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(X-Post) Crawl Space Condensation & Humidity Solutions

Posted by mydreamhome (My Page) on
Wed, Jun 11, 14 at 11:09

We're located in central NC & having condensation & humidity issues in the crawl space of our 2 year old home. It gets quite hot and very humid in the summer and we end up with a mess on our hands. The HVAC ductwork is in the crawl as well as our on on demand hot water heater.

I had posted a while back and we made some of the changes recommended--closing the vents, insulating the vents with rigid board insulation, running dehumidifiers. These measures have helped greatly, but the humidity level is still in the 64-70% range.

We've also done some more research. We keep hearing encapsulate the crawl space (to the tune of about $11K). But I also keep reading about odors over time and moisture under the encapsulating material. (Did I mention the $11K price tag?) But then the point is also made that in the south we didn't have these problems when we had greater ventilation under the house so the answer is to ventilate better.

The better ventilation end of the argument leads me to this company called Atmox. The have a sophisticated software system that supposedly measurea the humidity and dew point and turns the fans on or off depending on the readings. The end result is to bring drier outside air in to ventilate the space and keep things dry. They say a dehumidifier can be added to the system if needed. The cost is in the $2-3K range depending on if you do the install yourself or have it done turn key. I can't find alot online about them as a company other than someone on GW used them back on '07 or '09 and recommended them. I don't think its the company so much as the science behind it in any case.

We need to do something about this problem, but I don't want to waste money on a system that doesn't work or one that is overkill or one that I have to tear out a few years down the road.

Any insight into these options would be most appreciated.

Here is a link that might be useful: Atmox Vented Crawl Space Systems

Follow-Up Postings:

RE: (X-Post) Crawl Space Condensation & Humidity Solutions

I have never lived in NC, but my impression is the summers are not and humid. Is that not true? What are the typical summer highs and humidity levels? Bringing in hot humid air from the outside and then lowering its temperature does not sound like a good solution me.

Am I missing something?

RE: (X-Post) Crawl Space Condensation & Humidity Solutions

Yes it is hot & humid during the summer. The ventilation theory is that the vents open and the fans run when the heat & humidity drops based on the readings of the meters attached to the fan system allowing the space to breathe and ventilate. So you supposedly are only letting in & circulating drier air than what is in the crawl space. When the meters read out of range, the vents remain closed keeping the hot & humid air out.

The website link may explain it better.

RE: (X-Post) Crawl Space Condensation & Humidity Solutions

On what is the water condensing? If the ducts, then maybe you have to enclose the ducts to put them into the indoor space. If you are getting condensation on the floor joists or subfloor you probably need seal it in or enclose the crawl.

This is just a start. I know that there is a lot more to read at the LSU web site. There is a lot of information at

Here is a link that might be useful: Insulating Raised Floors in Hot, Humid Climates

RE: (X-Post) Crawl Space Condensation & Humidity Solutions

We have the same problems in our house in Chapel Hill:

Do you have plastic sheeting covering the ground in the crawlspace and are the seams taped?
Is there a drain that takes any standing water out of the crawlspace?
Are the ducts insulated?

There is no simple solution to this. NC code now requires all crawlspaces to be sealed - I'm surprised your crawlspace is not sealed if the house is only 2 years old(?)

If you go the ventilation route, be sure to get a demonstration of the fans first - they can be extremely noisy!

RE: (X-Post) Crawl Space Condensation & Humidity Solutions

Thanks for the replies. I'll do my best to answer the questions:

The condensation is on the ductwork & air handler (I think that's the right term) under the house. The ductwork is insulated and seams are sealed.

Yes, we have the plastic sheeting down and it is taped. There is no standing water in the crawlspace. We don't have a drainage issue (where the perimeter drains aren't installed correctly or not doing their job adequately). Its just the condensation problem.

We installed the rigid board insulation across the vents, closed the vents and started running the dehumidifiers last summer. They've been running nonstop since then. It no longer rains condensation, but the humidity levels are still too high.

That may be a county code you're talking about. From what I understand, NC goes by International Code and then the counties can further regulate from there. Plenty of homes in Randolph county are still being built & passing code with ventilated crawls.

Thanks for the heads up on the noise level of the fans.

We built our last house too and never had this problem. The only thing we can come up with that's markedly different is that the last house was heavily shaded by surrounding trees and this one is not.

RE: (X-Post) Crawl Space Condensation & Humidity Solutions

The Atmox ventilation system relies on the fact that the outside air will be dryer than the air in the crawl space. I would think this would not happen very often during the summer in NC. I am skeptical how well this will work.

What is the humidity level in the rest of the basement? What are you using for a dehumidifer?

RE: (X-Post) Crawl Space Condensation & Humidity Solutions

The Moisture has to be coming from somewhere. Either your crawlspace is not really sealed to the ground, or it is not air-sealed. You have to figure out which in step 1.

In humid environments, the air is a much greater source of water than the ground. Both are infinite in their quantity, but the air can move it onto your house faster. More ventilation can just mean more water to condense. In the olden days raised houses were common but they were not mechanically cooled. Add mechanical cooling to them and you add some condensation problems.

Adding ventilation AND a dehumidifier makes no sense. Adding ventilation makes sense only if the outside air has less ABSOLUTE humidity than the air under the house. Yup, it is cooler under there than out in the sun, so the RH is higher but the absolute humidity is the same. Ventilation will only help dry faster in times of lower humidity after times of high humidity. If the dew point is the same all the time, you gain little or nothing.

You have two choices. You can seal the crawlspace from the outside and make it part of the indoors, or seal the underside of the house, including the ducts, from the crawlspace. If you seal the underside of the house from the crawlspace, you have to be sure that the floor assembly can dry to the inside. IMHO, the first mistake was made putting the ducts outside of the house. I guess that that a third choice might be to move the whole HVAC system inside the house envelope, ducts and mechanicals. You could install mini splits. You have to do the arithmetic. Unfortunately, I don't think that fixing your new house will be inexpensive.

Read up from sources in your climate area. The local land grant universities in your state and ones with similar climate areas are a good source. is good too. Universal building codes are sometimes not specific enough and US building practices are often biased to cooler, less humid climates (vented attics). Common building practices change very slowly and are way behind the times when it comes to AC duct location.

You might want to call in a professional to guide you because this is important and it is expensive. I am not there to look at your house and I am not a building professional so I might be missing something obvious. What I do know is that I would not rely on anyone that is selling something to solve a problem like this. Unfortunately expertise of the kind you need might take some digging. Some home inspectors may be able to help ,other not. A lot of energy raters should be qualified, but may not have the experience you need. An engineering consulting firm that does a lot of residential work, including energy ratings and home inspections, might be a good source. I think that you'll have to educate yourself somewhat in order to even choose an expert. Talk to a few and outline your problem and see how they respond before you hire someone.

This post was edited by ionized on Thu, Jun 12, 14 at 11:28

RE: (X-Post) Crawl Space Condensation & Humidity Solutions

Have to agree with Ionized...but based on La. climate..
great LSU link..loaded with info, although climate specific.

how close to the ground is your house?
how close to the ground are ducts & plenums?
hard pipe ducting, not flex?

there is lots of conflicting information..even among
trades people.
open crawl
vented crawl
semi vented crawl
non-vented crawl.

the only thing that is the same in all of these
scenarios is that the vapor barrier has to be complete,
and sealed to pilars, or walls of crawl.

here where I live in La. we have open crawlspaces...and
houses with ducts in floors that are low to the ground...
like 2'..have wet ducts, plenums, equipment. just not
enough air flow in and around ductwork & equipment.

when you get to 3' crawlspaces the problems lessen...
again because of increased air flow.
4' and higher..usually no problems..some of our
houses were raised 8' due to flooding from hurricanes.

on another forum, I know a guy who's company does
enclosed crawlspaces in NC. great work from the pics he
posts & a very intelligent guy. if you'd like drop me an email
& I'll send you his user name & the website where we are
pro members. while I don't have issue with posting website
here, I don't want to post his user name. so

best of luck.

RE: (X-Post) Crawl Space Condensation & Humidity Solutions

For less expensive alternative check out Tjernlund Products, Crawl Space Ventilators

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