When is it ok to put your ducts into an attic, tight up against the Roof Deck?  With more conventional building techniques, I would have to argue: NEVER!  I do not care how the homeonwer pleads to keep their storage space, you cannot count on delivering cool air to the conditioned space with said branch duct.  Even if you did figure out a way to secure the duct to the peak without crushing the inside diameter, the CFM is not the problem.  The real issue becomes the dry bulb temperature of the air by the time it hits the terminal, significantly warmer than design.

Typical Roof Temp - http://www.fsec.ucf.edu

 

  As seen in Figure 1, during normal Summer conditions with a well sealed and insulated attic, the roof deck could be at least 25F warmer than the surrounding attic air!  I will not bore you with the radiant exposure and reasoning, the fact is that surface is significantly warmer than the space.  How do you think the attic temperature continues to rise during those hot Summer days?

  So, with this engrained in my head since the beginning of my HVAC time, I have preached this cardinal sin for duct layout for years: until today.   I have finally seen a situation that I could not argue with, granted a very expensive solution.  When the roof deck is spray foamed, the surface of that insulation is much closer to the attic, which is now within the building envelope!  I would still argue that this is not the way to secure the duct, as avoiding the use of flex duct in this type of layout could go a long way.

  What do you think about ducts in an attic?  I find they are a huge efficiency drain, sucking money from a homeowner's pocket year round.  Equipment must be larger, there will always be leakage, most attics are not ideally vented, and never mind the IAQ issues!  I was once with a contractor that found multiple dead animals in an attic that caused a stench like no other, sucked in through leaky returns and evenly distributed through out the home.  Is the answer to remove attic ducts as an option, via code?  Maybe eliminate attics?

http://excessair.blogspot.com/p/blog-page_23.html

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Comment by Hal Skinner on September 5, 2014 at 1:47pm

Hi Christopher,

Here is a discussion I started on how we use an RCC to insulate cooling  / heating equipment and ducts in attics.

Conditioned air in attics? Not me!

Comment by Dennis Heidner on September 4, 2014 at 2:01pm

In the picture above, it appears that the attic has been turned into conditioned space.  The temperature stacking chart presented is not likely to be what you would see.  

Turning the attic into conditioned space also helps with controlling the air leakage between the "actual" living space and the attic.  You've effectively moved that leak boundary to the roof.

I suspect that the reason for the longer ducts hung on the ceiling and back down... is someone is planning on storing lots of stuff in the attic.  For that it would seem, a better way would be to insulate the ceiling as had been done, keep the ducts low, foam them in again,  and build a platform floor above the ducting to hold items that would be used for storage.

It is also possible that the roof has enough height in future dormer would be added and perhaps a small office... nothing was said about how the attic is accessed... or the height.

Comment by John Proctor on September 4, 2014 at 11:56am

We found with the ducts on the floor of the attic, with smaller ACs, with shorter runs, with the ducts buried in lots of insulation, in hot dry climates, that there is almost no penalty for having the ducts in the attic and it is a lot easier than trying to manipulate the space to get them "in conditioned space". This is shown in slide 13 of this presentation http://www.proctoreng.com/dnld/Proctor-DatanotDogma.pdf  The red dots are the actual cooling energy use of the house system with the ducts in the attic as described above. The blue dots are the actual cooling use of the companion system that was installed totally within the envelope of the same houses.

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