What is the proper way to seal HVAC vents?

Hi, I am from Fort Worth, TX and I have some vents that are getting wet and mold at the ceiling.

During my last AC tune up, I asked the technician what was that and what was the cause of that. According to him, it was because my vents were not sealed and there were gaps between the ceiling and the ac boot. As the attic is very hot and the interior is cold, those gaps will let the hot air mix with cold air forming condensation.
Then, I asked how to fix that and he told me that was an easy job and it just need to use silicone to seal the gaps or he could do that for $70,00 each vent (probably he did not want to work on this). I thought it was expensive and decided to try to fix by myself.

In my visit to Home Depot, I saw a can of Foam Sealant (DapTex Plus http://www.homedepot.com/p/DAP-Daptex-Plus-12-oz-Window-and-Door-Foam-Sealant-18836/100357216) and I wondered if I could use that, once it will have a thick foam layer holding the heat from the attic. Is this foam good to seal the vents? Do you guys recommend use foam?

If foam is not good, what silicone should I use? I usually use DAP Premium Silicone (http://www.homedepot.com/p/DAP-Silicone-Max-10-1-oz-Clear-100-Premium-All-Purpose-Silicone-Sealant-08791/206046715) for small things. Can I use this silicone or should I use another silicone type? Thank you guys!

submitted by /u/johnbimbow
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