Vented Crawlspace: Dew Point Logic and a Borate-and-Fans Request
A 1970s ranch home, 1,400 sq ft over an 1,800 sq ft vented crawlspace. The vapor barrier is 4-mil poly over bare soil, with no lapped or sealed seams and tears over roughly 30 percent of the area. Eight 8 in. by 16 in. foundation vents are fully open. Floor joists are 2x10 at 16 in. on center; the north third shows white and black growth, and subfloor readings there are 19 percent MC against a 10 percent dry standard. Crawlspace air is 68 degrees F and 78 percent RH; outdoor air is 90 degrees F and 60 percent RH. Supply ducts cross the crawlspace, three sections have torn and hanging insulation, and the hallway return is partly open at a boot. The homeowner has asthma, runs the HVAC fan continuously, and wants to keep the vents open because ventilation dries it out, plus a borate spray on the joists and two box fans. The property is a rental, and the landlord wants a one-day, low-cost fix.
1. Use the temperature and humidity data to explain why keeping the vents open is adding moisture in this case.
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Convert both air masses to dew point and compare with the crawlspace temperature.
Crawlspace: at 68 degrees F the saturation vapor pressure is about 0.34 psi, so at 78 percent RH the vapor pressure is about 0.78 x 0.34 = 0.265 psi, which corresponds to a dew point near 61 degrees F. The crawlspace surfaces are at or below that; nothing condenses yet, but there is very little margin.
Outdoor air: at 90 degrees F the saturation vapor pressure is about 0.70 psi, so at 60 percent RH the vapor pressure is about 0.42 psi, giving a dew point near 74-75 degrees F.
When that outdoor air enters through the vents and cools toward the crawlspace temperature of 68 degrees F, it drops below its own dew point by roughly 6-7 degrees. The excess moisture condenses on the joists, the subfloor, the duct jacket, and any cool surface. Ventilation only dries when the outdoor dew point is lower than the crawlspace dew point, and here it is about 13 degrees higher.
This is consistent with the field data: 19 percent MC against a 10 percent dry standard, with the growth concentrated on the cool north side. The answer is to close and condition the crawlspace, not to open it further.
2. Set out the order of work and the criteria you would use to release the crawlspace.
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Sequence:
- Source control first. Correct exterior grade and roof or plumbing leaks, then fix the duct system: replace the torn and hanging insulation, seal the boots and the open hallway return, and verify that the return is not drawing crawlspace air into the living space.
- Change the operating condition. Until the crawlspace is controlled, set the thermostat fan to auto rather than continuous; a continuously running fan increases the pressure difference across the floor assembly and pulls crawlspace air into the house.
- Gross clean. Remove debris and any organic material off the soil, HEPA vacuum the joists and subfloor, wire-brush the growth, HEPA vacuum again.
- Dry to goal. Aim for wood at or below roughly 16 percent MC or within 2 points of the dry standard, and crawlspace air at or below about 55-60 percent RH with a controlled dew point.
- Treat only after cleaning and drying. A borate or an EPA-registered coating is a preventive; it does not remove a reservoir.
- Install a sealed 10-20 mil vapor barrier, lapped roughly 12 in., mechanically fastened and mastic-sealed to the foundation walls and piers, with penetrations sealed. Close and insulate the vents.
- Install a correctly sized dehumidifier with a condensate drain and a monitoring data logger.
Release criteria: wood moisture at goal, RH stable over 24-48 hours including a night cycle, no visible growth or dust, ducts sealed, and a written record with photos and readings.
3. The homeowner wants borate spray plus box fans, and the landlord wants it done in one day. Draft your response to both.
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To the homeowner: the borate is a reasonable preventive on clean, dry framing, but it does nothing about the water coming in. The fans make it worse in this weather, because they push 74-degree-dew-point air across 68-degree joists and drive condensation into the wood, and they also stir spores toward the return leak. The asthma in the household is a reason to avoid unnecessary chemical fogging and to stop the air pathway from the crawlspace into the living space, not a reason to skip the work.
To the landlord: one day will not achieve the goal. Cleaning can be done in a day, but drying and verification typically take several days and depend on the weather. The real cost drivers are the vapor barrier, the dehumidifier, the duct repair, and correcting the venting strategy; this is not a cosmetic job, and the growth will return if the moisture is not controlled.
Offer a phased scope so the landlord has a choice: Phase 1 covers source control, removal of the reservoir, HEPA cleaning, duct sealing, and temporary dehumidification with monitoring. Phase 2 covers the sealed barrier, vent closure, and permanent dehumidification. Put both in writing with prices, and state plainly what happens if only Phase 1 is bought: the growth will recur, and the report will say so.