Clean Crawlspace is Better For You and Your Home
Is 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 run scare away even the most brave DIY building owners! Often old construction materials and cardboard boxes are crammed down below, providing harborage 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 and wet air makes its way into your home. In 80% of the buildings we inspect we end up suggesting the crawlspace be professionally cleaned and upgraded. While not as sexy as a new roof or new flooring, 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 still have any doubts lift the hatch and take a look and sniff. If you wouldn’t want to spend 30 seconds down there – keep reading and consider having an industrial hygienist come to test the 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 crawlspace access moved or expanded.
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 have to periodically enter and inspect the crawl space. Sufficient sizing of the access points is also important, as is not blocking access with mechanical systems such as ductwork or drain pipes.
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 there are often visual signs of the carpet acting as an air filter for air entering the home via stack effect or other pressurization. Access hatches indoors should be air-sealed, and when significant work is done in the crawlspace the surrounding area within the home should be placed in containment with negative air and air-scrubbers running. When contractors are working significantly more dust is generated than on an average day, 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
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 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 under-building drainage improvements)
- Determine air barrier location: crawl space floor or walls and grade (where to install an air-barrier is an important decision 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 for opportunities (if not now, 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, not harborage or food for pests or mold, non toxic, 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 the moisture control strategy will be very different than if there is minimal soil dampness. Liquid water from rain, stormwater runoff, poorly located downspouts, or leaking plumbing pipes would be considered “bulk water,” and these are the worse offenders.
In the moderate climate of the San Francisco Bay Area condensation is not often 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. Lastly, evaporation of bulk water or soil moisture can 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 and planned for these common moisture sources.
What is Best Insulation for Crawlspace or Basement?
First determine the goals of adding insulation to a crawlspace or basement. The primary purpose of insulation is to improve thermal comfort and reduce energy consumption. In the San Francisco Bay Area we have a very 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 reinstalling 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) not negatively impacting 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 this 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.]
If you’re 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. When you extend the thermal envelope of the building to include the crawlspace you must now mechanically condition and ventilate the space so there is inevitably an energy cost, but there is also an energy and comfort benefit to air sealing creating less of a temperature difference between “indoors” and under the floor. For retrofits in this climate zone we often do not recommend insulating the crawlspace floor and perimeter foundation walls, and instead insulate the crawlspace ceiling (building floor between or over joists) and allow the space to ventilate naturally.
How To Locate the Air Barrier and Vapor Barrier in a Crawlspace?
This is one of those “it depends” answers. Seriously, where to install an air barrier or vapor barrier in a crawlspace 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. Suffice it to say that installing an air barrier is almost always a good idea as air leakage is one of the biggest energy losses AND ways contaminants enter a building. Infiltration is a very real phenomena. However, installing a vapor barrier is tricky business and if it’s in the wrong location it may very well cause significant damage to the building.
Crawlspace air barriers are usually sheet good products including sheet-goods that come in a roll, or a 4’x8′ solid sheet like plywood. If you can put your mouth to it and not easy 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 a bit more complicated and may involve sheet goods, expandable foams, caulks, etc. While it’s not rocket science, good air-sealing does take a lot of attention to detail and one must know the right product options for each situation, as well as 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. Using foam backer rod, caulk and expanding foam one can air-seal around plumbing, electrical, data and general framing cracks, holes, and gaps. Under kitchens and baths and around chimneys are frequently very leaky, and before new insulation is installed be sure to air seal these areas 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, most foam products, vinyl flooring or vinyl wall paper, glazed tile, etc.
So why are vapor barriers tricky? 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 is then trapped between vinyl and a wooden subfloor. That moisture cannot easily dry to the inside. The slow rate of drying may allow dry rot and microbial growth. In our climate zone vapor barriers are often either accidentally installed without much consideration, or intentionally installed on the crawlspace floor. Some building science experts believe the only good place for a vapor barrier in our climate is directly over the soil beneath a building.
Around the San Francisco Bay Area the most consistently right place to install a vapor barrier is over the soil under a building. It is hard to go wrong with installing plastic over the soil and/or below the slab or rat slab. The material should be thick enough to not be punctured, reinforced so as not to tear, light in color for good visibility and reflectance, 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 will work to minimize soil moisture (and gases) from entering the space.
As your designers or builder before installing any new barriers in your crawlspace. A fundamental concept in building science is to try and align all barriers (thermal, weather, air, vapor) from slab, to floor, to wall, to roof. This is not always possible, but know that improper placement of any of these layers may cause more harm than good for your building. So proceed with caution.
Signs You Need A Clean Crawlspace
If there is ever standing or running water under your building you should address the bulk water issue. Musty smells or frequent condensation on windows are other signs you likely need to address moisture in the crawlspace. If there is evidence of critters under the building and repeated efforts to rid them have failed, you must invest some time and money down below. If your feet are cold standing barefoot indoors, yup – 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.
Who is Healthy Building Science?
Environmental Testing Services at HBS
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