Crawl space ventilation requirements

[desdawg]

What is the formula for determining the amount of vent size required for a crawl space? I have a UBC book around here somewhere but I can't seem to lay my hands on it. I may have taken it to the other house.  [smiley=lolk.gif]

[MikeT]

Desdawg,

I have been reading and thinking about this issue.  I think you might find the discussion and the new research about NOT venting quite interesting:

http://www.advancedenergy.org/buildings/knowledge_library/crawl_spaces/

If you scroll down you can see some videos as well that I think illustrate some examples and the research findings behind them.

I do not know the code issues, but I imagine the code lags behind this research.  In my small crawl space, I am thinking of how not to vent it and what I would need to do to assuage the inspector.

mt

[MountainDon]

I know I can find that later.... have to run now.

The point of vent vs no-vent depends totally on whether or not you are subject to code inspection, and what the local code provision calls for. If it needs vents, you could always modify after inspection.

[ScottA]

I'm going to sheath the underside of my insulated floor and leave my crawlspace wide open. I'm of the opinion this will eliminate the moisture problem completly. Here in OK we have very humid weather in the spring and summer and I've seen way to many crawl spaces where water is literaly dripping off the floor joists in the summer with A/C running in the house. I've yet to see a floor properly insulated around here. I think the other best option would be a conditioned crawlspace but keeping the ground moisture out would take some serious site work to insure all water drains away from the foundation. If you are in a dry climate such as desert or mountains it probly wouldn't be an issue.

Scott

[williet]

There's lots of talk around here about radon gas. If you had no vents wouldn't that increase the r.gas problem?

Isn't there a trade off between a sealed crawl space and exposure to gas build-up?

[desdawg]

I will have to get it past the inspector so I am putting in vents. Since I am here in the desert I down't have to worry about freezing and I want to make sure the spiders and snakes have an adequate oxygen supply. I just wasn't sure what I would need to meet code. IIRC there is a ratio of sq inches of vent to sq ft of crawl space. When I am up north I do everything I can to avoid cold air infiltration and haven't had any moisture problems yet. Anyway does anyone know the ratio I am looking for?

[cedarglen]

Here in my area of CA it is 1 sq ft of vent for every 150 sq ft of under floor area.

[desdawg]

Thanks cedarglen. That is what I was looking for. I am skirting a 14 X 63 singlewide mobile home so I would need 5.8 (call it 6) sq ft of vent. I am hand mixing the footing, framing the knee wall with treated 2 X 4 and matching the RBB masonite siding that is on the home. My next rent-a-castle.  

[MountainDon]

IRC2003:
1 sq ft per 150 sq ft of under floor space minimum, just as cedarglen stated.

Also note: one such opening within 3 feet of each corner of the building. Opening must be covered with...
...perforated sheet metal .070" thick
...expanded metal .047 thick
...cast iron grill
... extruded load bearing brick vent
...hardware cloth .035 inch mesh wire
... all with max opening of 1/4"

There are bunch of other if, ands and buts. It's section R408.2 of the IRC 2003 which can be viewed here  http://www2.iccsafe.org/states/Seattle/seattle_residential/res_frameset.htm
It's seattle specific, but it is the entire IRC2003 with the sectuions not used by Seattle stamped over, but still very readable. Select Chapter Four and scroll down.

[Redoverfarm]

Don that sounds pretty much like the same figures for soffit/attic figures.