I own an older house (built around 1972) and it has a crawl space under it (approx 24 to 30 inches of space between sub floor and dirt). It has a 6 mil plastic vapor barrier covering the dirt. My question is i had a sub contractor come out and inspect the insulation. The contractor found that the insulation was the wrong type (no paper backing on the insulation) it is also the orginial insulation. The contractor pulled out all of the old insulation and then sprayed all of the joists, and wood with a moisture repelant. Then the contractor said i needed to have 2 low temp dehumidifiers installed into a couple of the vent openings (i have 17 vent openings for the crawl space and they stay open all year round). The low temp dehumidifiers are Ebac CD30 brand, but in reading there spec sheets they only dehumidify approx. 400 sqft. I have about 1500sqft of crawl space. Do i really need these dehumidifiers or are they trying to sell me something that does not work. Oh, they want $4k to install dehumidifiers. I also live in the Chesapeake are of Virginia.

A few questions: Was there a compelling reason to call someone to look at this crawl space? What was his logic for saying that the insulation was the wrong type? What size are the 17 existing vents? Better yet, how much open area is there to each vent?

The contractor is also our exterminator company (they are multi talented) they are reputable company and came highly recommended and they were out to do an inspection under the house. They indicated that the insulation should have been the type with a paper backing. The 17 vents are sized as such 20" long x 8" wide or tall. None of the vents are in an area that is closed off it is totally open all the way around each and every vent. No shrubs etc block the vents.

What Maintenance 6 was referring to is the net free area of each vent, which is usually anywhere from 50% to 70% of the gross area. The best way to find this is if there is a manufacturer's name on the vents, then look it up on the manufacturer's web site. Otherwise you just have to make an assumption. The total net free area of the vents should equal 1/150th of the floor area of the crawl space.

Your contractor is saying that the insulation should have a vapor retarder, If he replaces the insulation, be sure that vapor retarder is located on the winter warm side of the floor, which in a cold climate is up against the subfloor. Contractors frequently install the insulation upside down.

You may have a plastic vapor retarder on the dirt floor, but are the joints between sheepts lapped 6 inches and taped, and is the vapor retarder turned up and securely taped to all walls and all penetrations such as posts or pipes? If not, it is important to do that to maintain the integrity of the vapor retarder and truly seal off all moisture from the earth floor.

I'll check the vents for manufacturer. And no the old plastic retarder was not installed properly (did not go up the walls, was not over lapped and did not wrap around pipes). I will instruct contractor to do this upon replacing with new retarder. My real question is do you feel that i need to spend the 4k for the Ebac CD 30 low temp dehumidifiers or will installing the retarder properly be suffecient for moisture control?

Richard,
I could not locate the manufacture of my vents. But, the real question i have is what are you thoughts about the Ebac CD 30 dehumidifiers?

I have no experience with dehumidifiers, so I'm not qualified to answer your question. Perhaps some participants with real hands-on experience will offer an opinion.

Are you having some sort of an issue that would compell you to want to install de-humidifiers? Any mold found anywhere? Or moisture from an unknown source? Seems like a lot of money to spend if you don't have an issue to correct. If the vapor retarder on the insulation is wrong, then that deserves to be corrected. Anything beyond that, I would really dig into before investing money in it.

Maintenance 6:
I personnally do not want to install the dehumidifiers, my sub contractor is recommending them since the old insulation i had was the wrong type and then my 6 mil retarder was never installed properly. I feel that when we re-install insulation (with paper backing) and install it properly and then place a 6mil moisture retarder properly that this alone should take care of my moisture problems under my house........Your thoughts

You do not need to purcase these expensive products. Check out Humidex units. http://www.humidexhome.com/
Seal the crawl from the outside, install vent into the living area once you have the floor covered with heavy plastic. And install one of these vent units. They pull a small amount of conditioned air from the living area of the house and as this happens it draws any moisture out. I have put several of these units in and have in the past constructed my own with a radon fan and proper humidity controls. Works like a charm and cost very little to operate.
why would you want to keep the crawl cold in winter. It causes moisture where ever the insulation fails on the ceiling. By allowing a small amount of heat from the living area down into the crawl space it keeps the floors warmer and prevents mold and other ills from occuring.

I would have the work done to fix the insulation and vapor barrier. Then check the humidity level in the crawl space before spending the money on a dehumidifier you may not need. An inexpensive hygrometer can be used to test this. You should be able to find one at the local hardware store, usually combined with a digital thermometer. If you have a good cigar store nearby they should have one as well, they are used in a humidor.

Tom Pittsley
www.eebt.org

Assuming a net free area of 50% for the vents, each vent has 80 square inches of net free area. Multiply that by 17 vents, and you have a total net free area of 1360 square inches. That is far more than the minimum code requirement. If you install the vapor retarder properly as I described a few posts back, then if that doesn't cure your moisture problem, the source of moisture is probably the foundation walls. If they are properly dampproofed on the outside, moisture should not be present in them.

If all goes well with the insulation and vapor retarder, your problem could be solved. One key is to be certain which way to install the insulation for your climate. Up here in the chilly north, it goes with the vapor retarder side up against the subfloor. In the deep south, where it's always warmish, it goes in the other way. Your climate is in the middle and thus is to me a gray area. Ask several qualified people which is the best way to install the vapor retarder in your climate...local contractors, local architects, local insulation supply dealers, your local building department.

Hopefully it will solve your problem. Good luck.

Thanks to everybody on your comments and insite. I will make sure that we install everything properly this time and check the humidity with the hygrometer (i actually use these at my work)and we will go from there. Again thanks.