June 2011: Vented Crawl Spaces


Vented Crawl Spaces

Conditioning a crawlspace when living within in a mixed humid climate can be one of the largest factors in moderating and affecting indoor humidity levels. Obviously one of the major determinative factors is if a persistent moisture condition exists already.  One needs to make the call with a number of considerations in mind: can the source be diminished with a minimal financial investment, is it practically feasible to do so, and is one confident that the remedy will in fact work? The other health and safety consideration is the scenario one will be in effect creating. If there are combustion appliances in the vented or semi conditioned crawl space and one plans upon a total conditioning, then allowance for proper make up air must be taken into account (50 cubic ft for every 1,000 BTUs).This must always be built into the work scope on the front end and in proper sequence. If the moisture source is too great, then the pressure thermal boundary should remain as the floor and the space be intentionally vented with humdistatically controlled vents. Sometimes by adding a sump pump to the low spot in the crawl and diverting most of the ground source moisture there will be enough. There are Dranjer covers that can be added that will allow moisture to go one direction no vapor or radon gases to pass through in the other direction.

If the crawlspace is relatively dry and connected to an existing basement then the three exterior sides should be done. Depending upon the size of the crawlspace walls, sometimes doing the exterior can be one half or one third of the area when compared to the floor area. This approach either initially or upon retrofit, can reduce the insulation costs and remove the barrier from having to work overhead. That being said, if the walls are very tall only the first foot below the frost line and all area above should be foamed. If it is a separate zone under the dwelling, then all crawlspace vents should be blocked and all accesses and hatches addressed. If balloon framing exists and the wall cavities are open to the crawl then they should at least be blocked off or better yet utilized to dense pack the wall cavities. If the space is known to remain dry then open cell could be used but if there is any chance of exacerbated moisture levels or conditions then closed cell should be employed. The vapor barrier should be installed with the few seams possible and sealed at all the penetrations and follow best practices. The thickness of the vapor barrier can either be increased or decrease depending upon access, use, and amount of resultant moisture conditions. In any case humidity levels should be monitored and kept below levels where the dew point and condensation could occur. All other installation considerations should follow the Guidelines section on: Floor area and Crawlspaces.


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