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View Poll Results: I will leave my vents
shut over the winter. 30 60.00%
open over the winter. 20 40.00%
Voters: 50. You may not vote on this poll

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Old 10-26-2009, 10:37 AM
 
Location: Holly Springs
3,498 posts, read 5,733,449 times
Reputation: 2286
Please continue to leave crawlspace vents open in the spring and summer. It is very helpful to my crawlspace moisture business.
Seriously, all houses are different. Some respond just fine to leaving the vents open and some suffer. The best solution is to close them off and put in a dehumidifier that way all your bases are covered. You do not need a commercial DH like a Sante Fe, just a frigidaire from Lowes and pipe the condensate line to the outside. If you are relatively handy, this can be accomplished for less than $400 in most cases.
In the summer if you are leaving your vents open to dispel moisture (which comes from humidity unless you have intrusion issues....and in that case your vents do not matter), you are letting in just as much with the humidity content of the air. The humidity condenses on low dewpoint surfaces such as your ductwork and cold water plumbing lines leading to conditions favorable to mold and mildew. Our building code will change here to a closed foundation system in the near future for this very reason. The studies have been long performed, by changes to code can be a slow process. It takes even longer for people to let go of conventional wisdom lol.

Boardjnky4: Camel crickets only hang out in moist crawlspaces and they will move on when you take the steps to eliminate the moisture.

TrayandTom; if you close your vents, the only thing you need to be sure of is that you have a vapor barrier on the ground to prevent rising moisture. Other than that, no other changes are necessary.

Goat1of2: If you have intrusion issues then the vents are not going to make a difference one way or the other. The best approach is incrementally solving the problem. First you would bring in some clay soil and build the grade up along the foundation wall. The grade deteriorates over time and the foundation walls are porous. Clay is about $20 per cubic yard and a typical house can be regraded around the entire foundation with approximately 2-3 cubic yards. French drain work is relatively costly, but sometimes unaviodable. Of course, not fixing it will cost you even more if you have to start replacing subfloor structural members due to white mold that attacks the wood.

If anyone else has questions on this subject you are always welcome to message me here or email me at a1houseinspectors@gmail.com and I will be happy to answer them.


ps- fully sealed crawlspaces are nice, but overkill. You will not recoup the cost. Sealing the vents, 100% vapor barrier, and installing a dehumidifier will give you 90% of the benefit for less than 30% of the cost.
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Old 10-26-2009, 01:28 PM
 
698 posts, read 981,410 times
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what type of vapor barrier? Does something as simple as a sheet of plastic work?
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Old 10-26-2009, 04:41 PM
 
Location: The Charming Town of Fuquay-Varina
393 posts, read 167,685 times
Reputation: 131
Quote:
Originally Posted by sacredgrooves View Post
ps- fully sealed crawlspaces are nice, but overkill. You will not recoup the cost. Sealing the vents, 100% vapor barrier, and installing a dehumidifier will give you 90% of the benefit for less than 30% of the cost.
Can I just shut the vent doors and not do any sealing of them and not use a dehumidifier? Shutting the doors does not make it airtight. I do have a good vapor barrier.
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Old 10-26-2009, 06:59 PM
 
Location: Holly Springs
3,498 posts, read 5,733,449 times
Reputation: 2286
Quote:
Originally Posted by ApexIntruder View Post
Can I just shut the vent doors and not do any sealing of them and not use a dehumidifier? Shutting the doors does not make it airtight. I do have a good vapor barrier.
That will work fine. If you want to seal them, get a 4x8 sheet of Polystyrene insulating board (it is blue or pink in the insulation section of hardware stores) and cut to fit, place them in the cavity from inside the crawlspace, and seal the perimeter with expanding foam. Materials would run you $25 and probably will not take 45 minutes to do them all. Recommend using a carpenters square or other straight edge and a utility knife to cut the Polystyrene. The cavities typically measure out to 8"x16" but some foundation vent cavities have a variance from that number so may take a bit of trial and error. One sheet of the Poly will easily do a couple crawlspaces.

Boardjnky4: 6 mil poly is the norm. In Home Depot it is in the paint area with the painting tarps, at Lowes it is next to the insulation aisle. Recommend the 10x100 size as it typically takes less cuts. It is rare to have a 20' span between piers/foundation walls so the 20x100 can cause extra frustration. The 10x100 is $59 in black or clear (whatever you want to use is fine)
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Old 10-26-2009, 08:09 PM
DPK
 
1,017 posts, read 1,178,434 times
Reputation: 1141
My parents just replaced all their vents with auto-closing ones in their foundation/crawl-space. They're really not too expensive if you do it in batches and don't require a power source. They rely on a coil that expands/contracts depending on the temperature:

AIR VENT INC. at Lowe's: 16 x 8 Automatic Foundation Vent - Black
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Old 10-26-2009, 08:35 PM
 
Location: Holly Springs
3,498 posts, read 5,733,449 times
Reputation: 2286
Unfortunately they open in the summer. Great for the southwest, but not the southeast.
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Old 10-26-2009, 08:38 PM
 
Location: Wake Forest, NC
40 posts, read 74,460 times
Reputation: 11
Default Air flow prevents possible Radon gas buildup

If your home is built on a subfloor and the ground is exposed it is likely covered with a moisture barrier liner. You'll want to keep a few vebts open to maintain air circulation to reduce any chance of Radon gas building up.
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Old 10-26-2009, 08:42 PM
 
Location: Wake Forest, NC
40 posts, read 74,460 times
Reputation: 11
To prevent Radon gas buildup in sub floor areas. I have read and been told by home inspectors to leave some of the vents open to keep air circulating. Radon is a gas that is emmitted from the shale/rock sediment in the earth and common in our region.
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Old 10-26-2009, 09:31 PM
 
Location: near Cary High School/Cary Towne Mall
226 posts, read 616,767 times
Reputation: 324
Dear TrayandTom- If you are building and the foundation is completed: no problem; just seal the vents later with polystyrene panels cut to fit each opening and sealed with expanding foam insulation or sealants and install a dehumidifier- If only footings are complete and you're ready to lay block and brick, just tell the mason crewchief to leave them out. (they will be happy, finish sooner, and charge you less-)
Spend your money on proper grading, downspout extensions, vapor barrier, and keep the humidity-laden outside air out of your crawlspace ALL THE TIME, ALL SEASONS, AT ALL TEMPERATURES- Control the relative humidity with a basic, plain-jane dehumidifier and route the drain line outside the crawlspace into your favorite flowerbed and they will thank you for it! 8-)
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Old 10-28-2009, 04:06 PM
 
Location: New Jersey
2,510 posts, read 1,235,474 times
Reputation: 621
Quote:
Originally Posted by sacredgrooves View Post
They should be closed year around in our region. Frozen pipes are not too much of an issue here, but summer humidity sure is!
Foundation vents are installed to provide air circulation and allow moisture/dampness in the crawl space to escape......if you keep them closed your defeating the purpose for which they were installed.....your also preventing radon from escaping which will then penetrate into your living area above.....keep your crawl space vents open !!!
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