For crawl spaces with polyethylene, install at least one vent for every 500 square feet of crawl space. Temp Vent Automatic Foundation Vents. The non-powered, Automatic Foundation Vent will open and close automatically using a heat/cold sensitive coil.
http://www.allpestexpress.com/temp-vent-crawl-space-vents.aspx
For decades we were told to close our foundation vents in winter to prevent plumbing pipes from freezing. We were also told to open our foundation vents in the summer time to increase the air flow in the crawl space. Today, the State of North Carolina recommends that your foundation vents be shut year round.
http://trianglereconstruction.com/open%20vents
Crawlspace Conversion Process. Some of the structural alterations that will be done to convert a vented crawl space to a closed crawl space involve sealing vents and installing a sealed vapor barrier on the crawl space floor. This moisture barrier is extended up the interior foundation walls of the structure.
http://www.crawlspacedepot.com/closed-vs-vented-crawl-spaces/
Replace a Foundation Vent. Good crawlspace ventilation circulates fresh air through the crawlspace. For the best results, open foundation vents in the summer to allow moisture out and close foundation vents in the winter. Save Item.
https://www.lowes.com/projects/build-and-remodel/replace-a-foundation-vent/project
One vent should be installed for each 8 to 10 feet of foundation wall. Most building codes require 1 square foot of open ventilation area for every 150 square feet of crawl space. Temp Vent Automatic Foundation Vents have 50 inches of net free area per vent.
http://www.allpestexpress.com/temp-vent-crawl-space-vents.aspx
Automatic vents are less fuss. Air Vent makes ones that open at about 70 degrees F, close about 40, and don't require any electricity. You do not say whether your crawl- space floor is made of dirt or concrete, but if it is dirt, spread 6-millimeter plastic sheeting over it to help block that source of moisture.
https://www.thisoldhouse.com/ideas/crawl-space-vents-open-or-closed
For crawl spaces with polyethylene, install at least one vent for every 500 square feet of crawl space. The non-powered, Automatic Foundation Vent will open and close automatically using a heat/cold sensitive coil. The louvers open completely at approximately 72°F and completely close at approximately 38°F.
http://www.allpestexpress.com/temp-vent-crawl-space-vents.aspx
The closed-versus-vented question. Closed or “encapsulated” crawlspaces are now preferred to open, or vented, crawlspaces. Closed crawlspaces are consistently better at controlling moisture that enters via the damp earth or from warm humid air that would otherwise infiltrate from the outdoors.
http://www.myhomescience.com/encapsulated-crawlspace/
Open vents in your foundation create moisture issues that lead to musty odors, mold growth, and damage to insulation in the crawl space. When you turn on your air conditioner in the summertime, it changes the air pressure under your home. Warm humid air enters your crawl space through the foundation vents.
http://trianglereconstruction.com/open%20vents
These vents allow outside air to circulate under the floor in summer to prevent the moisture buildup that encourages mildew and rot. In winter, when the air is drier, the vents are closed to reduce the chance that the pipes in the crawl space might freeze.
https://www.thisoldhouse.com/ideas/crawl-space-vents-open-or-closed
when to open crawl space vents
Crawl space vents to be opened when temperature is about 70 degrees F. A crawl space vent is open at about 70 degrees Fahrenheit.