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.

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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.


Re Insulating HVAC Ducts

December 1, 2009

The US EPA reports leaking ducts reduce overall HVAC efficiency by 20% accounting for a loss of over $140 per year due to these leaks.

 As mentioned in earlier articles, HVAC ductwork has not received the attention it deserves. Take a look at  my HVAC Duct article to evaluate and inspect your existing duct work. If you have determined that it is in need of attention you have three choices, 1) do nothing, 2) have them replaced or 3) re-insulate them.

  Duct Replacement: Be prepared, HVAC contractors typically want to sell new systems.  Here is the SW part of the US, their big season is the summer, having this done during the cooler months may be a better choice. Duct replacement is not as profitable as system replacements, but keeping their technicians busy can be better than not working them at all.

Considerations when discussing this issue with a HVAC professional:

  1. Flex ducting is a widely used proven product but does not have the long-term performance rating found with a typical sheet metal product. But is widely used in the residential market and most professionals like to use it because it is easy to install and not as labor intensive as rigid duct work. This product  can easily be mis-installed by creating kinks and  sharp bends that can reduce the product performance. Read this flexible duct inspection method before you meet with the contractor to better understand the product and how it is applied.
  2. After an evaluation, most HVAC specialists will want to replace the ducting in lieu of  repairing or re-insulating. Why? It’s quicker, cheaper and it becomes a known value. This is not a bad thing, these folks are trying to make a living and attacking the problem by replacing everything may be an economical solution for both of you.
  3. If you allow a professional to replace the ducts, make sure to ask some of these questions. Will all the joints and edges be sealed with (paint on) mastic? What is the R value of the new duct? What is the life expectancy of the duct product? What is the product warranty? What preventative measures are followed to avoid kinking (of the ducts). Will the system be tested for leaks after completion?
  4. If the professional didnt mention it,  also consider having all the output and return registers (this is the box in the wall/ceiling where the vent cover is attached) replaced or re-insulated. Most new registers are sealed and insulated to insure a tight fit. Also, ask them to use spray foam insulation between the register protrusion and the sheet rock, this will further seal the interior  from the attic space. Most HVAC professionals do not address register penetration into the interior space.
  5. If you have any rooms that never seem to adequately heat or cool, make sure and mention this to the professional too as he may have to resize or reroute the new duct work to better balance the system.

Duct Re-insulation (for rigid ductwork): This is a great DIY project if you consider yourself cheap labor and you dont mind working in the attic. Consider this a mult-weekend project. Before you start, ask yourself these questions. 

  1. Does your system work reasonably well? If you have any rooms or areas in the house that never properly heat or cool, consider adding an additional output duct as this would be the time to address it. Consider using a professional to perform this work as you may need to rebuild some of the system to retain the system balance.
  2. Is your attic adequately insulated? If not, plan to do it, but after all you other attic work is complete.

 The process if fairly straight forward and here is an outline of the necessary work.

  1. Strip the existing insulation material
  2. Ensure all joints are secure and snapped and screwed together
  3. Use duct mastic to seal ALL seams and joints
  4. Use HVAC tape to complete any seals not treatable with mastic
  5. Use expanding foam insulation to fill any cracks between the duct registers and the sheet rock
  6. Use duct wrap to re-cover all the exposed duct, seal with duct tape and mastic

Material Required for the Job

  1. Duct wrap rated at an R value based on your region (see table below)
  2. 1/4,1/2, and 3/4 self tapping sheet metal screws
  3. HVAC Duct Mastic
  4.  Take adequate precautions while working in the attic. Avoid stepping directly on the sheet rock ceiling and wear protective clothing and dust masks.
  5. If a professional indicates the duct are under/oversized, you may ask them to provide  the Man L or Man J duct analysis supporting their position. (This is an engineering schedule that is used to properly size ducting.)