Crawlspaces & Ventilation: 10 Fun Facts & Random Observations

1. Inside the living space of a property, proper ventilation is absolutely crucial to the health, safety, and overall comfort of the occupants inside the home.

2.  Standards and guidelines have been established to specify minimum ventilation rates and other measures intended to provide indoor air quality that is acceptable to human occupants and that minimizes adverse health effects. More info here:  http://eetd.lbl.gov/ie/viaq/v_rates_1.html

3.  Improving the overall air quality inside a property can be achieved by two methods:

•    Exhausting air contaminants from the building
•    Removing contaminants from the air stream using filtration and/or absorption technologies (i.e. HEPA filtration and activated carbon)

Since most residential properties do not have access to proper filtration or absorption technologies, exhaust ventilation is most practical and commonly used.

4. In addition to improper ventilation, excessive or chronic water intrusion into the property, especially the crawlspace, will contribute to the growth of certain microorganisms.  This can lead to mold infestation impacting Indoor Air Quality, and even more destructive structural damage such as wood decay or dry rot.

5. Water intrusion into the crawlspace will often cause damage to flooring systems (i.e. cupping of hardwood floors, grout separation in tile floors, etc.), wood decay, and oxidation or rusting of metal strapping/hardware.

6. Water enters a crawlspace in either liquid or vapor phase by four moisture transfer mechanisms:

•    Liquid water (i.e. plumbing/sewer leaks, high groundwater table, drainage or exterior flooding)
•    Capillary suction or wicking (i.e. moisture being drawn through concrete footing from saturated exterior soils)
•    High moisture laden air (i.e. elevated humidity from atmospheric conditions entering the crawlspace through vents)
•    Vapor diffusion (i.e. moisture in the vapor phase moving through building materials)

7. Most properties are constructed with vents that are intended to remove moisture from the air in a crawlspace by cross-ventilation.   However, the introduction of moist air from outdoors can actually increase the relative humidity in a crawlspace.

8. Due to stack effect and vapor diffusion, which is a very powerful force, moisture in a crawlspace will seek dry areas.  When moist air comes in contact with  a surface that is colder than the  air, condensation will occur.  Condensation can develop on uninsulated plumbing pipes in the crawlspace, on the underside of a sub-floor, or even the attic roof deck.  Interestingly, many houses with exposed wet soil in a crawlspace also have mold and water damage due to condensation on the underside of the roof deck. 

9. If vapor diffusion from the soil, water intrusion from poor drainage, unmitigated plumbing leaks, or infiltration of moist air exist in a crawlspace, one or more of the following is usually observed:

  •   Surface mold growth, structural damage, and health issues
  •   Termite or other pest infiltration
  •   Accumulation of odors  
  •   Termite or other pest infestation

10.  The best way to mitigate crawlspace moisture is to treat the crawlspace as a conditioned space by (1) insulating walls with foam panels, (2) seal the crawlspace floor and walls with heavy gauge polyethylene or vinyl encapsulation system, with the seams sealed tightly at all edges and overlaps, (3) seal the rim joists with two-part closed cell foam.

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