Results From Sealing The Crawlspace

One of the many reasons people enjoy performing homeowner DIY projects is the satisfaction of a “job well done”. Sometimes, the reward can be a freshly painted wall or newly built tree house. For a project like sealing the crawl space, the results are a bit more complex. Tracking the results as I have served two purposes 1) it supports the steps I took to get the results and 2) provides you the data in a way to analyze your own situation as well as understanding the steps that may be required to achieve the intended outcome.  The overall objective was to bring the humidity level down to an acceptable level (60-65% range). Based on the numbers, I am not there yet, but looking at the trend, I should be there in a reasonable time frame.

As measured in my pre-sealed condition the following chart shows an RH average of about 81%. Through my inspection and sealing efforts, I found small accumulations of some mold (as expected). Additionally, at times it was noticeably damp.  With RH at this level it is too high to leave untreated.

As you can see in the second chart with the crawl space sealed, the temperature stayed about the same but the RH moved around a bit more. Overall the RH averaged about 80% .

In the third chart we see a noticeable reduction in RH.  On average, in this test cycle, the crawl space has averaged 68% RH. The spikes and elevations are occurring based on my decision to shut down the fan for about 4 hours a day.  As we have seen in various studies, the worst time for humidity is during the higher seasonal temperatures. Currently we are on a 14 day run of +100 degree days. I will continue to spot check the RH for the next year.

Even though I have not reached my goal of <65%RH, I believe we will continue to see a progressive downward trend as my measured RH in the house has been  in the low 40% RH.  

Conclusion: The following items reflect a snap shot of the steps I took to get to this final place. To see the details of the entire process read the following posts; Landscape Drainage, Landscape Drainage and Inspection, Crawl Spaces; Basements without Benefits, Sealing the Crawl Space, and  Crawl Space Sealed with Positive Ventilation.

  • Proper Drainage: Adding the proper drainage reduced the  opportunity to saturate the ground underneath the house by driving the water to the streets and storm drains. We were unable to totally eliminate the damp soil condition under the house but we were able to reduce the impact.
  • Sump Pump: Due to the amount of water accumulation in the crawl space, even after the drainage correction, adding a sump pump in the deepest part of the crawl space made sense.  (NOTE: In my region, it is not uncommon to receive large excessive amounts (3-6″) of rain in an afternoon). Without immediate remediation, this water can stand and a permeate the crawl space with excessive moisture for months.
  • Vapor Barrier: By sealing the crawl space floor with 6-8 mil plastic, the porosity of the dirt floor has been removed from the equation. 
  • Crawl Space Vents: As recognized in earlier posts, crawl spaces in regions with high summer time humidity levels are good candidates for sealed crawl spaces. My region is border line based on the studies, so by measuring the real-time humidity, I elected to seal the space. 
  • Positive Ventilation: As indicated in the chart, the positive ventilation of the low humidity air found in the conditioned space of the house caused the RH to trend downward. This is a low-cost solution compared to a dehumidier that can cost $1000.  To date, I have not recognized any negative effects of using this method to reduce the humidity in the crawl space.


  • Dehumidifier: Not that it couldn’t be required, as the current RH is still greater than 65%, a dehumidifier will bring the crawl space in compliance. Fortunately we did not have to go to that extreme.   By using  the PV Fan (found in Crawl Spaces-Sealed with Positive Ventilation) we were able to reduce the humidity levels.

As I mentioned in the beginning of this venture, this project is not for the faint of heart. It is very time-consuming and very labor intensive. If you contract it out, it will be expensive. However, if you have had mold and moisture problems this should eradicate the issues. Good Luck BOB.

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