Crawl space venting?

Can anyone tell me if a crawl space with a concrete slab that is under 100 sqr. feet need venting.

I looked in the code book and it says 1 sqr. foot per 150 sqr. feet.

The latest trend is to no longer vent crawl spaces to the out side and I agree with this trend . .
… Cookie

Roy, do you know if there is a code for this in the IRC?

Sorry I do not and I do not think we use the same codes .

One of many that do not recommend Venting .
… Cookie Canadian OBC

Thanks Roy, good article


Sometimes we have to challenge the code with newer research and the code guys with tact!! Most codes allow for innovation or else nothing would ever change. Sometimes though, it’s hard to affect change if the code folks are not up to the newest research.


I’m with you guys, here’s my problem, I am building a 10X10 attached breezeway with a 28X24 garage and the building inspector red flagged the crawl space. I have to talk to him but I would never put vents in something like this, plus, when I frame the floor I install 1" rigid foam insulation between the joists then fiberglass, then the sub-floor. Once this is sealed, and by the way we poured concrete for a slab, but once this is sealed why would you need venting?

IRC states 1 sqr. foot per evry 150 sqr. feet, my interpretation on that is anything under 150 sqr. feet does not need venting, per code.

Thanks for the help, when I talk to him I’m going to push the issue.

Print off a couple of the sheets listed above and give them to him so he too can learn.
Give him directions to this BB ( and tell him he is welcome to come on and see what a great group we are and would love to have him visit.
Let him know we love to trade information with all to help the industry . Even give him a copy of this letter so he too can see we really are an International association.

… Cookie

The IRC requirés ventilation unless the crawl space is conditioned, meaning insulated, heated and mechanically ventilated. Any crawl space under 150 square feet in area still requires one square foot of ventilation free area. Failing to ventilate the space, one way or the other, is simply asking for trouble, in the form of mold and/or structural decay.


Sounds like (as you describe building it) it’s not needed…

another website that I like…

Sorry I am on the other side of the fence have seen more mould in vented crawl spaces in my area then unvented .
I recommend to all my clients that I think and from what I have read Crawl spaces are better left unvented to the out side .

… Cookie

Read the stuff from Advanced Energy. They found lower RH’s with a properly prepared space. Why would we vent?? to make it wetter??

This new trend has been running through researched results not anectdotal based sections of code for 15-20 years now!!

Have seen too many crawlspaces vented to exterior that had more problems as a result.

Not venting is the way to go.

Treat the crawl like a basement. (you wouldn’t leave your basement windows open, would you?)

Well my site contractor told me that the red sticker meant it was OK, first time with this inspector, so I told him to forget the vent and back fill it.

I’m on the side of not venting.

Hey, Peter;

I always thought that

Red= stop

Yellow= proceed at your own risk.

Green,= Go for it and nail it. :mrgreen:

Marcel :slight_smile: :wink:

You all may be on the side of “not venting”, but unfortuately, the IRC requires that a crawl space either be mechanically conditioned or vented. There are no other choices.

I recently lived in a house where the owner had previously sealed the vents in the crawl space. A contractor went down there and came back out with his shirt soaking wet from touching the insulation, which was dripping with water. When the vents were re-opened, the moisture eventually left the crawl space. That is what the vents are there to do…allow moisture to leave when the humidity outside is low.

Hi Marcel, thats what I thought to but the site contractor who has work in this town before says red means he’s been there and the inspection is complete.

And what happens in August when the CS is 55 degrees and the air out side is 90 degrees and 90 % humidity. ( every thing is wet )
More dirt around the out side and extend the down spouts makes a huge improvement.
Why not cover the dirt with plastic and keep the moisture in the ground.
.If it is that bad it needs a dehumidifier to control the Humidity.
This also tends to make the floor warmer and warm floor makes mama Happy . Mama is happy every body is happy.
Unfortunately is not always correct they are usually a few years behind new methods.
… Cookie

So what you are saying, is that when you have vents and the humidity is low outside, the moisture leaves, But then again when the moisture outside is high, it would loggically come right back in, wouldn’t it?

That scenario, seems to indicate that the moisture barrier was not installed or not installed properly. A 100% vapor barrier would not allow that kind of moisture in an unvented crawl space unless it is coming from above. Which would mean that the interior of the house has a humidity level of in the upper 80’s, then there would be other issues.

Marcel :slight_smile: :smiley: