The county inspector dinged me on a couple of things for my bonus room renovation. One of these things is wrapping the register boots (endpoints) in insulation. I've seen registers with this insulation already installed, but he mentions that batted insulation is fine. The reason for this is to supposedly reduce the condensation leaking outside the boot and onto the wooden frame, joist, and sheetrock. Any idea what you folks do to accomplish this? I have some old R-6 flex duct insulation and some foil tape and was planning to wrap the batt around the boot and tape it up with foil tape.
How to insulate register boot
hvacinsulation
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New answer thanks to updated question.
Yes, the lip around the rim of the opening I'd call it a "ridge" but I dunno if that's right) on the boot is there to help ensure a firm connection. It's to keep the zip tie on the inner core from slipping free. This is necessary (the keeping the duct snug, not nessarily the tie) if your ducting is going to be pulling away from the boot due to gravitational stresses - such as when the boot is above the duct and the duct can be pulled down, such as along a wall or below a ceiling or floor. If your boot points down and the duct sits on a horizontal surface, you're good - no extra tension should happen.
Important note - neither mastic nor tape are intended to be structural - they don't serve to hold the ductwork in place, they serve to seal the ductwork so that there are no air leaks.
So the question really is - how do I properly secure my flexible ducting to the boot when there's no lip on the collar? Answer: Self-tapping screws.
Get yourself a box of those (the hardware aisle or the ductwork aisle). When you go to mount your ducting, pull the outer insulation back up away from the core, pull the core over the collar, and use TWO screws on opposite sides of the duct (180 degrees from each other - noon and 6'o clock (or 1 and 7, 3 and 9, you get the idea) to ensure that neither side of the duct can pull free.
Use the inner to snug up the connection (don't over tighten!), seal with mastic (I'd recommend tape though cause of the insulation layer...), slide the insulation up as far as it will go and snug it up with the outer zip.
A lot if things will happen because fluid dynamics is complicated.
If you reduce the opening of one register, you increase the resistance of the whole system. This increases the pressure of the whole system and reduces the volume of the whole system.
At the individual registers: The increased pressure in the whole system will cause more volume to flow out of all of the other registers (than was flowing before), because their resistance hasn't changed.
The increased resistance of the register you reduced will decrease the volume coming out of that one, but, the velocity (which is maybe what you meant by force?) will be higher, because of the higher pressure (Bernoulli's equation).
The actual degree of these effects will also depend on the system fan - as most HVAC system blowers have a very shallow pressure/volume curve (and sometimes even positive slope, yikes!), you will likely see only a very small increase in pressure and a more significant loss in volume. Nevertheless, the general relations I said above should hold. Unless you somehow push the blower into some unstable region (positive pressure/volume slope), then bad things will happen.
Always be careful naively applying the surface area relation in the cases. Going from 8" to 6" will reduce the surface area from 50 square inches to 28 square inches. Naively you could guess that this would reduce the volume to 28/50, a 44% reduction, or that it would increase the velocity by 50/28, a 78% increase, but when you consider the system, and the fan, you see that it does neither and both of these. It will decrease the volume, but by less than 44%, and it will increase the velocity, but by less than 78% (far less, in practice).
Also note that the degree to which these things happen will depend on exactly how you reduce from 8" to 6". A flat plate with high losses will make your volume loss closer to 44% and velocity increase closer to 0%, whereas a carefully designed nozzle with minimal losses will get much more velocity with less volume loss (keeping in mind that a nozzle, after all, is a device designed to convert pressure to velocity as efficiently as possible).
Best Answer
You're thinking about this too much. The goal is to reduce air contact with the metal. Simply get yourself some R-11 or R-13 fiberglass batts and pack the joist or ceiling space loosely around each vent component. Condensation is greatly reduced by doing so, and your obligation is fulfilled.