Insulating Supply ducts

[khllmrk]

I would like to get some input on the subject of insulating supply ducts which are located in return air spaces, perticularly in multi story applications. I Have seen installations designed with no insulation on the supply ducts where the ceiling was opened to the decking on single story buildings. I tend to believe that would be a bit risky. However it seems to be a much more common to see supply ducts not insulated on lower floors of multi story buildings. I am not sure the IECC would even permit this type of installation any longer. Any thoughts would be greatly appreciated.

Thanks
Khllmrk

 

[vannic]

You should check your local energy code as far as what insulation is required. As far as ASHRAE 90.1 is concerned, a supply ductwork in an "indirectly conditioned space" (aka return plenum with or without exposed roofs) does not need any insulation. I've never come across a sweating supply duct that was in a plenum. That being said, there is a benefit to insulating and not absorbing the plenum heat. That should be a cost benefit analysis on your part. This is usually a tremendous savings for the owner not to insulate up front.

 

[imok2]

It seems to me that the major determination to insulate supply ducts is based on the amount of cooling loss you are willing to accept and that of course is based on temperature difference of exposed duct run.
ASHRAE/IES Standard 90.1-1989, Energy Efficient Design of New Buildings Except Low-Rise Residential Buildings

Question 1: Does section 9.4.8.3 require duct insulation for a cooling supply air duct conveying air at a design temperature of 56EF where the duct is located within a ceiling return air plenum with a temperature of 77EF?

Answer: Yes. In the example given, insulation with a minimum R-value of 4 would be required since the supply air to ambient air temperature difference is between 15EF and 40EF.

 

[ChasBean1]

A good argument for insulating would be as follows.

A 55°F supply system is designed for light office work to maintain a 72°F db, 50% maximum summer humidity level. The uninsulated 55°F air duct should avoid condensation because the dew point is 52°F under the design conditions.

Five years later, the company does well, so there are more office machines and more people and the system returns 75°F, 60% humidity air during design conditions. The 60°F dew point air would then make the supply ducts sweat, leading to mold and other problems.

I would agree with your thought that it would be risky. The only supply ducts I've seen intentionally not insulated (inside or outside plenums) only appeared that way - they were actually lined on the inside.

 

[KenRad]

CB makes a great point. Especially in spaces where you are not doing active dehumidification (ie, with reheat), your return plenum dewpoint can easily be higher than your supply air temperature in regions like the Southeast US.

It takes only the thinnest layer of duct wrap to prevent condensation under these circumstances. I believe that the added cost for an insulated supply is well worth the knowledge that you are not creating moisture problems in your plenum.

---KenRad

 

[ChasBean1]

And we shouldn't skip the notion of infiltration. Some buildings become negative over time because people add task exhaust, or the building is not designed or balanced right to begin with. If there is infiltration of unconditioned air, it will always find its way into negatively pressurized plenums...