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 homeowner 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.
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 temperature 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?
Typical Roof Temp