There are already several visitors of my website that have contacted me with the similar question about heat runs located directly under the ductwork. Here is the last one:
“We want to drop the registers down from the duct work to the ceiling which will be located directly beneath. My husband tried using sheet metal with no luck. Just this evening, being fed up, he has resorted to using cardboard. I am worried about the safety issues of using cardboard.”
So, instead of explaining over and over again I’ve decided to create this page on my website dedicated only to this specific question:
How to Drop a Register to the Ceiling
1. List of materials needed for the project:
Single sheeting Silicon
a. Single sheeting
b. Silicon
c. Zip screws
2. List of the tools needed for the project:
Cordless drill Big snips Red snips Hammer
Hand simmers Tape measure 12” bar folder Green snips
Caulk gun Magic marker Screwdriver
In order to better organize explanation of execution of this project I would like to divide it on several steps:
Step #1 – Get rid of old registers and patch apertures in the ductwork:
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Step # 2 – Mark out an aperture(s) for the sleeve(s) on the bottom of the supply duct:
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Step # 3 – Cut an aperture in the duct:
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Step # 4 – Make a sleeve:
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Step # 5 – Installation sleeve in the aperture:
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This is a parent page. If you would like to read more you can buy an access to the secondary page for only 
2.49.You can pay right here on this page by using the PayPal button below.
If you need more information before you buy you can submit your question in the feedback module below. After I read your specific question I will evaluate if information on the secondary page or on any other pages are satisfy your search. If the answer for your question doesn’t present on this or any other pages of my website you may get it from me for free!
Currently I have ten pages on my website dedicated to the finished basement issues. Among them seven presented on the “PAQ” and three on the “Installation in Pictures” pages. Personally I understand that it’s very difficult to decide which one or two of them are more suitable in your particular case. So, I come up with an idea to help you out! For the price of only 9.99 you can buy all ten pages (23.38 value) and I will release all of them for you at once. On those pages you can find basically all the necessary information for your finished basement project!
Also, if you even have hired a heating contractor to do the job it going to be a very useful tool for you to check out the quality of his work !
I've decided to name this proposal “Finished Basement Bundle” and you can use the PayPal button on the "PAQ" page to pay for excess to the pages.