Crawlspace Vents

[Curtis Johnson]

I was wondering what the best way to install crawlspace vents in a standard concrete foundation wall is? Specifically, should the vent have a concrete lintel over the top? If yes, how thick and what reinforcement? Should the sides and bottom of the vents have special reinforcement? How much space should there be between the bottom of the vent opening and finished grade?
What is the size of vents typically used for this application (rough opening)? I know how to determine the # of vents required and that there needs to be at least one within 3' of all corners. I have references that show vent openings "open" on top (no lintel) and I have some that show a concrete lintel but I haven't been able to find anything in the code or elsewhere that tells me what is the best way to go.

The project I am working on will have SIP's ext. walls (except garage), 11 7/8" I-Joists, and frost requirement is 32". There shouldn't be any unusual point loads other than typical girder truss loads. I am leaning toward a concrete lintel at least 6" thick as it doesn't seem reasonable to have the vent R.O. open on top, especially where the joists are perpendicular to the foundation walls (only a single 2 x 6 sill plate will be used). I was planning to include a note to the builder to locate vent openings other than directly under a girder truss loads (where possible). Any help or suggestions is appreciated.

[Allen42acj]

You can add support in the deck=LVL box,to carry extra over the vents..

[Curtis Johnson]

Allen:

So then do you usually spec LVL's rather than 1-1/8" to 1-1/4" Rimboad over the sill plates then for added strenght and just leave the vent openings without a concrete lintel?

When your joists run parallel with the foundation wall, do you just have a single LVL on the outer edge of the sill plate or do you but up an I-joist against the rim as well?

[Allen42acj]

Curt
Depends on how big the opening is.If it's on the side with joist going over it,I just double the box or add an LVL over the opening..
Same with bulk heads=double box..
Or if you have a knee wall with out room for a header..
No idea how much the metal foundation window can carry..
Better to be safe..
This is from my framing,not drawing experience..

[Allen Brown]

The latest theory from the energy efficiency "guru's", is to not put any crawlspace vents in at all, unless you have water running up out of the ground. Having vents in your foundation will actually bring more moisture in from warm humid air in the summertime. If you keep it airtight and treat it like a basement, and make sure you have proper drainage outside, you will be drier and more efficient in the long run.

[custer]

The latest theory from the energy efficiency "guru's", is to not put any crawlspace vents in at all, unless you have water running up out of the ground. Having vents in your foundation will actually bring more moisture in from warm humid air in the summertime. If you keep it airtight and treat it like a basement, and make sure you have proper drainage outside, you will be drier and more efficient in the long run.

Fascinating....I've thought that for a long time....

ESPECIALLY when you have a "crawl space" access from the full basement. It's like opening the window in your house in the winter time to make it cooler because the heat is too hot.... If you really want to vent the crawl space - I think the vent should run up to the roof - like any other vent in the house....

In anycase - I use a little fixed window - no sash - 8"x16" at 8" plus the height of the floor system down from the first floor sub-floor in my CA houses.

[Allen42acj]

Is a foundation vent the same as a foundation window in this case?

[custer]

Allen B - could you verify that quote? I mean - where did you read, or hear, or see that about the vents letting in more moisture and all that....

I need some back-up for my theory

Allen C....I guess in my case - it's a window. I also know that there are some vents in the symbol library (that i've never used).....

[Allen42acj]

Thanks
You can close windows,there are some nice basement ones out there..

[slatta]

try researching an older fine home building article, they had a vent less crawspace too, it probably will not catch on because it is still not allowed by code

[custer]

http://www.builderonline.com/Industr...ticleID=209064

http://resourcecenter.pnl.gov/cocoon...r/article/1458
http://resourcecenter.pnl.gov/cocoon...er/article/131

http://www.ibacos.com/pubs/CrawlspaceInsert.pdf

http://www.contractingbusiness.com/2.../ServiceClinic

http://www.housingzone.com/topics/pr.../pr04ia006.asp

[Curtis Johnson]

Allen B:

I totally agree with you - The Journal of Light Construction has also had at least one article on doing crawlspace retrofits to eliminate vents. Of course you then need to insulate and condition the crawlspace with a min. amount of heated air. I got a little gun shy on eliminating the vents because a project I designed that way got horribly soaked during frame up in the fall and they had lots of trouble drying out the crawlspace because there was no good way to circulate air in it. There are provisions in the IRC for eliminating vents. My customer is one of the old fashion kind of guys so I thought I would include vents (or at least the option of vents) in the design even though he will end up with "cool" floors if he does it that way.

Allen C, what do you mean by "doubling the box"?

[kevin]

a builder friend says that he always pours what he calls a rat slab, just a few inches of concrete, and then isn't required to use vents. this is in eastern oregon, a high dry desert climate.

[Allen42acj]

Double box over windows or other openings in foundations.
Just cut joist shorter at that location or add a piece at openings if joist are running the same way.
Also works for over size doors/windows on the rest of the house with not enough head room.
If the header needs to be taller and it cut into wall above..

[jtcapa]

My personal experience with those pesky foundation vents installed in the concrete stem wall is varied. Most just don't understand the issues. Builders like to push those vents up to the top of the stem wall in order to allow grade on the exterior to rise as high as possible (to much stem wall in view is ugly and less marketable). Unfortunately, in our area of the Pacific Northwest, we usually need at least one rebar near the top of the stem wall and if the vent pushes it to far up or reduces the amount of concrete surround the rebar, it loses its effectiveness. The other solution is Rim Vents, which allow grade to get within 6 to 8 inches of the siding, making a much more appealing exterior, but those vents have to have insulation baffles installed to allow the proper air flow.

After reading the links on the need for crawlspace ventilation, I can see the validity to the arguement that no vents is better for certain portions of the country where heat and humity is a bigger issue than rain and ground water. Most rim or foundation vents don't seem to work well in general, since they often are not located correctly to encourage cross ventilation. There really is no good solution to uniform and continuous crawl space ventilation, which makes the idea of closing off and conditioning the space more appealing. I can only imagine the builders screaming at those costs though. Spending more money on hidden things the buyers won't ever see or notice is not very popular.

This kind of brings us back full circle to the idea of slab on grade instead of crawl spaces. This, like the rest of the idea's, is still not a great all purpose solution, since North America has such a wide and varied climate from region to region. I don't think there is ONE good solution that fits all regions, and thus the code should not dicate one to us.