# Exterior Duct - Insulation on the inside?



## Darren Emery (Aug 21, 2017)

Ground based HVAC unit for a gymnasium.  Plans called for exterior duct insulation and protective wrap. Contractor and engineer now saying the R-4 interior insulation on this duct is sufficient.  R-8 is required (2009 ECC adopted), but I'm also concerned about the insulation being on the inside of the duct, rather than the outside.  Seems to me that condensation and mold is a likely problem in the future.  Would appreciate thoughts on this one - we just recently began enforcing the ECC.

BTW - we are in Kansas, so we have both extremes: -10 to about 106.


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## Pcinspector1 (Aug 21, 2017)

Darren, might want to check "Thermaduct" it claims it handles the mold issue and has different R-values available.


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## tmurray (Aug 21, 2017)

If their product is not an R8 I don't know how you are going to approve it. Your condensation concerns are well founded.


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## Pcinspector1 (Aug 21, 2017)

2009 IECC, 503.2.7 Not sure how you interpret the exception #2 ?


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## Darren Emery (Aug 21, 2017)

tmurray said:


> If their product is not an R8 I don't know how you are going to approve it. Your condensation concerns are well founded.


Thanks for the input.  The question now is, if they somehow obtain R-8 on the inside, do I approve that?


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## steveray (Aug 21, 2017)

Darren Emery said:


> Thanks for the input.  The question now is, if they somehow obtain R-8 on the inside, do I approve that?



I don't see how you couldn't? You could certainly let them know your concerns, but I doubt there is a code basis for denial.


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## Pcinspector1 (Aug 21, 2017)

R5 min. unconditioned space
R8 min. outside the bldg.


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## tmurray (Aug 21, 2017)

steveray said:


> I don't see how you couldn't? You could certainly let them know your concerns, but I doubt there is a code basis for denial.



I would agree with Steveray. I would make sure I documented that my concerns were expressed to the engineer and that there was no violation of the code to prohibit their proposed design. 

For what it's worth, I don't know why anyone would ever insulate on the inside of a duct. As you mentioned, there is an increase in the likelihood that it will be conducive to mold growth, but it also introduces potential contaminants in the air stream through both material off-gassing and particles such as fiberglass, which are known respiratory irritants. Additionally, it completely screws up your friction calculations for your ductwork, you now have to buy larger, more expensive ductwork to deal with the reduction in area of the duct and friction loss. The only advantage I can think of is that the insulation is protected from damage.


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## ICE (Aug 21, 2017)

The insulation is on the inside to protect it from the elements.


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## steveray (Aug 21, 2017)

tmurray said:


> I would agree with Steveray. I would make sure I documented that my concerns were expressed to the engineer and that there was no violation of the code to prohibit their proposed design.
> 
> For what it's worth, I don't know why anyone would ever insulate on the inside of a duct. As you mentioned, there is an increase in the likelihood that it will be conducive to mold growth, but it also introduces potential contaminants in the air stream through both material off-gassing and particles such as fiberglass, which are known respiratory irritants. Additionally, it completely screws up your friction calculations for your ductwork, you now have to buy larger, more expensive ductwork to deal with the reduction in area of the duct and friction loss. The only advantage I can think of is that the insulation is protected from damage.



Kind of like insulating on the inside of the house..........


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## ICE (Aug 21, 2017)

https://www.jm.com/en/building-materials/hvac-insulation/duct-liner/

http://www.rockwoolasia.com/products+and+solutions/u/2011.construction/9794/HVAC/Internal+duct+liner

http://www.kflexusa.com/downloads/Applications/Duct Liner.pdf


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## mtlogcabin (Aug 21, 2017)

604.13 Internal insulation.
Materials used as internal insulation and exposed to the airstream in ducts shall be shown to be durable when tested in accordance with UL 181. Exposed internal insulation that is not impermeable to water shall not be used to line ducts or plenums from the exit of a cooling coil to the downstream end of the drain pan.

Needs to be R-8 minimum

http://ww1.owenscorning.com/literature/pdfs/HVACCatalog.pdf


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## Paul Sweet (Aug 24, 2017)

Internal duct insulation used to be used a lot for sound reduction, and still is to a limited extent, mostly when very low noise levels are needed.


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## JCraver (Aug 24, 2017)

I'd bet 90%+ of the houses built in the 80's - 90's around here have ducts in the crawlspaces that are insulated on the inside.  I spent a summer building them for a guy when I was in HS - lived that whole summer covered in contact cement...  I don't think they had too many troubles with them - or at least, I've not heard of many of them being replaced.

I agree with the other guys ^^^ up there, your guy needs to be putting in R8.


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## ICE (Aug 25, 2017)




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## Francis Vineyard (Aug 25, 2017)




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