insulation for ducts 5

[lemonmugua]

I am new here. I am currently distribute ducts from a roof top unit. I am told to add a 1" insulation from the output of the roof top unit in a distance of 25'. I donot know why it should be 25'. what should I do if my main duct are less than 25'???

 

[imok2]

The duct should be insulated as far as it is exposed to a temperature difference. Example: outside air, attic temperatures, the sun, etc.

 

[Waramanga]

Is the insulation internal or external? Internally insulated is often for sound attenuation.

 

[DrRTU]

You should review ASHRAE 90.1 for proper insulation required. The International Energy Code, sections of IMC and various state and local codes all have similar wording to 90.1. Check with your local municipality for the applicable code / standard. As other have pointed out, the designer has the option of internal vs. external insulation. Internal insulation provides sound attenuation but carries potential IAQ issues.

The codes / standards are based upon physical location of the building and location of the duct inside or outside the structure. The codes / standards have tables of minimum R value required.

 

[lemonmugua]

it is internal insulation. Thanks for your comments!

 

[stevenw]

On some jobs we internally insulate the first 25' of duct for sound control then go to external insulation (cost) for the rest of the duct run.

 

[sridhar1312]

If the ducting is from roof top naturally the duct will be exposed to out side ambient.As the duct carries cooled dehumidified air the duct need to be insulated to minimize the heat loss and also sweatimng on the duct when the ambient dew point is above the dew point of cooled air in the duct.

Depending on the length of supply air duct the tail end also need to be insulated to ensure the room temperature is same all around.This insulation is to limit the temperature rise to minimum.
All the above is external Thermal insulation .

 

[zintern]

The 2006 International Residential Code section N1103.2.1 and the federal Energy Code both require a minimum of R8 insulation of any outdoor ducts and ducts in vented areas. Some jurisdictions made federal Energy Code voluntary, but the IRC is the independent code requirement and most cities adopted it as their building code.
There is nothing to stop anyone from using better than R8 if that would provide a beneficial bang for the buck.
But the bottom line is R8 on the roof is a minimum, i.e. usually about 2" of good insulating material on the inside.