For an insulation and moisture control issues you should start with a description of your climate and HVAC system.

I am in Zone 5 in Missouri with fairly mild short winters and long, hot, humid summers.
My HVAC will be Geo Thermal vertical closed loop.

Our previous house had the vented crawl space with poly liner on the dirt floor and our new house has the sealed and conditioned crawl space and there is no comparison. The new house is much more comfortable, and we have practically no problems with the woodwork and hardwoods gapping and shrinking. We live in the SE, very hot humid summers, mild winters.

The Building America series shows a conditioned crawl in mixed-humid climates and unconditioned crawls are advised against by the US Department of Energy. See here, for instance. OTOH, there is research(www.ornl.gov/sci/buildings/2010/B11 papers/174_Hales.pdf) showing vented crawls work well in cold and marine climates such as the Pacific Northwest.

Zone 5 is a

You takes your choice!

Here is a link that might be useful: The SCARY Crawlspace

This post was edited by worthy on Sat, Dec 8, 12 at 14:55

worthy,
I appreciate the information. Most of the homes in my area are walk out basements built into the hillside and we own about 6,000 square feet on multiple levels and are downsizing and want a home all on one level. I have owned several homes with basements and they were great when the kids were younger and we needed more space but we just don't want a basement anymore. The problem I am seeing is that by the time I build a 4' crawl and condition/insulate the crawl to keep out moisture I might as well put a basement under it and not have any problems...but we don't want a basement.:(
btw...according to the map I am in Zone 4A

This post was edited by LOTO on Sat, Dec 8, 12 at 16:50

My major concern would be moisture...closed cell does not absorb moisture, but open cell does...

#2 contractor recently admitted he was incorrect in advising open cell foam in crawl...#2 is full of #2 and is out of consideration.

Another big decision is what to do about the floor of the crawl space(4 foot high). I do NOT want a basement (had several homes with basements and loved them but want a 1 level home now)but the dirt floor of a crawl space is not ideal either. I can get 4" of concrete poured in the crawl for around $6,000.00 and I haven't priced a crawl liner yet but even if it was 1/2 this price I still would rather have the concrete floor. My building site is not prone to water problems and I know concrete slabs don't cure all moisture problems but they go a long way if properly installed.

Anyone our there who went with a liner or dirt and opted for concrete later or wished the had?
btw...Merry Christmas !!

Here is a link that might be useful: Acreage Video

This post was edited by LOTO on Tue, Dec 25, 12 at 14:23

put a vapor barrier on the ground before pouring
the slab, just like you do for building a house
on a slab.

persoanlly I'd do 3" (not average fill but 3" everywhere)
closed cell. don't really
think you'll need a vapor barrier at all.

best of luck

Worthy, would you please clarify your response above: "The Building America series shows a conditioned crawl in mixed-humid climates and unconditioned crawls are advised against..." Does that mean that in a mixed-humid climate, conditioned and unconditioned crawls are both advised against? I'm seeking to warm up a bedroom.

I live in Anne Arundel County, Maryland, climate zone 4A, in an 1800 SF house, built in 1968, and heated with forced air oil furnace and a/c. It is a rancher with a developed lower level on a concrete slab, built with concrete block foundation walls about 8' high.

A bedroom was added on a separate foundation with foundation walls 4'4" high. The crawl space under the add-on bedroom has fiberglass insulation between the joist. has 2 small openings that I close in winter; dirt floor. It is very dry and shows no issues with moisture, bugs or mice/snakes/etc.

The add-on bedroom is really cold in winter. I will increase attic insulation. What are the options for improving the bedroom's winter temperature by improving the crawl space? Can the crawl space's cement brick walls have foam panels or should I add more insulation in the crawl space's ceiling? A 6 mil plastic sheet over the dirt floor?

There are two insulated duct pipes in the crawl space. Is it advisable to seal the crawl space's two little openings and condition the crawl space? However, if it is infeasible to install a return, is a conditioned crawl space not an option?

Thank you.

Does that mean that in a mixed-humid climate, conditioned and unconditioned crawls are both advised against?

In mixed humid climates such as yours conditioned crawls are advised by building scientists.

Here's a guide by the U.S. Department of Energy on how to do the job.

Since I posted up-thread in 2012, the argument for vented crawls in mixed humid climates has evidently gone down the memory hole at the Oak Ridge National Laboratory.

The Laboratory features a side-by-side study of identical homes, one with a vented crawl, the other without. The authors' main conclusion:

"is that sealing and insulating the crawlspace is the recommended strategy in a mixed humid climate like Tennessee. Further, in a sealed crawlspace, it may be advisable to monitor the relative humidity level and operate a dehumidifier periodically, especially during and after a period of heavy rainfall....if conditioning the crawlspace, care must be taken to prevent any unconditioned air from entering the crawlspace." (p.40)

A conditioned crawl doesn't necessarily need to be connected to the home HVAC system. The different approaches in a mixed climate are discussed here by physicist Allison A. Bailes. Another consideration if you are conditioning the crawl is radon collection and elimination.

Also see the U.S. Department of Energy publication, Energy Efficient Crawlspace Foundation Retrofit: Mixed Humid Climate.

Thank you, Worthy!

I'll seal the crawl space, close the little louvered window vents and insulate the walls. I'll get a humidity gauge with a remote probe to track whether a dehumidifier is needed.

If the crawl space is sealed, and the HVAC neither supplies nor returns air, how does air circulate in the crawl space? Would it become sort of "dead air"? Allison Bailes, the author of the very helpful article you referred to above, wasn't too keen on exhausting air with a fan.

The crawl space access (about a 3' x 3' opening) will be fairly well closed off to the only adjacent room, the downstairs recreation room. The recreation room doesn't have effective air circulation. I've considered installing a powered vent in the recreation room to use briefly to "air out" the room. Would that assist moving about air in the crawl space?

Love Alison Bailes website!

Great advice from worthy. Personally I think vented crawlspaces should be illegal for new construction in humid climates. Martin Holiday has a good article for those building UNvented crawlspaces.

Thicker mil plastic is certainly preferred, especially in high traffic areas but I think how well the plastic is sealed to itself, piers and walls is just as important. Consider yourself lucky to have a seemingly dry crawl. Radon and other soil gases are other reasons to better seal up that cave below your home.