Duct Work insulation question

This is my first post on this forum and I hope I’m doing things according to the rules. I have posted this question on another forum and as usual have not gotten a reply.

Here goes: After spending three long and hard days in my crawl space, I was able to replace the flex duct that was torn apart by rodents (before I move here).

While down there I noticed that what I think is called a Supply Plenum (long box where the ducts come out of) .
The furnace is located in a hall closet above. This box is not insulated at all.

Question is, should it be? And if so what is the best insulation to use and how to apply it. I think this box is making the banging nosie I hear when the furnace kicks on and the again about 3-4 minutes after the furnace kicks off.

Would appreciate any help at all.

Thanks in advance…

Fiberglass Duct Board Fiberglass Duct Wrap](http://www.industrialinsulation.com/air_handling.htm)

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simple answer…all supply plenums and ductwork, fittings, etc. is required to be insulated, and only after sealing all joints/connections with either approved foil tape or mastic material. In my area of the country we “generally” insulate the return ductwork system in Attic spaces and crawl spaces. Again, generally, not always.
I would ask a Prof. HVAC Contr. to inspect, and possibly give you a quote to fix, or tips on what is necessary, and how to do the work yourself. Again “generally”, insulating ductwork in crawl spaces is fairly undesirable work for the company technicians/mechanics, unless the company is really slow with work.
I’m sure I don’t have the total picture of your conditions, so really think about calling a Pro to look at your situation.
Good Luck, whatever you do.

Many thanks to those who replied. After hooking up the flex duct this week-end (around/over and in between the bath room plumbing where the ducts ran) insulating the plenum will be pretty much a piece of cake. I will get a bunch of duct wrap today.

I am assuming that I should insulate the ends first adding a couple of inches so I can then go over the top and around.

Tood a good look at my furnace yesterday in the hall closet. None of the connections were taped. I taped all the connections and seems with Mashua 324 Aluminum tape. There is one connection up high that I cannot get to.

In addition, I discovered a hole where the line and pipes enter the closet that was not caulked or sealed in anyway. Appx a 3" hole. It was very hard to see as there is not much room in there. I was able to stick my finger in the hole and could feel the draft from the crawl space. I was able to stuff some fiberglass insulation into the hole and then added some caulk.

What a difference it made. The furnace kicked on much quicker (without trying two or three time to catch) The banging noise is almost gone completely. This furnace has been like this for over 16 years…

Thanks again and if you think of anything to add, please do so

P.S. Is there a way to post pictures on this forum??

A lot of old metal ductwork here is insuslated on the inside.

As Brian said, they are insulated inside. This reduces motor noises.

Pull the bottom panel of the furnace and look down inside and see.

Thanks Dave, however the furnace is located in my hall closet so there is now way I can look down into my crawl space. As the unit is over 16 years old, were they even insulating the insides back then? Any other ideas on how I can tell? Also would it hurt to insulate outside even if is was on the inside?

Many thanks again…

Yes. Probibly more then than now!!

You should be able to see inside the return duct/plenum below the blower motor.

David’s right, I worked thru high school and college in the mid-late 60’s at an HVAC provider and have glued insulation on the inside of 1,850 miles of metal duct and plenums. When you replaced the flex duct you should have been able to see inside the supply plenum also. The insulation I used was about 1" thick and had a very dark, somewhat rigid outer coating.

The banging you mentioned was probably the sides of the plenum flexing outward as the air pressure rose when the fan came on and vice-versa as the fan went off. That should have been addressed when the plenum was constructed by placing small creases in the sides of the plenum in a X-fashion using a metal brake.

See this example.

The banging is also an indication of excessive negative static pressure in an improperly designed duct system.