Unvented Crawl Space

2012 Int. Res. Code requires unvented crawl spaces that are supplied conditioned air to receive a rate of 1 cfm for each 50 square feet of under floor area including a return duct to the common area with the perimeter walls insulated (R408.3). I am being questioned on air change rates for a crawl space. My understanding is that the air change rate is not relevant in this case. Rather, where I have 2300 sq ft of floor area in the crawl space, I would just need to show that I was providing 46 cfm. I had installed a 4 inch duct, and was receiving an air flow reading of 1700 ft/min from the duct. This results in 148 cfm. I don’t see a reason to even verify the air exchange rate, since the code only asks for 1 cfm per 50 sq ft. Any thoughts? Also, in areas with radon concerns, why would you add a return duct to the common area? The unvented crawl space may be considered sealed, but no space is air tight. The air from the crawl can leak into the common space, but at least there is a buffer with the crawl space. Thanks

Try this

I see allot of retrofitted crawlspaces. Your duct down there is plenty of air per the code. If your ducts are down there your also getting leakage. Even new homes have some duct leakage. The holes between the crawlspace and living space also allow air movement.Gaps between the subfloor and duct boot can be though of as a return if you added them all up. But if it is a concern you can add one grill someplace that is right to the crawl. The conditioned crawl space should have sealed plastic so it is less likely to allow soil gasses in the house. The only other thing you could do is open one foundation vent. The code allows 1 per foundation vent per 1500 sqft.

Complying with building code requirements is one thing (most code departments have no way of testing this ventilation rate in the first place).

However, if you are concerned about the functionality of this type of system (for the purpose of proper operation), Simply determined that the amount of ventilation into the crawlspace pressurizes the crawlspace above the outdoor atmospheric pressure.

Test the crawlspace pressure with a micro manometer with reference to the exterior. If your ventilation is not sufficient or your calculations are incorrect, you will not get the positive pressure you need.

Air change rates are about ventilation/dilution. An unvented crawlspace is designed to seal off from the exterior. A positive pressure within the space is required to prevent infiltration of anything (including radon gas). You can have a hole, but you won’t have a leak without a pressure differential.

Agreed David. Interesting that last year I found one that was over pressurized. When I opened the access it felt like I was standing in front of a big supply near a window. Very important to get just the right amount or you will not have a controlled condition and most likely higher fuel bills.
Always look for a damper to control the area flow. No damper means no one has properly balanced the system or is hoping they have got the calculations right.
The proper insulating of the skirting is very important if it is to prevent the cold air or hot air in that will effect the return temperature back to the furnace.