Without commentary:

NC Building Code WITH OUR COMMENTS / BOLD / {emphasis ours}

R409.1 Air sealed walls. Closed crawl spaces shall be built to minimize the entry of outdoor air into 
the crawl space. Specifically prohibited are foundation wall vents and wall openings to ventilated 
porch foundations. When outdoor packaged heating and cooling equipment is used, solid blocking
 and sealants shall be used to seal gaps between the exterior wall opening and the smaller supply and
 return ducts that pass through the opening. (The sheet-metal transition that connects the package unit to the masonry has been unanimously approved by code officials even though it doesn’t contact the “smaller supply and return ducts”, from Wilmington to Asheville in our experience, without any foam-board, plywood, etc.)

R409.1.1 Caulking and sealants. Air sealing caulk, gaskets or sealants shall be applied to 
the foundation wall and floor assembles that separate the crawl space from outside and other
 ventilated areas such as joints around access door and frame, between foundation and sill
 plate, at penetrations for plumbing, mechanical, electrical and gas lines and at duct
 penetrations. (This section should be expanded to mandate “floor system air-sealing”)

R409.1.2 Access panel/door. A minimum access opening measuring 18 inches by 24 inches 
(457 mm by 610 mm) shall be provided to the crawl space. See the North Carolina 
Mechanical Code for access requirements where mechanical equipment is located under 
floors. To minimize air entry, provide a tight fitting access panel/door with a latch mechanism.
 Access panels or doors shall be insulated to a minimum of R-2. (It would make sense here if they clarified if this applies to floor-insulated crawlspace doors. We insulate them all anyway.)

R409.2 Ground vapor retarder. Closed crawl spaces shall be protected from water entry by the 
evaporation of water from the ground surface.

R409.2.1 Ground vapor retarder.A minimum 6-mil (0.15 mm) polyethylene vapor retarder or
 equivalent shall be installed and secured to nominally cover all exposed earth in the crawl
space, with joints lapped not less than 12 inches. Minor pockets or wrinkles that prevent total 
drainage across the surface of the vapor retarder are allowed.The floor of the crawl space
shall be graded so that it drains to one or more low spots. Install a drain to daylight or sump 
pump at each low spot. Crawl space drains shall be kept separate from roof gutter drain
 systems and foundation perimeter drains.

R409.2.2 Liner. The ground vapor retarder may be installed as a full interior liner by sealing 
the edges to the walls and beam columns and sealing the seams. Single piece liner systems 
are approved. The top edge of the wall liner shall terminate (here I would add AT LEAST) 3 inches below the top edge of the
 masonry foundation wall. The top edge of liner shall be brought up the interior columns a
 minimum of 4 inches above the crawl space floor. The floor of the crawl space shall be graded
 so that it drains to one or more low spots. Install a drain to daylight or sump pump at each low
 spot. Crawl space drains shall be separate from roof gutter drain systems and foundation 
perimeter drains. WHAT DO THEY MEAN BY “SINGLE PIECE LINERS SYSTEMS? I have seen spray foam used for this. I would not call the way we do it “single piece” because we tape together many pieces. 

R409.2.1.1 Wall liner termite inspection gap. Provide a clear and unobstructed 3”
 minimum (here, 3″ is a minimum!) inspection gap between the top of the wall liner and the bottom of the wood 
sill. This inspection gap may be ignored with regards to energy performance and is not
 intended to create an energy penalty.

R409.2.3 Concrete floor surfacing. The ground vapor retarder may be protected against st
ripping and displacement by pouring an un-reinforced, minimum 2-inch thick, concrete surface
 directly over the vapor barrier. A base coarse of gravel or other drainage material under the
 ground moisture barrier is not required. The floor of the crawl space shall be graded so that
 the concrete surface drains to one or more low spots. Install a drain to daylight or sump pump
 at each low spot. Crawl space drains shall be separate from roof gutter drain systems and 
foundation perimeter drains.

R409.2.4 Drains and vent terminations. Drains (including but not limited to hot water tank
 pressure relief and drain pans, and condensate drain pipes) shall terminate outdoors, to crawl 
space floor drains or interior pumps, and shall not intentionally discharge water into the crawl
space. Crawl space drains shall be separate from roof gutter drain systems and foundation 
perimeter drains. Dryer vents shall terminate outdoors.

R409.3 Wall damp-proofing. Where the outside grade is higher than the inside grade the exterior
 walls shall be damp-proofed from the top of the footing to the finished grade as required by R406.1. IMPORTANT: This is the part of the code that allows a builder to dispose of waste-soils inside a foundation, aka back-fill grading, leaving workmen with no room to crawl, and the claim that the interior grade of the crawlspace is above the exterior and no damp-proofing is required. 

R409.4 Site grading. The building site shall be graded to drain water away from the crawl space
 foundation per the requirements of R401.3.

R409.5 Space moisture vapor control.Closed crawl spaces shall be provided with a mechanical
 drying capability to control space moisture levels. The allowed methods are listed below in R409.5.1 –
R409.5.5. At least one method shall be provided; however, combination systems shall be allowed.

R409.5.1 Dehumidifier. A permanently installed dehumidifier shall be provided in the crawl
space. The minimum rated capacity per day is 15 pints (7.1 Liters). Condensate discharge 
shall be drained to daylight or interior condensate pump. Permanently installed dehumidifier
 shall be provided with an electrical outlet. {not recommended-SHS}

R409.5.2 Supply air. Supply air from the dwelling air conditioning system shall be ducted into 
the crawl space at the rate of 1 cubic foot per minute (0.5 L/s) per 30 square feet (4.6m2) of
 crawl space floor area. No return air duct from the crawl space to the dwelling air conditioning 
system is allowed. The crawl space supply air duct shall be fitted with a backflow damper to 
prevent the entry of crawl space air into the supply duct system when the system fan is not 
operating. An air relief vent to the outdoors may be installed. Crawl spaces with moisture 
vapor control installed in accordance with this section are not to be considered plenums. {highly recommended-SHS}

R409.5.3 House air. House air shall be blown into the crawl space with a fan at the rate of 1
 cubic foot per minute (0.5 L/s) per 50 square feet (4.6 m2) of crawl space floor area. The fan 
motor shall be rated for continuous duty. No return air duct from the crawl space back to the
 dwelling air conditioning system is allowed. An air relief vent to the outdoors may be installed.
 Crawl spaces with moisture vapor control installed in accordance with this section are not to
 be considered plenums. {not recommended-SHS} We have done this dozens of times to avoid installing a costly dehumidifier. 

R409.5.4 Exhaust fan. Crawl space air shall be exhausted to outside with a fan at the rate of
 1 cubic foot per minute (0.5 L/s) per 50 square feet (4.6 m2) of crawl space floor area. The fan 
motor shall be rated for continuous duty. There is no requirement for make-up air. {not recommended-SHS} our system is piggy-backing off the existing HVAC supply trunk or distribution box so it naturally does more work the more you need it to do so. We then install a $25 monitor for the crawlspace that is placed next to the thermostat so that the homeowner sees the crawlspace conditions and interior conditions regularly. If a home is “moist” making it more negative is not going to help and pulling from a wet home will not make a wet crawlspace dry. 

R409.5.5 Conditioned space. The crawl space shall be designed as a heated and/or cooled,
 conditioned space with wall insulation installed per the requirements of R409.8.1. Intentionally
 returning air from the crawl space to space conditioning equipment that serves the dwelling
 shall be allowed. Foam plastic insulation located in a crawl space plenum shall be protected
 against ignition by an approved thermal barrier. {not recommended-SHS}

R409.6 Plenums. Closed crawl spaces used as supply or return air plenums for distribution of heated
 or cooled air shall comply with the requirements of the NC Mechanical Code. Crawl space plenums 
shall not contain plumbing clean-outs, gas lines or other prohibited components. Foam plastic 
insulation located in a crawl space plenum shall be protected against ignition by an approved thermal 
barrier. {not recommended-SHS}


R409.7 Combustion air. The air sealing requirements of a closed crawl space may result in a 
foundation which can not provide adequate combustion air for fuel-burning appliances; therefore, fuel burning
 appliances located in the crawl space such as furnaces and water heaters shall obtain 
combustion air from outdoors as per the NC Mechanical Code.

FIGURE A-2 
ALL AIR FROM OUTDOORS— INLET AIR FROM VENTILATED CRAWL SPACE AND OUTLET AIR TO VENTILATED ATTIC 

NOTE: The inlet and and outlet air openings shall each have a free area of not less than 1 square inch per 4,000 Btu per hour of the total input rating of all appliances in the enclosure. 

For SI: 1 square inch = 645 mm^, 1 British thermal unit per hour = 0.293 1 W.

There is another important issue, combustion. for a furnace of 50,000 BTUs you would need to have 50 cubic feet of volume per kbtu or 2,500 cubic feet of space... that gives you a space of roughly 700SF at 3.5' tall. We have never had the math on a crawlspace come back as "confined".

R409.8 Insulation. The thermal insulation in a closed crawl space may be located in the floor system 
or at the exterior walls. The required insulation value can be determined from Table N1102.1. 
Exception: Insulation shall be placed at the walls when the following condition exists: 
1. The closed crawl space is designed to be an intentionally heated or cooled, conditioned
 space.

R409.8.1 Wall Insulation. Where the floor above a closed crawl space is not insulated, the
 walls shall be insulated. Wall insulation can be located on any combination of the exterior and 
interior surfaces and within the structural cavities or materials of the exterior crawl space walls.
 Wall insulation systems require that the band joist area of the floor frame be insulated. Wall 
insulation shall begin 3 inches below the top of the masonry foundation wall and shall extend
 down to 3 inches above the top of the footing or concrete floor, 3 inches above the interior
 ground surface or 24-inches below the outside finished ground level, whichever is less. No 
insulation shall be required on masonry walls of 9 inches height or less.

R409.8.1.1 Foam plastic termite inspection gap. For outside wall Section R324 
governs applications. When expanded polystyrene, extruded polystyrene,
 polyisocyanurate, or other foam plastic insulation is installed on the inside surface of
 the exterior foundation walls, provisions R409.8.1.1.1 – 2 below apply.

R409.8.1.1.1 Earth floored crawl spaces. Provide a clear and unobstructed 3-
inch minimum termite inspection gap between the top of the foam plastic wall
 insulation and the bottom of the wood sill. Because Insulation ground contact is
 not allowed, provide a continuous 3-inch minimum clearance gap between the 
bottom edge of the foam plastic wall Insulation and the earth floor surface.
 Refer to N1102.1.7 to determine maximum allowances for insulation gaps.

R409.8.1.1.2 Concrete floor surfaced crawl spaces. Provide a clear and
 unobstructed 3-inch minimum termite inspection gap between the top of the 
foam plastic wall insulation and the bottom of the wood sill. Provide a 
continuous 3-inch minimum clearance gap between the bottom edge of the 
foam plastic wall Insulation and the concrete floor surface. Refer to N1102.1.7 
to determine maximum allowances for insulation gaps.

R409.8.1.2 Porous insulation materials. When fiberglass, rockwool, cellulose or
 other porous insulation materials are installed on the inside wall surface of a closed 
crawl space, provide a clear and unobstructed 3-inch minimum termite inspection gap 
between the top of the porous wall insulation and the bottom of the wood sill.
 To reduce wicking potential, porous insulation ground contact is not allowed in earth 
floored or concrete surfaced crawl spaces. Provide a continuous 3-inch minimum 
wicking gap between the bottom edge of the porous wall Insulation and the earth or 
concrete floor surface. Refer to N1102.1.7 to determine maximum allowances for 
insulation gaps.