Confused about my existing walls.

[davidm]

Hi all.
I have a 25x18' space that once was a shop of some sort. Cinder block walls, metal roof. My question is: The wall structure is, from inside to out, 1/2" Sheetrock, hat channel with no insulating that I can tell, affixed to cinder block. If I leave a 1" gap and build a 2x4 wall with insulation and 5/8 Sheetrock on that, have I set myself up for a 3 leaf system? Is the current wall 1 leaf or 2 due to the air gap created by the hat channel? I am at a place that I could pull the existing Sheetrock down to the cinder block and start fresh.
Any thoughts would be helpful.

[Soundman2020]

Look for a document called IR-586, put out by the Canadian National Research Council. It describes exactly what you are asking.

Is the current wall 1 leaf or 2 due to the air gap created by the hat channel? I am at a place that I could pull the existing Sheetrock down to the cinder block and start fresh.

The answer is "yes" and "yes".

Yes it is a two-leaf, and yes it is already reducing your isolation in low frequencies. Just by taking off that drywall, you would gain about 5 dB of isolation. If there is no insulation in there, then by taking off the drywall you might gain even more.

Thin cavities in walls can do some amazing damage to isolation. Here's a page from IR-586:

IR-586-loss-from-drywall-on-battens.jpg

Those graphs show how much improvement / deterioration you get from different "drywall on concrete block" strategies at different frequencies. The horizontal line across the middle is the "no improvement, no degradation" line. Anything above that is improvement, everything below that is worse.

In all four cases, the low frequency isolation is WORSE than having no drywall at all.

Your case is the top right graph. It clearly shows major degradation for all frequencies below about 400 Hz.

In other words, for the entire low part of the spectrum (20 Hz to 200 Hz) and for part of the mid range (200 - 2k), you have lost isolation like that. It's only in the upper part of the mid range and the highs that you get any improvement, but those areas already had good isolation.

So you already have a 2-leaf system, and it is so badly done that it is making your isolation worse, not better: light weight drywall on a thin air gap is a recipe for disaster.

If I leave a 1" gap and build a 2x4 wall with insulation and 5/8 Sheetrock on that, have I set myself up for a 3 leaf system?

Yes, and since your middle leaf is low mass with an existing degradation, you'd only make that worse.

Solution: Pull off the drywall and hat channel, paint the surface of the blocks to seal it properly (air-tight), then build your new inner-leaf walls such that you end up with at least a 4" air gap that is filled with insulation. For example, if you leave a 1" gap between the block and the framing, and you use 2x4 framing, then you'll have 4-1/2" air gap. which is good. Make it bigger if you can, and do make sure that you fill the cavity with suitable insulation, but without over-filling it.

- Stuart -

[davidm]

. Not what I wanted to hear, but I want it right.
Thanks again for taking the time to help me with my project. I value your insight and unselfish sharing of time.
David.

[Soundman2020]

Not what I wanted to hear, but I want it right.

Yep. Unfortunately, studio design and building involves a lot of "not what I wanted to hear!" It goes with the territory...

If you can, try to save the drywall as you take it off, and try to get it off cleanly, without breaking it too much. You might be able to re-use some if it (and maybe the hat channel too) if you are careful. Ditto if there is any insulation in the cavity.

On the bright side, taking all that off will give you good access to the concrete block behind, so you can carefully inspect it all over and seal any holes, gaps, or cracks that you find, then paint the surface with masonry sealant, to close up the surface porousness. That alone will help considerably in your final isolation.

- Stuart -