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Q.We are about to begin construction on an addition. Code requires venting the crawlspace under the wood floor. However, the foundation will be surrounded by concrete sidewalks on three sides, limiting the use of passive vents. Are there other alternatives for venting?

A.Everything depends on which section of the code you are reading. The 1989 and 1992 editions of the CABO One and Two Family Dwelling Code requires that foundation walls extend at least 8 inches above the finished grade to prevent moisture problems from splashback that can rot wood siding and framing. An exception can be made if the house is sided with a brick veneer. In this case the foundation must extend at least 4 inches above grade. So, if you are planning on wood siding, there should be enough room above finish grade (in your case the concrete flatwork) to install crawlspace vents. If you have brick veneer, you may have to vent into the band joist area with short ducts into the crawlspace.

Most codes require at least one square foot of free vent area for every 150 square feet of floor area. Remember, the floor framing must be at least 18 inches above the floor of the crawlspace, or treated wood framing is required.

Alternatively, the code often allows underfloor ventilation by an "approved mechanical means." However, approval of any system will require a lengthy code review, which will probably require design and certification of the ventilation system by an engineer.