Crawl Space – Sealed With Positive Ventilation

July 24, 2010

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 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.

Landscape Drainage – Inspection and Maintenance

June 4, 2010

The last article I wrote on this subject was about the result of poor drainage and the impact on your property. Assuming you have reasonably good drainage, it is still important to inspect and maintain all the elements associated with the drainage system for proper performance.  This article assumes you have some or most of the solutions mentioned.  If you have some of these elements and still have drainage issues, and you havent read Landscape Drainage, take a look to see how you can improve the situation. 

  1. Gutters: As a late winter or spring reminder, gutters must be cleaned of tree leaves and debris. The water that reaches the gutter must flow freely to exit the down spout. Allowing the gutters to accumulate debris will not only clog the gutters and the flow of water, but also clog the downspout. Allowing the leaves to breakdown in the gutters will cause the material to turn in to dirt. The accumulation of this dirt and new leaves  will cause water to back up. All of this adds up to a lot of weight, which in turn can cause the gutters to fail. If you leave the condition for an extended period of time, the weight and deterioration will cause the gutters to pull away from the house and fall to the ground….It really happens.
  2. Gutters with concealed drains: With the accumulation mentioned in item 1, some of this same material can eventually clog up the buried pipe. The clogged pipe will cause the water to back up till it finds a place to get out. This is usually at the transition fitting between the downspout and the underground drain. This exit of water will virtually defeat the purpose of the extended drain. If the concealed drain is clogged up; using a Drain Snake or power drain auger and a lot of water will be the solution to completely flush the pipe.
  3. Surface grade of Landscape: As mentioned in “Landscape Drainage”, maintaining the slope away from the house is important. Keep debris and buildup to a minimum. Rake and grade the landscape material to ensure the proper grade (4 degrees) and fall from the structure.
  4. Keep landscape material away from sill plate: Keep the landscape material build up to a minimum as not to accumulate to the point that it is level-to or above the sill. As mentioned in #3, rake or grade the landscape material away from the structure.
  5. Surface Drains: Depending on the system design, you may have drainage grate(s) to receive water runoff as well as an exit drain(s). It is critical to keep these opening free and clear of debris. Most of these drains work purely on slope, gravity and volume. If the exit drain is covered with grass or obstructed to impede the flow of water, it will easily reduce or stop the water flow away from the structure.
  6. Dry Creeks: Even though dry creeks can be designed to be attractive, it is important they retain the design slope and path to remain clear of any obstructions that might block, back up or divert water in a way that defy the design intent. Keep the creek clear of weeds and landscape refuge. If necessary, flush water through the creek to ensure proper flow.
  7. Sump Pump: Sump pumps can be located in basements, crawl spaces as well as pits associated with surface drains. Test them by flushing water through them to make sure they activate and pump the water out of the pit and off the property as designed.  Inspect the exit drain for clear free flow.
  8. Moisture Barrier: Once a year, crawl through the space looking for voids and rips in the material.  Look for rodent penetrations and any damage they may have caused. Tape and reseal any breaches in the barrier. Keeping this material intact and resistant to moisture is the key element.
  9. De-humidifier: Any de-humidifier placed in the crawl space should include an exit drain pipe just like the sump pump. Test the de-humidifiers ability to remove the water through the exit drain in the same way as the sump pump. There may be filters to be cleaned. Refer to the owner’s manual for any specific maintenance.