Crawl space encapsulation and crawl space conditioning are relatively new technologies used to treat chronic humidity, mold and unhealthy conditions in crawl spaces. The process completely isolates the crawl space from the ground and outside air by lining the structure with a high-performance vapor barrier, specifically designed for this purpose and installing either a dehumidifier or a crawl space conditioning system.
The objective is to include the crawl space area as a part of the internal envelope of the building.
Dirt and Vented Crawl Spaces: A Housing Epidemic
Crawl spaces, as the name suggests, are low-height foundations within which one cannot stand up. Built as an alternative to conventional basements and concrete slab type foundations, crawl spaces are normally used to allow easy and convenient access to pipes and air ducts and often to house utilities and combustion appliances as well. Unsuitable as living space, they might sometimes be used for storage once encapsulated.
However, because of the way they are built, crawl spaces are extremely prone to moisture and moisture-related problems, and their low height and overall conditions make them hard to inspect and maintain. The results of this lack of proper maintenance can be disastrous, especially for homes with dirt crawl spaces.
Differences in temperature between the crawl space and the surrounding areas, cause moisture present in the air or rising from the dirt floor to condensate in every cold surface in the crawl space. As a result, organic surfaces within the crawl space will develop mold, decay, and rot, sometimes to the point of compromising the home’s structural integrity.
The moisture, warmth, and decay will attract termites, carpenter ants, dust mites and vermin and cause additional problems. Wildlife, such as snakes, raccoons and feral cats are know to seek shelter in vented crawl spaces, and with them the risk of rabies, parasites and diseases. Some animals, die and decompose inside the crawl space, and the stench will plague the home, sometimes for months.
Crawl Space Venting
The concept that generated many building code specifications concerning vented crawl spaces seemed sound enough to combat crawl space moisture. Vents were installed to let air circulate and dry it up. After all, air is known to dry things. We open windows to let the carpet dry after shampooing, and wind make the clothes on a line dry as well.
So why wouldn’t this work in the crawl space?
First of all, the air will not circulate within a crawl space the same way it would in the upper floor or outside.
When put to the test, the building code specifications, many of which still in practice in some areas of the US, made no sense from a scientific standpoint. Studies concluded that the assumptions concerning the relationship between size, number and positioning of crawl space vents and the air exchange rate within the space, which are the basis for the code specification, are incorrect.
Furthermore, as specialists began to look at the issue more closely, they found that there was not a single shred of scientific evidence supporting such recommendations when they were first enacted either.
In other words, the specifications concerning the size, number and positioning of crawl space vents are, at their best, pure guess work.
What was never considered when those building codes were formulated is that crawl spaces are very peculiar structures with equally peculiar thermal characteristics. The temperature in the crawl space is always different from the temperature of its surroundings. Bigger, larger, and more numerous vents will not improve the air exchange rate. Likewise, changes in the temperature and humidity in the outside air will impact the crawl space differently than they would any other area of the building.
Crawl Space Vents in Hot Summer Days.
Now let’s examine the effect of venting the crawl space in a hot summer day. Let’s say it’s 85 degrees with 75% humidity. That air is being allowed into the space by the vents. The temperature in the crawl space is 66 degrees, however the surfaces within the crawl space (walls, dirt floor and floor joists) is a bit lower: 62 degrees.
For every one degree that the air is cooled, the relative humidity is increased 2.2%, as cool air holds less water than the warm air.
The temperature difference between the warm air coming through the vents (84F), and the crawl space surfaces (62F) is 22 degrees. Multiply that by 2.2%, which is the rate in which relative humidity in the air increases for each degree that it cools down and we have an instant 48.4% increase in relative humidity for the air in the crawl space.
However, the humidity in the air coming in was already 75%. Add to that another 48.4% and the relative humidity in the crawl space will be of 123.4%.
The air can only hold 100% humidity before it starts to give up its moisture, which means condensation on cool surfaces. The wooden structures will absorb that water, and moist wood favors mold growth and rot, sometimes compromising the structural integrity of the floor joists and subfloors.
Even in cooler spring and fall days, when the relative humidity is not as high, if the relative humidity in the crawl space goes above 60%, there is still the risk of mold growth and decay.
Crawl Space Vents in Cold Winter Days
In many regions homeowners close the crawl space vents during summer and leave them open during winter so that the crawl space will dry out.
After all, by bringing 35°F air with 60% RH into a crawl space that is around 60 degrees, theoretically the relative humidity in the crawl space goes down.With this dry air, the crawl space can then dry out.
However what seems like a good idea can be disastrous for the crawl space and your home. First of all, the wood that is rotten and flaky will remain so. The mold, which lays dormant when the RH is at or below 60%, will resume its growth and go on damaging the structures as soon as the temperature rises.
In addition, keep in mind that very cold air is brought in, which will make its way into the house through openings in the floor, thanks to a physics phenomenon called the Stack Effect.
Several studies conducted by reputable organizations such as Advanced Energy, Habitat for Humanity, and Building Science Corp. on the impact of vented crawl spaces on a building’s overall energy efficiency, concluded that encapsulated, sealed crawl spaces can make a home in average 18% more energy efficient. In homes in which the air ducts run through the crawl space, energy losses can be as high as 36%.
The Stack Effect
Stack effect refers to a common physics phenomena that explains why everything happening in the crawl space has a direct impact on your home’s indoor air quality and energy efficiency.
As heated air rises inside a building, it exits the structure from any openings in uppermost levels: attic vents, windows, and wall and ceiling gaps. As that happens, the indoor air supply is consistently being replenished with air sucked in from the lowest levels of the building: the basement and crawl space. Therefore, at least one third of the air breathed in a home is coming from the home’s crawl space or basement.
If the crawl space is moldy and rotten, mold spores and dust mite waste pellets are being consistently sucked in with that air. Mold spores and dust mite pellets are the two main indoor allergens, known to cause mild to severe upper respiratory allergy symptoms in sensitive people, the elderly, and small children.
Conditioning the Crawl Space
To date, one of the most effective ways to treat and prevent crawl space moisture problems is a process called Conditioning or Encapsulation.
This process includes:
The objective is to include the crawl space area as a part of the internal envelope of the building.
Dirt and Vented Crawl Spaces: A Housing Epidemic
Crawl spaces, as the name suggests, are low-height foundations within which one cannot stand up. Built as an alternative to conventional basements and concrete slab type foundations, crawl spaces are normally used to allow easy and convenient access to pipes and air ducts and often to house utilities and combustion appliances as well. Unsuitable as living space, they might sometimes be used for storage once encapsulated.
However, because of the way they are built, crawl spaces are extremely prone to moisture and moisture-related problems, and their low height and overall conditions make them hard to inspect and maintain. The results of this lack of proper maintenance can be disastrous, especially for homes with dirt crawl spaces.
Differences in temperature between the crawl space and the surrounding areas, cause moisture present in the air or rising from the dirt floor to condensate in every cold surface in the crawl space. As a result, organic surfaces within the crawl space will develop mold, decay, and rot, sometimes to the point of compromising the home’s structural integrity.
The moisture, warmth, and decay will attract termites, carpenter ants, dust mites and vermin and cause additional problems. Wildlife, such as snakes, raccoons and feral cats are know to seek shelter in vented crawl spaces, and with them the risk of rabies, parasites and diseases. Some animals, die and decompose inside the crawl space, and the stench will plague the home, sometimes for months.
Crawl Space Venting
The concept that generated many building code specifications concerning vented crawl spaces seemed sound enough to combat crawl space moisture. Vents were installed to let air circulate and dry it up. After all, air is known to dry things. We open windows to let the carpet dry after shampooing, and wind make the clothes on a line dry as well.
So why wouldn’t this work in the crawl space?
First of all, the air will not circulate within a crawl space the same way it would in the upper floor or outside.
When put to the test, the building code specifications, many of which still in practice in some areas of the US, made no sense from a scientific standpoint. Studies concluded that the assumptions concerning the relationship between size, number and positioning of crawl space vents and the air exchange rate within the space, which are the basis for the code specification, are incorrect.
Furthermore, as specialists began to look at the issue more closely, they found that there was not a single shred of scientific evidence supporting such recommendations when they were first enacted either.
In other words, the specifications concerning the size, number and positioning of crawl space vents are, at their best, pure guess work.
What was never considered when those building codes were formulated is that crawl spaces are very peculiar structures with equally peculiar thermal characteristics. The temperature in the crawl space is always different from the temperature of its surroundings. Bigger, larger, and more numerous vents will not improve the air exchange rate. Likewise, changes in the temperature and humidity in the outside air will impact the crawl space differently than they would any other area of the building.
Crawl Space Vents in Hot Summer Days.
Now let’s examine the effect of venting the crawl space in a hot summer day. Let’s say it’s 85 degrees with 75% humidity. That air is being allowed into the space by the vents. The temperature in the crawl space is 66 degrees, however the surfaces within the crawl space (walls, dirt floor and floor joists) is a bit lower: 62 degrees.
For every one degree that the air is cooled, the relative humidity is increased 2.2%, as cool air holds less water than the warm air.
The temperature difference between the warm air coming through the vents (84F), and the crawl space surfaces (62F) is 22 degrees. Multiply that by 2.2%, which is the rate in which relative humidity in the air increases for each degree that it cools down and we have an instant 48.4% increase in relative humidity for the air in the crawl space.
However, the humidity in the air coming in was already 75%. Add to that another 48.4% and the relative humidity in the crawl space will be of 123.4%.
The air can only hold 100% humidity before it starts to give up its moisture, which means condensation on cool surfaces. The wooden structures will absorb that water, and moist wood favors mold growth and rot, sometimes compromising the structural integrity of the floor joists and subfloors.
Even in cooler spring and fall days, when the relative humidity is not as high, if the relative humidity in the crawl space goes above 60%, there is still the risk of mold growth and decay.
Crawl Space Vents in Cold Winter Days
In many regions homeowners close the crawl space vents during summer and leave them open during winter so that the crawl space will dry out.
After all, by bringing 35°F air with 60% RH into a crawl space that is around 60 degrees, theoretically the relative humidity in the crawl space goes down.With this dry air, the crawl space can then dry out.
However what seems like a good idea can be disastrous for the crawl space and your home. First of all, the wood that is rotten and flaky will remain so. The mold, which lays dormant when the RH is at or below 60%, will resume its growth and go on damaging the structures as soon as the temperature rises.
In addition, keep in mind that very cold air is brought in, which will make its way into the house through openings in the floor, thanks to a physics phenomenon called the Stack Effect.
Several studies conducted by reputable organizations such as Advanced Energy, Habitat for Humanity, and Building Science Corp. on the impact of vented crawl spaces on a building’s overall energy efficiency, concluded that encapsulated, sealed crawl spaces can make a home in average 18% more energy efficient. In homes in which the air ducts run through the crawl space, energy losses can be as high as 36%.
The Stack Effect
Stack effect refers to a common physics phenomena that explains why everything happening in the crawl space has a direct impact on your home’s indoor air quality and energy efficiency.
As heated air rises inside a building, it exits the structure from any openings in uppermost levels: attic vents, windows, and wall and ceiling gaps. As that happens, the indoor air supply is consistently being replenished with air sucked in from the lowest levels of the building: the basement and crawl space. Therefore, at least one third of the air breathed in a home is coming from the home’s crawl space or basement.
If the crawl space is moldy and rotten, mold spores and dust mite waste pellets are being consistently sucked in with that air. Mold spores and dust mite pellets are the two main indoor allergens, known to cause mild to severe upper respiratory allergy symptoms in sensitive people, the elderly, and small children.
Conditioning the Crawl Space
To date, one of the most effective ways to treat and prevent crawl space moisture problems is a process called Conditioning or Encapsulation.
This process includes:
- Proper grading and drainage of the dirt to divert rain water away from the space. In some cases, a sump pump is included to help collect moisture from condensation, eventual plumbing leaks, and heavy rains.
- Completely isolating the crawl space from both ground moisture and outside air by lining the whole crawl space with a thick and sturdy vapor barrier, preferably a puncture resistant, multi-layered 20 mil poly sheet (some contractors will use 4 mil 6 mil and 10 mil generic poly sheets, but they are known to rip easily). The liner is then secured to the walls and structures by mechanic fasteners. Mastic tape will seal the seams between the sheets and a special caulk will seal and the openings in the walls and structures. Professionally installed systems will include a features such strips and caps that will allow monitoring of the structure for termites and carpenter ants.
- Conditioning the crawl space by using a crawl space conditioner system, that basically blows air from the floor above into the crawl space, keeping it dry, or a crawl space dehumidifier.
Incorrect treatments for crawl spaces
References:
Crawl Spaces Learning Center
Dirt Crawl Spaces Information Center
Advanced Energy Crawl Spaces Website.
Building Science Corp - RR-0401: Conditioned Crawl Space Construction, Performance and Codes
Building Science Corp – RR-0509d: Crawlspaces
RLC Engineering - Crawl Space Fallacies
Builder Online – Closing the Crawl
Washington State University Energy Program - Crawl Spaces: Energy, Moisture &
Soil Gas Impacts
Crawlspace Insulation. Office of Building Technology, State and Community Programs (BTS) Technology Fact Sheet.
Crawl Space Insulation
Section R202 Definitions: Closed Crawl Spaces
US Department of Energy- EERE - Crawl Space Insulation
US Department of Energy- EERE - Best Practices: Consider the Crawl Space.
US Department of Agriculture – Forest Services – Crawl Space Ventilation
US Department of Energy – Building Energy Codes Resource Center - Does the Energy Code Allow an Unvented Crawlspace?
US Department of Energy – Building Energy Codes Resource Center – Crawlspace Insulation
- Installing vapor barriers or rigid foam insulation boards on the ceiling of a crawl space.
- Closing the crawl space vents and leaving the ground uncovered
- Sealing the ground with a vapor barrier and leaving the vents open.
- Using fans to blow outside air into the craw space
- Using fans to blow air out of the crawl space, when the make up air supply is coming from the outside.
- Installing a liner or a concrete floor without removing all the organic debris and insulation from the crawl space floor first.
References:
Crawl Spaces Learning Center
Dirt Crawl Spaces Information Center
Advanced Energy Crawl Spaces Website.
Building Science Corp - RR-0401: Conditioned Crawl Space Construction, Performance and Codes
Building Science Corp – RR-0509d: Crawlspaces
RLC Engineering - Crawl Space Fallacies
Builder Online – Closing the Crawl
Washington State University Energy Program - Crawl Spaces: Energy, Moisture &
Soil Gas Impacts
Crawlspace Insulation. Office of Building Technology, State and Community Programs (BTS) Technology Fact Sheet.
Crawl Space Insulation
Section R202 Definitions: Closed Crawl Spaces
US Department of Energy- EERE - Crawl Space Insulation
US Department of Energy- EERE - Best Practices: Consider the Crawl Space.
US Department of Agriculture – Forest Services – Crawl Space Ventilation
US Department of Energy – Building Energy Codes Resource Center - Does the Energy Code Allow an Unvented Crawlspace?
US Department of Energy – Building Energy Codes Resource Center – Crawlspace Insulation
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