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Our home was built with a dirt crawl space that is about 2' from ground to bottom of floor joist. The floors are insulated with fiberglass held up (where it hasn't fallen down) with wire rods. The AC/heating duct runs through the crawl space and has insulation that looks worn out. There are vents all around the perimeter thru the concrete blocks.

Have had trouble for years with dampness in the crawl space. A few months ago after it had rained for about a week I was under the house and it was so damp it was dripping from some of the floor fiberglass. Also, the AC ducts always drip in the summer.

Currently I have a fan running all summer long to help keep it dry down there.

I'm considering encapsulating the crawl space thinking this should solve the moisture problem. From what I've read it should also eliminate the need to rehang all the fiberglass that keeps falling down, as well as eliminate the need to redo the duct insulation.

Would like to hear from someone who has done this before as to how it turned out.

Update, 10/3/13: Just want to thank littleturtle not only for your comprehensive info on this but your cited source. I read through it last night and am still trying to digest it. But one thing I picked up on was that you don't have to vent the crawl space to the inside of the house to exchange air. That's what seems best for me because I'm thinking that during the winter I'd have to pay to heat the crawlspace, and heating KWHs are expensive. Incidentally, I'm near Charleston, S.C. (Zone 3) where the cooling cost exceeds the heating cost and the humidity is typically high. Anyway, I'm going to keep plugging along on this topic until I get a good feeling about doing it, or not. I'm retired and have time to think this through carefully.

Update, 11/14/13: Have decided not to encapsulate for now. Why? Doesn't make sense to spend thousands of dollars when the little box fan seems to work okay. In other words, running a box fan in the crawl space all summer does an adequate job of removing the condensation, at a low cost. Keep in mind that there has never been a problem with the dampness actually rotting any of the floor joists, or sill plate. Since the condensation is the only problem, other than that the insulation keeps falling down (another story), it just doesn't seem necessary or smart to spend that kind of money. If, on the other hand, there was ample evidence that there was a payback from the reduction in the KWH used for heating and cooling, then I would do it.

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Does your AC ducts have to be in the crawlspace? –  DMoore Sep 25 '13 at 16:43
    
Is the crawl space floor at/below/or even with the surrounding ground level around the house? –  HerrBag Sep 25 '13 at 19:39
    
Based on our answers, I think we need clarification on encapsulation: are you planning on also conditioning the crawlspace? –  DA01 Sep 25 '13 at 20:05
    
I'm going to have to check re whether the ground slopes down (i.e., the outside ground is higher) on the west side of the house. –  getterdun Oct 3 '13 at 10:39
    
I'm new to encapsulation and I'm unsure about whether you have to also connect duct work between the house and the crawlspace to exchange air. From what I've read though, it sounds like that's the only acceptable way for it to be approved by a building inspector. What are your thoughts? –  getterdun Oct 3 '13 at 10:43
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4 Answers

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Yes, bringing the crawl space into the conditioned space is generally recommended as a way to prevent crawlspace moisture problems (I assume this is what you mean by encapsulate).

This means:

  1. sealing vents to the exterior
  2. sealing the dirt floor with polyethylene sheeting (at least 6 mil), or concrete
  3. creating vents between the living space and the crawlspace (ideally supply air to/from the space with HVAC ductwork), and
  4. insulating the interior side of the crawlspace walls on (not necessary in climate zone 1 or 2; and preferably with board foam or spray foam because fiberglass doesn't do well with moisture).
  5. probably a good idea to remove existing floor insulation

This will prevent humid outside air from coming into the crawlspace and condensing on cold surfaces, and prevent/reduce moisture from coming up through the dirt floor or walls. Your AC will also deliver cold air more effectively. Good site drainage is also key.

There are many good references on how to do this properly:

http://www.housingzone.com/business-management/crawlspaces-vent-or-not-vent

http://www.buildingscience.com/documents/reports/rr-0401-conditioned-crawlspace-construction-performance-and-codes

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To your point 3, I would emphasize active air exchange (through the HVAC used in the rest of the house) is to be preferred over passive, per Joseph Lstiburek. –  HerrBag Sep 25 '13 at 19:52
    
HerrBag: This is something I still have questions about. If you seal up the crawlspace, and only go down there when you have to (to run cable wires, electric wires, inspect plumbing lines) I don't understand why it should be necessary to exchange air between the house and crawlspace. Plus, I expect that heating the crawlspace would significantly increase my electric bill in the winter. Maybe though it would be offset by the reduction to the cooling bills in the summer? Your thoughts? –  getterdun Oct 3 '13 at 21:52
    
@JohnGleason Exchanging air between the house and crawlspace is necessary for moisture reasons—to keep the crawlspace air conditioned in summer (to keep humidity levels down) and to keep it warm in winter (so that moisture in air doesn't condense on cold surfaces). Your heating bill may go up a bit, but you are already indirectly heating and cooling the crawlspace to some degree because your ducts are down there. As a side benefit, the floor will feel warmer, which will improve comfort. –  littleturtle Oct 7 '13 at 18:06
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If you live in a humid region, mechanical venting between the crawlspace and outside won't necessarily improve things, as you'll just be bringing in moist air to then condense in the cool crawlspace. In this situation, the moisture problem is in the air.

If the issue is moisture in the ground, then what you want do do is seal the dirt...typically with sealed plastic.

That said, you mentioned dripping AC duct work, which makes me believe you live in a humid region...so you likely do not want to mechanically vent the crawlspace to the outside. What you do want to do is replace and improve all the insulation under there...especially around your duct work.

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I think you are right, if the OP doesn't close the exterior vents. I think the 'encapsulating' aspect of his question would mean closing off the exterior venting and follow what @littleturtle and Joseph Lstiburek are saying.. –  HerrBag Sep 25 '13 at 20:01
    
I agree. If we're talking about conditioning the crawlspace, then sealing it off completely does make sense. –  DA01 Sep 25 '13 at 20:04
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I am in coastal North Carolina. I encapsulated my crawl space since the neighbors on either side of me had incurred thousand of dollars in repair of the floor joists in their homes older than mine, which was about twenty years old at the time of encapsulation. I have a basically flat lot but the water does not necessarily drain away from the house. I also installed underground drains that the gutters drain into, diverting water to a ditch. The house has one section that is on a slab (and it is not encapsulated, of course), but most of the house is about three feet above the ground with some additional space on a second floor.

The main floor is very cold when temperatures get in the thirties or below. This was not a problem before encapsulation.

I use heat pumps with supplemental heat from a small portable electric heater in the master bathroom and a portable oil filled heater in the slab area. Encapsulation did not lower my electric bill. I think the underground drainage helps with moisture too but can definitely be a mosquito breeding area.

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See http://www.advancedenergy.org/portal/crawl_spaces/pdfs/Moisture%20Solution%20Becomes%20Efficiency%20Bonanza.pdf

Using the matched pairs approach, the houses are divided into three study groups of four houses each. Energy use for each of the houses is being recorded from sub-meters attached to each heat pump. The findings indicate a 15% or greater average savings in heating and cooling energy use in the houses with properly closed crawl spaces compared to the houses with traditional vented crawl spaces.

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