HVAC return in crawlspace

Inspected this house this morning. There is an HVAC supply and return in the crawlspace. I found the code that allows for zero crawlspace ventilation if there is a proper vapor retarder, insulated perimeter foundation wall and an HVAC supply and return. In this case, the return is in the return chase downstream of the filter and the OSB subfloor is saturated at the supply. The house is 1 year old and the OSB subfloor is already coming apart where it is saturated. My question is, In the code book it states that the return needs to be to a “common” area. Is the return chase considered a “common” area? In the photo, the PVC pipe is the HVAC supply, directly from the plenum and the hole covered with hardware cloth is in the return chase. Thanks for any help or thoughts.

Please cite the code that allows a return in the crawlspace. I’ve never seen one there.

I know this is from the 20016 IRC and it may assist your search:



We had a bunch of units built around here using complete crawl as the return air supply with registers in the appropriate places in the living space. they were built with 4 to 5 foot high floor trusses on washed stone w/vapor barrier and a pt wood perimeter foundation. Somewhat odd but passed.

It’s on page 26 of the Code Check Complete, 2nd edition. It says “Ok to omit ventilation openings if Class I vapor retarder on soils with 6 " overlaps and 6” lapped and sealed to stem wall and one of the following: Conditioned air supply 1 cfm/50 sq. ft and return to common area through duct or air transfer opening & perimeter insulated." The house has a sealed crawlspace with vapor barrier and perimeter insulation. I don’t feel right about the situation though and don’t know if the return chase, downstream of the filter is a “common” area. Because the HVAC is pumping warm air into the crawlspace, the OSB subfloor around the supply air pipe is saturated and starting to come apart, and it’s only a year old. Thanks for your help.

It sounds like regardless of whether you feel it is technically allowed or not that there were enough observed issues to recommend further evaluation by a qualified individual.

Codes aside, something is very wrong if there is that much moisture in a conditioned crawl space. From the looks of the wood in the crawl space, a mold inspection may be called for.

Based on the pictures it is all wrong. If a crawl space is unventilated, it must be conditioned, ie, heated or cooled the same as any room in the home, so larger and properly designed supply and return ducts will be needed.

I guess they should condition all garages as well.
Screw the IAQ.

If there is this much problem with a conditioned CS, it was not built right.

This crap is going on around here and it is destroying houses and lives. And it is a Code.
I make a fortune on this issue, but I can only protect just so many people.

There are better ways. This one is just a dirty band-aid put on an open wound.


I have seen many problems with the conditioned crawlspaces as well. It sounds like the HVAC supply was not sized properly for the crawlspace. Conditioned crawlspaces seem to be the rave right now. At least in my area, contractors nor HVAC guys have it figured out. They were developed in the High Humidity area of the country. Was the vapor barrier sealed at the seams and completely sealed at the foundation walls?

I’ve inspected 3 new homes in my area in the last month and there are problems with every one of them. It has been proven thus far that when you close off the crawlspace (at least in my part of the country, high radon) the radon levels in the house can go up by 10 times.

There are solutions such as mechanical venting, etc. Report and recommend a company that’s familiar with conditioned crawlspaces.

It is acceptable to permanently close foundation vents provided the joints of the vapor retarder overlap by 6 inches and are taped or sealed and the vapor retarder extends at least 6 inches up the foundation walls.

Additionally, one of the following is required:

  1. a continuously operated mechanical exhaust vent at a rate of 1 CF/min. for each 50 SF of crawl space floor area and the perimeter walls or floors must be insulated, or;

  2. a conditioned air supply discharges into the crawl space from the HVAC system at a rate of 1 CF/min. for each 50 SF of floor area and the perimeter walls or floors must be insulated, or:

  3. a mechanical dehumidification system and the perimeter walls or floors must be insulated.

(Whichever system is chosen, I recommend a sensor with a display inside the home to monitor the humidity conditions in the crawlspace.)

I don’t see any requirement for an air RETURN in the code. Did the house meet the requirements I noted above??? I know the thread it getting old, but again I have never seen a return in a closed crawl space system.


M1602.1 Return air. Return air shall be taken from inside the dwelling. Dilution of return air with outdoor air shall be permitted.

IRC 408.3 states “Conditioned air supply 1 cfm/50 sq.ft. & return to common area through duct or air-transfer opening & perimeter insulated”. I’ve recommended that the buyers bring in an HVAC specialist with design experience and a contractor to evaluate the sub floor. The house is just a year old but it’s in Bastrop County which does not have any kind of a residential inspection or code compliance department, so home builders can do whatever they want. For example, the electrician used 2004 AFCI standards even though the house was built in 2016. I’ve punted this one. Thank you and the others that chimed in on this.