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Old 06-29-2010, 02:33 AM   #1
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What Type Of Subfloor Is This?


Hi Everyone, my name's Frederick. I'm a newcomer to the forums. I've been a long DIYer, but this is my first time trying to modify a house.

I'm looking to do a bit of additional soundproofing in my second floor condominium. Over the last 2 years I have noticed that the sound transmission through the structure of the building (as well as airborne) is fairly high.

I'm already looking into adding a layer of drywall with Green Glue over some existing walls, but I'm at a loss as to what to do with the flooring. I pulled up a corner of the carpet and I found what I believe is Gypcrete underneath, but I'm not certain. Can someone help me identify the material in the pictures below? The carpet already installed had its tack strips and pad stapled to whatever this is, which leads me more to suspect it is Gypcrete (unless you can staple into lightweight concrete?). I can also score it using the edge of a flathead screwdriver without too much difficulty.





Assuming it is Gypcrete what can I do with it? I was hoping to put another layer of OSB down with Green Glue and recarpet over it. However, I don't know the best way to attach the OSB to the Gypcrete. I don't think Green Glue alone is enough, but I'm wary of screwing into the Gypcrete directly. It does look like it has some sort of spray/sealant over the material to protect against moisture at least.

I've sent an email to the condominium association asking what it is, but they're somewhat less that diligent with the speed of their replies if they decide to reply at all.

Thanks!

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Old 06-29-2010, 08:37 AM   #2
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What Type Of Subfloor Is This?


Hi Frederick,

That could be gypcrete, or a concrete slab. You can't damp (Green Glue) a slab. Too massive.

Same with block walls, in case that's what you have.

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Old 06-29-2010, 10:07 AM   #3
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What Type Of Subfloor Is This?


Thanks for your insight Ted.

Fortunately the walls are standard drywall on 16" OC studs, so Green Glue and added drywall is still an option there.

Is there nothing that can be done with the flooring? I think there's a fundamental issue with the structure itself as it seems to transmit vibration quite readily; when there are heavy footsteps below I can feel it in my floors. I'm not sure if there's any quick solution for that.

I'm also concerned about the amount of airborne noise transmitted through the floor. I can definitely tell when those below are talking although I cannot make out what is said. I'm certain that given a loud enough shout I could understand it. I don't think it's all being transmitted through the walls either as the sound is more pronounced when I put my ear to the floor.
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Old 06-29-2010, 10:13 AM   #4
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What Type Of Subfloor Is This?


Doesn't sound like a slab. You would need to verify the floor construction to determine the best solution.

Concrete slab? Joist system? Drywall applied?

That sort of thing
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Old 06-29-2010, 10:33 AM   #5
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What Type Of Subfloor Is This?


I see. Without suddenly developing my X-Ray vision I think I'm at the mercy of the association for that information. I'll see if I can't get them to provide that detail.

If I had to venture a guess I would say it is a joist system with a plywood or OSB top with Gypcrete poured over it. I know that the walls go directly to the wood subfloor; they are not above the poured material. On one section where I pulled back the baseboard trim I was able to dig through a section of drywall to find the bottom of the wall's stud embedded in the poured material. To add to that I can definitely hear it when things are dragged along my neighboor's wall below. I suspect I would not be having these issues if my walls were atop a concrete slab.

Thanks.
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Old 06-29-2010, 10:39 AM   #6
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What Type Of Subfloor Is This?


You have vibrations being conducted as well as airborne noise. You have the worst of both worlds.

Ideally, sound isolation is done at the source, to reduce the vibration before it travels. That would mean treating your neighbors' walls and floors, so thats's out as an option. In your case you likely have sound coming in from some number of walls as well as the floor. Possibly the ceiling.

You could try treating one surface at a time and see how that works out.

If the floor is a joist system with wood subfloor and gypcrete, you can damp that. The exact procedure would depend on the desired finished floor material.
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Old 06-29-2010, 11:08 AM   #7
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What Type Of Subfloor Is This?


Thanks Ted.

If carpet were to be the finished flooring what would you recommend as far as damping the floor?

As a side question is there a cheap material you would recommend for filling a wall cavity between studs? I am planning to put an interior doorway in the middle of an open hallway. The ceilings are 9' so the top of my doorway frame has a rough 1'x2' opening to be filled. I hear Roxul is the best filler, but I'd rather not buy a full roll for a 2 sq ft opening.
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Old 06-29-2010, 11:15 AM   #8
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What Type Of Subfloor Is This?


Insulation in walls only helps a little. Sometimes very little.

Fiberglass IN a wall cavity will work as well as mineral wool, cellulose, etc. If you want to insulate, consider cellulose. Don't dense pack it.

If you are carpeting, you might consider adding a layer of subfloor and damping material between the gyp and wood subfloor. Then carpet and pad.
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Old 06-29-2010, 11:33 AM   #9
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What Type Of Subfloor Is This?


Would my original idea of Green Glue and OSB on top of the Gypcrete work? My only concern was with how to fasten the OSB to the Gypcrete. So far I haven't heard of any fasteners that are rated for Gypcrete. I've heard Tapcons for concrete, but I don't think Gypcrete will hold.

I've also heard of drilling through the Gypcrete to the wooden subfloor below, but I've a sneaking suspicion that the association won't take kindly to that.

I don't suppose rubber mat would be of any use? I image it would lessen any vibration I would feel in the floor as well as provide some airborne noise blocking if heavy enough.

Sorry for all the questions. It seems like the more research I do the more questions I end up with.

I appreciate all the input you've given!
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Old 07-14-2010, 09:04 AM   #10
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What Type Of Subfloor Is This?


How thick is the gypcrete? I suspect damping will be limited due to the significant difference in stiffness between the subfloor+gypcrete combo and the new OSB

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