Crawl Space Insulation

Apr 27, 2012 by

Best Practices to Build Energy Efficient Crawl Spaces



Crawl Space insulation is one of the hottest topics in home improvement and home efficiency these days.
Homeowners are actively seeking for the best return for their investment, whether they are struggling to diminish the impact of soaring fuel prices in their lives, or racing to cash in government incentives and the Federal Tax Credit for Home Energy Efficiency.
As they try to decrease their home’s energy consumption, lower their utility bills and improve comfort, insulating the home’s foundation is becoming a top priority – especially for homes built on vented crawl spaces that have dirt floors.
Justifiably so: A dirt-floored, vented crawl space is a huge energy waster. For a home with ductwork running in a vented crawl space, HVAC system efficiency can be reduced by as much as 50%.
According to Advanced Energy, energy loss in these homes is so substantial that they might as well ignore all other attempts to save money and energy until the crawl space problem is solved.
That said, it is necessary to understand that effective crawl space insulation can not be achieved without proper measures to control moisture in that environment, or without using waterproof materials to to seal out and remove the moisture.
The following is a step-by-step guide to proper crawl space insulation.

1. Provide adequate drainage

  • Grade the terrain around the house to slope away from the foundation. Keep gutters clean or install them if the house has none.
  • Extend the downspouts at least eight feet (the typical backfilled area) away from the house.
  • Plants and bushes that need regular watering should be kept away from the foundation, and care must be taken to prevent hoses and outdoor faucets from leaking.

Sometimes, the above measures will not completely keep the water from entering the crawl space. Interior drainage pipe combined with a sump pump might be recommended in these cases, as well as the use of drainage mats underneath the ground cover.
Drainage mats, as shown in the picture to the right, will keep water from getting in and provide a desirable thermal break as well.
The ill effects of a vented, dirt crawl space can be felt in many other ways besides high heating and cooling costs. Cold drafts come through the floor in the winter, while uncomfortable indoor humidity enters the house during the summer. Moist atmosphere is unhealthy and destructive – a place where mold thrives and spreads.

2. Control ground moisture with a vapor barrier

A crawl space with adequate drainage will still have moisture problems, coming from two major sources:
the ground and the outside air.
Ground water will evaporate into a dirt-floored crawl space and soak the joists and wooden structures. It will also infiltrate concrete walls and floor causing them to be damp most of the time. It is not possible to effectively insulate a craw space without eliminating these major moisture sources.
Most building codes and contractors will recommend a waterproof ground cover, which will keep some of the ground water from evaporating into the crawl, but will not address the water that seeps through the walls.
To completely control moisture in the crawl space, both the floor and the walls should be lined with a vapor barrier that completely isolates the area from ground and wall moisture. This process is commonly called crawl space encapsulation.
In warmer areas, the entire space, including the walls, can be lined or encapsulated with a flexible poly sheet vapor barrier and, if the space is properly air sealed and conditioned, there will be no need to add insulation.
Drainage mats also provide a thermal break under the vapor barrier.
We will examine this process a bit further in this article, when discussing different insulation approaches.
While the norm is to use generic 6mil poly sheet to line crawl space , better, long-lasting results can be obtained by using a 10mil or 20mil crawl space vapor barrier instead.
These liners were developed specifically for this application. If mechanically fastened to the walls, a sturdier liner will not rip as easily when people crawl on it, during inspections and maintenance visits.

3. Chose the proper insulation method.

The most popular and widespread method of crawl space insulation – applying fiberglass insulation bats between the floor joists of a vented crawl space – is also the main cause of most crawl space mold disasters.
Fiberglass soaks moisture like a sponge, and in contact with water it sags, also supporting mold growth on its paper facing. To make matters worse, damp fiberglass loses it is R-Value, becoming useless as insulation, and as it sags, it opens holes in the insulation blanket, allowing the air to leak into the living area.
The U.S. Department of Energy recommends that rigid foam board insulation be used on the craw space walls. Rigid foam is waterproof, does not absorb moisture, and will not support mold growth, so it’s an excellent choice in moist areas like basements and crawl spaces. It’s also recommended to install insulation against foundation walls rather than between floor joists, because this extends the “building envelope” to encompass features like ductwork, water heaters and HVAC equipment.
Advantages of insulating the wall, rather than the floor include:
  • Although foam board is more expensive than fiberglass bats, less insulation is required for the walls.
  • The crawl space becomes part  of the building envelope of the house, so there is no need to insulate pipes and ductwork running beneath the house for energy efficiency or protection against freezing.
  • Air sealing between the house and crawlspace is less critical.
The boards should be attached straight against the walls, using the proper adhesives. The ground vapor barrier should overlap the board at the base and taped over with mastic tape.
A protective membrane should cover the the top of the block wall to allow for termite protection and it should also overlap the board a few inches as well. Professionally developed crawl space encapsulation systems include a transparent cap instead of this protective membrane, to facilitate termite inspections.
It is not always necessary to insulate the floors, specially if you are using a drainage mat, which provides a thermal break.
In colder climates, a lower R-Value foam bord can be placed between the mat and the vapor barrier.

4. Chose the right insulation type

Fiberglass insulation:

As we mentioned before, fiberglass insulation is quite popular because it is the most inexpensive insulation
material available. However, it is the worst possible choice for a crawl space, because it absorbs moisture and favors mold growth.

Cellulose Insulation:

Although modern cellulose insulation is chemically treated to be mold resistant and handle moisture better than fiberglass it is still made with organic materials and there is not enough data to support that it can withstand chronic moisture conditions.

Closed Cell Spray Foam Insulation:

Closed cell spray foam, when properly applied is an excellent choice for crawl spaces. It expands after application, air sealing gaps and fitting snugly around pipes, wiring, ducts and fixtures. When incorrectly applied, however, it can lead to problems.

The insulation blanket should, for example, end a few inches before the top of the wall, and include a protective, removable protective membrane, or cap, to allow for termite inspection.
When applied on wood, installers should make sure that the wood is dry and healthy (mold free), otherwise you will seal the problem but the mold and rot can still cause the wood to decay under the insulation layer.
Another problem with spray foam insulation is price. It costs from 2 to 3 times more than any other type of
insulation in the market.

Rigid Foam Board Insulation

The U.S. Department of Energy recommends using rigid foam board to insulate crawl spaces. Less expensive than spray foam, it is impervious to water, inorganic and non absorbent.

There are basically 3 types of foam board insulation with R-Values ranging from 3.8 to 8.7 per inch:  Extruded polystyrene (XPS) , Expanded Polystyrene (EPS) and Polyurethane.
Foam board comes in different thicknesses, and board sizes are typically 2x8ft. or 4x8ft. Polyurethane foam has the highest R-Value per inch.
To find out which is the best R-Value for the area you live in, refer to the DOE insulation guidelines found in this map (zip file). Insulation boards are attached directly to the wall with mechanical fasteners, adhesives or a combination of both.
A vapor barrier, lining the floor, usually overlaps the base of the board a few inches, and is secured and sealed onto it with mastic tape. Any gaps and holes are then sealed with tape, caulk or spray foam.
For improved energy savings, a lower R-Value foam board, can be placed under the ground cover or in between the liner and the drainage mat.

5. Control outside air and moisture infiltration by air sealing the crawl space.

On the warmer days, the outside air, if allowed to enter the crawl space through the vents, will cool down.

As it does, relative humidity levels will increase 2.2% for each degree that the outside air is cooled.
A temperature difference of only 10 degrees will cause an increase of over 20% in RH once the outside air enters the crawl space. When the RH levels in the crawl space rise to or above 60%, mold is likely to develop. And that happens quite often during summer.
During the winter, RH levels are not much of a concern, but if you have ducts and pipes in the crawl, you can have some condensation issues. The biggest problem during cold weather is the infiltration of cold air into the living space. Often homeowners complain of cold drafts that can be felt coming from the floors above a crawl space.
This is why air sealing is fundamental, if the goal is to increase energy efficiency. Crawl space insulation
without air sealing is not as effective. The encapsulated crawl space should be thoroughly air sealed and the access to the crawl space should be through an outside entry that can be effectively air-sealed when not in use.
Note: in areas in which radon is a concern, such a crawl space encapsulation system acts as a passive mitigation system, but tests should be conducted to determine if there is the need for an additional radon mitigation system, which can be easily installed to work with the vapor barrier.

6. Control moisture from condensation with a dehumidifier

With the space is completely isolated from ground and outside air, some condensation may still occur due to temperature differences between the crawl space and its surroundings. That moisture should be addressed with a crawl space dehumidifier. A dehumidifier will draw any moisture present in the air before condensation happens.

Good crawl space dehumidifiers are energy efficient and require little maintenance, as they monitor RH levels, turn on and off as needed, and empty the collected moisture into the sump pit.
An encapsulated and properly insulated crawl space can significantly decrease a home’s energy consumption, protect its structural integrity and improve the lives of its inhabitants.
Crawl space insulation is one of the home improvements that bring the best results and the most savings per each dollar invested.
Good dehumidifiers for crawl spaces, discharge the collected moisture into the sump pit , saving homeowners the trouble of emptying the trays regularly.

Sources and Information

Advanced Energy
US. Department of Energy – Energy Efficiency and Renewable Energy
US DOE – Energy Savers
Forest Products Laboratory
RLC Engineering
Best of Building Science
Building Science Corp
Basement Systems Inc
Doctor Energy Saver


  1. We’re considering buying a home close to a creek. No basement but we can do a crawl space. I’m glad we came across this article. The damage from moisture looks really bad. I’d hate to have that happen to my new home. I’ll be taking precautions. Thanks for a well written article.

  2. David Dunphy

    After Hurricane Sandy, we gutted my mother in laws single story beach house in Lavallette NJ. The H VAC and duct work are in the crawl space and the floor was never insulated. Researching other sites it sounded like the most economical way to insulate was to use batts w/ the vapor barrier up……that is until I read your site. She is close to the water and has a sand based crawl space, about 2 feet from the sand to the bottom of the joists. As I understand, you do NOT recommend batts due to moisture infiltration but rather suggest rigid. Should the foam boards be pushed up to the the floor or leave an air gap. As she lost everything in the house and is a senior citizen on a fixed income, we have to be frugal, but smart at what we do. BTW, we will use batts on the exterior walls. Thanks in advance for your help. Dave.

    • DRESblog

      I’m sorry to hear about the damage to your mother-in-law’s house. Normally the treatment recommended for a crawl space (to improve energy efficiency and control moisture levels beneath the house) is encapsulation, which involves installing a moisture barrier and insulation against crawl space walls, along with a moisture barrier on the crawl space floor. However, in a beachside environment, this strategy may not be appropriate because of the increased chance of flooding. The alternative is to insulate and air-seal the crawl space ceiling, using rigid foam or spray foam insulation. Though economical, fiberglass batt insulation is not recommended because it doesn’t form an air barrier, and because in moist crawl space conditions, batts compress, lose R-value and promote mold and wood rot by retaining moisture.

      Rigid foam and closed-cell spray foam don’t absorb moisture, compress or lose R-value. Installing either type of insulation between your crawl space ceiling joists will cost a bit more, but (unlike fiberglass batt insulation), it’s a permanent energy-saving improvement. After you make sure the ductwork in the crawl space has dried out, all crawl space ducts runs should be sealed and insulated as well.

  3. Bill A.

    In response to high moisture, I encapsulated our crawl space three years ago in our 2600 sq ft home in Chesapeake, VA. I replaced the vapor barrier, and installed a Santa Fe Advance dehumidifier. The RH stays at about 50% year-round. Now I wonder if I need to retain our termite contract, since I don’t see how a termite could survive in this crawl space. I have not found any relevant info online. I make quarterly inspections in the crawl, looking for termite mud tunnels, and look for standing water, etc.

    Feedback is appreciated.

    • DRESblog

      Hi Bill,

      Although I am not a pest control expert, lowering the relative humidity, and thus the moisture content in your home’s wood, should make your home less favorable to termites. I’d be hesitant to say that you don’t have to worry about termites ever again, especially as you’ve had them before. A pest control expert that understands the connection between termites and moisture content may be a better source of knowledge here.

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