crawl space vent cover?


The humidity in the crawl space is high during rainy season (winter) here, and I want to set up a dehumidifier. This means I need to close the crawl space vents temporarily. In the summer I won't need the dehumidifier and the vents would have to be opened again.
What's a good way to close the vents that is easy to re-open? I prefer being able to open and close them from the outside so I don't have to crawl into the crawl space.
Some of my house vents are "sunken" similar to this pic (not my house, just using it for convenience). Actually mine is sunken even deeper.
http://www.pugetsoundhomeinspections.com/yahoo_site_admin/assets/images/DSC03349.92143554_std.jpg
I search the web and found only one closable crawl space vent. Is there something I can pick up at a local hardware store?
http://www.iaqsource.com/product.php?p=tjernlund_fv1&product 4938
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I made covers for one house from sheet Styrofoam. I used a circular saw to cut it, as no other tool left as clean an edge, or was quite as accurate. we used them for 10 years and they were still there when we moved.
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Sounds good to me, a wooden cover would absorb moisture and the styrofoam would be much more inert.
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That's very intersting. Our ranch house built between 1950 and 54 in Indianpolis had them. I think they were cast aluminum, or maybe iron. There were vertical slots and a handle on the outside that only had to move 3/4 inch sideways to close all the slots. We opened or closed them every spring and fall.
Painted outdoor plywood wouldn't soak up much humidity and if one ever rotted in 20 or 30 years, you could make a new one. But I guess his endorsement of styrofoam is very good.
That's styrofoam itself, right Eric? Not the newer stuff that is made with some sort of little balls squeezed together, that breaks so easily. I have a winter cover for my hose bib that is made of that stuff, and the other one broke.

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My material was left over drop in ceiling 2' X 4" tiles. It was high density, & had a pattern pressed into it making it even tighter. There are high density 4' X 8' sheets that would do as well. The only problem I ever had was the neighbors grandchildren taking them off and playing with them. A scolding solved that, but I had to make a couple of replacements. My vents were concrete and had a stud in the middle for an accessory cover. That held them on nicely even though it didn't protrude enough to put a nut on it. On my present house they were wood, and very hand made. I found some plastic ones with thermal activated louvers at a habitat store, and am changing them out as I rebuild the walls in hardie-board. I'm not sure just how effective the thermal part is, but they look a lot better.
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