Not to offend anybody but, ductboard is not a good option because the inside of the duct is easily boken off in small fiberglass paticles and is distributed throughout the house, and you will find this in homes with this product 5 or more years old it also accumulated like a dust on furniture. Your better option is a acuostical duct liner that have a thick face and is resistant to breaking apart with velocities of up to 2500fpm, this the the common product installed in commercial projects. It is glued and pined inside of metal ductwork and is a little more expensive but consider the life of the product and less fiberglass in you and your childrens lungs.
I also wanted to mention in your video you use way to long of flexible round ductwork, it has been proven that 25' of good straight flex has as much pressure loss as 100' of metal duct so homeowners beware just because it is easy to install it does not give good airflow if it kinks, sags, makes 90 degree turns or runs longer than 5' ( usually the maximun amount needed to make and easy connection)
