In looking though some material on how wood roof decks may suffer decay from the underside, I’m looking at a book that talks about high interior humidity and the potential for near 100% humidity in “unvented ceiling plenums”.
My question is:
If the purpose of the ceiling plenum is to distribute conditioned air, why would it be vented?
You’d either be venting conditioned air to the exterior, or introducing unconditioned air to the system designed to distribute conditioned air.
Kenton you are getting no response here because you are talking a foreign language. What are you calling a ceiling plenum. To me a plenum is the metal or duct board connection to the supply and or return air side of a AHU. Occasionally one can find a remote plenum in a attic that has a 16 inch supply duct from the AHU that has several 8 inch ducts connected traveling to the drops (ceiling registers)
Can ya be a little more defining in your question.
I think Kenton is referring to using ceiling space for air returns to roof top units. Pretty much every commercial building I have ever been in does this.
To answer Kenton’s question, the term ‘unvented plenum’ is redundant if my guess of the meaning is correct, and as Kenton points out the term makes no sense. The ceiling space is ventilated by conditioned air. Condensation under the deck or any other cold surface would definitely be an issue if humidity was high.
Ahhh I am with you now I was thinking residential. It would be very unusual to see a attic chase here used as a plenum in commercial. We normally have a dropped acoustical ceiling grid to hide the duct from view, supply and return is brought through the grid. I am seeing a lot of spiral round duct exposed and the roof decking visible primarily in restaurants
Are you sure what you read was the ceiling plenum used to distribute conditioned air (supply air) with a wood roof deck exposed. That would never be allowed at any humidity level because wood is never to be used on the supply or conditioned air side of a A/C system due to condensation which results in mold. Now if you are talking on the return side wood is allowed because the return air is not considered as conditioned air.
Commercial building are required to have at least 10% outside air drawn into the return air side of the system usually at the motorized dampers on a roof top unit
“Wood decks can rot when exposed to high, unrelieved humidity. In the normal air-conditioning range of 34- to 50 percent relative humidity (RH), wood’s moisture content generally runs 8 or 9 percent, far bellow the 35 to 50 percent range required for active rot-fungus growth. But in unvented ceiling plenums, where periodic condensation can keep relative humidity close to 100 percent, wood decks can rot.”
They don’t further explain what they’re calling a “plenum”.
I’ve been in multi-story commercial buildings where the conditioned air was distributed through the space between ceilings and floors, but that’s all I know.
I’m used to residential where the plenum is basically a manifold from which various supply ducts are routed to different parts of the house. I don’t understand it’s use here, discussing commercial buildings.
I can not testify as to how air delivery is accomplished in other areas but in my area and what I was taught for 40 years supply air is to make no contact with wood what so ever. The words " Unvented ceiling plenum" makes no sense to me