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Old 07-02-2009, 04:39 PM   #1
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insulating the inside of square ducts


I have a duct that is 20" by 8" and runs about 15 feet and I want to insulate it on the inside. I want to use 1" thick so that it becomes a 18" by 6" effective duct.

I can buy a roll of 1" 100' duct insulation for 300 dollars and only use about 20 feet of it, or I can try to use another material.

Can you use the silver backed Styrofoam insulation for lining the inside of square ducts? Home Depot sells this for much less and I can cut the 4x8 sheets to size. They have the pink and the white silver backed foam. Is it not code to use these products in ducts?

-Jeff

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Old 07-02-2009, 05:23 PM   #2
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insulating the inside of square ducts


If you have any local HVAC wholesale houses in your area, contact them and see if they carry "Duct Board". It is a scrim faced insulation board that is designed to be duct when broken and formed. Most supply houses will not sell equipment to HO, but often will sell some of the ancillary items. They are hungry like everybody else.

I would not know why the other would not insulate, but I would be concerned about how it is effected by moisture.

Another option would be to bend the ear of a few local HVAC contractors and see if they would sell you the amount to roll insulation that you need. Offer them cash, maybe?

Good luck

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Old 07-05-2009, 11:30 AM   #3
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insulating the inside of square ducts


you are going to lose about 35% of the capacity. don't do it.
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Old 07-06-2009, 01:20 AM   #4
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insulating the inside of square ducts


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Originally Posted by tinner73 View Post
you are going to lose about 35% of the capacity. don't do it.
I forgot to mention the duct isn't in yet.

The duct is over sized to support the 1 inches less on each side. Also it is only heating/cooling one room. It will have plenty of flow. 108 sq inches of duct for 300 sq feet is going to have plenty of capacity. I want lots of cold air to get to the loft otherwise it will all just flow over the railing.. There will also be dampers everywhere off the trunk.

Ok, now that we're past that, have you ever heard of people using duct board as insulation rather then just making ducts out of it? What about plain old Styrofoam? Is it a mold problem? It is 10x less costly then duct insulation, I want to understand why.


-Jeff
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Old 07-06-2009, 07:14 AM   #5
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insulating the inside of square ducts


I have been reading up on foam insualtion for my basement reno.

Styrofoam and foam insualtion can give off toxic gas in the event of a fire. Installation instructions state the board should be covered by fire resistant material when installed to prevent the transfer of fumes in a fire.


This is a potentially DEADY idea!!!!


Are you getting this ducting free. Why not just have it sized correctly for the application...
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Old 07-06-2009, 11:15 AM   #6
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insulating the inside of square ducts


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I have been reading up on foam insualtion for my basement reno.

Styrofoam and foam insualtion can give off toxic gas in the event of a fire. Installation instructions state the board should be covered by fire resistant material when installed to prevent the transfer of fumes in a fire.


This is a potentially DEADLY idea!!!!


Are you getting this ducting free. Why not just have it sized correctly for the application...
Cool, just the info I was looking for!

I'm planning a remodel and want the extra capacity.

Last edited by zootjeff; 07-06-2009 at 11:22 AM.
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Old 07-10-2009, 02:00 AM   #7
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insulating the inside of square ducts


OK,

Basically to run the 205 inches of 20x8 insulated duct with the insulation on the inside from the do-it-yourself sheetmetal house, it is going to cost me 450 dollars!

If I buy the same 205 inches in 60 inch pre-fab sections it costs me 148 dollars without the insulation.

Basically I really don't want to spend 450 bucks. I'd like to go with route B and spend 100 bucks on some kind of insulation solution. Half of it is in the crawl space above so having it insulated on the inside isn't worth it up there.

Here are some types of insulation for ducts I have seen. All are different:

Type A: This is what lines my furnace, there are no welded pins. The main part is a dense fiberglass, there is a stiff liner that comes in contact with the air. This seems like the most rigid of the types. It is only glued on and isn't mechanically supported on the edges. If I can figure out what this is, it would be my first choice.



Type B:

This is a fiberglass insulation with a reinforced foil liner that come in contact with the air. It lined my cased coil. It appears to only be glued on the inside and held in in place on the corners via mechanical wedging-in-place. This would be my second choice.





Type C:
This is the typical furnace liner material that I can by in rolls of 100 feet for 300 dollars, and it is what my fabricator used on my custom sheet metal. It isn't very dense and if it is only glued in place I would worry that it would separate the liner from the fiberglass body. The welded pins are helpful for this reason. Since I can only find it by the 100 foot roll and I can't get away with just gluing it in place, this is my last choice.





If anyone has any ideas of part numbers or the trade names of insulation that I should look for, please help me out!

Thanks,

Jeff

Last edited by zootjeff; 07-10-2009 at 02:02 AM.
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Old 07-10-2009, 08:53 AM   #8
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insulating the inside of square ducts


Call your fabricator and ask if you could bring your pre-fab in and have him cut and glue your example C. That is the most common duct liner that most use. That way you would not need to buy a whole roll. Or, as I said above, go to your fabricator and buy the amount of that insulation that you need and DIY, though he can probably do in about 15 minutes with his cutting table and spray gun and banger pins.

This should be the easiest part of your project so far.

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