GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Taming a 51 year old vented crawl space

User avatar
Steve Robertson | Posted in Energy Efficiency and Durability on

Based on the Martin’s article and related research in the links he provided, unventing my crawl space in the hot-humid southeast (Climate Zone 3A) seems like a good idea.

But if so, I would like to decouple the crawl space from the living space to hopefully improve the indoor air quality. That means IRC Option 1 which requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area”. That is a 20 cfm fan for my 1000 square feet of crawl space. Even with the potential problem noted in Martin’s article, the fan will “eventually conk out”, I’m still interested in this option first. I have two reasons for this but would appreciate your feedback on them. The first reason is that the house is located in an EPA radon Zone 1 (highest potential) county. (A short-term radon test is underway.) I believe the research has shown that the unvented crawl space has higher radon levels. But unless I missed it, none of the test houses had radon mitigation. I’m willing to install a radon mitigation system in my house. Closing up the crawl space seems to include most of what is required for radon mitigation anyway. Although to complete the mitigation requires building a chase and installing the EPA recommended 4” PVC pipe. Finding a location for that is the hard part.

The second reason for the external exhaust fan is that the crawl space has been vented for the last 51 years. Although it is not mold ridden, how clean can it be made after living in the wild for that amount of time?

Assuming my reasoning is correct, should I try to seal the subfloor? The subfloor is 1 x 6 planking with gaps between the boards of 1/4 to 1/2 inch. The finished floor in most of the house is 3/4 x 2 1/4 oak over felt paper. On GBA, I have seen suggestions from taping and caulking the gaps to installing 1/2 inch plywood over the existing subfloor. The later of course requires removing the finished floors in my case. Also, the crawl space studies prefer no floor insulation so does that rule out close cell spray foam? I would prefer not to use spray foam just from a cost standpoint.

I was looking for a source for the crawl space exhaust fans. So far only the WSU study used crawl space exhaust fans. But it didn’t include any of the details on the fan.

The Building Energy Codes Resource Center has the following recommendation for the exhaust fan:

Type of fan: The code does not specify details about fans used in crawlspaces. One possibility is to follow the Colorado amendments to the IRC. Colorado requires a fan rated for 44,000 hours (five years) of continuous operation with flex connections, or other installation detailing to reduce vibration and noise associated with the fan. The fan must be connected to a trouble light or an alarm to signal occupants when the fan fails.

Are there any sources for such a fan?
Tjernlund has crawl space exhaust fans but they are intendend for vented crawl spaces.

1300 square feet single story house
2×4 walls, brick veneer
1000 square feet of crawl space, R19 fg batts between floor joists
Climate Zone 3A
HVAC ducts in crawl space
gas furnace in crawl space (furnace is 25 years old and will be replaced)

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. User avatar GBA Editor
    Martin Holladay | | #1

    I suggest using a Panasonic FV-05VK3 bathroom exhaust fan. It draws a tiny amount of electricity -- only 4.3 watts at 50 cfm -- and I think it is adjustable to allow for lower air flows.

    If your crawl space is depressurized, you probably don't have to air-seal your subfloor. But if you want to attempt to air seal it, the easiest way would probably be to install thin aluminum-faced polyiso insulation -- I think you can get 1/2-inch thickness -- and install it at the underside of the floor joists. Tape the seams with housewrap tape.

    You should test for radon -- both in the crawl space and your first floor. If you are concerned about radon, you need a layer of crushed stone and perforated PVC pipes under your crawl space floor, as well as a vertical vent through your roof.

  2. User avatar
    Steve Robertson | | #2

    Thanks for the tip on the Panasonic bath fans. One interesting feature is these fans sense the static pressure and adjust the fan speed to maintain the specified flow rate. FYI, the FV-05KV3 has only one flow rate, 50 cfm. The FV-08KVS3 has selectable flow rates of 30, 40, 50, 60 and 70 cfm for continuous venting. The maximum flow rate is 80 cfm. See p. 6 in the installation manual:
    For Panasonic's full line of ventilation fans:

    If only had an air flow switch.

    I'm not interested in sealing the subfloor unless it would help. I'll spend the time and money improving something else. If the crawl space perimeter is sealed and the hvac duct leakage is minimized, would there be a reason to seal the subfloor? That seems like one of the benefits of exhaust ventilation in the crawl space.

    If I did seal the subfloor with aluminum-faced polyiso insulation, would there be a modest energy penalty in the summer based on the results of the Advanced Energy field studies?

  3. User avatar GBA Editor
    Martin Holladay | | #3

    1. If you do a good job with your sealed crawl space details, you don't have to air seal the subfloor.

    2. If you do air seal the subfloor, I don't think you would notice any energy penalty.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |