Not Use Rectangular Ducts Above Roof

Duct temp gain when inside doghouse with r30 is less than 5 at 50.

Not use rectangular ducts above roof.

Transitions and offsets shall follow figure 4 7 of smacna hvac duct construction. Air extractors and splitter dampers are not permitted. Rectangular ducting can come in various sizes from 3 x 7 up to 35 x 40. It has both rectangular and round ducts.

And round ducting can be anywhere from 4 to 41 in diameter. And as you go through the exercise you can imagine how this duct support system would be different if you used a 60 inch round duct instead of the 96 inch x 32 inch duct shown. Above is an example rooftop duct layout. If you live in an area where frost occurs add an insulated duct sleeve to the portion of pipe in the attic.

This proportion works well on low country and traditional styles. The heat registers and return grilles are also available in different sizes depending on the opening and can be made of metal wood or plastic. Cut the hole for the duct attach it then install the roof cap. Finally install the hood.

These mimic the proportion of the windows in the home that are 3 wide by 6 tall. I like the proportion of rectangular vents to be twice as tall as they are wide. Craftsman style elevations can use a stouter proportion say 18 by 24. Where a rectangular duct changes in size at a square throat elbow fitting use single thickness turning vanes with trailing edge extensions aligned with the sides of the duct.

The absolute best way to run the ductwork above the roof is to use r8 insulation on the ductwork internal and build a doghouse outside the ductwork with minimum r30 insulation on sides and top of duct.