According to the U.S. Department of Energy, the purpose of insulation is to provide resistance to heat flow and lower heating and cooling costs. In our decades of experience in Middle Tennessee, an air-sealed, conditioned, encapsulated crawl space maintains a temperature range in the 60s for 99% of the year without adding insulation.
Adding foam board insulation to the crawl space walls would not improve the crawl space temperature or the home’s temperature, reduce energy usage, or make cold floors feel warmer.
Check out our website’s crawl space encapsulation page for more info related to insulating crawl spaces in Nashville, Tennessee.
What is the most common crawl space in Middle Tennessee?
Most crawl spaces in the Nashville area have open foundation vents, bringing in outside air for ventilation.
The open vent covers ventilate the crawl space with cold air in the winter and warm air in the summer. The vapor barrier is usually a thin, recycled plastic sheeting barely better than a dirt floor. Blown cellulose insulation on the walls is now falling off.
Portions of the fiberglass insulation batts are hanging out of the joist bays due to moist air. In the summer, the air conditioner supplies cool air through the ducts in the humid crawl space. Condensation occurs on the cool surfaces of the duct insulation.
Sweaty ducts (as we call them) diminish the duct insulation’s thermal barrier and r value.
This common crawl space usually has groundwater leaks and standing water on the dirt floor. We usually see a little to a lot of mold growing on the joists and subfloor.
Meets codes. Doesn’t work.
This common crawl space described above meets codes but doesn’t work. Crawl space insulation will not help. This crawl space doesn’t need repair. It needs a do-over.
In our experience, the crawl space above never worked, even though it met the building code.
Open vents let in the outside moisture in the summer and cold air in the winter, and outside air brings in much more moist air than a dirt crawl space floor in the summer.
Crawl space insulation using fiberglass insulation, foam board, or different insulation types will not make the crawl space more energy efficient.
Let’s get a few questions out of the way before we explore why we can maintain a crawl space temperature in the 60s all year without adding insulation.
Should foundation vents be opened or closed?
The most common questions we hear from owners of this broken crawl space is
- Should I close the vent openings in the summer or the winter?
- What is the right crawl space insulation to reduce energy costs?
- Do building codes require crawl space insulation?
- Why do spray foam insulation companies recommend spray foam insulation?
- Will insulating the subfloor make my floors warmer?
Crawl Space Insulation and Building Codes
The International Residential Code R402.2.10 clarifies how to insulate a crawl space where it is installed (Davidson County is climate 3a). It also gives a choice of whether to use open or closed vents.
It explains how a plastic vapor barrier should be installed on the dirt crawl space floor. On the other hand, it gives instructions for installing insulation material with no vapor barrier on dirt floors.
Interestingly, the code states its intent “to provide flexibility to permit the use of innovative approaches and techniques to achieve this objective” (for the effective use and conservation of energy).
Many homeowners have experienced home problems even though their houses passed code inspections. Subfloor fiberglass insulation in the joist bays of a vented crawl space can cause moisture buildup and lead to mold growth.
Most building codes also don’t require waterproofing of crawl space foundation walls, which can create severe water intrusion problems. Solutions include a drainage system and a sump pump on the dirt floor.
A crawl space encapsulated without insulation includes sealing off air leaks, a high-quality vapor barrier sealed to the walls, sealed seams, exfiltration between the dirt floor and vapor barrier, and a dehumidifier that complies with the intent of the building code.
Spray foam insulation companies recommend spraying foam insulation in crawl spaces.
More insulation is always better, right? Not when it is a waste of money or causes more problems.
Does crawl space insulation on the wall cause more problems? Not necessarily, except if sealing the wall leaves no exposure for a termite inspection, which might be a code violation.
Closed cell spray foam insulation on the crawl space subfloor would seal air infiltration into the house, but unless all of the wood is covered with spray foam, it might trap moisture in a vented crawl space and potentially cause mold growth on the exposed wooden components.
If water damage occurs in the house and affects the insulated crawl space subfloor, the insulation around the joists and subfloor must be removed to expedite drying.
Is crawl space spray foam insulation a waste of money?
Not when it is used as a foam sealant to make a tight seal around the sill plate and to seal rim joists. Air sealing with spray foam is the best sealant at those locations to seal the gaps.
However, spraying it thicker or adding foam board to achieve a specific R-value wastes money. Spray foam and foam board insulation on the foundation walls will not change the crawl space’s temperature. More on that later.
Will foam board insulation on my crawl space walls make my floors warmer?
We’ve discussed how spray foam insulation and rigid foam board on the crawl space walls will not provide a thermal barrier and make the crawl space warmer.
Let’s talk about what makes floors feel cold. The carpet feels warmer on our bare feet because it is a poor conductor of heat.
If your home’s conditioned space is 72 degrees, everything in the room is 72 degrees, including your floor. Dense floors, like tile or hardwood, feel colder because your body is around 98 degrees, and your body transfers heat more quickly to dense surfaces.
Which means it pulls the heat out of your body more quickly. That’s also why toilet seats feel cold. Tile floors will feel cold even on the second floor.
If you insulate the crawl space with fiberglass batts between the floor joists will not make tile floors feel warmer.
Installing a radiant floor heating system is a much more efficient and effective way to make your floors feel warmer than insulating your crawl space with foam board.
Basement Wall Insulation
If you insulate a crawl space, does it solve the same problems it solves in a basement wall? Actually, no. A finished basement with uninsulated walls may grow mold on the back of the drywall due to water vapor coming through the block or concrete foundation wall, which raises the dew point in that wall cavity, which creates condensation on the back of the colder drywall.
Placing rigid foam insulation or spray foam insulation on the basement wall moves the dew point condition away from the drywall. It creates a gradual change in temperature rather than abrupt, possibly at the dew point, at the back of the drywall.
A better way to build a basement wall is to leave an air gap between the basement foundation wall and the drywall with a transfer grill at the top and bottom of the drywall.
The transfer grills allow conditioned air to circulate between the drywall and foundation wall, but that is a topic for another time.
Crawl Space Wall Insulation
In a crawl space encapsulation, there is no drywall, only a vapor barrier.
So if condensation occurs on the back of it, the water runs down to the ground and is absorbed, which is another reason we recommend the radon and earth gas system under the vapor barrier.
A Crawl Space Encapsulation Creates a Cave Temperature
How do we maintain an average crawl space temperature in the 60s without insulation?
So far, we have discussed insulating a crawl space by adding insulation to a conditioned crawl space. However, the earth naturally insulates a crawl space like a cave.
What temperature is a cave? The cave temperature is very consistent and is dependent on three main things:
- Its depth
- Proximity to the mouth
- The average temperature of the region
The cave’s temperature rises closer to the earth’s core. The closer you are to the cave’s mouth, the more it reflects the outside temperature (reference our discussion on air sealing). The cave’s average temperature will approximate the region’s average annual temperature.
Tennessee Cave Temperature
Tennessee is in a mixed humid climate zone. The average annual temperature of Tennessee is challenging to make meaningful because it is so varied in topography, but the mean temperature of Nashville is 59 degrees Fahrenheit.
The most famous cave in Tennessee is the Cumberland Caverns located in McMinnville, TN, which maintains a temperature of 56 degrees. The Lost Sea Cave in Sweetwater, Tennessee, is 58 degrees year-round.
A Crawl Space is Already Insulated by the Earth
We can maintain a constant temperature in a conditioned, air-sealed, encapsulated crawl space without adding foam board insulation because the earth provides the crawl space insulation using geothermal energy and absorbed solar radiation.
The dirt floor needs no insulation, and the ground mostly insulates the crawl space walls. The ceiling of the crawl space keeps the crawl space warmer than the interior of a cave in the winter.
The outside conditions make the crawl space a little warmer than a cave in the summer.
Additional Crawl Space Insulation is Unnecessary
There may be a handful of days in the winter that cool crawl spaces a few degrees, but those days do not affect the temperature of the floors in the house, cause loss of energy efficiency, or heat loss.
Crawl space insulation on the walls to control the temperature assumes that cold crawl space walls make the crawl space cold by the process of conduction, which is false.
Would You Insulate a Crawl Space Dirt Floor?
Crawl space insulation on the wall makes as much sense as insulating crawl space dirt floors.
Air Sealing is the Key
Crawl spaces are cooled in the winter by convection through air leaks, which is why we seal vents, the rim joist joints, and sill plate gaps. But this is not done for thermal conductivity or radiation reasons.
This information is not based on codes but on research, decades of experience, and observing what works.
This discussion has focussed on insulating crawl space walls, which can be a needless insulation process. Air sealing is entirely different.
Save Your Money
Don’t insulate the crawl space if you’re concerned about saving money on your energy bills. Focus on other areas of your home, such as sealing windows and doors, adding weather stripping, sealing your ceiling lights and attic opening, and adding attic insulation.