My cargo trailer has a standard size vent in the top. it's maybe 6 years old. I had it parked in the back yard for the summer, and last Thursday I looked out the window at it and noticed something off-white sort of sitting at an angle on the roof. Wondering, I went out and stood on the tire to see onto the roof... and found that my vent cover was hanging by just a small strip to the vent area, and was sitting lopsided at an angle, completely off the vent!
Mind you, when I parked the trailer there around June 1st I had just encountered a bit of leakage at the vent during rainstorms. Examination from inside the trailer revealed what looked like a tiny chip out of one corner. Other than that I thought it was ok. Wrong!
Got the ladder out and took a closer look, and found that the cover, while shiny and new looking from the inside, was rough and pitted on the outside. And it was incredibly brittle! I was snapping pieces off the edge without even trying.
Well, I ordered a Maxx Air vent cover and installed it yesterday. Very easy. Now I can have some air movement while driving to take out the heat buildup. It's my work trailer and, now that I'm back to working, I was in & out of it about 20 times today (104* outside) and I think it helped. Seemed less stifling than usual, anyway.
So the moral of the story is, get out the ladder and check your vent covers from the outside. Don't rely on how they look from inside, because all the UV damage is on the side you rarely see. I sure learned my lesson!