Problems & Solutions

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This article is called Crawl Space Issues and not Crawl Space Insulation because when we inspect crawl spaces for insulation, we are also looking for evidence of rodent infestation, moisture problems and potentially any other issues that might contribute to indoor air quality problems or comfort complaints. Based on our experience inspecting thousands of crawl spaces in Northern Arizona, odds are that your crawl space has an issue. This article is broad overview of the problems we routinely find in crawl space foundations and our approaches to fixing these problems.

Problems we find in crawl space

The old cliché “out of site and out of mind” rings true for crawl space foundations. Who wants to crawl through a tight access to get underneath their home? We know from firsthand experience that some crawl spaces can be pretty claustrophobic and disgusting environments. Over the years of we have worked on homes with the following problems in crawl spaces in Northern Arizona.

  • no insulation,
  • improperly installed and falling down fiberglass batt insulation
  • rodents (pack rats, mice, skunks, raccoons, etc) and their feces
  • moisture – damp soil to standing water
  • mold growth
  • rotting rim joist and floor joists
  • plumbing leaks
  • termites
  • radon

Although not all crawl spaces have these problems, a surprising number have one or a combination of these them. It is rare that we inspect a crawl space that is free of issues. These types of problems can affect your homes indoor air quality, building durability, comfort and energy efficiency.

Our crawl space inspection policy

As a result of our experience dealing with these types of crawl space problems, when we inspect a crawl space for insulation problems, we are also looking for any other issues that might be compromising your home. We don’t want to solve one problem and neglect another that is a major concern. For example if there is a rodent infestation, we would recommend the rodent problem be addressed before doing any insulation work.

Crawlspace solutions

In this article we provide a “broad” overview of problems and solutions. Your estimator can discuss any of the solutions to the types of these problems after his inspection of your crawl space. At Advantage Home Performance we have a solid track record for resolving the types of issues that we are discussing in this article.

Problems with conventional fiberglass batts in crawl space

The biggest problem we find in crawl space insulation is that it was never insulated when the home was built or more likely that the batts installed in the crawl space were installed poorly and over time started to fall to the ground. To work effectively, the fiberglass batts need to be in contact with the subfloor. We routinely go back into homes to push the batts in contact with the subfloor, replace missing pieces or damaged pieces of batt insulation, install wire batt hangers, …

Air leakage from crawl space into homes

We observe numerous air leakage paths from crawl spaces into homes. A surprising amount of crawl space air can find its way up from the crawl space to the house due to stack effect. This is the impact of warm rising and leaking out of the top of the home and drawing air in from the bottom of the house which of can be the crawl space. The air leakage occurs at chases for flue pipes, ducts and plumbing. It also occurs where electrical wires and plumbing lines are cut through bottom plates. As part of our insulation repair process we can include or option our air sealing package.

Duct systems in crawl spaces

Often the problems with crawl spaces are magnified if the crawl space has an air distribution system located in it. Invariably we find duct leaks in the return portion of the air distribution system. These return leaks are under negative pressure because the fan is sucking air back to the unit sort of like a vacuum cleaner sucks air. Moisture, pesticides, dust, radon and musty odors are what get sucked up by these return leaks and then distributed throughout the house. We recommend to all of our customers who have air distribution system in crawl spaces to have them sealed and tested with a duct blaster or blower door and pressure pans.

Rodents in crawlspaces

We have firsthand experience with the damage pack rats, skunks, javelina and raccoons can do in a crawl space. These rodents will pull down batts and live in them. Their feces can litter the crawl space floor. Sometimes they move into your dust system. Sometimes these creatures go into crawl spaces to die. A rodent infested crawl space can really be disgusting.

The first thing we do is work with home owners to keep these rodents out of the crawl space. We may install a heavy duty wire mesh over the crawl space vents. We may fill entrance points with rocks or concrete. In some cases we turn over a house to a rodent proofing specialist. There is no point in doing any work in a crawl space until the rodent problem is dealt with.

Moisture problems in crawl spaces

Moisture problems are more common in crawl space foundation than you would guess in Northern Arizona. Monsoon rains and melting snow are the culprits. When the surrounding grade is higher than the floor of the crawl space the ground often gets wet and some cases water starts to puddle. What we find is a cycle of wetting and drying. The crawl space gets wet and eventually it dries. The longer the crawl space stays wet the more likely problems will occur. The photos below show evidence of we rim joist and ledgers in crawls spaces.


A wet crawl space creates indoor air quality issues, building durability and energy efficiency problems. A poorly designed and constructed vented crawl space will have will have standing water, damp ground, mold, and in the worst situation decay fungi occur. Wet lumber also attracts termites. Venting crawl space is not the solution if the rate of moisture entry is greater than dilution rate of venting. We have seen many vented crawl spaces with significant problems with the code required ventilation.

Moisture control strategies

Preventing moisture entry through foundations is not difficult. This is especially true if you are familiar with moisture transport mechanisms. Moisture moves in four principle ways: liquid flow, capillary suction, air flow and diffusion. In any successful design these four flows must be dealt with. This is especially true of liquid flow and capillary suction because they can contribute so much moisture to the indoor environment if left unchecked.

It is critical to control the liquid flow of moisture around the home. Ground water and rain water must be dealt by sloping surfaces away from the house, gutters, vapor retarders, water proofing, damp proofing, and underground foundation drainage system. If the lot doesn’t permit the ground sloping away then extra measures in the crawl space need to be taken to keep the crawl space such as a sump pump or drain to daylight.

Capillary suction occurs when moisture rises against the force of gravity in porous materials. As moisture evaporates from soil it creates a pumping action for more moisture to be drawn to the surface. The vapor retarder shuts down the drying and thus the flow of moisture into the crawl space through capillary suction. We have had a lot of success with our vapor retarder installations.

Vapor Retarders

Advantage Home Performance has installed many vapor retarders in damp crawl spaces. We know from some of our retrofit jobs that we have immediately stopped condensation on windows and moisture condensing on rim joists in the crawl space. The tear resistant poly is extremely durable so it doesn’t get damaged when workers walk on it. We mechanically fasten the vapor retarders to foundation stem walls and tape every joint throughout the crawl space. Our installations also look very clean and professional.

Radon problem and solutions

We have had a great deal of success working on homes with crawlspace foundations that had elevated radon in the indoor environment. Many of the ways we deal with air flow and moisture flow in crawl space also help us to reduce radon levels. We have a whole case study on radon mitigation in the home performance section of this website. We employ several different strategies to deal with radon, but they are dependent on the specifics of the house.

Unvented crawl space with two pound foam on perimeter walls

The ultimate treatment for a crawl space is to eliminate of the vents, put a vapor barrier on the ground and spray the walls with two pound foam. This especially makes sense if there is a duct system in the crawl space. The downside of this strategy is that it is expensive.

The building code allows unvented crawl spaces insulated with two pound foam on the exterior stem walls of the foundation. Unvented crawl spaces are a superior way to go “when executed correctly.” Executing correctly primarily means controlling moisture, and doing a great job on the insulation. We have retrofitted several crawl spaces with this approach and we the anecdotal feedback has been very positive.

There are several code requirements that you must follow, if you are going to go with unvented crawl spaces insulated with two pound foam on the exterior stem walls of the foundation. In an unvented crawl spaces you must have a sealed combustion gas furnaces, which are typically 90% or greater. A regular 80% efficient gas furnace is not acceptable. The code also requires a vapor retarder on the ground to of unvented crawl spaces to control the entry of moisture through capillary suction. You also can’t be using the crawl space for storage.

There are several exceptional websites dealing with all the issues surrounding and unvented or closed crawl space. The best source of information on is put out by Advanced Energy. See their guide called Closed Crawl Spaces: An Introduction to Design Construction and Performance.

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