have lots of humidity here) and have a ventilated crawl space; my HVAC system is located in the crawl space as well.  I want to condition the crawl space in order to both make the system run more efficiently as well as improve air quality.  I have done some research on through building science.com but, didn't find any information on inexpensive ways to seal a crawl space.  Thanks!

The best way to seal your crawl space would be with spray foam insulation. However, that would most likely be your most expensive option. If you want to do the job yourself, a combination of foam board and expanding foam would probably be your best option. Don't forget to insulate the floor of your crawlspace and upgrade your vapor barrier with an additional layer of poly. Also make sure that your have drain tile under the vapor barrier to handle any ground water that might come up during the rainny seasons.

jmkbash,

If you insulate the foundation walls inside the crawl space, you should leave a small strip undone so that an inspection for termite activity can be done.  Talk to your pest company to see what they require.

I assume you've gone through this document, then:

http://www.buildingscience.com/documents/reports/rr-0509d-crawlspaces
 
By "inexpensive", just how cheap does it need to be?

Using recycled or factory-seconds foam board you can cut the material cost by more than half.  (If you can't find it locally, see if insulationdepot.com has anybody working your area.)

Using unfaces EPS or fiber-face ISO instead of XPS for crawlspace wall insulation is also just fine. (Don't use foam board with poly or foil facers though.)  Sealing he seams& gap with caulk/tape/spray foam isn't a huge expense. If there's a lot of it, buy a proper foam gun for 1-part foam and use the 22oz+ cans, not cans from the box-store.  (EFI sells it online, as does Grainger, if you can't find it elsewhere.)

If there are any combustion appliances (gas/propane/oil-fired   water-heaters/furnaces/boilers) in the crawlspace, you DO need to make provisions for combustion air however.  This might require you to reduce, but not eliminate the size of the crawlspace vents, but in many or most cases a properly sized grill to the condtioned space above can work, which will usually be more efficient. Some amount of air communication between the crawl an the living space will be required in any case (burners in the crawl or not).

Search for crawl space threads by Dmaceld who includes photos as well. Here's one

http://www.greenbuildingtalk.com/Forums/tabid/53/view/topic/forumid/4/postid/46417/Default.aspx
Dave

jmkbash;

to retro fit an existing home is more difficult than from new, but I can outline a method we successfully used in new construction:
waterproof inside of crawlspace foundation, pour 2 inch thick "rat slab" over termite treated soil with 6 mil vapor barrier, install sump at a low spot in case of water intrusion
Insulate side walls with borate treated EPS, Do NOT insulate floor

on the particular home we did this;  the cold air return (at air handler) was cut into the floor deck and low wall returns in each room utilized thru deck as well. Crawl space was used as one big retrun air plenum.

This was done in the Southeast US, but I have seen similar methods used in the NE as well

Chris - That is an interesting concept! Would not have worked for me in the home that I owned in Portland, Oregon, however as we had a spring in the crawlspace with a drain hole through the foundation at the low point. It's tough to get away from water in the Pacific Northwest other than move to AZ which is what we did!

glenfotre;

springs around a foundation are hard to overcome, I had a full foundation in NW PA that had a spring problem

Chris, your description of the crawlspace as a return-air plenum is spot-on. However, it has been my experience that the same house in a northern climate will suffer with an uninsulated rat slab in the winter. My own house went through this conversion several years ago and I poured an uninsulated slab. The same year, I was amazed at how efficient the air-conditioning was and how inefficient the heating was. The rat slab became a huge heat sink that was both a plus in the summer and a negative in the winter. I now have a combination of foam board and insulating blanket that goes on and off with the change of seasons. The results have been very interesting. Having said all that, I agree with an uninsulated slab in Atlanta.

panelwright;

the similar method I saw done (late 70's) in western New York was done with: insulated wood crawl space walls, no slab, no floor insulation, a 10 mil liner (joints sealed) on earth and up sides of wood foundation.
There was no AC in the house, instead the furnace was a down draft into the crawl space using the area for a forced air supply plenum with floor cut outs for supply grilles, no duct work was used.
It was a very nice warm floor in the winter

from Build America program, upper left pic is conditioned crawl space with 2" rat slab