Replies

1. 
   woodway | Nov 05, 2006 02:45am | #1

   There should be foundation/crawl space vents all around the perimeter of the house to allow for cross ventilation and prevent long standing high humidity situation under the house. Each vent should be about 6in X 16in and covered with wire to prevent vermin from going below the house.

   Since your on a hill, I would find out if a subsurface French Drain system was installed to take water away from the edge of the house, on the uphill side, and drain it off to the low side without first draining into your crawl space. The drain system may or may not be needed but if you've got high ground on one side of the house, then subsurface water will naturally flow up to and under your house. If the hill is very slight, there may not be much water to begin with and it shouldn't be a serious problem.

   If you have vents around the perimeter, you could have standing water in the crawl space for 6 months out of the year and not have a problem with mold on floor joists. In your situation, putting down a moisture barrier will do nothing to prevent water from getting to the crawl space. From your description, it doesn't sound like you've got huge amounts of water to contend with but because you observed mold under the house and dry earth in areas not covered with plastic, it sounds like simple vents will solve your problem. I would not remove the plastic however, it may be required in your area.

   There have been discussions here about this topic many times. In many areas of the country, they say that high humidity and cold crawl space means trouble. I, being slightly contrary by nature, don't hold to that premise but argue that getting good air movement will alleviate the problem with time. You might have high humidity situations for periods of time but sooner or later, the humidity drops and the crawl will dry out sufficiently to prevent any mold or fungus development.

   Down in Louisiana, Fort Polk area in particular, all the buildings are on stilts and they don't experience any problems with mold. Being 18 to 24 inches off the ground and completely open, you can't get any more ventilation then that and their humidity is high most of the year.

   Now, stand clear of the slings and arrows!

1. 
   BillB82 | Nov 05, 2006 10:12am | #2

   Thanks for your thoughts on this. There are already several vents installed. I may consider installing fans in one or two to increase airflow. Or maybe I'll just a few more vents.

2. 
   Matt | Nov 05, 2006 04:29pm | #3

   First, some background:

   You said: >> A moisture barrier covers most of the dirt with approximately a foot of uncovered ground around the edges. <<  Prior to 2002 we in NC used the CABO building code which stated something like: Crawl space earth floor will be at least 85% covered with a 6 mil polyethylene plastic film.  I think CABO is/was used in Georgia too.  In one code enforcement jurisdiction in particular that I was building in prior to 2002, the building inspectors (BIs) were forcing us to only cover the ground by 85%, and normally the method was to leave a ~1' strip around the edges.   I discussed this with 1 or 2 BIs and they told me some BS (building science :-) about it was bad for the house to be to dry as it was bad for hardwood floors, etc.   I built several houses in which I folded the plastic under so that the 1' was left open and then after the final inspection I pulled the plastic out for 100% coverage. 

   The reason the soil is dry in the areas of your crawl space that are not covered with plastic is that this moisture is evaporating up into the CS air, and potentially into the house framing, etc.   Obviously in the plastic covered areas the soil moisture is unable to evaporate - it just condenses on the bottom of the plastic.  Which is what the plastic is supposed to do, although there should not be excessive moisture.  BTW - the plastic should really be black - the problem with clear is that it degrades over time, especially when exposed to sunlight - granted there is little sunlight in a crawl space...

   When you purchased the house did it get a termite inspection?  I would guess it did.  Termite inspections in our part of the country often include the guy checking the moisture content of the joists, etc with a moisture meter.  Here is an example.  You could contact the termite company and ask if they did a moisture check and what the readings were.  Sometimes they even write it on the joists with a marker, etc.  ~10% or less is good, >~15% is bad.  The type of termites we have in the SE, like to eat damp wood - that is why they check wood moisture content.  Either way,  I'd recommend you find out what the joists moisture content is as it sounds like you have an issue.   Home inspectors often check wood mositure contents too.

   So in terms of resolving the problem, the above poster had some good advice regarding perimeter drain systems, but in addition a few things to start with are: 
   1) fully cover the CS floor with plastic.  You might even want to put it up the walls about 6" or so and glue it in place.  If you want to spend $ :-), there is a high performance plastic called Tu-Tuff that you can find with an internet search.
   2) Extend your downspouts so that they are not depositing water near the foundation.
   3) If there is already a CS perimeter drain system search around for the place where the drain tail(s) exit to daylight.  Be sure these are clear so that water can flow out them.
   4) resolve any grading issues to get the water to run away from the house.  You may want to post some pics to help us BTers get a better idea of how much of a problem there is in this area.

   Regarding ventilation of CSes, first, someone here will probably respond with a recommendation to go with an un-vented or sealed CS.  Actually "unvented" or "sealed" is a bit of a misnomer, as actually the CS is vented into the house living space.  Essentially the CS becomes a conditioned (heated and cooled) space.  The problem with these is that they are particularly hard to implement, and now that you have black stuff on your joists I don't think you want to seal that stuff up in your house and vent it to the living space. 

   As far as how much ventilation is necessary, again in my area, ventilation requirements have changed over the last 5 years or so.  Our building code has always (in recent history) required X# of sq ft of ventilation for X# of sq feet of CS floor area.  The new addition though is that a vent be placed within 6' of each corner, meaning that in a simple box house, there could be no fewer than 8 vents.  This thinking seeks to eliminate dead spaces caused by attached masonry porches and garages, which effectively block off one side, or a portion of a side from being vented.  the point being that a lot of ventilation is now required.  Adding vents after the house is completed can be rather difficult in the case of a masonry foundation, unless it happens to be pier and curtain construction which is somewhat popular in the SE.  I'll guess though that you do not have pier and curtain.  So, because of the level of difficulty, I like the idea of a humidistat controlled fan - they sell them for this exact application.

   BTW - One question that comes to my mind is does this CS ever actually flood.

   This whole thing is really a FAQ so try using the "advanced search" function here at BT.

    

   
   

   ---

   Edited 11/5/2006 8:38 am ET by Matt

1. 
   BillB82 | Nov 06, 2006 12:36am | #4

   Thanks for all of these things to think about. I have not purchased the house yet but have it under contract at a decent price. I am trying to determine how expensive the fix may be or if it is better  for me to walk away now. (I am paying approx $140,000 for the house. It's not a steal but a very decent deal.) The house foundation is cinder block. I wonder if I may cut vents in this myself. 

1. 
   Matt | Nov 06, 2006 12:56am | #5

   The problem you describe doesn't sound serious at this point, but it does need to be dealt with.  Like I said, condensation under the vapor barrier is ok.  Black discoloration on the joists needs to remedied though.

   The only problem with cutting vent holes in the cinderblock foundation is that you will then likely need to install metal lentils above the holes which would be a PITA or you might get off so light so as to just double the (2x10?) 3' section of rim board above the new vent hole.   Doubling the rim would work in the case where the joists are parallel to the foundation wall where you are cutting in the vent.

    As a aside, IMO, just about anyone should get a home inspector when buying a house, if for no other reason just to get another set of eyes on the prospective property.  They have usually comprehensive checklist they go through.  Might be better not to get one through a reality company, as these guys may have agendas implanted by the RS company. 

1. 
   BillB82 | Nov 07, 2006 04:46am | #6

   When you said that the black discoloration (mold?) should be remedied is it possible that a dehumidifier would be sufficient? How much would that cost?

1. 
   andy_engel | Nov 07, 2006 04:54am | #7

   If you're using a dehumidifier, you'll want to completely cover the ground with plastic and close the vents. Otherwise, you'll just run up your power bill attempting to dehumidify your entire county.Andy

   "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

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

2. 
   Matt | Nov 07, 2006 05:47am | #8

   I don't think a dehumidifier would help, for the reason Andy cited.  Start by fully covering the soil floor, increasing ventilation and checking the moisture level in the joists.

3. 
   BillB82 | Nov 07, 2006 08:08am | #9

   Thanks guys. To increase the ventilation can I just add more vents? If thcinder blocks are "on their sides" can I just cut out the hollow parts between two of them and make a few big vents.

4. 
   andy_engel | Nov 07, 2006 03:29pm | #10

   You should really read the information on vented and unvented crawlspaces at http://www.buildingscience.com, and http://www.pathnet.org before adding vents. Go to the guys who do the science.

   I would not vent a crawlspace in any but a desert climate, and there's no good reason to do it there, either. It's just that doing so there would be harmless. The argument about pier foundations is specious, because they're all vent, and don't retain moisture the way a crawlspace does. They aren't good for other reasons though, mainly protecting plumbing from freezing (Did you you know that most burst pipes happen in the south? That's 'cause houses aren't built for the rare deep freeze.), and energy usage.Andy

   "Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein

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

5. 
   BillB82 | Nov 15, 2006 08:25am | #11

   Thanks. I'll check out these web sites.