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 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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Crawl Spaces; Basements Without Benefits

June 12, 2010

I never saw a residential basement until I was about 12. I always wanted one as a kid, young adult as well as an old guy. The true man cave!! Dark dank musty, cob webs…. the whole thing.  Unfortunately for me, basements are a phenomena found mostly in the colder regions of the US and I don’t see me moving there anytime soon.   This article is  for all of us that have crawl spaces (not basement) under our houses, as the DIY mainstream media rarely addresses the subject adequately.

A Crawlspace is a Basement Without Benefits: Crawlspaces provide the negatives of a basement without providing the benefit of usable space for water heaters, HVAC’s shop space and storage. If you live in the South, Southeast, some parts of the Southwest and your house was built before 1975, there is a reasonable probability your house may have a crawl space. Even though crawl space construction continues today, slab type foundations are less expensive to construct and are used more often in these same regions.

Similar to a basement, crawlspace construction allows access to the vital organs of the home including plumbing and some electrical services. The code has changed over the years requiring a minimum height, but older structures can have crawl spaces with limited working room. NOTE: Some tradesmen may charge extra fees for working in the crawl spaces. If you are having major work in your house that includes re-working the floor, consider allowing them to open the floor for better access. This may save you some money.

Vented Crawl Spaces: Since about 1950 ventilating crawl spaces have been mandated by most building codes. For the most part we were told to open our vented crawl spaces in the summer and close them in the winter. In their inception of early construction,  these houses were built with little to no insulation and in many cases (prior to 1965) were built without air conditioning. Letting the house breath or move air was a good way to keep the overall temperature of the interior space to a tolerable level (during the summer) as well as a way to remove the moisture from the crawl space floor. All this was fine until we started insulating the floors, caulking every crack, adding air conditioning and pretty much sealing the house to the point that little if any air flow is occurring.   By trapping this air/humidity under the house we have caused problems including  rodent/insect infiltration, mold and wood rot. Vented crawl spaces work well in dry climates with low humidity.

Even though most building codes continue to support  “vented” crawl spaces,  there is some compelling data that may cause you to consider sealing your crawl space. But be warned, if you choose to (properly) seal your crawl space when you go to sell the property, a home inspector may dispute the decision. NOTE: If you have any appliance (floor furnaces, gas heater, etc) installed in the crawl space; they require input air, so sealing the crawl space is not recommended in those applications.

Here are just a few of the reports and white papers that have been written on the subject:

Non-Vented Crawl Spaces: If your crawl space is not vented and was properly sealed during construction and you have no moisture, drainage or mold/mildew problems you are in good shape. However, based on the numerous studies; the South, Southeastern and some parts of the Southwestern regions of the United States appear to be good candidates for non-vented crawl spaces. These areas typically have high humidity most of the year, combined with high heat in the summer. Here are some reasons to consider sealing the crawl space:

  1. Reduce humidity and moisture  in the crawl space
  2. Control mold and mildew
  3. Reduce exposure to rodents and other 4 legged creatures
  4. Reduce drafts
  5. Impact the floor temperature in the winter by 3-5 degrees
  6. Reduce heating and cooling demands

Candidates for sealed Crawl Spaces: As a general rule, if it’s above 75F outside, there is more moisture in the outside air than in the 70 to 75F crawlspace air; so, if you ventilate the crawlspace, you’re bringing more moisture into the crawlspace than you’re removing. If you drop the air temperature by 20 degrees, the relative humidity (RH) approximately doubles. When it gets to 95F outside and that air is brought into the 75F crawlspace, the RH doubles. If it’s 60% RH outside, the relative humidity in the crawlspace is at the dew point. At dew point conditions, water vapor turns back to liquid. Kevin O’Neill of HVAC at Carolina Cooling & Plumbing, Inc.

 In laymen terms; the high humidity found in the ambient air outside the structure will naturally seek places of lower or less humidity to normalize or equalize the humidity. When this occurs underneath a house,  the crawl space acts like a wick and brings more humidity in the space. To make maters worse, if you have added insulation to create a barrier between the crawl space and living area, you are creating a stagnant nest for humid air to reside.

 After much studying and consideration, I have decided to seal my crawl space. Look for my future article on creating a moisture barrier and sealing up the space.