Foundation Vents

[O'BrienConstruction]

When we build on a crawlspace and have porches that we back fill and pour slabs on we will install foundation vents in the porch wall and run two sections of 6" PVC pipe back into the crawlspace to maintain airflow to the crawlspace under the porch. I was just wondering if anyone else was doing it differently. Inspectors have no problem with it as long as we're maintaining 1 vent for every 150 sq ft of crawlspace floor area. On a small porch we sometimes eliminate the vents and catch up on the requirement elsewhere, but normally that doesn't seam like a good idea. Especially on full front porch or wrap around porches.

[Dick Seibert]

Re: Foundation Vents

Rob:

I hear you and agree, I've had the same problem over the years, the inspectors don't make us create cross-ventilation, they just check for the 150 square foot number. What I've done to get cross ventilation is have my tin-man build me 14 ¼" "Z" boots, I build stem walls and cast the tin boots into the stem walls (they probably rust out in 10 to 20 years but they are non-structural and will leave the ventilation area in-tact. This subject came up here about 6 years ago, I made a rough diagram with my limited computer skills back then, here it is if you can understand it, if you can't, ask and I'll try to help you understand what I tried to convey.

[SolarPowered]

Re: Foundation Vents

Have you read up on Joe Lstiburek's "conditioned crawlspace" approach to crawlspaces? That's a very different take on it (no vents).

[O'BrienConstruction]

Re: Foundation Vents

Dick,
Thats interesting. You put a lot more thought into this than I have. I like your idea.

You ever heard any good arguments for changing the 150 sqft rule. I took a class here for contractor's given at a Community College by a green building group. They were preaching that to properly ventilate a crawlspace that we should be figuring vents based on 150 cubic feet. I ran the numbers, the one I just finsihed the foundation on had 2100 sq ft of crawlspace 4' high. Thats 8400 cubic feet, or 56 vents instead 14. I'd have a vent like every third block, or have to stack them every other course of block and like every 5th block in the run. I liked the flow rate numbers they showed in a 5-10 mph breeze, but that is so many vents I think it would start to compromise the strength of the foundation walls.

[O'BrienConstruction]

Re: Foundation Vents

Solar,
I have read into that some, I asked my inspector during my last footing inspection and he quoted our codebook. It says:
R408.1 Ventilation.
The under–floor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement or cellar) shall be provided with ventilation openings through foundation walls or exterior walls. The minimum net area of ventilation openings shall not be less than 1 square foot for each 150 square feet (0.67 m² for each 100 m²) of under–floor space area. One such ventilating opening shall be within 3 feet (914 mm) of each corner of said building.

So he wasn't willing to take the conversation any further after that.

[Dick Seibert]

Re: Foundation Vents

Rob:

I think the 150 square feet is adequate, you don't need the larger quantity, but I give you credit for figuring out on your own that cross-ventilation is the problem, and it's not really addressed in the codes. I came up with the tin boot idea to solve the problem, but I've built many houses prior to that where I just ganged up additional vents in available areas to compensate for those we were unable to install. I saw problems in areas of "dead" air in some older homes, and started fabricating the tin boots.

I've been following Lstiburek's sealed crawl and attic space theories, but have decided that they are too dangerous. I even ran them by Frank Woeste (University of Virginia) and he also is scared, if something goes wrong with no ventilation, there could be too many problems. What if you get ground source moisture, plumbing leaks, mold, radon gas, termites, or any number of other problems, are you going to want that air pumped throughout the house? Let's leave the innovative theories to others until they are proven over years, let's protect our own butts from the liability which might occur, and stick with tried and proven products and methods. Every house you build, you are putting everything you have at risk, corporations and insurance can only give you limited protection.

[BEWhittier]

Re: Foundation Vents

Rob,

Check your codebook, as North Carolina does allowed sealed crawlspaces. R408.2 exception #5.

Also check www.crawlspaces.org

JLC has run several articles on sealing crawlspaces, October 2003 and August 2004, by Jeff Tooley who is based in NC as well.

Brian

[O'BrienConstruction]

Re: Foundation Vents

Brian,
To seal a crawlspace according to that section. The ground surface has to be covered with an approved vapor retarder, the space is supplied with conditioned air and the perimeter walls are insulated. That article is pretty good. I'll admit, I'm a believer of vented crawlspaces. I've been in many old ones and other than the cobwebs, things seamed alright in ones that weren't dealing with drainage issues. I like the idea, but would want to see it in person before I would attempt to switch. Its like anything new that comes along. It sounds like a good idea, I never believe one persons research though becuase it can be targeted. I'll have to do more research on it. Everything here is either vented crawlspace, slab, or on pilings.