AC ducts in attic accumulate water in winter

I have posted this question in the past, but never received a satisfactory answer. Perhaps this time someone smarter than I can come up with a good solution...
Question: How to stop AC ducts in the attic from accumulating water during the winter
Environment: Ranch house, baseboard hot water heating, whole house air conditioner with AC registers in the ceiling. Floor of attic insulated to at least R40. Ceiling of attic not insulated. AC air handler in attic attached to rigid duct surrounded by 4 inches of insulation. Rigid duct traverses the center of the attic for about 2/3 the length of the house. Flexible, insulated 6 inch duct rated R4.2 attaches the ceiling registers to the rigid duct. AC ceiling registers have internal dampers that can be closed in the winter.
Problem: When the AC ceiling registers are closed for the winter, the internal dampers do not affect a perfect seal. Some warm, moist air from the house will leak through the ceiling registers into the flexible duct and into the rigid duct. Even though the rigid duct is insulated, on very cold winter days, the internal temperature of the flexible and rigid ducts will be low enough to condense the water vapor in the warm air that leaked from the house into the ducts. This condensed water accumulates in the rigid duct, causing rusting and if there is enough water, leaking back into the house through the ceiling registers.
My solution: Not perfect. Every fall, I enter the attic and disconnect the flexible ducts from the ceiling registers. I cover the ceiling registers with lots of insulation. In the late spring, I go into the attic and reverse this process by reconnecting the flexible ducts to the ceiling registers. This process is a royal pain. I would like a more permanent solution.
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David wrote:

Other than taping saran wrap over the grills, how easy would it be to slip something behind the grills? If they are plastic and held in place with 4 screws like mine, remove one or more screws and loosen the others just enough so you can slip something inside to block the vent, like a piece of cardboard, or other thin material, cut to the shape of the vent grill.

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There are magnetic register covers that can be applied to the registers in the ceiling that should do the trick.
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snipped-for-privacy@optonline.net (Chet Hayes) wrote in message

Good question, I struggle with this also. One option I have actually done. Leave the registers open and heat the whole AC ductwork. Kinda Pricey because the insulation is not that thick, not to mention the vapor barrier is on the wrong side.

A decent amount of work, also the way my registeres are anchored, this is a two person job. 'There must be an easier way'(?).

I admire your dedication, but this is a lot of work!

Ahhh, I had a sheet magnet and tried this on one register, only to find it was not magnetic!
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Zaf wrote:

Then how about constructing some sort of cover made from a thin sturdy material and use clips (I'm thinking plastic automobile type panel push clips) that would push into the slots in the vent. With the right size clips, it would be an easy snap-on snap-off arrangement. ( I wonder if someone doesn't make these already?). Google on - ac vent covers -, you'll find some other ideas like, http://www.energyfederation.org/gwne/default.php/cPath/86_742_111

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Thank you all for your suggestions. Let me add additional details about the registers in the ceiling. The resiters are not the rectangular type on which you can attach the magnetic type covers. Rather the registers are circular, that is, there are 4 concentric round metal rings. In the center there is a know that opens/closes the internal dampers. While the rectangular registers are somewhat flush with the ceiling, the circular registers protrude about 2 inches and their construction prevents a cover from being attached to the register to close the opening.
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David wrote:

Then my only solution would be my first, and that was to tape clear plastic wrap, or whatever, over the vent. You can use the blue 3M painters tape which claims not to leave a residue when taken off. The blue stripe around the vent should add a nice contrasting accent color to the rooms. :-)
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On 19 Oct 2004 11:49:46 -0700, snipped-for-privacy@yahoo.com (David) wrote:

Close the dampers with the knob. Cut some overlapping (1/3 round in my case) partial circles out of duct wrap and slid them in over the braces holding the knob (or, in other words, next to the dampers) and see if that works. Once cut to the proper size, you could always pull them and spray paint the aluminum foil some appropriate color---then reinstall. Replacing several feet of the duct closest to the register (or the entire duct) with flexible duct should also work---may be more than you want to do.
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(David) wrote:

Actually the duct that connects to the register is flexible and insulated (R4.2). THis makes it fairly easy to disconnect the duct work in the fall and reconnect in the spring. I am thinking ahead to when I am old and not as nimble as I am today and do not want to crawl around in the attic. Which is why I am looking for a solution that involves doing something in the house and not the attic.

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In alt.home.repair on 17 Oct 2004 11:45:59 -0700 snipped-for-privacy@yahoo.com (David) posted:

That's normal, to not make a perfect seal. You mean "effect".

If the louvers don't stick out (too much?), and are steel, maybe you can get some of that magnetic sheeting, such as are used on truck doors to temporarily label a truck with the business name, and put them over the registers every fall. Are brass registers steel underneath, or are they solid brass? You can tell with a magnet.

Replace this with aluminum? That will cause more cooling loss when cooling the house with a hot attic, but you have insulation around it anyhow.

Meirman If emailing, please let me know whether or not you are posting the same letter.
Change domain to erols.com, if necessary.
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