Improper Operation of a Crawlspace

Over the last few weeks, I’ve focused on the sealed crawlspace and what goes into one. This week, I will review why you shouldn’t leave a crawlspace open and what is not a proper solution to a vented crawlspace. The reasons for sealing a crawlspace have already been discussed in the previous post; however, here's a review of what a vented crawlspace will typically have:

1. Higher moisture content in the wood
2. Increased fungal growth
3. Higher relative humidity
4. Overall less energy efficient for the home
5. The need for more pest control
6. Lower indoor air quality in the home

Since most homeowners know about these issues, some are looking for alternatives to the sealed crawlspace. While there are possibly individual solutions to each one of these problems, none of them solve the problem like a sealed crawlspace.

There is one solution - the foundation fan - that I have seen used  time and time again in an attempt to control moisture. However, it not only fails to eliminate the problem, but will actually increase the issues in the crawlspace. There are several variations to this product, ranging from simple humidity-controlled fans that install in the vents to larger, more powerful fans that are run on timers. Most individuals will sell these fans on the premise that the crawlspace needs more ventilation to “dry” the space. This is the concept on which the foundation vent was based: By providing ventilation under a home, you are able to keep it dry. Here in the South, this is not the case, especially since we began installing plumbing and HVAC in the crawlspace. By exposing the crawlspace to the exterior air and having surfaces in the space with lower temperatures, you are just providing an optimum situation for condensation.

Condensation occurs when the temperature of the surface is below the dew point of the air. This is the same thing as dew in the morning. When we have surfaces such as plumbing pipes and ducts that will consistently be below the dew point of the outdoor air, we will get condensation of these surfaces. This condensation can wet the other surfaces and cause even greater problems.

Wood surfaces can also be affected, and since fungal growth can occur with a humidity over 60%, you increase the risk of providing an environment conducive to growth when introducing the outdoor air. This brings up another aspect of introducing outdoor air, because as you lower the temperature, the relative humidity rises. We can all assume that the crawlspace will typically be cooler than the outdoor air in the summer time, so as the outdoor air comes into the crawlspace, you will actually increase the air's relative humidity.

Originally, the idea of a foundation vent fan was to increase the ventilation of the crawlspace; however, all it does is actually increase the moisture content of the space by pulling in more exterior air. Therefore, the fan actually increases the relative humidity and the potential for condensation. Many will argue that the air will help remove the condensation, but as you lower the temperature and raise the humidity, you will actually decrease the amount of moisture the air can even hold. Adding more moisture to it will only increase the dew point temperature. There are several cases I've seen in which a vent fan has been installed to combat a small amount of fungal growth, and actually ended up causing the floor structure to rot!

So, if someone tells you that a sealed crawlspace is a bad idea, and that you should instead install a foundation fan, take the time to consider the negatives.

Reduce your air leakage.

If you have a leaky home, think of it as spilling money through the cracks. Most of your loss is through the ceiling, so start looking there first. Seal all ceiling penetrations you can; these include recessed lights, attic stairs, and even standard lights. Next, look to your walls and floors. Last should be your outlets, as they only account for about 2% of the leakage in your home.

Need help finding the leaks? As a homeowner, you can feel for drafts, use a candle, or have a professional set up a blower door.

What Goes Into Sealing a Crawlspace?

Last week I discussed why to seal a crawlspace, so how about now looking at what goes into properly sealing the space? There are a large number of variations on how a sealed crawlspace is installed; while most just vary by quality, not all of them are proper. A sealed crawlspace is intended to provide 5 things:

1. Moisture management
2. Pest control
3. Combustion safety
4. Fire safety
5. Radon safety

There are a few items that are required to make sure your crawlspace is properly sealed and that you will get all the benefits that are intended:

1. Installation of a vapor barrier with sealed joints on the floor, piers, and foundation walls.
A minimum of a 6-mil vapor barrier should be installed, with all joints sealed via mastic or an approved tape. It is highly recommended that a higher mil plastic be used because the 6 mil will tend to tear and wear easier.

2. Seal all foundation vents.
No vents or openings to the outside should be present, other than an access door. Porch penetrations should also be sealed with an access panel or permanent material.

3. Provide a foundation drain.
A foundation drain is needed with a backflow preventer. It is a good idea to allow the drain to accept moisture from above and below the vapor barrier.

4. Insulate the floor or foundation wall.
Insulating the foundation wall is more efficient, but either the floor or wall is acceptable. Evaluate the situation and choose accordingly.

5. Air seal duct and floor penetrations.
Air sealing the ducts will help improve your home's efficiency, and sealing them and the floor penetrations help prevent air transfer between the home and crawlspace. This also helps with fire safety.

6. Vent appliances to the exterior.
Gas appliances should be direct vented or two-pipe vented in the crawlspace. Most vented spaces do not have enough makeup air for single vent appliances, and a sealed one definitely won’t. Direct venting is recommended.

7. Mechanical Moisture Control
There are several variations on this, including the use of HVAC ducts and dehumidifiers. If an HVAC supply duct is to be used, a backdraft damper is needed and no return ducts. This can affect the efficiency of the system, so check with an HVAC contractor prior to doing it. I tend to recommend a dehumidifier, as it is the least problematic.

Additional Item:
A relative humidity monitor is good addition to help you know that the system is operating properly!

Use CFLs!

This is pretty rudimentary when it comes to energy savings, but there are still a lot of people who are not aren't doing this! Incandescent bulbs convert 90% of their energy to heat and only 10% to light, so they are good heaters but poor light generators. CFLs not only use less energy, but they last longer, too. A 14-watt CFL is equivelent to a 60-watt incandescent. The CFL uses around 75% less energy, and therefore costs 75% less to operate. Also, because the CFL lasts 10x longer, you buy one CFL for every 10 incandescent, which increases your savings. Many will argue that you can't use a CFL everywhere, but they are also becoming more versatile in their uses. So look at your light use and install CFL’s where you can!

To Seal or Not to Seal, That is NOT the Question!

There are still individuals in the construction industry who will question a sealed or conditioned crawlspace, but this argument should be put to rest: Studies have shown significant improvements in a home's efficiency and environment with the installation of a sealed crawlspace! According to a study conducted by Advanced Energy, sealed crawlspaces, when properly installed, will perform better with relative humidity, wood moisture content, and energy efficiency than a traditional vented crawlspace.


The study has shown that during the summer of 2003, the studied sealed crawlspaces only exceeded 70% relative humidity only 5% of the time, while the vented crawlspace stayed above 70% almost all the time. The wood moisture content in the sealed crawlspace stayed below 12% during the study, while the vented crawlspace wood moisture varied greatly. Also, the sealed crawlspaces showed an annual energy use reduction of 15% for sealed crawlspace with insulation on the floors and 18% for sealed crawlspace with insulation on the foundation wall.

Here are some of the main benefits to having a sealed crawlspace:

1. Improved Moisture Control
A sealed crawlspace improves the moisture content of the space by preventing transfer of moisture through three of the main entrance points, the foundation wall, the soil floor, and the foundation vents.

2. Pest Control
By removing moisture from the space, the appeal to subterranean termites is reduced.

3. Improved Efficiency
As already stated from the study, results a sealed crawlspace can improve the efficiency of the home.

4. Improved Indoor Air Quality
A good number of contaminates can come from your crawlspace. By cleaning up the space, you may be able to improve the air quality of your home. A sealed space can reduce the mold content in the crawlspace and can be used or modified to prevent radon intrusion.

5. Improved Maintenance
This is an unintended benefit, but due to the increased visibility and improved environment, homeowners are more likely to “visit” the space, leading to better home maintenance. Also, if the insulation is installed on the walls, the floor structure is visible and individuals are more likely to find problems around plumbing or HVAC.

Increase your ceiling insulation.

Most of your heat loss is through the ceiling of your home; by adding insulation in the attic, you can save energy. Insulation in the attic is relatively easy to install when compared to insulation in other areas, and it can have a greater impact. Here in North Carolina, we are required to have R-30 insulation in the ceilings, but I can see a benefit by increasing this to R-38 or beyond. Be careful, though - there will come a point in which the cost of the additional insulation will not outweigh the benefit of installation!


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