The first winter after we replaced our old, nonfunctional rangehood and fan with a very functional microwave/rangehood/vent-fan, we discovered that the fan worked quite well when kitchen ventilation was required, but unfortunately the vents also ventilated much too efficiently during cold weather, sending gusts of frigid air swooping down into the kitchen even when the fan was off.
As my toes curled up in protest despite two pairs of wooly socks and a pair of fuzzy slippers, I realized that I had some figuring-out to do. After a fairly short period of contemplation, I came up with the following solution, which I feel is one of my very best:
I bought a 12-by-24-inch roll of Magnetic Adhesive Sheet at a non-denominational arts and crafts store, and cut two 8-by-12-inch pieces off the roll, which allowed plenty of magnetic overlap when stuck over our 5-by-7.5-inch vents. In order to straighten out the semi-permanent curl of the sheeting, the pieces were weighted down and left to straighten out overnight.
Because it only sticks well to flat surfaces, the magnetic sheeting must be trimmed to fit the situation. Do not remove the paper backing that covers the sticky adhesive surface on the sheeting — after a couple of years, when the backing paper is good and greasy, you can peel it off and stick a fresh piece of paper to the newly exposed adhesive surface.
This weekend I replaced a set of vent covers because after several years, they had become untenably brittle and sticky. But first I had to degrease the vent filters and the undercarriage of the microwave/rangefan, because rather astonishingly, magnets are not magnetically attracted to thick, sticky layers of grease.
If you decide to make your own magnetic kitchen vent covers, remember that they must be removed when you need to turn on the kitchen fan. I stick our vent covers to the refrigerator for safe-keeping while the kitchen fan is in use, because I am so absent-minded that if I tried putting the vent covers anywhere less obvious, I’d probably lose them.
Update, Dec. 24, 2012:
Our son Dmitri came home for Christmas, and thought up an important and valuable innovation: The paper backing had begun to curl and separate from the magnetic sheets, so he suggested that we peel off the backing and stick aluminum foil to the sticky side of the sheets.
In short order the backing was peeled, the magnetic sheets were laid down on aluminum foil and carefully smoothed on, and the foil was trimmed even with the magnetic sheeting with a sharp knife. So now our vent covers are reflective!