[Greenbuilding] Crawl Spaces Again

elitalking elitalking at rockbridge.net
Tue Apr 26 12:55:19 PDT 2011


I am looking for advice on crawlspace humidity issues when the floor defines the thermal barrier.  


I have a client that lives in a low spot in an urban neighborhood.  Recent heavy rains and flawed city storm drainage resulted in his crawlspace being flooded.  Fortunately it did not reach the floor.  However, the original installation aspired to be a sealed crawlspace.  It had plastic on the ground and coming up the walls stapled to anchor plate below floor. Walls were covered by fiberglass that was trashed in flood.  


The crawl space has ducts and a 15 year old forced air gas furnace.  It appears to have survived the flood.  I understand how to install a better sealed crawlspace using foam instead of fiberglass.  I also am encouraging him to insulate the ground if he goes that route.  However, we are also considering removing the old furnace and ducts to replace with a high efficiency mini split heat pump.  This avoids the need HVAC equipment in crawlspace.  Therefore, the thermal barrier can be defined at the floor.  I have seen Journal of Light Construction report that floor installed thermal barrier is more energy efficient than sealed crawl space.  The installation they described did not include insulation on the ground, however. With floor defined thermal barrier, the efficiency would be even greater if no HVAC is located in crawl space.  Also, this puts the installation out of harms way if another similar event occurs again.  The authors of the study article were advocates for sealed crawl space that were dutifully reporting the results of their study.  The main advantage for sealed crawlspace is that the humidity is better managed with house HVAC system than ventilation with outside humid air during hot time of year.  


I am considering recommending the floor bottom be sealed with closed cell foam.  This would provide a vapor barrier to protect the floor framing.  Currently the house is not cooled.  However, the mini split heat pump would provide this option.  Therefore the conductive U value would need to be low enough to avoid condensation on the warm humid crawlspace side (bottom of floor) during the cooling season.  If all framing is covered with layer of 1or more inches of foam, are they protected from humidity?  My assumption is that the masonry foundation is not vulnerable to damage from humidity or similar flooding?  


As always, your thoughts are appreciated. 


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