Cutting A Concrete Basement Floor For Isolation

Posted: Mon Jan 05, 2004 1:53 pm

Wes and Rod,
I am considering cutting my basement floor concrete slab in order to isolate a section of floor from the rest of the house. I have a "control joint" crack along one section of the floor, and I have performed a very simple & crude test to confirm that this crack does create significant isolation between the two sections of floor divided by the crack. However, my main concern is not isolation from one area of the basement to another, but from the basement to the main floor of the house.

So, the sound travel path of concern is from the basement floor to the basement walls to the main level floor joists (the basement walls are cinder block on top of the concrete footer, and the floor slab was poured over the footer and butts up to the cinder block wall). It seems that in order to de-couple this path I would need to cut the concrete floor far enough away from the block wall to disconnect the floor slab from the foundation footer. I think this would cause me to loose about 6 inches on each side of the room enclosed by a basement wall (3 sides of my room would be affected in my case).

So, my queston is - will the isolation improvement beteen the basement floor and the main floor be enough to make it worth loosing 6 inches on 3 sides of a room that is approximatley 24 feet by 14.5 feet?

(I will already be building a "room-within-a-room" with its own ceiling separate from the floor joists to maximize the air-borne sound transmission to the main floor)

Thanks!

Posted: Mon Jan 05, 2004 2:16 pm

Scott,

I've cut a few slabs in my time, that's for sure. The thing about your case is that I don't feel that is your main path of sound transfer. Your biggest problem is going to be sound transfer directly through the air and up through the floor, and, even worse, footfall noise from above that is structure-borne and will rock the house.

I'd need to know more about your overall budget and plans before recommending cutting the slab in this case.

--Wes

Posted: Mon Jan 05, 2004 2:23 pm

Scott,

My gut reaction to this is that you will never gain enough isolation to make it worth giving up the value of the real estate.

Carrying noise through the slab and then up through a dampened exterior concrete wall assembly to the area above and carrying it through a slab into an adjacent room on the same level are 2 very different animals.

Isolating the walls and ceiling make more sense to me........

But still, I agree with Wes that you need to fill us in more on what your long term plans are.

Rod

Posted: Mon Jan 05, 2004 3:07 pm

Thanks guys for your fast replies!

My current plan is as follows:

1) Stuff insulation in between currently existing floor joists, and attach 2 layers of drywall (of different thicknesses) via Resilient Channel to these floor joists. (These are the main level floor joists, which is also the current basement "ceiling". The basement is currently unfinished so the joists are exposed).

2) Build a self supported room with its own ceiling supported by new walls built on top of the basement concrete floor. This new ceiling will also be constructed of 2 layers of drywall (but I'm not sure if I should attach this drywall via Resilient Channel or directly to the new ceiling joists). These new ceiling joists will probably be 18ga 3-5/8" steel joists placed at 12" on center in order to maximize ceiling height.

By using the construction indicated above, I was hoping that sound transfer directly through the air, and footfall noise from the main floor would be minimized well enough - but if you think I need to do something else here, please advise.

I plan to record a variety of music in this room, from acoustic folk/bluegrass, to rock and roll.

I will perform all the labor myself, so my budget only needs to account for materials, and I hope to keep the material cost to under $5k.

Thanks again.

Posted: Mon Jan 05, 2004 11:00 pm

Scott,

Quote:

I'm not sure if I should attach this drywall via Resilient Channel or directly to the new ceiling joists

No need for RC here - the isolation takes place with the new joists.

Quote:

These new ceiling joists will probably be 18ga 3-5/8" steel joists placed at 12" on center in order to maximize ceiling height.

Be sure to check your span tables for steel joists. I don't have them in front of me, and you didn't specify the span. By the way, I like wood for it's combination of compressive and tensile strength. You need both in a beam situation. (The top of the beam is in compression; the bottom of the beam is in tension.)

Quote:

By using the construction indicated above, I was hoping that sound transfer directly through the air, and footfall noise from the main floor would be minimized well enough - but if you think I need to do something else here, please advise.

You're doing the right thing.

Quote:

I plan to record a variety of music in this room, from acoustic folk/bluegrass, to rock and roll.

Should work fine, except for the bluegrass. Just kidding.

Quote:

I will perform all the labor myself, so my budget only needs to account for materials, and I hope to keep the material cost to under $5k.

Good luck. If you have any money left over, I figure I got 2 cents comin' my way.

--Wes

Posted: Tue Jan 06, 2004 6:35 am

Wes,
Thanks for your comments.
So, regarding the original question - do you think I should cut the concrete slab basement floor to de-couple it from the basement walls, which in turn de-couples it from the main floor joists?

By the way, my material budget is somewhat flexible; I can "steal" from my equipment budget if the improvement in isolation is worth it.

(Also by the way, I was never turned on by Bluegrass music either until I realized how difficult it was to play some of that fast pickin' stuff. Surprised

)

Posted: Tue Jan 06, 2004 6:51 am

Max Span on 3 5/8 18ga. is about 8'-6" @ 12" centers carrying 6psf (2 layer of 5/8" gyp = 5.25 psf)

So as long as you don't load anything else onto it you should be fine up to that span.

After hearing everything I've heard - I don't believe it's worth it to bother cutting the slab.

Rod

Posted: Tue Jan 06, 2004 9:13 am

Rod,
Your comment about the max span for that steel joist has me concerned. I planned to use that particular joist spec based on a recommendation from Dietrich Industries (www.dietrichindustries.com). I sent a technical assistance e-mail request asking for a joist that would span 15 feet and support 2 layers of 5/8" drywall, and I received the following reply:

"Scott, Thanks for your interest in our product. The stud you will need is a
18ga 3-5/8" CSJ3 placed at 12" on center. This was calculated using L/240
for the deflection and a 10 pound load. The 2 layers of board are a load.
You would need to put in blocking every 8' at mid point and strapping on the
top flange at midpoint."

Is there something I'm not understanding here? Do you think Dietrich's reply was a mistake?

Thanks for your help!

Posted: Tue Jan 06, 2004 9:32 am

Scott, Rod,

By "stapping" do they mean a strongback? I've seen these used to increase spans, but still, 15' seems like a very long span.

Also, there is no allowance for a live load of any type? Just 10 lbs. dead load?

Scott, if you intend to hang a cloud you will need to leave some room in those figures as well.

But I'm very curious to hear Rod's response.

--Wes

Oh yeah, and shouldn't the deflection be 1/360?

Posted: Tue Jan 06, 2004 10:38 am

Quote:

Originally posted by Scott Hill:
"Scott, Thanks for your interest in our product. The stud you will need is a
18ga 3-5/8" CSJ3 placed at 12" on center. This was calculated using L/240 for the deflection and a 10 pound load. The 2 layers of board are a load.
You would need to put in blocking every 8' at mid point and strapping on the
top flange at midpoint."

Ist of all - i have to apologize - I read the property values for the wrong steel joist from the table.

My eye gravitated to 18 mil rather than 18 ga (43 mil)

A 362S162 43 mil (according to SSMA this is the largest guage 3 5/8" member manufactured) is rated for 14'-5" without lateral support of the compression flange with a design for L/240 - and actually the same span for L/360.

You should be fine at 15'. But this only if you don't hang anything else from the ceiling.

My data is coming straight from SSMA's (Steel Stud Manufacturing Association) Product Technical Information and Guideline.

They do not show an allowance for additional loading because of the installation of blocking or bridging.

The only alowance they give is for center support, and you don't indicate that you intend to support this from structure.

But - as i stated - this will only support the drywall - and not the 10 psf indicated in the communication to you - that is unless the engineering data provided to you has taken this down to the level of "Unity = 1" (or possibly less) - which is to say - there is no safety factor being used in the calculations provided to you.

The span tables typically have a safety factor involved seeing as they are not judging every installation individually - but are rather trying to establish a safe generalized installation guideline.

Wes,

I agree that I would use L/360 for design purposes - even for simple ceilings - I don't use the L/240 formula for anything - however it does meet code.

The design for 10psf is acceptable without live load being added for ceilings (attics) without usable space above (less than 42") and a reduction to a 5psf dead load (or the actual -whichever is greater) is allowed for ceilings with not more than 30" between them and the rafters/joist above. (BOCA 1606.2.2)

The "strapping" they are refering to is not a strongback.

The details for this at SSMA indicate a light guage channel section - and indicate this with or without a blocking below made from track section.

I will field any more questions after lunch.... which for me is coffee and a cigarette. Very Happy

Rod

Posted: Tue Jan 06, 2004 3:17 pm

Wes,
I haven't heard your opinion yet on whether or not it is worth cutting the concrete slab - what do you think?

Posted: Tue Jan 06, 2004 8:59 pm

Scott,

I'll have to agree with Rod that cutting the slab would not be on my list of top priorities in your case. There are just so many other things to think about, and they all take time and they all cost money...

--Wes

This topic sponsored by: