Why is our ceiling wet?

Humidity can a real problem while living in an RV. We have been traveling east, officially leaving the dry desert and entering bayou country. On the first night of 90% humidity, we wondered when we would be dealing with complications of humidity in our RV life.

The next morning, while making my morning tea, I noticed a six inch round spot on the ceiling with condensation forming.

view of condensation formed on the ceiling with an a/c vent close by

On rare occasions in the past, I have seen condensation forming on the air conditioner vents because they are so much colder than the air. We have also noticed condensation on the windows from time to time, but this was rare and short-lived.

On this morning, however, the condensation was directly on the ceiling, a foot away from any a/c vent. I pulled off the nearby a/c vent to explore what might be causing the cold spot.

view of the a/c vent with the cover removed

The a/c ducting consists of a foam channel running through the metal roof rafters. The channel is about 2 inches tall and 10 inches wide.

view down the a/c vent duct

I used my cell phone camera to get a better picture of what was happening near the cold spot.

It appears the duct continues past the last vent for about a foot, and ended about where the cold spot was. It looks like the installers closed off the end of the duct with aluminum foil tape. The tape must be leaking, allowing the cold air to blow down on the 1/4 inch plywood that is our internal ceiling, causing the cold spot where the condensation formed.

We needed to close off the end of this duct without ripping our ceiling apart. There was limited access though the small a/c vent opening, so we needed something flexible and expandable enough to fill the space without a lot of work. I was about to head to the local hardware store to find some foam when Jen came up with the brilliant idea to use a sliver of our brand new memory foam mattress topper. It was a little oversized for our bed, so a small edge would not be noticed.

View of the foam mattress topper with a small 2 inch by 10 inch section cut from the edge

This piece was easy to slide into the opening and quickly expanded to close off the duct right after the a/c vent.

view of the a/c duct with the foam plug closing off the leaky end

After reassembling the a/c vent and sleeping through another humid night, the ceiling cold spot was not noticeable, and we were condensation-free.

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