I'm trying to finish an en-suite bed/bath that currently has a capped 8" rigid duct coming off the main trunk. What is the proper method to supply air to both the bedroom and bathroom?

The bedroom is about 135 sq-ft and the bathroom is about 35 sq-ft. Can I just split the 8" rigid into two 6" flex? Or do I need to come off the trunk line with two 6" lines, one for the bedroom and one for the bath?

Thanks in advance!

  • 1
    Hard to say without knowing more about your HVAC system and home. Are the other rooms fed with discrete 8" ducts?
    – isherwood
    Nov 6, 2019 at 21:19
  • 2
    Definitely use flex only where necessary. It adds quite a bit of resistance to the line. Semi-rigid is better than the light foil stuff, and rigid isn't that difficult to work with anyway.
    – isherwood
    Nov 6, 2019 at 21:20
  • We need to know: HVAC system size in Tons, size of each return and their location(s), room sizes, size of all branches/outlets to give you a reasonable answer. Do you know why that rigid was capped?
    – peinal
    Nov 6, 2019 at 21:48
  • Hello, and welcome to Home Improvement. We'll need to know more info before we can help. And, props for taking our tour before posting; few newbies do. Nov 6, 2019 at 22:00
  • Is there a reason using rigid ducting for the branches isn't an option? Nov 7, 2019 at 1:03

1 Answer 1


Yes splitting an 8” duct to 2 separate 6” ducts is an appropriate size transition, since it is a master the air will be going to the same areas while the bathroom door is open. I usually add dampers to all my vents it costs more but makes balancing easier.

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