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A Clean Crawlspace is Better For You and Your Home

Clean Crawlspaces and Basements are Good for Health

Is a Clean Crawlspace Really That Important?

Crawlspaces are generally not what anyone would consider “clean.” When was the last time you went into a crawlspace? It’s typically a dark, dank environment where critters and odors scare away even the bravest DIY building owners. Often old construction materials and cardboard boxes are crammed down below, providing nesting and food for mold and all sorts of furry critters. The stack effect dutifully draws air from crawlspaces up through small holes, gaps and cracks in the building and that nasty, wet air makes its way into your home. In 80% of buildings we inspect, we end up suggesting the crawlspace be professionally cleaned and upgraded. A clean crawlspace can significantly increase the durability and healthy enjoyment of your building, by eliminating unwanted heat loss, moisture, pests and odors.

It’s possible you’re in the <20% of buildings with a clean crawlspace or basement already, but the odds are stacked against you. If you have any doubts, lift the hatch, take a look and sniff. If you wouldn’t want to spend 30 seconds down there keep reading and consider having an industrial hygienist test your crawlspace.

Safe and Secure Crawlspace Access is Important

Crawlspaces have often been treated with legacy pesticides to prevent termite and rodent infestations. Mold is also a likely inhabitant of crawlspaces and unconditioned basements. If professionals feel compelled wear protective clothing and respirators into your crawlspace to protect themselves from biological and chemical contaminants, do you really want them entering and exiting the crawlspace via the floor of a kitchen pantry or bedroom closet? If access to a crawlspace is so constricted it would add days of labor time for excavation or make bringing in necessary building materials impossible, it often pencils out to have your crawlspace access moved or expanded.

Safe and Secure Crawlspace Access

Children and pests aren’t often uttered in the same sentence, but a solid and secure access hatch will keep both out of the crawlspace. The above photo illustrates a prefab metal access hatch with hydraulic hinges, air-sealing gaskets, and a padlock. This access hatch will last longer than most of the house, and be a luxury for service professionals who must periodically enter and inspect the crawl space. Sufficient sizing of the access points is important, as is not blocking access with wires, mechanical systems such as duct work or drain pipes.

Air Migrates From Crawlspaces into Living Zones. Carpet acts as air filter.

Air migrates from crawlspaces into living zones. Carpet acts as air filter.

If access is indoors, careful consideration must be made to not to cross-contaminate the building while work is ongoing below. Where carpet is covering an access hatch we often find visual signs of the carpet acting as an air filter for air entering the home via stack effect or other pressurization. Indoor access hatches should be air-sealed. When significant work is done in the crawlspace the surrounding area within the home should be placed in containment, with negative air machines and air scrubbers running. Significantly more dust is generated than on an average day when contractors are working, and that dust may well contain legacy pesticides, biological contaminants (feces, mites, mold spores, bacteria), heavy metals and hazardous fibers (fiberglass or asbestos).

Crawlspace Upgrade Opportunities

Air ducts on disconnected and on the ground in crawlspace.There are always economies of scale to consider with any renovation project. When the project involves tight spaces and specialty contractors dealing with access issues, it’s especially important to take a step back and consider the big picture. The last thing you want is to complete your crawlspace remediation and realize you missed a golden opportunity to accomplish another important objective that would have been easy and affordable during the previous push.

Goals for a crawlspace upgrade usually involve moisture management, improved air quality, pest protection, long-term durability of structural elements, improved thermal comfort and energy efficiency.

During a crawlspace inspection our industrial hygienists generally include the following:

  • Identify access and egress issues (can we get in and out without danger of being trapped)
  • Identify any threats to life and safety (rattle snakes, standing water with live electrical wires, etc.)
  • Determine if the space is dry enough to seal and insulate (may require additional site or drainage improvements)
  • Determine air barrier location: crawl space floor or walls and grade (where to install an air barrier is an important decision best left to your licensed design and building professionals)
  • Determine what grade of ground cover you’re going to install (common vapor barrier or advanced radon mitigation material)
  • Survey mechanical, electrical and plumbing systems for opportunities (consider low hanging fruit future proofing before installation of any new insulation or sheet-good products that will cover up access)
  • Determine appropriate insulation for the surfaces considered (enough R-value, easy to install properly, within budget, non toxic, not nesting or food for pests or mold,  etc.)
  • Collect measurements and pictures to estimate any suggested improvements

Water Control Strategies Vary Depending on Primary Sources

Strategies for enhancing a basement or crawlspace will vary depending on how much water and moisture are present. If there is an artesian well springing up under the building moisture control strategies will be very different than if there is minimal soil dampness. Liquid water from rain, storm water runoff, poorly located downspouts, or leaking pipes would be considered “bulk water”.  These are the worst offenders.

Water Control Strategies and Priorities in Damp Crawlspaces

Compliments of Gavin Healy

In the San Francisco Bay Area condensation is often not a significant problem, unless there is excessive bulk water evaporating in a confined and poorly vented space. Capillary action, or the ability of a material to transport water vertically (think about a towel left hanging over a bathtub), is frequently problematic when concrete or wood are in direct contact with wet soil. Evaporation of bulk water or soil moisture can also create dampness problems sufficient to form condensation and support mold growth. Of all these things controlling bulk, liquid water is the most important function of a well built basement or crawlspace

It is critically important that the primary source of water is addressed in your crawlspace work. This may involve fixing plumbing leaks, installing french drains outside the home, installing submersible pumps under the building, etc. No crawlspace solution is 100% until you have considered a plan for these common moisture sources.

What is the Best Insulation for Crawlspace or Basement?

The primary purpose of insulation is to improve thermal comfort and reduce energy consumption. The San Francisco Bay Area is a moderate climate zone where floor insulation for existing buildings is often considered optional. However, if you’re about to embark on a crawlspace improvement project this is an opportune time to consider proper insulation materials and installation techniques.

Crawlspace renovations almost always involve removing old insulation (usually poorly installed fiberglass), air-sealing, and installing new insulation. While the primary goal of insulation is to resist temperature transmission, secondary goals should include a) not providing habitat or food for unwanted critters, b) not creating an opportunity for trapped moisture to support microbial growth, and c) protecting indoor air quality. For these reasons we generally do not recommend large applications of rigid foam products or anything with a paper (mold food) or foil (vapor barrier) backing. Blown-in dense pack cellulose is a good choice for our climate zone. [Note: There are some applications where rigid or spray foam is the best option available, but these scenarios seem to be the exception and not the rule.]

When building from scratch it’s an excellent opportunity to better insulate and extend the thermal envelope of the building to include the entire crawlspace or basement. Extending the thermal envelope of your building to include the crawlspace requires mechanically conditioning and ventilating the space so there is inevitably an energy cost. The energy and comfort benefit to air sealing is that it reduces temperature differences between “indoors” and the crawlspace. For retrofits  we recommend insulating the crawlspace ceiling (building floor between or over joists) and allowing the space to ventilate naturally instead of insulating the crawlspace floor and perimeter foundation walls.

Where to Install the Air and Vapor Barriers in a Crawlspace?

Where to install an air barrier or vapor barrier depends on far too many variables to have a simple answer. Building science is an expertise in itself and generally this decision is left to the licensed design and construction professionals on the job. Installing an air barrier is almost always a good idea as air penetration is one of the biggest energy losses AND primary ways contaminants enter a building. However, installing a vapor barrier is tricky business and installed in the wrong location, it may very well cause significant damage to your building.

Crawlspace upgrade for Marin County.

Photo compliments of Gavin Healy.

Crawlspace air barriers are usually sheet good products that come in a roll, or a 4’x8′ solid sheet like plywood. If you can put your mouth to it and not easily blow through (e.g., drywall, Tyvek, plywood, foam, caulk, etc.) it is probably considered an air barrier (or “air retarder”). Any of the common sheet products commercially available will do.  The objective is simple: reduce air transfer between the crawlspace and the occupied portions of the home. The how-to is more complicated and may involve sheet goods, expandable foams, caulks, etc. While it’s not rocket science, good air sealing requires attention to detail, knowledge of product options for each situation, and how to affix each product with a compatible material or fastener without compromising the air barrier.

If there is no insulation between floor joists and crews are already working below the building it’s an opportune time to accomplish other air sealing tasks. One can air-seal around plumbing, electrical, data and general framing cracks, holes, and gaps by using foam backer rods, caulk and expanding foam. Be sure to air seal around kitchens, baths and chimneys before they are made inaccessible by new insulation.

Vapor barriers are more complicated. A vapor barrier (or “vapor retarder”) will stop vapor (moisture laden air) movement from one side to the other. Examples of vapor barriers include sheet plastic, foam products, glazed tile, vinyl flooring or vinyl wall paper, etc.

Since vapor barriers restrict moisture movement they also inadvertently trap moisture. If the underside of vinyl flooring gets wet due to a plumbing leak or condensation, moisture gets trapped between the vinyl and a wooden sub-floor. The moisture’s slow rate of drying may facilitate dry rot and microbial growth. Vapor barriers are often installed without much consideration, or intentionally installed on the crawlspace floor.

The right place to install a vapor barrier in the San Francisco Bay Area is over the soil under a building. It is hard to go wrong with installing plastic over the soil and/or below the slab. The material should be thick enough to resist puncturing, reinforced so as not to tear, light in color for good visibility and reflectivity, and properly rated as a vapor barrier or vapor retarder. Materials rated for radon mitigation are guaranteed to get the job done, but there are plenty of other 3-ply materials available on the market that work to minimize soil moisture (and gases) from entering living space.

A fundamental concept in building science is to align all barriers (thermal, weather, air, vapor) from slab, to floor, to wall, to roof. This is not always possible, but improper placement of any of these layers may cause more harm than good for your building. The process must be intentional and cautious.

Insulation and vapor barrier in crawlspace.

Photo compliments of Gavin Healy.

Signs You Need A Clean Crawlspace

You need to address the bulk water issue if there is ever standing or running water under your building . Musty smells or frequent condensation on windows are other signs you may need to address moisture in your crawlspace. Evidence of critters under the building or repeated failed efforts to rid them mean you need to invest some time and money down below. If your feet are cold standing barefoot indoors…yup, it’s time to look under the floor. It’s not a fun job and you cannot see the improvements, but trust me when I tell you that you’ll sleep better at night (or work better during the day) knowing you’ve got a clean crawlspace.

For residential projects in San Francisco, Marin County, and Sonoma County, let Four Season Stewardship help you with the planning and execution of your clean crawlspace!

Special thanks to Bill Hayward and Carl Grimes of Hayward Healthy Home, and Gavin Healy of Balance Point Home Performance for their contributions to my knowledge on this subject, and some photos for this blog. Also gratitude to the PG&E Energy Center for hosting great seminars on high performance crawlspace upgrades.

5 Responses to “A Clean Crawlspace is Better For You and Your Home”

  1. Alex says :

    If attic or crawlspace access points are too small, nobody ever goes in voluntarily. And if you don’t inspect these out-of-the-way areas in a home it can lead to deferred maintenance and major problems down the road. If you have the luxury of designing from scratch – put in an awesome door!

  2. Alex says :

    And too often nobody goes to inspect a crawlspace for years… until there’s a bad smell or odor complaint… or a contractor goes down to do a job… and then they report major water damage or pest infestations. By that point the damages can be very extensive. So we suggest that folks inspect their attics and crawlspaces at least twice a year, and ideally quarterly. For Marin, Sonoma, Napa and San Francisco counties we have a sister division that focuses on residential general contracting. offers a unique quarterly maintenance service where our two-person team inspects the attic and crawlspace so as to avoid long-term deferred maintenance problems.

  3. Absolutely! Find and fix minor drips before they cause major damage.

  4. jarboes says :

    Every plumbing leak has the potential to severely damage the interior of your home or business. If left unchecked, even a slow drip can cause serious problems including health issues for some people and animals. A hidden leak in the building may take quite some time to notice. As the leak continues, water from the pipe will begin to soak the floors and walls.

  5. Tiff Gregers says :

    How great that you talk about how an access hatch is important for kids and pests and crawl space. I am building a new home this year and I want a crawl space. I will find a good crawl space insulation service as well in the area.

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