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A properly constructed crawlspace should be clean, free of moisture, and free of potential allergens and contaminants that will rise into the house with the air from the crawlspace. That probably doesn't sound like a description of the crawlspace currently under your house!
Unfortunately until recently, almost all the crawlspaces built have vents placed in the walls in an effort to allow air to "ventilate" the crawlspace and dry them out. Although this seems intuitive, it doesn't follow the laws of physics. Humidity is relative (hence the term "relative humidity"). In short, warm air can hold more water in suspension than cold air can. So if you have 90 degree air at 70% humidity (typical summer conditions), and cool it down to 70 degrees (typical crawlspace temperature), the humidity level in that same air changes to 100%. While your crawlspace may not look as bad as the one in this picture, the mechanism described above is still what is happening. Research studies using tracer gas have shown that about 40% of the crawlspace air enters the house. Aside from any moisture issues the affect of this on your indoor air quality is significant, especially for anyone in the home who suffers from allergies, asthma, or any other respiratory issues.
The only path to a truly dry and healthy crawl space is to seal it entirely form the ground and outside air. This leads to big improvements in indoor air quality and annual energy savings typically between 10% - 15%.
With our sealed or closed crawlspaces, 100% of the ground is covered with a thick layer of plastic, the vents and mud sill (wood at the top of the foundation wall) are sealed from the inside, and a high quality access door is built with treated wood and provided with proper latching mechanisms and weather stripping. From the outside, everything looks as it did before (except for a nicer looking access door).
To control humidity levels, your crawlspace is then provided with a small amount of air from a supply on the HVAC system or a small dehumidifier.
Finally a web-enabled humidity sensor is installed. This picture is from our dashboard of closed crawlspaces and closed attics that our clients have shared with us. From your login you can see the temperature and humidity of the space which is monitored along with historical data. But who has time for that, right? This is where this system shines. It will send you (and us if you share your sensor data with us) a text message and/or email if the humidity in that space gets above the level we have set before it results in a moisture related problem. This feature is an extremely important component of this system! Without early warning you will not realize you have a problem until it's noticeable from inside the house. By that point it's too late to prevent moisture related damage.
This excellent monitoring system will also tell you if it loses communication for more than one hour or if the batteries are getting low. So the situation of having a moisture problem surprise you because the batteries have not been replaced or the system has gone offline is avoided. We have had numerous occasions where we were alerted that there was a problem before there were any signs of trouble, including a plumbing leak discovered through one of these systems in the crawlspace of a newly-built house just weeks before writing this.
"What's the difference between what we call a sealed and a closed crawlspace?" Though both liners are durable, with a sealed crawlspace the ground liner is 14 mil. thick vs. ten mil. thick for a closed crawlspace. To give you an idea of what that is, most poly sheating you may have seen in a crawlspace is six mil. In a sealed crawlspace the seams of the liner are sealed together with caulk and tape, and the liner is sealed to the foundation walls and piers. In a closed crawlspace the liner is overlapped and staked down, but not sealed.
"Which one should I get?" Due to the labor involved and the difficult working conditions, sealed crawlspaces are much more expensive than closed crawlspaces. They are fantastic looking crawlspaces! The image at the top of this page is of one of our sealed crawlspaces. They are just gleaming in their shiny white beauty. You may want to have a party down there! They are the best choice for people who just "want the best!", or for people who have chemical sensitivity or extreme respiratory issue and need the ground completely and hermetically sealed off from the crawlspace and house above. For the rest of us a closed crawlspace is the better option. Both sealed and closed crawlspaces result in a dry and clean space below your house, which is the goal.
"I'm in a flood zone so I can't have a sealed crawlspace." We regularly install sealed and closed crawlspaces in houses in flood zones. The design needs to be such that the water, if it comes, will be able to pass through the sealed vents. It's not difficult to have vents that keep air out but let water in if there is a flood.
"So and So Crawlspace Company gave me a quote and they want to install a super great special crawlspace dehumidifier." In our experience you are wasting money on the super great crawlspace dehumidifier. They really are great. They typically have a large capacity for moisture removal (much more than you need if your closed crawlspace is installed properly), they are very efficient (they will not run much if your closed crawlspace is installed properly), and they are more durable. Again, if you are a person who "wants the best!", go ahead and buy one. We will even install one for you. For the rest of us a low capacity, Energy Star rated, regular portable dehumidifier is a better choice. We have them in crawlspaces all over Eastern NC and 99% have been running for years with no problems. If they stop working we know it from our monitoring systems and a replacement is easy & inexpensive. One important tip learned the hard way: Do not install a digital dehumidifier in your crawlspace unless you want to go down there and reset it every time the power goes out & comes back on. And never install and run a dehumidifier in a crawlspace that is still vented! You are dehumidifying the great outdoors! Believe it or not, we see this fairly often.
"What should the humidity level be in my closed crawlspace?" We commonly see crawlspace dehumidifiers set lower than necessary, and sometimes so low that they cause problems with hardwood floors. We set our dehumidifiers to control the relative humidity in our closed and sealed crawlspaces at 68% - 70%. This seams high to some people, and you can certainly set the dehumidifier to maintain levels down around 50% or so if you want, but you are more likely to have issues with your hardwood floors, dehumidifiers are expensive to run, and you are not living down there. We have seen crawlspaces run above 75% (where we set our notification alarms) for weeks at a stretch with no ill affects. You do not need to make it a pleasant place to have tea in the afternoon. You just need to keep levels low enough to control the wood moisture content and levels in the high 60's, even with spikes into the 70's is perfectly acceptable for that.
"What should I do about the mold in my crawlspace?" Ah, this is a difficult one. First, we all have mold in our crawlspaces. At least you do if it's been vented. Even if you can't see it, it's there. In fact there is mold all around us all the time. Especially here in this humid climate. Did you know that there is no specific level of mold that is considered safe? When mold testing is performed they measure it against the levels already in the air around your house. If the mold levels in your house are at or below the levels around your house you are considered safe. The reality though, is that mold affects everyone differently. So it's not an easy question to answer. Of course there are situations where things are very extreme and mold remediation is obviously needed. But there are many more situations where the homeowner is the only one who can make that personal decision for themselves, based on their particular sensitivities and comfort level. Just make sure that any remediation that is done is performed properly and completed before the crawlspace is sealed that the permanent liner is installed. We work with an industrial hygienist to perform pre and post testing and write a remediation plan, which is then followed by a certified remediation contractor.