Ventilating Crawl Spaces: The Wrong Way and the Right Way

The EPA’s Indoor airPLUS program says ventilating air entering unconditioned crawl spaces from outside during warm, humid seasons actually wets these spaces.

They note that this problem can happen “even if effective damp-proofing and drainage systems are installed to keep water out, and even if a vapor retarder is in place on the earth floor.”

To avoid such scenarios, they say, conditioned crawl spaces should include the following features:

  • Sealed perimeter walls (to prevent air infiltration). Air sealing should include doors and hatches, joints between sills and joists and between joists and subfloor and around ducts, pipes and other penetrations.
  • Insulated walls (to reduce heat transfer). Insulation can be applied either to the exterior or interior; however, don’t use interior insulation products that include material that can act as a vapor barrier, as it may trap moisture from the foundation.
  • Floor covered with vapor retardant membrane (to control soil moisture).
  • Floor drains that lead either to the outside or to a sump pump.
  • Either mechanical exhaust ventilation or conditioned air supply.

These strategies are illustrated below.

Mechanical Exhaust

An exhaust fan located in a crawlspace wall removes a small amount of air from the crawlspace; this is replaced by conditioned air that is drawn in from the living space through cracks and other small penetrations.

Conditioned Air

In this scenario, a small amount of conditioned air (minimum 1 cfm per 50 square feet of horizontal floor area) is blown directly from the HVAC system into the crawl space. Although this diagram shows conditioned air coming from equipment located in the living space above, HVAC equipment can also be located in the crawlspace or basement.

Source: EPA Indoor airPLUS

Addendum: Here are a bunch of articles old and new about crawlspace ventilation and insulation:

  1. Building Science Corporation: Unvented Crawlspaces and Conditioned Basements - This document provides an understanding of basements and crawl spaces, including unvented crawl spaces. It covers the principles of moisture behavior, air movement, and temperature, and how they interact with foundations.

  2. Energy Vanguard: The Science of Crawl Space Encapsulation - This article discusses the science behind crawl space encapsulation, including unvented crawl spaces. It explains the benefits and the process of encapsulation.

  3. Green Building Advisor: Building an Unvented Crawl Space - This article provides detailed guidance on building an unvented crawl space. It includes information on code requirements, insulation, vapor barriers, and other key considerations.

  4. ResearchGate: Performance of Sealed and Insulated Crawl Spaces: A Study in the Mixed-Humid Climate - This research paper on ResearchGate examines the performance of sealed and insulated crawl spaces, including unvented options, in a mixed-humid climate.

  5. NCBI: Impact of Crawlspace Conditions on Air Quality in the Home - This research paper explores the impact of crawl space conditions, including unvented crawl spaces, on indoor air quality. It includes scientific findings and recommendations.

  6. Fine Homebuilding: Sealing a Crawlspace - This guide provides step-by-step instructions on sealing a crawl space, including considerations for unvented crawl spaces. It includes practical tips and illustrations.

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