Attic Space Vs Plemum Space

A two room addition on the 1st floor has limited attic space and a 4 crawl space.

Attic space vs plemum space.

Cabling between the many devices and to the users runs through this open floor space. Unventilated attics can help to mitigate energy loss through leaky ducts or ceiling fittings. The plenum truss bulkhead adds only the amount of inside conditioned space required to house the ductwork by modifying the roof framing to create a boxed in area in the otherwise unconditioned attic. However if moisture is able to enter the unvented attic space it may lead to roofing panel issues.

It will increase your load and contaminate your house air if it has been converted from a ventilated attic to a sealed attic. The energy savings touted by unvented attics can be as high as 20. It s easy to see your point about eliminating ductwork from the attic. However i have two questions if i want to centrally air condition my entire house.

Plenum trusses create a reverse bulkhead in the center of the attic space where ducting can be housed in an insulated area. A basement is below and an attic is above the main part of the house. So when we have a choice we prefer hvac systems located under the house. Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum.

The velocity of the overall air movement within the two spaces would be substantially different at the same cfm. That dead air space is designed to be a buffer between the roof. That space typically has about a 10 12 degree difference than the conditioned space below it. Rarely will a sealed foam insulated attic deviate more than 15 degrees from the living space temperatures below it.

This by definition is a plenum air space and requires plenum rated cable. I wonder if the nec definition of the word plenum would actually apply to the entire attic space. Think about the volume of space normally associated with return air ducts vs the volume of space involved in an attic space. Don t use the foamed attic space as a conditioned plenum.

Compare that with the daily 140 degree temperatures found in vented attics.