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Wet crawl space - More ventilation or seal it off?

Wet crawl space - More ventilation or seal it off?

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  #1  
Old 06-16-08, 08:54 PM
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Wet crawl space - More ventilation or seal it off?

Sorry, I know this is long, but I'm trying to give all the relevant info. I'd really appreciate your input!

I recently bought a house in Knoxville, TN, and now that the weather's gotten hot and humid, I've noticed some problems with high humidity and condensation forming on the HVAC ducts in the crawl space. They get wet enough to drip down on the floor and cause the plastic on the floor to get slightly wet under the ducts. I'm worried that this will lead to mold and rot.

The foundation has the regular block vents around the walls, except where the garage slab is. The floor is currently covered with black plastic (6 mil), that's not sealed in the joints, and small gaps around walls and columns, nothing on the walls. So, the first thing to is probably to make that seal better. However, I'm guessing that's not going to be enough, so what else should I do? Seal the vents and get a dehumidifier (seems to be a popular route), or get mechnical ventilation to increase air flow?

The average outside conditions are 80F, 47% RH this time of year, and my crawl space is 68F, 78% RH right now. Inside is 71F, 41% RH. Does it make sense to vent the crawl space under those conditions? For example, with the numbers above, 80F 75% air cooled to 68F is 73% if I'm not mistaken! Doesn't seem very useful for drying. Also, is it normal for my crawl space to be cooler than the living space, or do I have a duct leak or something? I would've guessed that the crawl space temperature would fall somewhere between inside and outside temps.

What kind of humidity should I be content with? Do I have to take it all the way down to 50%, or is eliminating the condensation enough?

Also, about the dehumidifier route... I understand that unvented and unconditioned crawl spaces don't meet code. Does that mean that they are unhealthy? Can it actually lead to more mold spores despite low humidity? I've seen putting an A/C register in the crawl space being mentioned, but circulatig the crawl space air through the whole house doesn't seem like something I want to do. Even if there isn't a return in the crawl space, the positive pressure will push crawl space air up into the house, right?

Also, for the mechnical venting alternative above, I had something like this in mind... What do you think? http://www.atmox.com/Systems/Compact...7/Default.aspx

Thank you!
 
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Old 06-17-08, 07:44 AM
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Hey, I would say you should seal up the crawlspace. There is some really good information on this website (www.yourcrawlspace.com) Check out thier DIY Kit. They have a 20 mil liner that works great, and an easy way to attach it all with just an adhesive. I used it, and it works great. I have a dehumidifier put in with it sealed now, and the humidity stays below 60%. I'm in costal SC, and have pretty humid conditions outside most of the year.

I also had a 6 mil plastic liner, and it wasn't doing anything, got holes in it, and didn't attach to the walls. Nothing good at all from it. They also have pretty good tech support there, and are willing to help and answer all your questions. It was a pretty easy DIY project, alot easier than the 250' of wooden privacy fence I just put up. Haha.
 
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Old 06-17-08, 01:17 PM
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Originally Posted by SC_fisherman View Post
Hey, I would say you should seal up the crawlspace. There is some really good information on this website (www.yourcrawlspace.com) Check out thier DIY Kit. They have a 20 mil liner that works great, and an easy way to attach it all with just an adhesive. I used it, and it works great. I have a dehumidifier put in with it sealed now, and the humidity stays below 60%. I'm in costal SC, and have pretty humid conditions outside most of the year.

I also had a 6 mil plastic liner, and it wasn't doing anything, got holes in it, and didn't attach to the walls. Nothing good at all from it. They also have pretty good tech support there, and are willing to help and answer all your questions. It was a pretty easy DIY project, alot easier than the 250' of wooden privacy fence I just put up. Haha.
Thanks for the tips and the link! Looks good, I'll definitely look into that DYI kit. If you don't mind, about how big is your crawl space, and how long did it take you to install the vapor barrier?
 
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Old 06-18-08, 08:54 AM
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hey, no problem. My crawlspace is about 1200 sf, about 3 feet tall. I didn't have much clean up to do, as the house isn't that old. It took me about a day and a half, but thats cause my wife wouldn't get under the house with me. haha. to many spiders and stuff.
 
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Old 06-18-08, 06:19 PM
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Your existing problem is not enough ventilation. Do not close up your crawl space. You might call someone in to add more venilation and do check your ducts for leaks.
Thanks for listening.
 
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Old 06-18-08, 07:39 PM
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Only way to prevent mold is to seal crawl space vents, 6mil poly on ground and some will need a dehumidifier.
 
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Old 06-19-08, 03:17 AM
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Aaah, at least I have one ally. If you are in the Southeastern US, crawlspaces are required to have ventilation methods installed. Install 6 mil poly on the ground, for sure. That will help alleviate the moisture from eeking up into the area. Cross ventilation will help move the air in a constant flow. We agree to disagree, so you will need to really research the best method for your area.
 
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Old 06-19-08, 06:43 AM
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crawlspaces are not required to have ventallation, if they are conditioned.

Thats the old thinking rearing its head again. Many codes have already changed to allow the closing of crawlspaces. Check the 2006 IRC and see. Also, check out AdvancedEnergy 's website about closed crawlspaces in NC. The did a very scientific study, and proved that closed and conditioned crawlspaces are much more efficient, and healthy. I did plenty of research before doing mine.
 
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Old 06-19-08, 11:29 AM
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I happen to know many of these "Building Science Organizations" (BSO's) and the people who belong to them. Furthermore they have my deepest respect for their efforts and work. This by no means that I agree with them. And YES, I'm the "Old School" thinking guy who they say doesn't accept change or new "IDEAS".

This concept of sealing crawl spaces has been around for decades. In the late 1970's a paper came out proposing this concept. One of the reasons for this concept was "Partially Conditioned Spaces" where heating/cooling ducts were noted. In the heat of this debate, I was given the opportunity to make a statement. I simply stated that one either wants to condition a space or does not. Hence there is no such thing as a "Partially Conditioned Space". Mind you it took them more than ten years to accept this. And they say I'm the old guy who won't accept change or new ideas.

However, this concept didn't go away. Rather it came back with "Conditioned Crawl Spaces". This saves energy based on "Pressure Induced Moisture Flow". The "Gas Law" states that for a liquid to change states to a vapor, the liquid must be absorbing heat. For a vapor to condense into liquid, the vapor must be losing temperature or if you prefer, giving off heat energy.

To apply this to a wet crawl space is during the winter the moisture in the crawl space evaporates. It uses the heat energy inside the crawl space to cause the evaporation, thereby lowering the temperature of the crawl space. We know that the colder the winter is that season, the higher your energy bills will be. As well as the greater the temperature degree difference between your crawl space and inside your home, the greater the heat loss or higher energy bills.

During the summer the moisture vapor inside the crawl space condenses. And the Gas Law states that the vapor must be giving off heat energy. Which raises the temperature of the crawl space. Which increases the Heat Gain or if you prefer, increases your cooling costs. In both instances the increased energy costs can be measured and verified.

This is actually well known within the energy conservation industry. In fact I have lectured on it for decades and instructed others on how to avoid the energy loss due to moisture throughout the structure. Their concept uses the crawl space to avoid this energy loss by pressurizing the crawl space. If you have a supply duct blowing air into a sealed crawl space you create a space of high pressure. This pressure influences moisture flow or if you prefer, moves the moisture away from the crawl space. Thereby avoiding the energy loss created from evaporation or condensing of moisture.

While their position appears to have some merit, when we compare the costs and saving of ventilating, ground cover, floor insulation and duct/pipe sealing and insulation to sealing the crawl space, outer wall insulation, ground cover and pressurizing the crawl space, the former wins hands down.

Furthermore we energy conservationists all agree that we should close off rooms we do not use in order to save energy. What message are we sending if we tell people to heat and cool a crawl space where no-one will ever live.

Lastly, there is growing concern among BSO's and energy conservationists that people are reading these papers and attempting to apply it without fully understanding the concept. And what has been said here does not even scratch the surface of the number of concerns.
 
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Old 06-19-08, 04:07 PM
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I'm speechless to say the least. Thank you for your insight on this Resercon.
 
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Old 06-22-08, 09:14 PM
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Thank you very much for the input everyone.

I certainly wouldn't mind saving money by going with ventilation, but what I can't seem to find is information about the parameters that make up a healthy crawl space. If I may quote myself:

The average outside conditions are 80F, 47% RH this time of year, and my crawl space is 68F, 78% RH right now. Inside is 71F, 41% RH. Does it make sense to vent the crawl space under those conditions? For example, with the numbers above, 80F 75% air cooled to 68F is 73% if I'm not mistaken!
So am I right that during this part of the year, even optimal ventilation and ground cover, the lowest humidity I can expect is 73%? Is that acceptable, given that you want less than 50% indoors? Is occational condensation expected/acceptable, or what would be the highest humidity where this does not occur? In July and August it will be hotter, so there will be more water content in the outside air, so I'm pretty sure the problem will be worse.

Thank you!
 
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