How to supply air in 2 rooms through a single joists cavity?

In my basement, I have a bathroom of 8'X5' and next to it there is kid space of 10'X15' both sharing one wall. I don't have much space in ceiling to run 2 flex ducts in their own separate joists cavity.

Just above in the upstairs, have two bedrooms and one bath room which took almost entire ceiling space for plumbing/flex duct. so I have only one cavity left between 2 joists in the ceiling to supply air to both of these rooms/spaces.

But How ?

Thanks

• Heat supplies in my area have been 6" round for many years (though lately they're using 8"). Is there some problem that keeps you from putting two 6" or 8" round ducts in that cavity? I'm not sure I understand the question. May 5 at 21:17
• the cavity is just 15" wide and I am planning to put flex duct. you can't put 2 either of the size. Also I can make only single hole a big enough in trunk. I can install a tee duct in kid area. so one duct can supply air to both the rooms. May 5 at 21:53
• 2 @ 6" ducts = 12" cross section. 1 @ 8" duct plus 1 @ 6" duct = 14" cross section. Either should fit into your 15" cavity. Considering that flex duct is, well, flexible, you should be able to squish it in a bit to make it fit. It may make each duct a bit more square and less round, and that may effect air flow a bit, but slightly reduced air flow is better than none at all. If using 2 sizes, run the smaller duct to the smaller room. May 6 at 12:05
• @Freeman, What size of take off , if you have space only for 1 hole. Can I use wye duct ? May 6 at 13:35
• I'm not an HVAC expert, but I'd think a wye duct would work fine. May 6 at 13:41

Without being there to visually see just what you have to work with is almost impossible. Try looking at an 8"X 6"X 6" WYE duct at Lowes, Home Depot, or on-line to see if that will solve your problem. Can't think of anything else without being there. Don't forget to add a return air grill mounted near the floor if you want to add heat to the game room.

One possibility would be to install a heat duct for the game room and a small electric wall heater for the bath room. A better drawing or even a picture or two would also be helpful.

• so I have to ask the great OZ ; why the mark down? May 6 at 16:07
• No reason given so down vote reversed. May 6 at 18:10
• Ops Sorry @D. george, I am not sure how ?. May be by mistake that went unoticed. thanks for your input man! thanks Ed Beal for reversing the vote. May 6 at 18:35

I would run an 8” line to the first room and at the register location drop to a 6” and continue to the bathroom .

This will allow a large register in the play room and a smaller one in the bath.

If you are concerned about the flow in the bathroom run the 8 the entire way but you will want a smaller register in the bath than the play room.

We do things like this all the time, I always use adjustable registers I like the butterfly ones when using round ductwork easy to add and less noise than the face closure type those tend to whistle and the adjustments are corse compared to a ~10 turn butterfly.

• @ED_Beal, how about 8" take off, run 8" flex to a 8" tee. Get a 8x6 reducer for the play room, and a 8x4 reducer for the bathroom ? May 7 at 2:34
• Yes that would work great, I usually have several boxes of different sizes, this way you can do it with one box of 8”. May 7 at 13:08