Ceiling T-joist isolation question

[romanvm]

Hello,

We are trying to isolate ceiling between T-joists. We have two options (see attached photo for detail):

1. Add layers of 5/8 drywall directly/flush to subfloor. However above flooring was screwed directly through the subfloor and has large amount of screws that came through the subfloor that would not allow to add drywall flush to subfloor.

2. Adding drywall over the "lip" of the t-joist. However this will create about a 3/4 gap between drywall and subfloor. Will this gap negatively affect isolation properties? One option is to add additional "foam board" to fill the gap as it is soft enough to be pressed against the screws in the subfloor plywood.

Your feedback will be much appreciated!

[TomVan]

snippers.jpg

Roman,
Anyone that has beefed up their sublfoor/joist area will tell you its hard work. To add insult to injury the screws are just one exciting addition to your work load.
You want option one so it is tight and no cavity. It will take some time. I hired a couple guys on 1000 square feet and it took them two days to complete and they were cussing near the end.
Snippers to cut off screws or a metal cutting blade on a pea grinder. Safety glasses will be your friend just from all the crap that will fall in your face while installing
Best
T

[Gregwor]

I second what Tom has said here. From personal experience I can say that it truly is a nightmare. I've had to use an angle grinder to remove some of the big screws that were sticking through the floor and of course it set off the smoke detectors and pissed my wife off. I snipped thousands of nails before beefing up. After you remove the nails/screws, caulk all the seams where the joists join the floor. Then add drywall with Green Glue Compound that is held up with home made cleats. Caulk everywhere except around the cleats. After that drys, remove the cleats and move them to an already caulked section of the drywall. Then caulk where the cleats were initially.

You're clearly doing this for isolation. If you have any HVAC duct work or other mechanical running through any joist bays you're going to have to box those in. I used 1" MDF for my build. But, that will just cover up the bottom of the joists. The joists are probably made out of something like 3/8" OSB so you're going to have to beef those up to truly box in your mechanical. I added 3/4" MDF to the "sides" of the boxed in joists to get the surface density to match the rest of the system.

This took me a very very very long time to do mostly because it feels like it never ends and depression sets in. Stick with it and chip away at it over time. It'll get done and it will be worth it. Please keep us updated with photos!!

Greg

[romanvm]

I second what Tom has said here. From personal experience I can say that it truly is a nightmare. I've had to use an angle grinder to remove some of the big screws that were sticking through the floor and of course it set off the smoke detectors and pissed my wife off. I snipped thousands of nails before beefing up. After you remove the nails/screws, caulk all the seams where the joists join the floor. Then add drywall with Green Glue Compound that is held up with home made cleats. Caulk everywhere except around the cleats. After that drys, remove the cleats and move them to an already caulked section of the drywall. Then caulk where the cleats were initially.

You're clearly doing this for isolation. If you have any HVAC duct work or other mechanical running through any joist bays you're going to have to box those in. I used 1" MDF for my build. But, that will just cover up the bottom of the joists. The joists are probably made out of something like 3/8" OSB so you're going to have to beef those up to truly box in your mechanical. I added 3/4" MDF to the "sides" of the boxed in joists to get the surface density to match the rest of the system.

This took me a very very very long time to do mostly because it feels like it never ends and depression sets in. Stick with it and chip away at it over time. It'll get done and it will be worth it. Please keep us updated with photos!!

Greg

Yep, I know this is lots of work hence asking about adding drywall to the joist lip instead... guess there is no shortcuts in the sound isolation work!!!