The best crawl space in Nashville, TN:

1. Keeps the relative humidity 50%-60% at all times
2. Maintains a consistent, moderate temperature
3. Prevents earth gasses and odors from entering
4. Prevents mold growth
5. Prevents condensation
6. Manages or prevents water intrusion
7. Prevents rodent and insect intrusion
8. Is easily accessible
9. Is easily monitored
10. Inspected regularly to check for plumbing leaks and other mechanical problems, structural problems, wood rot, and termites. 

The most reliable way to ensure crawl spaces perform optimally is to build what is known as a crawl space encapsulation or sealed, unvented, conditioned crawl space. By definition, crawl space encapsulations are closed, unvented crawl spaces. They include

• A water management system, if necessary, 
• A high-quality vapor barrier fastened to the crawl space walls and piers above the outside soil grade level with an airtight seal, 
• Seams that are air and water-tight, 
• No air and water infiltration gaps, 
• Negative air pressure created below the vapor to exhaust earth gasses and water vapor before entering the crawl space, and 
• A dehumidifier that controls relative humidity. 

This guide will also include some DIY encapsulation tips. 

How Much Does a Crawl Space Encapsulation Cost?

The average crawl space encapsulation can cost anywhere from $5,000 to $15,000 or more depending on the size, difficulty of work conditions, water management system, mold, and several other factors. No two crawl spaces are the same, so you should request a site visit and quote to get the actual price for your situation. Many homeowners might think professional crawl space encapsulation companies are just trying to sell everything on their wagons. It’s important to understand that every crawl space problem has a corresponding solution. Still, you don’t need to “over-fix” it. If the proposal is more money than your current budget, consider breaking up the recommendations into phases to make it easier on your budget. Financing could also help. Of course, DIY encapsulation is the cheapest alternative.

The Pros and Cons of Crawl Space Encapsulation

Why is my house built on a crawl space? Wouldn’t a slab or basement system be better? Crawl space construction can be the most economical way to build a house or building, depending on the weather and site conditions. To make it perform more reliably, it should include a conditioned crawl space encapsulation. It is easier and cheaper to install and repair the plumbing, electrical wiring, framing, heating, and cooling systems in crawl spaces.

So why build on slabs or basement systems?

A slab foundation is good when soil conditions are favorable and you don’t want to step up to the house or building. What is the downside to a slab? With the plumbing or electrical wiring installed in the slab, you could be jackhammering the slab to make repairs. Plus, surface and groundwater could more easily enter the interior.

Basement systems give you more usable space in the foundation. Still, it is also a hole in the ground that water wants to fill, so waterproofing the foundation walls and floors must be designed to last the lifetime of the house. 

What is the Warranty on a Crawl Space Encapsulation?

A properly installed and maintained crawl space encapsulation should last the lifetime of your house. Warranties from reputable crawl space specialists range from 10 to 25 years to a lifetime transferable guarantee, usually depending upon an annual service agreement. 

There are usually exclusions for the equipment, like a dehumidifier, sump pump, fan, and condensate pump, which carry the manufacturer’s warranty. More expensive service agreements, which include equipment replacement, are just amortizing the equipment replacement costs over time. Additional exclusions include catastrophic events like 

• Flood 
• Fire, 
• Damage from animal infiltration or other workers, and 
• Consequential damage. 

The plastic vapor barrier, sealants, and adhesives used in a quality crawl space encapsulation are not biodegradable. So what is not suitable for the landfill is good for a permanent solution in your house.

How to Insulate a Crawl Space

Crawl space insulation is controversial. We don’t speak for other climate zones, but in Tennessee, we have seen crawl space insulation cause more problems than it claims to help. With open crawl space vents, subfloor fiberglass batt insulation keeps the floors warmer in the winter. Still, in the summer months, the humid air from outside enters the crawl space through the open crawl space vents and is trapped by the subfloor insulation. The wood subfloor absorbs moisture to the level that supports mold growth. 

On the other hand, an encapsulated crawl space that is appropriately air-sealed and creates conditioned air with a dehumidifier needs no insulation in the floor, ceiling, or walls. The temperature inside the crawl fluctuates from around 60 in the winter to approximately 75 in the summer. Crawl space encapsulation creates a cave-like temperature range. 

Crawl space insulation isn’t harmful in a conditioned crawl and doesn’t make anything worse. You will get a much better return on your investment by spending that money on adding proper insulation to the attic.

How to Stop Crawl Space Air Infiltration

An encapsulated crawl space includes sealing all possible openings to the outside air. 

The most logical place to start when creating an unvented crawl space is with open vents. But just closing them doesn’t make a tight enough seal. Unvented crawl spaces should be airtight with spray foam, foam board, caulk, or any other material that serves to seal. 

Another source of air infiltration is where the HVAC ducts go from the HVAC equipment outside through the foundation wall into the crawl space. The same sealing techniques are applied here; the band joist or rim joist can be another source to seal.

For a final check, if the temperature outside is different than the crawl space temperature, use a thermal imaging camera to find sources of air infiltration. A “low-tech” way to look for gaps is to close the crawl space door so you are sitting in the dark. Crawl around with a flashlight and turn it off to look for incoming light.

The Best Crawl Space Door

The purpose of a crawl space door is to provide access while also sealing against air infiltration. The standard metal access doors installed on most houses are marginally effective when new. They are ineffective when falling apart, rusted, bent, and closed with a block or stick leaning against them. Wooden site-built doors are okay if they include weather stripping but are not acceptable when the hinges fall off due to wood rot. 

Crawl space doors cannot provide waterproofing when they are below the yard’s grade because they need to be as tight as a submarine door, which is probably out of your budget. So a pit is usually dug around the door opening in the yard. Install a block wall above the yard’s grade around the perimeter to hold the dirt back. Put gravel in the bottom to make it less muddy. Cover the pit with a board or simple lean-to to keep surface water from running in and rainwater from filling the hole. A fabricated crawl space entrance product that does that is called a Turtl.

How to Install a Crawl Space Earth and Radon Gas System

Installing a perforated pipe under the vapor barrier, connected to a radon fan, has several benefits. 

1. Any radon coming up through the ground is collected and pumped out. 
2. The system Pumps out soil moisture, reducing the work for the dehumidifier. 
3. Negative air pressure under the vapor barrier pulls the makeup air from the crawl space, which in turn pulls conditioned air as its makeup air from the house. These create the healthiest indoor air quality for your home and crawl space.

Installation is simple. Before laying down the vapor barrier, put some corrugated and perforated pipe around the perimeter. This pipe will create the pathway for installing the radon and earth gas system. Now you can connect the perforated pipe to one that is non-perforated, which connects to a radon fan outside, which exhausts it from the crawl space. Sealing any gaps around the pipe penetration is next. Install any necessary electrical wiring and turn it on to run continuously.

Additional Crawl Space Encapsulation Options

Just like buying a car, a crawl space encapsulation has options for your convenience.

• Lighting

Installing motion-activated lights adds convenience for you and service providers. Plus, having them turn off automatically when done prevents them from burning out when someone forgets to turn them off.

• Water monitoring

Installing water sensors can detect and alert water leaks or sump pump failure

• Motion sensors

How many people have been surprised that a skunk or raccoon has destroyed their crawl space? I won’t mention the customer who discovered someone living in their crawl space.

• Cameras

Why not? If you already have a security set up on wi-fi, adding a camera to keep an eye on things could save you a trip to the crawl space. Of course, it would have to have infrared or night vision features.

How to Control Crawl Space Humidity

Keeping your crawl space between 45% and 60% Rh has several benefits

• Mold prevention, your 
• Framing lumber will maintain its proper moisture content, 
• Eliminated moisture problems, and 
• Hardwood floor cupping will not be a problem in the summer. 

Over the years, we have tried several different methods for crawl space humidity control:

• Pulling conditioned air through a transfer grill inside the home into the crawl space. (Sometimes it worked, but not when the crawl had high humidity)
• Putting dryer vent registers on the HVAC supply trunk. (Sometimes it worked, but usually, it would just create condensation and grow mold around the opening, which could affect indoor air quality)
• Using a dehumidistat-controlled fan to pull air into the crawl space when it was dryer outside than in the crawl. (Sometimes, it worked, but in the summer months, the outside air was so humid the unit wouldn’t operate all day and only on a few nights. So the humidity in the crawl space would stay high).
• Powered foundation vents that increased air movement in vented crawl spaces. (Sometimes, it worked, but sometimes it made it worse. Drawing more humid outside air into vented crawl spaces doesn’t make it drier.)
• Drawing humid exterior air across a coil of metal which uses the cooler temperature of the crawl space to reach the dew point and condense the incoming air (It worked when the crawl space air temperature was low enough, but we couldn’t predict if the crawl space temperature was going to be cool enough to get the coil down to dew point)
• Just about everything in between. 

Speaking of dryer vents, sometimes we find broken vents exhausting in the crawl space. Other times we learn there has never been a duct installed to discharge the clothes dryer outside. Most homeowners are surprised when we show them because they don’t spend much time in their crawl space. Exhausting hot, humid air from the clothes dryer into the crawl space will raise the relative humidity.

Despite a dehumidifier being a piece of equipment that needs maintenance and will eventually fail, we have found it to be the most effective way to control humidity. It sucks in humid air and blows out conditioned air based on the dehumidistat setting. Closed crawl spaces, along with the rest of the encapsulation features, work much more reliably.

Crawl Space Dehumidifier Costs

Online, a crawl space dehumidifier will cost anywhere from $1,000 to $2,500. The range depends on the size, energy star rating, included condensate pump, and installation materials. To have it professionally installed, you can add from $800 to $1,500. They require a GFI-protected circuit, which will add to the installation costs. A dehumidifier designed for crawl spaces will typically last longer and perform better than the ones in a big box store. 

We have found that running an Energy Star-rated dehumidifier will only sometimes cost more money for electricity. It reduces the load on your HVAC equipment and doesn’t have to work as hard. It’s similar to having a basement underneath your main floor.

Crawl Space Mold

Mold will quickly grow on floor joists in crawl spaces when the relative humidity stays above 60% or when subjected to water damage. Crawl space encapsulation will keep the relative humidity around 50%-55%, preventing crawl space problems. The exception would be if a plumbing line broke and sprayed or leaked onto the wood components in the crawl space or a catastrophic flood submerged the wood framing. Water intrusion, which leaves standing water to evaporate, can also raise relative humidity and subsequently create moisture problems.

Water intrusion can come from downspouts discharging too close to the foundation, surface water running through the foundation wall, and a high water table rising above the crawl space floor. As water evaporates, the moisture is absorbed into the wood and raises the moisture content, or water activity (Aw), to a level capable of supporting mold growth. The more toxic mold species require a material with a higher Aw. Mold spores are microscopic and float everywhere, looking for a nice place to live. If you were a mold spore, humid crawl spaces would be a great place to raise a family.

Is Crawl Space Mold Unhealthy?

The only way to know if the mold in your crawl space affects your indoor air quality is to do an air test for mold spores in your house and a surface sample for your crawl space. However, the mold in your home could be coming from the outside. Or it could be growing from moisture problems somewhere in your house. However, it is always good to start with the obvious. 

If you see mold, remove the mold. Some people grew up in moldy houses and never had health problems. Many homeowners have health problems because they live in moldy homes with poor indoor air quality. For most people, an elevated mold level in the air is not as healthy as a low concentration of air-borne mold. 

There are three ways mold can affect your health. Mold can be:

1. Allergenic 
2. Toxigenic
3. Pathogenic

Crawl space mold can be a significant source of poor indoor air quality because of the laws of physics. The stack effect happens when warm air rises; it pulls the crawl space air into the home. Nature seeks equilibrium, so when you run your clothes dryer, bathroom exhaust fan, fireplace, or kitchen exhaust, you create a negative air pressure in your home. The relatively higher air pressure in your crawl space forces air movement into your home to equalize the pressure.

If your HVAC ducts are in your crawl space, leaks in the return duct could suck poor air quality into the ducts. On the supply side, the venturi effect pulls air into the system. All of these situations could affect your indoor air quality. 

Your immune system, the toxicity of the mold, the indoor air quality, and the energy efficiency of your home will determine if the crawl space mold is unhealthy.

How Do You Remediate Mold in Crawl Spaces?

Not to be confused with mold treatment, which is just spraying the mold with a chemical formulated to kill it, mold remediation is the actual removal of mold. No professional organizations nor chemical manufacturers recommend applying a fungicidal coating over the mold to “encapsulate” it as a mold treatment. If you kill mold and it is still there, it can irritate allergies and respiratory sensitivities. The IICRC S520 Guidelines recommend a “HEPA Sandwich,” which means HEPA vacuuming, spraying and wiping with a fungicide, and then HEPA vacuuming again. 

Other methods include media blasting, like soda blasting or dry ice blasting, pressure washing, and sanding. The gold standard is to pass a 3rd party visual inspection along with a surface sample without creating cross-contamination during the remediation process.

How Much Does Crawl Space Mold Remediation Cost?

Crawl Space professional mold remediation can cost anywhere from $600 for spot cleaning to $6,000 or more. Many homeowners do not regard crawl space mold remediation as a do-it-yourself project. However, if you are not claustrophobic, not afraid of spiders and snakes, healthy and fit, able to work while wearing a respirator and other PPE, willing to research and follow industry best practices, and ready to have your work inspected by a qualified professional, in that case, you could save money doing it yourself. 

Several factors make the price of professional crawl space mold remediation go up or down, including the size of the crawlspace and conditions that affect the production rate. In Tennessee, the Department of Agriculture issues a C-14 license to contractors applying an anti-microbial to kill mold. Certifications do not guarantee quality work, but you should check. Protect your risk by checking their commercial liability insurance, workers’ compensation, and pollution insurance for working with mold. Reviews, referrals, and testimonials are best to screen for a quality contractor. 

The work is labor intensive and challenging to do well, so it costs more than many homeowners might think. Still, it is an investment in your home’s value, indoor air quality, and your family’s health.

What Is a Crawl Space Vapor Barrier?

Unless bone dry, all soil has a certain amount of moisture, evaporating over time. Hard rain can increase the soil’s moisture content. Without a vapor barrier, moisture evaporation can increase the humidity. This excess water vapor supports mold growth as the moisture absorbs into the wooden subfloor and framing members. The vapor barrier material must cover the floor 100% and have an airtight seal above the exterior grade at all walls and piers. The seams should be overlapped at least 6 inches and taped, as is done in a crawl space encapsulation. 6 mil poly should be the minimum thickness considered. A vapor barrier is not a water barrier, so water intrusion solutions should be installed under the vapor barrier when necessary. 

A vapor barrier only removes some moisture if it has open crawl space vents. Even with a vapor barrier, humid climates like Middle Tennessee can have higher relative humidity in the crawl space than the outside air. Open crawl space vents let in moist air and trap moisture in the fiberglass batt insulation. Then the floor joists absorb the condensation and start growing mold. So while a vapor barrier is a good idea, it only solves some crawl space problems.

What is the Best Vapor Barrier?

The purpose of vapor barriers is to stop the soil’s water vapor from entering crawl spaces from the ground. How thick does the poly sheeting have to be to do that? Which is better, 6 mil, 10 mil, 14 mil, 16 mil, 20 mil, or even thicker? The main question is the vapor permeance, which measures how much water vapor passes through the material. Without regard to thickness, most high-quality, non-recycled vapor barriers have a good enough permeance rating to stop water vapor. A thicker material will protect against wear if you anticipate higher traffic. 

The installation methods and fastening materials significantly impact the water vapor intrusion from the ground and foundation walls. An underlayment like a dimple board will prevent punctures and tears when the vapor barrier is installed over gravel or sharp rocks. Fully reinforced vapor barriers are generally more resistant to tears than thread reinforced. Still, the underlayment would offer the best protection if rips or tears are a risk. A 20 mil vapor barrier may not give you more protection than a thinner material. Buy the best crawl space encapsulation you can afford and have it properly installed with an underlayment.

How Much Does a Crawl Space Vapor Barrier Cost?

A vapor barrier for average-sized crawl spaces will cost $1,200 to $3,600 or more. Factors that make the price of a professional crawl space dehumidifier installation go up or down are 

• Quantity of materials needed
• Quality of the materials chosen
• Site preparation
• Working conditions

Not surprisingly, larger crawl spaces with taller foundation walls require more materials and labor. Thicker, more expensive material will only sometimes perform better or last longer. The underlayment and proper installation will impact performance and durability more than the thickness. Site preparation includes removing stored or abandoned materials, leveling the ground or filling in holes, and even digging out soil to access the areas. Floor joists close to the ground restrict movement and reduce the production rate. The HVAC system trunk often lays on the crawl space floor without the 4″ clearance it should have. You must hang up the trunk or dig out the ground, so there is enough room to put the vapor barrier underneath it.

What is the Cheapest Vapor Barrier?

The cheapest vapor barrier is a DIY 6 mil poly, recycled sheeting, costing around $100 per 1,000 square feet. It should be overlapped 6 inches, taped at the seams, and fastened to the walls at least 6 inches up. Is it good enough? Many new construction builders think so. We have found that it falls apart over time and doesn’t even have a good permeance rating when new. But, it checks the box when selling a house.

For some homeowners, the cheapest vapor barrier works fine; for others, it’s not enough. When we sit down to discuss your needs we can look at all the options available to you so you can make an educated purchase.

Crawl Space Waterproofing

Did you know that in most places, the walls around a crawl space don’t have to be waterproof like the walls around a basement? Waterproofing basement systems include coating the outside perimeter wall with waterproof, asphalt-like material. Plastic sheeting is installed over that surface, extending from the top of the foundation wall down to the bottom footer. French drains are installed around the perimeter of the foundation to collect water and prevent flooding. The pipe carries the collected water away from the house. The basement slab floor also has a vapor barrier to keep moisture out. Crawl spaces have none of that. So, it’s no surprise that rain water and a rising water table create a water problem in the crawl space. 

How to Prevent Crawl Space Flooding

There are two approaches to preventing crawl space flooding: 

1. 1) Keep the water from coming in
2. 2) Let the water come in, then direct it to a positive drain or sump pump to get it out. 

The best way to solve a water problem is to take care of it before it becomes one. This “upstream” approach can entail the following:

• Grading the yard so that water slopes away from the house, 
• Constructing a swale or berm to reroute water away from the building, 
• Elongating all gutter downspouts so they lead further away from the foundation,
• Excavating around the property line to expose foundation walls and waterproofing them as you would with basement systems.

 The last option is the most reliable way, but it doesn’t come without its problems: 

1. It usually costs more money
2. Many things might have to be moved, like a sidewalk, HVAC units, a porch, landscaping, and maybe a driveway.
3. It does nothing to prevent a high water table from entering the middle of crawl spaces. 

If you cannot waterproof the whole house, you can still fix some of the places water might be coming in. If the foundation vents are below the grade level of the yard, water can enter through. In that case, a foundation vent well can divert water away. Also, the door can be a source of water intrusion if it is below the yard’s grade, and creating a well using concrete blocks can work. The prefabricated Turtl will prevent water intrusion at the door. Water can come into your home through holes in the walls around pipes or electrical wiring conduit. To stop water from coming in, seal the gaps with hydraulic cement.

How to Install Crawl Space French Drains

French drains are not from France; they are drainage systems that remove water from an area. Henry French promoted their use to help with farm irrigation and drainage in the mid-1800s. Since most crawl space walls are not waterproof like basement systems, the most economical way to manage water is through crawl space interior french drains. 

Most water intrusion happens at the point where the foundation walls sit on the footer. To manage water intrusion at that point, you dig a trench just inside the footer around the perimeter wall of the house and install a pipe that allows water to drain into it. Then it follows the path of least resistance inside the drain to a positive drain or sump pump, where it is then drained or pumped outside. 

The solution for a rising water table is installing french drains throughout the crawl space area to catch the water from below.

How to Stop Crawl Space Flooding with a Sump Pump

All crawl spaces should have a system to evacuate standing water in case of flooding. The most common approach is a positive drain. Typically a corrugated plastic pipe is installed in the lowest corner, then continues through the crawl space foundation wall on top of the footer and drains to “daylight” in the yard. 

Unfortunately, positive drains fail over time. The positive drain is sometimes higher than the crawl space ground level, and water will pool until it gets high enough to reach the footer. Since muddy water runs to the positive drain, it usually gets clogged. We typically have difficulty finding where the positive drain exits because the ground has covered or crushed the exit over time. Perhaps installing a thin plastic pipe underground was a good idea at the time. 

The most dependable way to expel water is by using a sump pump. In the lowest area where water pools, a pit is excavated for the sump basin (not a 5-gallon bucket). Approximately 3-5 inches of gravel is placed underneath and around the sump basin. The sump basin is either perforated or has holes drilled into it on the bottom and side, allowing water filtered by the gravel to enter the sump basin. The pump discharges the water out of the basin with a float that triggers the pump when it reaches a certain level like a toilet float. A check valve is installed on the discharge line to keep the water from running back into the basin when the pump stops. The discharge line routes through the foundation wall underground to terminate away from the house.

Comparison of Vented Crawl Spaces vs. Crawl Space Encapsulation

Every so often, we will inspect a crawl space with 

• Open vents, 
• No vapor barrier, and 
• No evidence of water damage or mold growth; 
• Dry soil, and 
• The moisture content of the wood is perfect. 

They do not need a crawl space encapsulation; that is the exception. A crawl space encapsulation is usually necessary to prevent mold growth on the floor joists, wood rot, and other moisture problems.

Traditionally builders installed vented crawl spaces to reduce high humidity. The idea was that if we left vents open, air would flow in and dry the wood components. But this only works well in places with lower humidity. The humid months in Middle Tennessee vent the crawl with moist air, and this causes water to form on cool surfaces.

Encapsulated crawl spaces eliminate this problem by creating a sealed barrier between your home and the outside environment. Unvented crawl spaces allow for less heat loss and good humidity levels without inviting moisture and contaminants into your crawl space. Water management systems prevent water intrusion that can cause wood rot and other structural damage.

Maintaining dry crawl spaces is key to keeping your home in optimal condition. Whether you choose open crawl space vents or encapsulation, consider doing a yearly inspection and maintenance. Doing so will help ensure the health of your home and keep problems at bay.

The decision to continue with your existing crawl space with open crawl space vents or change to an encapsulated crawl space comes down to personal preference, existing crawl space performance, and budget constraints; both systems can effectively control moisture levels, but crawl space encapsulation may provide more reliable benefits overall. Ultimately, it is up to you to determine which system works best for your home and situation.