Do the following situations and solutions fall in line with what you are doing to comply with IRC 408.3? There seems to be discrepancies between code officials. All I need is the practical solution.


Force Air Ducts in Crawlspace

– If 80% Install Louvered Vent to Air Supply, Install 6” External Combustion Air Vent

– If 90% or Air Handler not in Crawl - In the Crawlspace, Install Louvered Vent in Air Supply and Return

Nothing Mechanical in Crawlspace or Seal Combustion Boiler

– Input & Output from Conditioned Space through Two Internal Wall Cavities with Vent Grills

Unsealed Combustion Boiler in Crawlspace

– Install Two 6” External Combustion Air Vents Extended to Boiler

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First, just so everyone is on the same page – the applicable section…

R408.3 Unvented crawl space. Ventilation openings in under-floor spaces specified in Sections R408.1 and R408.2 shall not be required where:

1. Exposed earth is covered with a continuous Class I vapor retarder. Joints of the vapor retarder shall overlap by 6 inches (152 mm) and shall be sealed or taped. The edges of the vapor retarder shall extend at least 6 inches (152 mm) up the stem wall and shall be attached and sealed to the stem wall; and

2. One of the following is provided for the under-floor space:

2.1. Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7m2) of crawlspace floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.9;

2.2. Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.9;

2.3. Plenum in existing structures complying with Section M1601.5, if under-floor space is used as a plenum.

Personally 2.1 is a joke, 2.3 is a mess, which leaves one with 2.2 – insulate the Crawl Space walls (Crawl Space wall insulation reqs. vary from a U .477 for climate zones 1 & 2 / .136 for zone 3 / .065 for all others) & add in one supply & 1 return duct sized properly. Now there is nothing that says the grills have to be open, just that it can supply the air to help eliminate any moisture issues. If you are going this route, the HVAC designer needs to know this to appropriately size it when they are doing the JST&D calcs.

As for the AHJ, well they have the final say on what the codes actually mean & what they will accept, but a properly detailed diagram with the calcs should make them happy & alleviate them from pushing for their “rule of thumbs”

Now onto your list - the items you listed do not really fall into this section, and instead fall into the Mechanical section. Personally I don't want anything in the crawl space, but if anything mechanical is in there it would be a sealed combustion unit or electrical only. As for the others, find an area that you can place them, seal it off from the rest of the house, insulate it & supply combustion air from the outside only

Thanks and to clarify some more, I'm taking about sealing the crawlspace from the outside with expanding foam along the rim joists, vinyl faced fiberglass on the walls, and sealed plastic on the floor.
Uh, what? I can see doing some air sealing on the outside, but the rim joist needs to be insulated on the inside of the crawlspace with CC Foam (you can also run some foam on the exterior to cover it, but I would still insulate inside also). Maybe you can get away with FG in your crawls, but I wouldn't recomend it - I would generally  go with sprayed on CC foam or foam panels (assuming all water issues have been dealt with)
Right. Air sealing the keep outside air from coming in. But the foam, insulation, caulk or however you want to do it is on the inside of the crawlspace. My concern is meeting the ventilation requirements of IRC.


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