plaster for sure

I can think of no advantage of plaster over drywall except hardness of the finish and there are more important issues involved.

The separation wall between the two units must have a fire-resistance rating and that is much easier to obtain with drywall.

Here is the code requirement:
"R302.3 Two-family dwellings. Dwelling units in two-family dwellings shall be separated from each other by wall and/or floor assemblies having not less than a 1-hour fire-resistance rating when tested in accordance with ASTM E 119 or UL 263. Fire-resistance-rated floor-ceiling and wall assemblies shall extend to and be tight against the exterior wall, and wall assemblies shall extend from the foundation to the underside of the roof sheathing."

That means the wall assembly must have been tested by an agency like UL have have a published test number. These tests are expensive so there are few that are for metal lath and plaster.

If the studs are wood, in bearing and don't contain plumbing, etc., the UL test number will be U 300 and U 399. The 1 hour rating can be achieved with a layer of 5/8 Type X drywall (U305).

For metal lath and plaster, the the lath would need to be 3/8", 3.4 lb per sq. yd. diamond mesh expanded steel and the plaster would need to have a scratch coat of 2 c.f. of vermiculite aggregate to 100 ibs of fibered gypsum and 100 lb. of unfibered gypsum for the brown coat. (U315).

The approved manufacturers of the materials used for each of the tested assemblies are listed for each test. there many more tests for drywall than for lath and plaster.

It would be far cheaper and easier to construct and get approval for a drywall assembly with no loss of fire protection or sound separation.

To improve the sound separation with drywall (I don't how to do it with plaster) add another layer and perhaps hang it on horizontal resilient channels. Seal the perimeter well and avoid back to back electrical outlets or other penetrations. It can help if one side is different from the other.

There are many alternate ways to increase sound separation but the 4 principles are:
1. mass - the heavier and thicker the materials the better
2. resiliency - materials that move with the sound reduce it's energy
3. absorbency - materials that are porous slow and trap sound
4. no cracks - cracks and doors defeat the other principles

To get the hardness of a plaster finish with the advantages of drywall you can use a veneer plaster system (blue board) if it is available in you market. It's just a thin coat of plaster on a special drywall board and far superior to taped drywall usually at no additional cost.

This post was edited by Renovator8 on Thu, Feb 21, 13 at 9:39

Would definitely do plaster - your insurance is playing for it - do
It right. Way better soundproofing and fire resistance - it is like stone.

"I can think of no advantage of plaster over drywall except hardness of the finish and there are more important issues involved."

The higher mass and stiffness of plaster walls greatly affects sound transmission.

Even 2-coat plaster on gypsum lath is far superior to 1/2 inch drywall.

You end up with about 3/4 inch of VERY stiff wall material.

This post was edited by brickeyee on Sat, Feb 23, 13 at 13:32

Recently completed a series of confidential rooms,our method as follows.
2 layers of 5/8" sheet rock attached perpendicular to each other on both sides on 2x4 metal studs and cavities filled with insulation.
These are smaller conference rooms that abut a common area that by code require double 5/8 on the outside. In our case a very large manufacturing area.
I can tell you these rooms are virtually sound proof.

Sound transmission occurs via two paths:

--Airborne sound
--Structure borne sound

To effectively create a sound-resistant assembly, one must address both of these issues. In addition, as Reno says, in the case of multiple unit dwellings, they must be seperated by a fire resistive assembly meeting local code requirements.

Generally speaking, airborne sounds are mitigated by mass, while structure borne sounds are mitigated by seperating air spaces. There are numerous sources available on the Internet for options on how to accomplish both types of mitigation. One of the simplest for airborne sound is to use sound asorbent batts in the stud spaces, and a double layer of gyp board fastened to the studs. For structural borne sounds, the use of resilient clips and another layer of gypboard is effective. For maximum effectiveness, you will need both types of solutions.

Good luck with your project.

I 'd say plaster over drywall hands down. The only thing about plaster it's expensive.