Return Plenum Insulation Questions

[SVibs]

I have a new gas furnace and AC system on the way and hope begin installing in the next week or so. I end up having to order a new base can/return plenum that looks like this:



The plenum is uninsulated and will be located in an unconditioned space. I believe code calls for a minimum of R6. What are the acceptable means of insulation in this case? Can I wrap the outside of the plenum with 1.5” duct board? I looked at plenum lining such as Armaflex but it’s crazy expensive and I can’t seem to find it in small lengths.

 

[fitter30]

Do see why not but there are other products just do a search. Don't forget the bottom.

 

[danski0224]

Rigid foam board, foam adhesive, foil tape, long screws and washers.

 

[Smokeem]

There is also a foil bubble insulation that has either an R6 or R8 rating that I have seen used. Is this plenum going in the crawl space or attic? I know in our area code says R6 in unconditioned space and R8 if in attic space.

 

[SVibs]

Do see why not but there are other products just do a search. Don't forget the bottom.

I have done plenty of searching but haven't found anything specific to a base can that sits on the floor. And while duct wrap would work on the sides, putting it on the bottom would be useless since it would compress the fiberglass flat.

 

[SVibs]

Rigid foam board, foam adhesive, foil tape, long screws and washers.

Is foam board rated for that application (I pulled permits)? And if so, can I used it on the bottom where it will bear the weight of the furnace and cased AC coil?

 

[SVibs]

There is also a foil bubble insulation that has either an R6 or R8 rating that I have seen used. Is this plenum going in the crawl space or attic? I know in our area code says R6 in unconditioned space and R8 if in attic space.

Plenum will be in the garage. From what I've seen, the foil bubble wrap needs a spacer strip to get the full R value. Again, not an issue on the sides but problematic on the bottom.

 

[PoorUB]

Is foam board rated for that application (I pulled permits)? And if so, can I used it on the bottom where it will bear the weight of the furnace and cased AC coil?

I don't see an issue with foam board on the outside of a return plenum. A return plenum can be wood. You might have to cover the foam to satisfy fire code.
Angle iron legs bolted to the corners, or wood blocking to keep the weight off the foam.

 

[Smokeem]

Ok where this is going in the garage there is a required min of 26 gauge metal for the duct work.

 

[Smokeem]

For the insulation code requires a flame spread of not more than 25 and smoke development of not more than 50.

 

[PoorUB]

For the insulation code requires a flame spread of not more than 25 and smoke development of not more than 50.

I assume, only if it is left exposed. Cover it with tin, perhaps even foil tape and you probably will met the code.

You really need to talk to the code enforcement in your area.

 

[SVibs]

Ok where this is going in the garage there is a required min of 26 gauge metal for the duct work.

Can is 24 gauge so all good there.

 

[SVibs]

For the insulation code requires a flame spread of not more than 25 and smoke development of not more than 50.

Looks like this foil faced Polyisocyanurate Board Insulation might fit the bill. I need to look into it further but I'm hoping foil side out satisfies code.

 

[SVibs]

I assume, only if it is left exposed. Cover it with tin, perhaps even foil tape and you probably will met the code.

You really need to talk to the code enforcement in your area.

Agreed--just hoping to have a couple of options to run by them.

 

[danski0224]

Is foam board rated for that application (I pulled permits)? And if so, can I used it on the bottom where it will bear the weight of the furnace and cased AC coil?

Up to you to find out specifics for your location. Some foam boards cannot be used inside without some sort of exterior fire resistive covering on the foam. The foil faced foam should be suitable, but check for yourself.

That said, rigid foam board attached to the exterior of the metal box will be the fastest and easiest way to achieve the desired R value.

The bubble wrap stuff does not work without an air gap.

Fiberglass duct wrap does not have the rated R value if it is compressed.

Bubble wrap and fiberglass would require your assembly of parts to be supported.

If the foam has a PSI rating, that's how much weight it can support in pounds per square inch. Go to the store and try to squish it.

You may want to put something between the foam and concrete to prevent moisture from getting into the foam.

 

[SVibs]

Good info--thanks everyone.

 

[dscheidt]

There is also a foil bubble insulation that has either an R6 or R8 rating that I have seen used. Is this plenum going in the crawl space or attic? I know in our area code says R6 in unconditioned space and R8 if in attic space.

The foil covered bubble wrap is about r1. To get r6 with it, you either need six or 8 layers, or you have to build a frame to create a substantial air space between two layers of the stuff. No one does that, they just pretend it's not snake oil sold by liars.