Replies

1. 
   barmil | Oct 24, 2009 10:51pm | #1

   Do you actually have mold, or do you just think you might in the future? What you've just described sounds normal and not a harbinger of mold. Dehumidifiers are merely a form of air conditioner, and they don't operate well at lower temps, like 50 degrees or less, so I wouldn't expect good performance during the winter much above the Mason Dixon Line.

2. 
   6bag | Oct 24, 2009 11:01pm | #2

   a 2" concrete slush coat in the crawl will go a long way to reduces the humidity.  make sure you have a 1' over lap on any seems in the poly.  is the dirt in the crawl wet?  if it is you may consider a internal drain tile and sump pit.

1. 
   trekka | Oct 25, 2009 11:58am | #3

   It was bone dry until it rained 4 inches in two days. Now it is damp but not wet under the plastic in a couple of areas.    No mold issues, I just want to keep it that way.  I have read different opinions on sealing up foundation vents to prevent high humidity air from coming into the crawl area from outside the house.  Any comments on this? 

1. 
   rjw | Oct 25, 2009 02:34pm | #4

   >>I have read different opinions on sealing up foundation vents to prevent high humidity air from coming into the crawl area from outside the house. Any comments on this? It depends to some extent on your location/climate, and whether you can send some conditioned air into the crawl.Check out the building science corp web site.On the firs question, it doesn't sound like you have moisture issues if you just got some under the vapor barrier after 4" of rain.fwiw; I do home inspections in NW Ohio

   "Man's capacity for justice makes democracy possible, but man's inclination to injustice makes democracy necessary." Reinhold Neihburh: 'The Children of Light and the Children of Darkness'http://rjw-progressive.blogspot.com/

2. 
   Dogmeat12 | Oct 26, 2009 03:25am | #6

   I have a crawlspaceunder my summer home with a plastic vapor barrier and four inches of concrete. I found that leaving the crawlspace vents open in the summer allowed moist humid air to enter and condense on floor joist and floor insulation. I sealed all vents and put in a dehumidifier set at 2 (out of a high of ten) and drained it to the outside. I turn it on in May and turn it off in late September. No problems, everything is nice and dry with no musty smell. I might add that the house has no central AC so I can't condition the space.

3. 
   andy_engel | Oct 26, 2009 08:36pm | #7

   If you run a dehumidifier with the crawlspace vents open, you're essentially trying to dehumidify North America.Andy

   "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)

   "Get off your dead #### and on your dying feet." Mom

   "Everything not forbidden is compulsory." T.H. White, The Once and Future King

1. 
   trekka | Oct 28, 2009 10:11am | #8

   I will fill them in with foam blocks.  All the nearby rental places carry the Dri Eaze 1200 dehumidifier- about 80 lbs.  I will try one out and decide if it is a worthwhile investment.  Any one have a favorite brand of humidity guage?.   I may use some left over decking/ fence paint to coat the underside of the ply subfloor.  I will of course be putting tar paper on top of it before the wood floor goes down but a little paint on the underside might help to keep the moisture content down.

1. 
   andy_engel | Oct 28, 2009 05:00pm | #13

   You should check out the National Wood Flooring Manufacturers web site for specific moisture content info before installing a wood floor over any questionable crawlspace. And you might want to use a better vapor retarder than tar paper - Bostik makes several, including a trowel on membrane and more traditional ones on a roll. The trouble with tarpaper is that the perm rating isn't necessarily consistent. In most installations, it'll probably work, but over crawlspaces I'd err on the side of caution. Moisture, btw, is the number one cause of wood flooring failure.,Andy

   "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein (or maybe Mark Twain)

   "Get off your dead #### and on your dying feet." Mom

   "Everything not forbidden is compulsory." T.H. White, The Once and Future King

2. 
   BillHartmann | Oct 29, 2009 08:49pm | #19

   They used to have lots of good info on their web site. Basic, but full installation manuals. But they have since split it into a consumer and pro side and you have to be a member (or possibly cash buy, don't remember) to access the info.BTW, I used the idea of using spacers to hold the ledger board off the house from an article in your mag "Don't Build Decks that Rot". In fact I used many of the ideas, but those I had already designed into before I saw the article..
   William the Geezer, the sequel to Billy the Kid - Shoe

2. 
   DanH | Oct 28, 2009 02:38pm | #9

   I was going to say trying to empty the ocean with a teaspoon, but same difference. Even with vents closed it's something of a fool's errand. Better to set up a fan to draw in outside air.
   A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It's a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. --Jimmy Carter

   Edited 10/28/2009 7:40 am by DanH

1. 
   Dogmeat12 | Oct 28, 2009 03:23pm | #11

   How does bringing in warm moist air into a cool basement with a fan  help? All you will get is more condensation. Even with only a plastic vapor barrier on the dirt floor and sealed vents it will be easy to run a dehumidifier and dry out the crawlspace. You would only need to do it in the summer months.

2. 
   DanH | Oct 28, 2009 07:40pm | #14

   Like I said, depends on the climate. Generally the outside air is much drier than the air in the crawl, since the crawl is getting moisture from the ground below. It's all in the dewpoints.
   A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It's a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. --Jimmy Carter

3. 
   Dogmeat12 | Oct 28, 2009 09:31pm | #15

   No doubt there will be some moisture from the ground depending on how well his vapor barrier is installed but if the oustside air is warm and moist like it is here in the Mid-Atlantic states in summer and you are bringing that hot moist air into a cool crawlspace  you are going to get moisture condensating on the walls, floor joiost, and insulation. Think of a glass of ice tea in the summer time.

4. 
   DanH | Oct 29, 2009 01:41am | #16

   IIRC, we never got any info from the OP on his location or climate. Here in the midwest dewpoints are rarely above 60 and remain below 50 (often very far below) for most of the year. Yet the humidity in an unventilated dirt-floored crawl will be near 100% much of the time, and the temp in the crawl is, I would guess, generally about 10 degrees above the outside dewpoint most of the time. So around here I don't think you'd lose, on average, by ventilating. (At least not until the pipes freeze.)
   A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It's a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. --Jimmy Carter

5. 
   trekka | Oct 29, 2009 01:01pm | #17

   Thank you for all the responses and the comments re the wood floor.   The sub floor and joists are fir ply/fir 2x10's and are currently fine.  I just want to keep them that way.  The house is on the water in the pacific northwest.  We are talking lots of rain and fog.   Current outside humidity is 87 percent with showers and a dew point of 37 degrees.  41 degrees air temp. (Per weather service).  I will get my own humidity meter soon but expect the humidity to be much higher outside the crawl than in it.  That is how it feels at least when one sits in the crawl next to the open foundation vents, but before I do anything I will get some numbers.  The crawl space is not wet but the dirt under the plastic is damp in a couple of places.  The lot is fairly flat, the septic guy thinks the soil has reasonable drainage - I would say average.  There is a foundation drain but no curtain drain.  I use this term to mean the foundation drain is near the footing, wereas a curtain drain would be several feet further away and extend deeper than the bottom of the foundation footings.    

6. 
   DanH | Oct 29, 2009 03:27pm | #18

   Unfortunately, I can't find a graph of dewpoint over time for the NW coastal region, to know what your range is. It sounds like your crawl is reasonably dry, but assessments like "average drainage" are highly dependent on locality and personal experience.Keep in mind that when comparing humidity between inside and out you must compare dew points, not relative humidity. So you need to measure temperature when you measure humidity. (And most humidity meters are crummy -- try to find one that's halfway accurate.)
   A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It's a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. --Jimmy Carter

7. 
   BillHartmann | Oct 29, 2009 09:13pm | #20

   "Here in the midwest dewpoints are rarely above 60 "Define "midwest"?BTW, you aren't in the midwest. The term has been corrupted, but orginally it was the the states that where been settled in the early 1800's. Many of the veterans or their families from the Revolutionary War.You are in a Plains State, Upper Plains state, And believe me the central plains states had dewpoints above 60. And with the cool ground it is easy to get conditions where the moisture level is well above 80%66F with a 60F dew point gives you 80% RH, mold city. And with a typical AC installation is is not hard to get 66, in fact that is reachable even without AC..
   William the Geezer, the sequel to Billy the Kid - Shoe

8. 
   DanH | Oct 30, 2009 01:45am | #21

   Whatever you say. IMO, in most of the country you're better off ventilating the crawl than trying to dehumidify it. Certainly cheaper.
   A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It's a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. --Jimmy Carter

9. 
   Dogmeat12 | Oct 30, 2009 01:51am | #22

   in most of the country you're better off ventilating the crawl than trying to dehumidify it.

   IMHO you are incorrect. I still can't see how bringing warm moist air into a cool crawlspace is going to lessen the humidity in that crawlspace. But anyway like you said " IMO".

10. 
    DanH | Oct 30, 2009 01:57am | #23

    Note that I wasn't pretending to be humble. ;)
    A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It's a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. --Jimmy Carter

4. 
   DanH | Oct 28, 2009 02:43pm | #10

   Where do you live? (Hint: Fill in your profile.) Unless you live in the humid south the dewpoint of the outside air is lower than inside the house most of the year.
   A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It's a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. --Jimmy Carter

3. 
   renosteinke | Oct 25, 2009 07:45pm | #5

   I have seen a different arrangement used to address the same issue.

   Namely, fresh air is let into one side of the crawl space and removed at the opposite end, with the fan controlled by a humidistat.

4. 
   Matt | Oct 28, 2009 04:02pm | #12

   An important pre-rec of a sealed (unvented to the exterior) CS is that it is bone dry.  If you think you may have moisture infiltration issues, they must be dealt with first.