help needed with wall cavity at the top

chippy · Post by **chippy** » Sat Apr 12, 2003 8:09 pm

Hello I love reading the help and advice on this very good site.
I know there are a lot of experienced people here, so I am keeping my fingers crossed that there is an easy solution to my problem.

From looking at other peoples pics most twin walls start at the floor and finish at the roof but unfortunately mine doesn’t. What is the best way to seal the cavity at the top of the two walls and still keep them unconnected to each other?
Or is it better to leave the gap?

Daz

knightfly · Post by **knightfly** » Mon Apr 14, 2003 12:57 pm

Is this already built?

Can you give more detail on how each wall is constructed, what layers where, sealing method between ceiling and walls, etc? Be very specific, and try to draw to scale, or at least give dimensions of everything -

If not necessary for strength, I wouldn't connect the two walls at all - if you have to, don't use a solid link but make sure there is isolation provided by rubber, neoprene or possibly other ways ???

If the ceilings of both rooms are well-constructed, probably the best way would be to just add more insulation over each, to further absorb any leakage either way, before it can get to the other ceiling. This area can act as a substantial bass trap with enough absorbent... Steve

chippy · Post by **chippy** » Sun Apr 20, 2003 11:20 pm

each wall is x2 plasterboard 12.5mm, 50mm flexy rockwool on 2x4 stud then an air gap of 4''. the twin Walls are sitting on 2'' rubber.

The ceiling is 2x6 timber with 12mm chipboard on top and will be filled with rockwool and covered in cloth.

Above the ceiling is 4-5 foot air space then the roof.
As you can see this air space links the air gap in my twin walls and also the hall way.

chippy · Post by **chippy** » Mon Apr 21, 2003 12:03 am

the layout

knightfly · Post by **knightfly** » Tue Apr 22, 2003 2:17 pm

The maximum allowable span for 2x6 on 24" centers, assuming extra sheathing, is 10'8" - for 16" centers, this increases to 13'0" - so, if the ceiling joists run the short dimension of the rooms you could (and should) add at least one layer of 5/8" sheet rock, preferably two layers, over each ceiling. Putting a heavy layer of rockwool over that would help mid-high frequency isolation - if it would show around the edges (no inner ceiling) you could frame around the perimeter to contain the rockwool.

You didn't mention how solid the roof is - but the extra mass in both ceiling leaves would improve that path also.

If you do all that and there is too much leakage out of the area, you may need to look at extra layers on walls and better door treatment... Steve

chippy · Post by **chippy** » Tue Apr 22, 2003 8:15 pm

thanks Steve, that gives me a lot of help with the ceiling.

What should I do to seal the 4'' (cavity)air gap in the twin walls.
I don't want to fill the cavity, I just need to seal the top so there is no air connection to the roof and the hallway. My main concern is, I do not want to make the twin wall solidly joined together.

knightfly · Post by **knightfly** » Wed Apr 23, 2003 5:17 am

One way to do that would be to contact cement 1/2" neoprene or rubber on top of each wall, then use two layers of sheet rock/gyp rock/wall board, whatever it's called in your locale - then rock wool over that, same as the ceilings. For the vertical opening by the hallway, same minus the rockwool... Steve