Crawl space retrofit: three zones, low clearance
We are pursuing a remodel of a 2-story house in climate zone 3, coastal California. The first floor was built around 1920, and the second floor was added in 1980.
In addition to changing interior layouts, we are trying to make it “tight, well insulated, and mechanically ventilated”, with a primary goal of higher IAQ due to dust/mold sensitivity. Thermal comfort is important too, but the climate is pretty mild.
The foundation is a perimeter type, but it is separated into three areas (there are center walls that divide the crawl space up into three zones. Access is via a side cut on one area, and through floor hatches on the other two areas, but we are losing the two interior access points. The crawl space is currently vented, and has about 14″ of clearance between the dirt and the floor joists (sketch attached). I was in there on Friday, a few days after rain, and the dirt was moist but no signs of standing water.
The original plan with the crawl space was to encapsulate with a poly lining (floor and up walls), add R-5 or -10 insulation to the interior of the perimeter walls, seal and insulate vents, air seal between floor, and mechanically ventilate.
My designer suggested that we add spray foam insulation underneath the floor, and leave it ventilated, but I’ve never seen that approach. I have higher confidence in the encapsulation method, but 14″ doesn’t seem like enough room to work.
Also, the three zones makes it a bit tougher to ventilate. If we use an HRV (HVAC system is not designed yet), it would require three duct connections. Heating now is met through a single downstairs floor furnace, and remodel will likely change this heat source to direct vented gas fireplace. No existing AC or mechanical ventilation.
What do people recommend I do with this crawl space?
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