Sustainable Home Solutions

Homes For Humans

CRAWLSPACES

Most homes in the United States are built over either concrete slabs or raised crawlspaces.  If your home is built over a crawlspace, chances are you have little idea what's down there.  You may have peeked inside, and found it dark and creepy.  I've spent a fair amount of time in these places and can tell you there's a lot of stuff you should know, and a lot of stuff you'd rather not. 


A crawlspace usually has a bare dirt floor, though we'll occasionally find a rough concrete (aptly named) "rat slab" poured over the surface.  A raised perimeter foundation, most often concrete in our area, supports the exterior walls. On the interior, concrete piers may support wood posts which support wood beams that together support the underfloor framing.  Crawlspaces have traditionally been ventilated at the rate of one square foot per 150 square feet of underfloor area.  The idea was to provide adequate passive ventilation to allow the crawlspace to dry when it got wet.  Unfortunately, it was a bad idea.


Moisture in the crawlspace may come from a number of sources: 
  • Ambient levels (found in all soils)
  • Relative humidity
  • A high water table
  • Storm or irrigation water not effectively directed away from the foundation (finding a way in, but no way out)
  • A plumbing leak
  • Or a combination of any of the above.  
The moisture itself is bad enough, promoting mold growth and wood rot.  Toss in natural soil off-gassing (radon?), fiberglass insulation fibers, critters, and bugs, and you have the potential for a toxic soup that can undermine your health and safety, as well as the durability of the structure.


All of this is taking place in an area you never visit.  What you don't know can hurt you.


Most of that under-structure I described above is made of wood.  When wood gets wet, and isn't allowed to dry, it can decompose while creating a perfect breeding ground for mold and bacteria.  Neither is good for the durability of your home, or the health of your family.  The floor you thought separated your living space from your crawlspace is punctured with a multitude of holes of varying size, from plumbing and electrical penetrations, as well as framing openings.  These penetrations provide a convenient pathway for that crawlspace moisture to infiltrate your comfy home, and undermine the health and safety of all inside.  In addition, if your furnace and/or ductwork are located down there, leakage on the return air side can pull that moisture directly in with no obstruction.  None of this is good for your Indoor Air Quality.


Fortunately, we have solutions that will not only improve Indoor Air Quality for the occupants, but the durability of your home.
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A HEALTHY SOLUTION
The building science behind crawlspaces has led us away from the archaic passive ventilation approach, to a more effective solution.  That is, we understand they are an integral part of the house as a system, and can have a dramatic impact on our built environment.  As a result, we now think of them as mini-basements, connected to the house rather than a separate, isolated part.  We have to make them healthier and safer, or the building and occupants will suffer.

To accomplish that, we have to take an aggressive approach.  Proper grading of soils away from the foundation, as well as effective gutter and downspout placement, to reduce potential moisture intrusion, is essential.  Grading the crawlspace floor to a collection area, and installing a sump pump may be necessary.  Installing an effective vapor barrier to contain the moisture, and sealing all penetrations in the subfloor is of primary importance.  If ductwork is installed in the area, it must be properly sealed and insulated.  And finally, we want to close off all passive ventilation and insulate the perimeter foundation.  A nice, tight, comfy space, instead of the alternative.

Here are examples of a crawlspace with a properly installed vapor barrier.
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