Plenum Truss—Keepin’ the Ducts Inside – GreenBuildingAdvisor

Building Science

A plenum truss is one method to keep ductwork inside the conditioned space of a home

The first article I ever wrote for GBA was back in 2020. Titled “Keeping Ducts Where They Belong,” it was all about the issues I see when ducts are installed outside the conditioned space of the home. I even mentioned in the article that of the new homes that fail the code-required blower-door testing I conduct, most have ductwork outside the building enclosure.

So what’s changed since I wrote that piece? Not very much. A couple months ago I had another failed blower-door test because of ducts in the unconditioned and vented attic. As a matter of fact, it failed its second test as well.

Why ducts outside the envelope cause problems

When ductwork is located in unconditioned and vented attics, vented crawlspaces, attached garages, or any other location that is outside the thermal and air boundaries of the home, there can be a whole host of problems:

• Holes through the air barrier are needed to extend the duct system into the conditioned space.
• Insulation may be displaced where the duct moves through the thermal boundary.
• Air leaks in supply and return ducts located outside the building envelope can create pressure imbalances inside the home.
• Cold ducts can condense, causing water damage and mold on ceiling and walls.
• DOE Building America Research shows 10% to 30% HVAC energy loss when ducts are outside conditioned space.

When ducts remain inside the building envelope, most of these concerns are solved.

Strategies for keeping ducts inside the envelope

There are a lot of different ways to correct this issue.