As you no doubt know, the fogging phenomenon you describe only happens in the spring or fall, when weather conditions are warm in the daytime and cool at night. The morning sun hits the housing and drives moisture onto the dome. First, I'd make sure no water is inside the pipe. Duncan housings drain into the pipe they are mounted on, so we have 3/8" weep holes in all our pipes that help keep water away from the housing. Duncan has also suggested a number of ventilation holes than can be drilled, but doing this hasn't been effective here.
We did some experimenting a few years ago with a product from a company called A+ in Baton Rouge LA. They manufacture a self-regenerating dessicant packet, about tea bag size, that we found quite effective in the spring. Our observations indicated that fogging can be reduced by 90-100%. The packet acts like a sponge to absorb up to six ounces of liquid. When the temperature warms up later in the day, the packet dries out and is ready to work again. Some of our packets have been in place for 4 years. However, they only work when the daytime temperature rises. In the fall that doesn't happen as much, so the packet doesn't help with fogging because it can't release the moisture it absorbed overnight.
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