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A previous owner put a small addition on the house and the little room is not well heated. (I have forced air HVAC.) One wall of the small addition is shared with a larger room. On the side of the shared wall is a sheet metal plenum 8"x8" in cross section that brings HVAC to the larger room and a foyer. I want to open a vent from the plenum in the larger room into the small, colder room. Here is the problem.

The plenum is right next to a supporting beam, so I need to run a path for the air around the beam. There is no easy way to get into the crawl space above the two rooms in question. So, I need to run the path below the beam.

My question: Do I need to build a sheet metal duct or can I just make a path for the HVAC air using plasterboard?

Thx.

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If the addition that you mention has 3 side walls and a ceiling, heating and cooling this room from the same furnace as the rest of the house is nearly impossible. The 4 surfaces exposed to the weather will loose heat in the winter and gain heat in the summer at a faster rate than the rest of the house. If this is the way the addition was built, then a separate heating and cooling unit or the existing system should have been zoned to provide it's own temperature control. Adding too much air supply will cause large temperature swings in the addition. As far as giving advice on running the duct, I would have to be there to see the actual space and items you mention. And "no" I would not use plaster board for a duct. At this point, I would add it's own heat/cool system or better yet have a professional in your area give their advice.

  • TY. Pls clarify. What are the downsides of using plasterboard for a (short) HVAC duct? – Yehuda_NYC Nov 21 '17 at 12:04
  • There is probably no down side if done correctly. However,; you won't find any respectable HVAC company installing it this way – d.george Nov 21 '17 at 15:37
  • If there is no downside, why would no HVAC company do it that way? Plasterboard is easier to work with than sheet metal, even thin sheet metal. Also, what do you mean by "done correctly"? What could I do wrong? – Yehuda_NYC Nov 21 '17 at 16:02
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    HVAC companies work with sheet metal all the time not drywall. For an HVAC company it's easier and faster to do it with sheet metal. If you plan to do this yourself the big box stores sell various sizes and lengths of ducts. They can be easily cut with good tin snips and bent as needed. A better way of doing it than drywall and less work if you can find a size duct that works. As d.george said additions like this with 3 outside walls would probably need something more than just tapping into an existing duct. – Platinum Goose Nov 21 '17 at 18:11
  • Thanks for alerting me that just tapping into an existing duct might not do the trick. But it seems like a cheap and easy approach and it just might work. My HVAC circulates air every 15 min or so, which might be beneficial. – Yehuda_NYC Nov 21 '17 at 18:24

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