Answer This Perplexing Crawl Space Question?

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Old 11-06-07, 07:06 PM
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Answer This Perplexing Crawl Space Question?

O.K. so I bought into this whole "seal your crawlspace" idea. Sealed my vents, installed a dehumidifier and am now installing a new vapor barrier over the dirt floor, up the walls, etc. With all this, my humidity in the crawl so far (we live in the South East) has been running about 66-70%. I am hoping once I get the vapor barrier down and maybe a bigger dehumidifier, I can get the humidity in the crawl closer to the magical 50% target. Well, the outside humidity tonight is running 45%! Doesn't that argue against the entire idea of sealing up my crawl?? Seems like with humidity that low outside, I should be OPENing up my vents. Can anyone explain this to me? Am I doing all this work in vane?
 
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Old 11-06-07, 07:13 PM
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I live in the southeast, also, and have NEVER been an advocate of sealing up the vents. Cross ventilation is absolutely necessary to keep the crawlspace at least as dry as the surrounding atmosphere. The Southern Building Code calls for this ventilation, and we are required to install them. There are arguments against this theory, and to do exactly what you have done, but it don't work. In other regions of the country, it may help, but not here. One word....ventilate!
 
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Old 11-06-07, 07:19 PM
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Originally Posted by Lukester1980 View Post
Am I doing all this work in vane?
No. The idea of the vapor barrier once down is that it will retard the moisture from ever getting high enough for you to feel like opening the vents. Btw didnt you already put down a new VB weeks ago? Or are you still working on it for the last few weeks?
 
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Old 11-06-07, 08:16 PM
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The reason your RH is still high is because their is still moisture coming into the crawl. Once you get it sealed up you will be fine.With vents open you will never be able to maintain 50% RH.
 
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Old 11-07-07, 04:35 AM
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With vents open, the RH will be whatever the outside RH is, and will remain stable. But, we've been here before. I can't understand why a code would call for the vents, give specific locations, sizes, require their installation,etc. and not have done extensive research on the matter. They may not be an ideal situation in other parts of the country, but in the Southeast, they work just fine. You seal up that crawlspace and you will create so much humidity, you will have stalactites hanging from your joists. Dehumidifiers are cute, but can't replicate what you need to rid a crawlspace of moisture.
 
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Old 11-07-07, 05:53 AM
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http://www.buildingscience.com/bsc/ Codes are slowly changing. You said the crawl will be what the outside is! 76% is way to high. It must be under 55% to keep mold from growing. Only way to accomplish this is to seal the crawl and add dehumidification.
 
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Old 11-07-07, 08:04 AM
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I have not put the new V.B. in yet. I got sidetracked cleaning up the mold. I am now dealing with fixing my rain water intrusion problem. Looks like I will have to bring in fill dirt to build up the level of dirt along the inside of the crawl. After that, I will do a french drain outside to keep water away from the crawl. Once all that is done, I will put the new V.B. down. My concern and confusion is trying to wrap my brain around the idea that if the humidity outside is lower than inside the crawl, why am I sealing up the crawl? It makes sense in the summer when the outside humidity is very high to seal it up to keep the humid air out. This morning the outside humidity was 42% and I know the humidity is around 66% on the inside of my crawl with the vents closed and dehumidifier running. Makes me want to scratch my head.
 
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Old 11-07-07, 08:39 AM
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The simple answer is that you only have half the job done. You have sealed it from the air but not the ground where the majority of the moisture is coming from.
 
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Old 11-07-07, 03:53 PM
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Why are we wanting to seal it from a drying agent (air)? Moving dryer air will tend to dry out a crawlspace many times better than leaving it a sealed tomb with a pukey dehumidifier. You are already experiencing the validity to the claim that sealing the tomb won't lower the humidity. Let the already lower humidity from the outside, circulate and equalize the humidity underneath. Codes are changing, daily, and it may be the best solution to use the seal-it-up technology in certain climates, and I can see where it would work. But in the South, it won't fly, due to the varying temperatures and humidities we experience throughout the year. One day it can be 65 degrees with a relative higher humidity than the next day which will top out at 45 degrees and 20% RH, so keeping these crawlspaces as equal as possible to the outside air is what they are trying to achieve.
 
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Old 11-07-07, 04:04 PM
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Originally Posted by chandler View Post
You are already experiencing the validity to the claim that sealing the tomb won't lower the humidity. .
Did you miss the part where he stated he doesnt have the VB down yet? Sealing doesnt mean just cut off from the air...the ground is a bigger source of moisture so you have to seal a crawl from that as well.
 
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Old 11-07-07, 06:53 PM
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Chandler I know we have been around on this many times before. So I think we are going to have to agree that we disagree. One thing you said got me think about away I can bring you to the Right side.

(Why are we wanting to seal it from a drying agent (air)? Moving dryer air will tend to dry out a crawlspace many times better than leaving it a sealed tomb)

Anything over 60%RH is not a drying agent. True RH will be lower in the winter months. Remember their are 12 months in a year not just 3 cold ones (At least not in VA. It is true that their are areas that do not need sealed crawl spaces Green grass states are not those areas. Those areas are areas that rarely see RH levels above 60%
Anyway had fun and best of luck tell we do this again my friend.
 
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Old 11-08-07, 04:30 AM
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D00bs, the way I read it, was "I haven't installed the NEW vapor barrier". This just indicated to me there was already vapor barrier down, and that he was just bolstering it up a little in order to perform the seal. I agree, the moisture must be stopped from the ground up. Thanks. Airman: Yeah, it is a good thing that we have differing viewpoints. If we all agreed on everything, there would be no need for the forum. At least it gives the OP the option of making an informed decision. We'll be here again, and someday, maybe you will repent, who knows!! Take care, friend.
 
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Old 11-08-07, 12:31 PM
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I do have a V.B. down right now, but it is ratty, torn, has open spaces, etc. In addition, the dirt in my crawl is mostly clay and is very moist. Much of my problem is rain water intrusion along one wall. I am going to add fill dirt to raise the level of the dirt on the inside of the crawl to a level higher than the outside. After that, I am going to put in a french drain system on the outside. Hopefully that will solve my water intrusion problem. After that, I will put down the new V.B. I noticed this morning that the outside RH was 62%. I have no idea how many days of the year that the outside RH in Charleston, SC is above 50%, but I would guess it is a good bit more than 180 days. My father-in-law who lives 25 miles inland thinks I am absolutely nuts to close up my crawl and install a dehumidifier. I looked into his crawl last weekend and it was dry as moon dust. Go figure!
 
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Old 11-08-07, 04:15 PM
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dehumidifiers work but i prefer simpler systems,,, have done a couple of crawls here in augusta & aiken, sc,,, 6mil plastic & power'd vents wired into humidistats,,, no one's complained & we've not chang'd telephone numbers ;-)

many times we're seen crawls so dry the dirt just fluffs into dust,,, leading me to think its temp variations that might be causing higher humidity levels,,, but i don't have data to support this hypothesis.
 

Last edited by so-elitecrete; 11-08-07 at 04:18 PM. Reason: more inf
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