Sealing the Crawl Space

Crawl-space construction has the greatest likelihood of having mold, mildew and wood rot issues than any other residential construction foundation. It is very common to find this space musty and damp, partially due to the dirt floor.

In most cases; if you have a moisture problem in the crawl space, it will be relatively obvious once you go under the house. Damp, musty conditions will be apparent. If you prefer, testing the humidity levels will validate your suspicions as well as bench mark your progress. You can use a RH test probe or Hygrometer, the  test probe can be purchased for under $100. Here are a few numbers and calculation that turn into real facts that will be valuable along the way:

  • Wood rot occurs when moisture content is greater than 27%.
  • Relative humidity (RH) of 100% equates to a moisture content of 23 – 28%
  • To cultivate active mold in building materials;  the presence of mold spores and a moisture content of 20% or greater is required

Common conversions % Relative Humidity to % Moisture Content:
0% RH = 0% MC

30% = 6%
50% = 9%
65% = 12%
80% = 16%
99% = 28%

Insulating and sealing the floor of the house  may actually increase the humidity in the crawl space. So if you are planning to insulate the floor, go on and seal the dirt floor.

The Do Nothing Approach: If you find the temperature and RH at an acceptable level,  no mold, air quality or pest issues;  the best decision may be to leave it alone. On the other hand, if you are planning to insulate the floor, this may actually cause you to have a moisture problem. By nature low humidity areas will draw moisture from high humidity areas. So, by placing insulation between the dirt floor and the conditioned space, the humidity in the crawl space can increase because the insulation will block the humidity migration from the crawl space to the conditioned area. This same principle applies where the crawl space is lower in humidity than the ambient air outside the building envelope. When insulating and sealing the house you may actually increase the humidity in the crawl space. So if you are planning to insulate the floor, go on and seal the dirt floor.

 Sealing the Dirt Floor Crawl Space: As a DIY project; this is not for the faint of heart. This requires many hours under the house. Having a helper for this project will greatly reduce the amount of time. This process is not about laying down some plastic sheets and calling it done. This purpose of this is to seal the crawlspace from the dirt floor. The plastic should be placed smooth and flat to the dirt floor. Sealing all the joints, seams and obstructions. The professional charge for this can be as much as $4 a square foot. Since I did this by myself, I figure I have over 100 hours in the task with a lot of sore muscles and scratches on my head. Granted, I have not been in a hurry, and I work on it when time permits.

  1. The first step in controlling the RH and moisture content of the crawl space is to ensure you have adequate drainage.  Read my article on Landscape Drainage and take the appropriate steps.
  2. Clean the space. Remove all construction debris and any sharp objects.
  3. Grade the soil as best as you can to create a dome in the center with no low pockets in the surface area. This will keep the moisture near the perimeter of the structure.
  4. Sump Pump Option. If you see that you may be adding a sump pump as  needed for severe conditions, you create water channels to the lowest spot in the crawl space. The sump pump should be located here. Adding the pump can be performed before or after sealing the floor. TIP: If you have found water completely covering the crawl space floor, even after addressing drainage issues, adding the sump pump is inevitable.
  5. Layout and install 6-8mil plastic sheeting on all exposed ground surface. There has been a lot written by professionals saying 6-8 mils is too thin and that a 20-22 mil product should be used. So far, in my application, I have not seen any drawbacks mentioned by using the 6 mil product, but if you foresee a lot of traffic or circumstances that may warrant it, you may consider using the thicker product. As you have already figured; its a lot more expensive. The bottom line is reducing the humidity and thicker product is no more impervious than the 6-8 mil but is less likely to be damaged by activity.
  6. Overlap all plastic sheets together, use landscape stakes to anchor the material on the crawl space floor. Use a good quality duct tape to seal all the seams.
  7. Place the sheets with an excess of 6-12″ of material at the exterior beam. Use a mastic (liquid nail) to seal the plastic sheets to the foundation beams.
  8. Cut plastic around all protrusions (concrete piers, plumbing pipes, etc.) and seal the plastic around the obstruction with duct tape.
  9. You can use 3/4 insulated fiber board  to insulate the perimeter beam to add insulation value. In some cases this may create greater access for termites. At a minimum, completing item 7 will help with the moisture issue.  Use a liquid nail type adhesive to attach the fiber board to the perimeter beam.

As you figured, this is dirty work and you may now understand why professionals are paid a premium for this service. The goal is to achieve RH of 60% or less; anything greater will increase the opportunity for termite infiltration. If you purchased a RH probe, you should notice instant results. As mentioned earlier, if you live in an area with high humidity you will need to take further steps to get below 60% goal.

If further remediation is required, there are several options. Your specific application may dictate your choices:

  1. Seal the crawl space vents: Controversial in some municipalities, but gaining acceptance in areas that experience high humidity in the summer months. By sealing the space from an exterior environment you may reduce this humid air  from its natural migration into the crawl space. This alone may resolve the remaining humidity problem. However some codes require that you add some type of positive ventilation (see below; “By the Code”). If you seal the vents from the outside, plan to fill the grate with a  foam board then use the vent covers to seal the deal to leave a finished look. Also read my post Crawl Spaces: Basements without Benefits.
  2. Conditioned Air: Some studies have shown that introducing air from the conditioned (living area) space to the crawl space will adequately reduce the humidity. If you have chosen to seal the crawl space, most codes that recognize sealed crawl spaces require them to have some form of positive ventilation or dehumidification.   I found an interesting product that does this, but it appears (so far) they only sell it as part of an installed service.  Crawl O Sphere System. I created a DIY solution to provide simular results to the Crawl O Sphere. Go to this  link to see my solution. 
  3. Dehumidifier: (Optional) Adding a dehumidifier will draw additional water out of the air. A crawl space rated dehumidifier for 1500 sq. ft is approximately $1100. This is an expensive step, but if you are having severe problems this may be your only choice, it will work.  As an alternative to the dehumidifier look at #2 or #4 as your other options. One of the three should be sufficient to bring the space into compliance.
  4. Duct warm dry air from the attic space to the crawl space:  (Optional) Other studies have shown that by pushing dry air into the crawl space from the hot attic may be enough to make the necessary adjustment. To date, I have not seen a fan system to perform this correctly to transfer the air based on the correct conditions. This would require a fan with both stationary and remote temperature and RH monitors.  The primary difference between these two are that #3 draws the moisture out of the existing air where #2 pushes dry air into the crawl space to dry the space. The Crawl O Sphere could probably be adapted to an attic space type application. Since I do not have a good avenue to get the attic air to the crawl space this is not an option for me.

By the Code: I have attached the excerpts from the IRC code that has been adopted by my city of residence. So for this reason, I should not have  code issues down the road due to an inspection. For greater details, take a look at this site; Building Energy Code Resource Center.

International Residential Code (IRC 2006), Section 408.3 Unvented crawl space.Ventilation openings in under-floor spaces specified in Sections R408.1 and R408.2 shall not be required where:

    1. Exposed earth is covered with a continuous vapor retarder.
    2. One of the following is provided:

2.1 Continuously operated mechanical exhaust ventilation at a rate equal to 1.0 cfm (0.47 L/s) for each 50 ft² (4.7 m²) of crawlspace floor area, including an air pathway to the common area, and perimeter walls insulated in accordance with Section N1102.2.8.

2.2 Conditioned air supply sized to deliver at a rate equal to 1.0 cfm (0.47 L/s) for each 50 ft² (4.7 m²) of under-floor area including a return air pathway to the common area, and perimeter walls insulated in accordance with Section N1102.2.8.

2.3 Plenum complying with Section M1601.4 if under-floor space is used as a plenum.

 Next time, I will provide details on sealing the exterior vents and building the positive flow fan for the air supply.

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14 Responses to Sealing the Crawl Space

  1. [...] the series of articles; Landscape Drainage, Crawl Spaces – Basements without Benefits and Sealing the Crawl Space  we stepped through identifying potential problems and the process of correcting the problems just to [...]

  2. [...] Landscape Drainage, Landscape Drainage and Inspection, Crawl Spaces; Basements without Benefits, Sealing the Crawl Space, and  Crawl Space Sealed with Positive [...]

  3. dave frase says:

    why is it better to seal the crawl with plastic rather than a slush coat of concret?my crawl is real tight with a lot of vents and plumbing and it mite be easier to flow a watery mix over it rather than spread plastic and seal around pillars and pipes. will i create a water presure problem that way and just crack the coating? any comments would be appreciated thank you, dave

    • homeownerbob says:

      Dave, Two things here. 1) Even though the concrete will provide you a more desirable surface, it remains porous, so it would still require a plastic vapor barrier under the concrete to combat humidity issues, 2) Access; if you put concrete over the surface and you have plumbing leaks, you will be chipping it up to find them, then you will have to refloat the concrete. Additionally, most any contractor will use this as a reason to charge you more money to work on you plumbing. BOB

  4. Curt says:

    What is the best way to seal around block piers in the crawlspace?

    • homeownerbob says:

      Assuming you are trying to create a vapor barrier as part of sealing the crawl space, you can take the same 6mil thick plastic and wrap around the blocks. 1) Pre cut the 6 mil sized to wrap around the pier with about a foot or more of overlap. 2) Use a liquid nail type construction adheasive to apply beads of caulk liberaly on all sides of the pier. 3) Wrap the 6mil around the pier. 3) Smooth out the plastic to spread out the caulk. 4) Then use high quality duct tape to secure the plastic film to itself at the overlap seam. 5) Secure wrap at the top and bottom of the pier with the duct tape. Then, assuming you are sealing the crawl space cut the plastic around the piers then bridge the floor plastic with the plastic around the pier with the duct tape and extra plastic sheeting. BOB

  5. I bought this house 4 months ago, after a super downpour I found some of the ground sinking, I flooded it and checked the crawlspace, I seen some water leaking from the crackes where the blocks had been joined, what should I seal the blocks with before trying to seal the crawl space?

    • homeownerbob says:

      Sealing the crawl space is not really about stopping water, but reducing moisture from the space. First off, make sure and read It’s important to have good surface drainage that naturally moves water away from the foundation. Solving that problem should fix most of the common items. Even if you seal the wall perimeter, that will not stop water from perking up underneath the foundation. Fix the drainage first then if water is still coming into the crawl space, you may have to consider adding a sump pump.


  6. Victor Demar says:

    I would like to know where to purchase a Crawlosphere ventilator and how much SF they cover. The space is currently sealed. There is very little moisture comeing in during rainy periods. There is a radon mitigation system exhausting any moisture under the barrier to the outside. The sill and Ban board aren’t sealed yet. They will be done soon along with the the vents. The vents are closed off but not sealed with caulk as. Yet. Thanks Victor

    • homeownerbob says:

      Victor, From my investigations, the Crawlosphere has to be purchased as part of an installation. I can not find that it is purchasable as a DIY type item. As you may have figured, that is why I documented and built my own. Good Luck BOB

  7. Hi there! We just had the termite company here (no termites), and the guy told me our crawl space humidity is 23-25%. Is that relative humidity? As he took the measurements, the rain came down — HARD. He says we have a serious moisture problem, but if I understand your post correctly, it’s the wood’s reaction to the moisture in the air due to the rain? Please help! Thank you.

    • homeownerbob says:

      Karen, we may be caught up in the correct terminology. 20-25%RH is great, that is about 5% moisture content. On the other hand 20-25% moisture content is bad as that is about 99% RH. Overall, you want the crawl space to be below 65%RH. As you pointed out, it you are seeing heavy moisture during rains, you have a drainage problem first. At 20-25% moisture content, you are probably seeing a lot of mold. Make sure and read all the articles I wrote starting with drainage all the way to result. Good Luck. BOB

      • Karen says:

        Thanks Bob. I got a second opinion from a restoration company. Our RH was only 17% (low humidity day) and the moisture content was only 13%. I believe the termite company used a relative humidity rating to spook me. Your article was quite helpful, because I knew to ask for both meter readings. Thank you.

      • homeownerbob says:

        Awesome!! Glad I could help. BOB

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