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Dirty and Moldy Crawlspace

Clean Crawlspace For Better Moisture Management and Air Quality

March 25, 2019

Why Bother with a Clean Crawlspace?

When was the last time you went into a crawlspace? It’s generally a dark, dank environment where critters and odors run free! Too often old construction materials or storage boxes are crammed down below, providing habitat and food for critters and mold. Due to the stack effect air from crawlspaces is sucked up through small holes, gaps and cracks in the building and that nasty and wet air makes its way into your home. That’s why we suggest having your crawlspace professionally cleaned and “rebuilt.” While not as sexy as solar panels on the roof or a remodeled bathroom, a clean crawlspace can significantly increase the durability and healthy enjoyment of your home.

You may be living in the <1% of homes with a clean crawlspace already, but the odds are stacked against you. If you still have any doubts lift the hatch and take a peak and whiff. If you wouldn’t want to spend 30 seconds in your crawlspace – keep reading.

Crawlspace Renovation 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.

Goals for a crawlspace renovations often include moisture management, improved air quality, long-term durability of structural elements, and energy efficiency.

During a preliminary crawlspace inspection we aim to 1) Identify access and egress issues; 2) Identify any threats to life and safety; 3) Determine if the space is dry enough to seal and insulate; 4) Determine air barrier location: crawl space floor or walls and grade?; 5) Determine what grade of ground cover you’re going to install; 6) Survey mechanical, electrical and plumbing for opportunities; 7) Determine appropriate insulation for the surfaces considered; and 8) Collect measurements and photos to estimate any improvements.

Prioritize Water Control Strategies

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 your home, the moisture control strategy is very different than if there is just a little soil dampness. Liquid water from rain, stormwater runoff, improperly designed downspouts, or leaking pipes would be considered “bulk water,” and these are by far the worse offenders.

Water Control Strategies and Priorities in Damp Crawlspaces
Compliments of Gavin Healy

Condensation can be problematic, but in the moderate climate of the San Francisco Bay Area (Marin County and Sonoma County) condensation is not often a significant problem. Capillary action, or the ability of a material to transport water vertically (think about a towel left hanging over a bathtub) is often a problem when concrete or wood members are in direct contact with wet soil. Lastly, evaporation of bulk water or soil moisture can create dampness problems sufficient to support microbial growth. But of all these things controlling bulk, liquid water is the most important function of a well designed crawlspace or basement.

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.

Safe and Secure Crawlspace Access is Important

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 your 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.

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.

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 within the home careful consideration must be made to not to cross-contaminate the home 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 folks are working below a home 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).

What is Best Insulation for Crawlspace?

First one must determine the goals of insulation in a crawlspace. Generally insulation is installed to improve thermal comfort and reduce energy consumption. In the San Francisco Bay Area we have a very moderate climate zone where floor insulation 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.

Crawlspace renovations often 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 shy away from 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.

New foundations and crawlspaces offer an excellent opportunity to better insulate and extend the thermal envelope of the building to include the entire crawlspace. When you extend the thermal envelope of the building to include the crawlspace you must now mechanically condition and ventilate the space so there can be an energy cost, but wow is it wonderful to be in a clean and conditioned crawlspace! 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 (household floor) and continue to allow the crawlspace to naturally ventilate.

Where to Put the Air Barrier and Vapor Barrier in a Crawlspace?

It depends. Seriously, where to install an air barrier or vapor barrier in a crawlspace depends on far too many variables to have a short 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 home. However, installing a vapor barrier is tricky business and if it’s in the wrong location a vapor barrier can cause more harm than good.

Crawlspace upgrade for Marin County.
Photo compliments of Gavin Healy.

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 for each situation, as well as how to affix each product with a compatible material or fastener without compromising the air barrier.

When insulation is down and crews are already working in the crawlspace it’s an opportune time to accomplish other air-sealing tasks. Using backer rod, caulk and expanding foam one can air-seal around plumbing, electrical, data and general framing holes, gaps and cracks. Under kitchens and baths and around chimneys are frequently very leaky, and before new insulation is installed be sure to air seal the low-hanging fruit.

Vapor barriers are significantly more tricky. A vapor barrier (or “vapor retarder”) will, as its name implies, stop vapor (moisture laden air) movement from one side to the other. A sheet of metal is a vapor barrier, as is a sheet of plastic or rubber, most foam products, vinyl flooring or vinyl wall paper, glazed tile, etc.

So why are vapor barriers so tricky? Since vapor barriers restrict moisture movement they can inadvertently trap moisture. If, for instance, the underside of vinyl flooring gets wet due to plumbing leaks or condensation, then moisture is trapped between vinyl and a wooden subfloor. Since that moisture cannot easily dry to the inside… in will slowly dry to the outside. But the rate of drying may be too slow to stop wood decay and mold growth. In our climate zone vapor barriers are often either accidentally installed without much consideration, or intentionally installed on the crawlspace floor.

In our climate zone the most consistently right place to install a vapor barrier is over the soil under the home. You want a material that is thick enough to not be punctured easily, reinforced so as not to tear easily, light in color so it reflects light and is easy to inspect, and obviously a material that is officially rated as a vapor barrier. Any material rated for radon mitigation is guaranteed to get the job done, but there are plenty of 3-ply materials available on the market that will work to minimize soil moisture (and gases) from entering the crawlspace.

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.

Insulation and vapor barrier in crawlspace.
Photo compliments of Gavin Healy.

Signs You Need A Clean Crawlspace

If there is standing water under your home you must address the bulk water issue. If there is a musty smell or frequent condensation on windows in the home, you likely need to address moisture in the crawlspace. If there is evidence of critters under the home and repeated efforts to rid them have failed, you need to invest some time and energy down below. If your feet are cold standing barefoot indoors, yup – time to look down below.

Let Four Season Stewardship help you with the planning and execution of your clean crawlspace upgrade!

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 to the PG&E Energy Center for hosting great seminars on high performance crawlspace upgrades.

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