Crawlspace Ducts, Attic Exhaust, and Combustion Air
A 2,400-square-foot house in Climate Zone 5 has a vented crawlspace. The builder installed the supply and return ducts in the crawlspace but left them uninsulated except for a 6-mil vapor barrier on the soil. The bathroom exhaust fan discharges into the attic, 3 feet from a ridge vent. A natural-gas furnace is in a 6-foot by 8-foot closet with a louvered door; the closet also contains a gas water heater. The house has 2x6 walls with R-21 batts and a Class II vapor retarder on the interior. The homeowner complains of high humidity, cold floors, and a faint exhaust smell in the attic. The builder says the insulation and vapor retarder meet code because R-21 is greater than the R-20 wall requirement.
1. What is wrong with the crawlspace ducts and floor insulation, and what should be required?
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Uninsulated ducts in a vented crawlspace are not acceptable. The energy provisions require ducts in unconditioned space to be insulated and sealed; the exact R-value is set by the applicable IRC energy table, with R-8 commonly required for ducts in unconditioned space. A soil vapor barrier is not duct insulation and does not fix cold floors. Also, if the crawlspace is vented, the floor above it must be insulated to the required R-value, often R-30 in Climate Zone 5, or the crawlspace must be conditioned and the walls insulated. The high humidity and cold floors point to duct leakage and missing floor insulation. Require sealed, insulated ducts and the correct floor or crawlspace insulation detail, then verify by inspection.
2. Is the bathroom exhaust fan installation acceptable?
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No. Bathroom exhaust must discharge outdoors, not into an attic. IRC M1507 requires exhaust to be ducted to the exterior, and the termination must have a backdraft damper and be located away from intake openings per the manufacturer and code separation distances. A 3-foot offset from a ridge vent does not cure the problem because the attic is not outdoors and moist air will condense on roof framing. Require a smooth, insulated duct to a properly flashed exterior termination, with the fan rated for the duct length and static pressure.
3. Does the furnace and water-heater closet have adequate combustion air? What information is missing?
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It cannot be approved from the information given. The closet is 6 by 8 feet; assuming an 8-foot ceiling, the volume is 384 cubic feet. The standard indoor-air method allows 50 cubic feet of volume per 1,000 Btu/h of appliance input if the space is not unusually tight and the appliances are not fan-assisted in a way that changes the method. The inspector needs the input ratings of both the furnace and water heater. If total input exceeds the allowable volume, permanent openings or outdoor combustion air are required. A louvered door is not automatically adequate; the free area and opening size must be calculated. Also verify the water heater ignition source is elevated or protected as required.