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.

Sealing the Crawl Space

July 3, 2010

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.

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.

Landscape Drainage

May 22, 2010

When it rains it pours. Here in my home state you don’t complain about rain very often as we rarely get enough of it. You can go an entire month without rain, then get  a months worth of rain in a day…..

Proper drainage is generally dealt with when the house is constructed, however changes in the property such as landscaping, pools, concrete surfaces and new structures can create or increase drainage issues.  Many times, proper drainage is the cause, but not recognized as the problem. Look at the following items to see if you have any of these problems. If you do, proper drainage may be the culprit.

Results of Poor Drainage:

  1. Flooding: This one is pretty obvious, but during heavy rains, water seeks low spots and paths of egress. A good heavy rain of  1-3″ of rain in an hour will identify locations of standing water, pooling  and recognizable paths the water  may be following to enter the structure.
  2. Rotting Building Material: House built  prior to 1980 were constructed without any form of treated wood where the framing comes in contact with concrete foundation. Most  issue show up in interior/exterior walls, sill plates and framing structures.  These sill plate can degrade over time due to excessive moisture. Wood rot can occur in wood with a moisture content of  28% or greater which translates to RH of approximately 100%.
  3. Structural Movement: If the sill plate has deteriorated due to water exposure,  it can break down and create structural movement that may result in shifts and cracks sheet rock.
  4. Foundation Movement:  Excessive water under a foundation can cause the piers and beams to move. Extreme movement can cause permanent damage by cracking concrete slabs and beams. Large cracks can even act like a hinge resulting in constant movement.
  5. Moisture Content: High moisture content in the soil under the house (crawl spaces and basements) can cause #6, 7, and 8.
  6. Pest Control: Water and moisture attracts bugs and insects of all types;  the worst of all are termites. By controlling the moisture content in and around the house will positively affect the amount of insect infiltration. The probability of termite infiltration will be high when the moisture content of wood is greater than 20%.
  7. Mold: Much like #5, moisture content will impact this problem. Houses (with crawl spaces and basements) that have a relative humidity of 70% or greater can  have a mold problems. Expect to find mold where the moisture content of wood structure to be at 20% or greater.
  8. Air Quality: Associated with #5, 6, and 7 will impact this item. Unpleasant, musty or certain health conditions could be due to the presence of mold, animals or  animal feces  can negatively affect the quality of air in the conditioned space of house envelope.

Top Ten List of Construction tasks to encourage proper drainage courtesy of Clemson University.

Correcting Drainage Problems: For the water that reaches the structure surface, it will need a natural or man-made method of  moving water away from the building. The severity of the problem will determine how much drainage work is required.  The best way to deal with it is to address the obvious and monitor the results. If you continue to have the problem, take the next counter-measure to attack the problem. Look at the following solutions to determine which solution best matches your problem.

  1. Surface landscape should have at least a 4 degree grade away from the structure:  This will encourage water to move away from the house not toward it. This may be accomplished by regrading the surface material or possibly adding surface material to create the grade. Do you best to avoid the 6″ restriction mentioned in #2.
  2. Sill plates: Should be approximately 6″ vertically from the exterior surface grade. Besides the obvious, keeping dirt away from the sill plate will reduce the opportunity for bugs and insects to enter the house through cracks, crevices and weep holes.
  3. Crawl Space Height: In a perfect world, the height of the crawl space should be above the exterior surface grade. Adding material in the crawl space can be difficult after the house is built. If possible, dome the grade of the material so it is higher in the middle and lower around the edges. If  you choose to add this material ensure you retain a 24″ clearance for the crawl space. Note: Dont view this item as a requirement as it may be very difficult to complete without negatively effecting other items.
  4. Paths and Driveways: Adding these surface as concrete or impervious surfaces will add more water to the problem. If you are considering adding additional paths or driveways, consider using natural porous materials such as gravel, crushed stone or granite.   What ever you do, ensure you retain the same 4% grade away from the structure mentioned in #1
  5. Roof Gutters: Ugly as they may be, they serve a purpose. During a heavy rain fall the surface area of the roof takes on a lot of water. Naturally it will run down the roof to the edge. Without gutters this water and the lack of a proper grade can cause excessive water to pool under the house.  Assuming you have the natural grade mentioned in #1, much of the surface water will flow away from the house in a path of least resistance. Hopefully its path takes it to the street, alley or drainage ditch. Additionally, the gutter downspout should extend away from the house at least 10′  with a fall of 6″ over the 10′ distance. This will ensure the water moves away from the structure.
  6. Roof Gutters combined with sub surface drain pipes: To reduce the ugliness of the roof gutter downspout, adding sub-surface drains can help. Granted, this can be expensive as these pipes must be ditched into the ground and run far enough away from the house for the water to naturally drain.
  7. Dry Creeks: This may be a stone or rock drainage ditch that is normally dry until the rain water needs a place to go. Creating a Dry Creek to channel the water will assist in moving the water around or away from the residential structure. Read more about how to build a dry creek.
  8. French Drains: Similar to sub-surface drains, but the pipes are normally perforated. These 4″ pipes pick up the water that is seeping into the ground; like the sub surface drains, these pipes must be ditched away from the house with a fall  and run far enough away to allow the water to drain naturally. TIP: It is not unusual to have both sub-surface and french drains, but they should not be mixed; in other words do not combine them into just one pipe system. This item is easily a topic in its self. Read more about how to build a french drain for more details.
  9. Sump pump: This may be considered drastic, but if you have a serious drainage problem and you find that you continue to have large amounts of  water pooling; creating a sump under the house and collecting the water in a pit then pumping it outside the structure and away from the house.
  10. Vapor Barrier: More moisture control then drainage, but they go hand in hand.  With most crawl spaces, the floor is dirt. This is very normal and in the early days of construction, houses were built to allow the flow of air to pass through the house crawl space. However, as we spend more money and time to insulate and tighten up the house we can create a worse problem in the crawl space. The best way to deal with the dirt floor and the moisture associated with it is to cover it with plastic (aka: moisture barrier). By sealing the dirt floor from the crawl space you will dramatically reduce the moisture content that is transmitted through the dirt. Look for a future article on this topic as I am currently completing this project myself.
  11. Dehumidifer:  Besides the moisture barrier mentioned, you may also consider using a dehumidifier in the basement or crawl space to further reduce the humidity factor. This is a common solution; some new home builders even require (as part of the home warranty) the homeowner adds a dehumidifier in a basement to keep the warranty intact.
  12. Rain Water Collection: This is an alternative method of dealing with drainage issue. In concept, instead of pushing the water away from the house, the water is collected in cisterns, barrels or tanks using the gutters and drains. The water can be treated or used to water the landscape when its dry. Look for future articles on this subject.

Proper drainage can be a difficult nut to crack.  For me, having the correct exterior grade, crawl space height and sill plate clearance has been a struggle if not impossible and I am still working to find the complete solution that normalizes the condition. Look at all these items as a “system solution” and even though you may not be able to fix everything perfectly, the collective impact of solutions can address the problems associated with  poor drainage.