Crawl Space – Sealed With Positive Ventilation

This series of articles actually started with drainage. But as most everything goes, they are all inter-related. Just to catch up; a dirt floor crawl spaces can be problematic, they present many opportunities for excessive moisture, mold, bugs, termites.. and the list goes on. As we continue to attempt to make our houses more energy-efficient, problems can be created by sealing and insulating. This is especially an issue in regions that have naturally high humidity. Within 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 get to the point of sealing the space (to keep from creating additional problems).  After sealing the dirt floor, the final steps are to seal the perimeter vents and create a positive air flow of conditioned air from the interior of the house. As I previously mentioned,  a product such as a Crawl-o-Sphere is designed to pull in this conditioned air into the crawl space. As an alternative, I created a homegrown device with similar results. I am tracking the before and after results to validate the change. For test gear, I used the Onset dataloggers for temperature and humidity recordings. 

Sealing the Vents : This is pretty straight forward. The age of the house will dictate the style of vent, but the process is still the same. Here is the material and the process to compete this phase.

  • 3/4 inch fiber board cut to fit.
  • Expanding spray foam to fill in the cracks around the edges
  • Some type of decorative or nondescript type cover. Older house like mine were provided with metal covers as pictured

PV-Fan Design and Installation: The fan will drive this low moisture air into the crawl space to further reduce the moisture content. Based on the calculations in my last article, the 8″ fan should exceed the building code requirements. The fan will be scheduled to run 4 hours a night between 1 & 5AM (TIP: With the timer, I can adjust this accordingly to meet the stated moisture content). My RH datalogger showed this to be the timeframe with the lowest amount of humidity in the house during a 24 hour period. The following pictures show my home brew version (aka PV-Fan).

As much as I thought about it, I couldn’t bring myself to cut a hole in the hardwood floors. Many houses with crawl spaces include some form of trap door from the interior of the house. If that is the case, you can remove the existing door and replace it with something similar to mine. I cut a piece of furniture grade plywood to fit, drilled with holes for ventilation. NOTE: You only need holes that equate to the surface area of the size of the  fan. The reason I ended up with holes throughout the new door was based on earlier experimentation with passive air flow to see if that would work without forced air…. it did not. I used cardboard and aluminum tape to seal the holes around the fan.  You can also stain and seal the new door to match the floor.

  1. Gang box and 6′ 3-conductor (W/B/G)  jacketed wire with 3 prong plug.
  2. 24″x 24″ furniture grade plywood with 3/4 holes throughout the surface.
  3. 1 – 8″ boost fan, typically used to boost air flow in existing duct systems, they are available in many sizes. I purchased mine from Smarthome.com. Use the calculation in the previous article to determine the amount of CFM you need for the size of house. This will tell you how large of fan is necessary.
  4. Add a metal gang box to the side of the fan to attach the factory wiring to a plug cord.
  5. Also add a flange (picture 2) allowing a lip to secure to the new vented door.
  6. Mount the fan (flange) with short screws to the new plywood door. The screws must be less than 3/4″ long.
  7. Install a GFI outlet near the crawl space to plug in the fan.
  8. The fan can be run continuously, but I am trying to make the numbers based on a limited run schedule. To do that, I purchased a simple 24 hour timer to plug into the GFI outlet for the fan 

 

In addition to the positive flow fan, the code also states ….a return air pathway to the common area N1102.2.8. Fortunately for me I have a second hatch door in the new part of the house that works out perfectly. My situation is a bit unique so if you are looking at creating a PV-Fan system, look for an inconspicuous location to place a vent equal to or greater than the dimension of your PV-Fan. NOTE: So far, I havent experienced it, but initially, you may find a mildew smell in the house based on the return air. Assuming you sufficiently sealed the crawlspace; this should pass.

I have completed the job and am currently in test mode. I want to “let er cook” for a couple more weeks. So look for the data in the next article.

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3 Responses to Crawl Space – Sealed With Positive Ventilation

  1. b says:

    Nice job, Im sure I missed this part ….so when the “suck-o-fan” is resting does this mean that the natural Aromas from under the house will seep thru the orfis of the still fan into the house ? and when fan is on PUSHING the dryer air under the house where does this air go? or do you have lift off??

    • homeownerbob says:

      Technically, if the crawl space has been completely sealed the aroma should mostly be gone. Granted, it may take some time for it to flush out but over time it becomes part of the conditioned space. Since my wife can smell anything, I am figuring she will tell me as soon as she smells anything new or different. If necessary a presure activated vent could be installed that would seal the return vent while the fan is not running.

  2. […] 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 […]

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