Re: How to soundproof ... floors/ceilings
From: Michael Black (
Date: Mon, 28 Jan 2008 17:36:33 -0800 (PST)
At Yulupa Cohousing we used 2 separate 2x4 walls separated with 1" min. of
air space.  This is more effective than staggered 2x4's on a 2x6 plate.,
especially if there is plumbing in the wall. We can hear a hum from the
bathroom fan and some very minor plumbing noises from our bath which is
backed up to our neighbor. Though we wanted blown in cellulose insulation,
we settled on a no-formaldehyde fiberglass insulation, because of a jump in
construction prices at the time of bidding. This system is supposed to offer
better sound control than masonry, even when the masonry is solidly filled
with concrete or utilizes poured-in insulation. Reflective sound is more of
a problem with masonry than wood studs.

It is very helpful to not have symmetrical walls. Consequently, we only have
the acoustical channels on one side of the party wall. Two separate stud
wall are actually required if you have a townhouse situationl. They are not
necessary for a condominium, but well worth the cost.

A bead of acoustical sealant should be placed between the drywall and the
floor and at the corner of the walls and ceilings.

We have 2" thick lightweight concrete slabs with radiant heating. These need
to be covered with another material. The lightweight concrete adds
acoustical value. The concrete slab should be installed over a layer of
insulating material made for the purpose. We also have the gypsum-board
ceiling connected to resilient channels and full depth insulation between
the floor joists.

We do have a cork flooring alternative here. Cork is softer than wood and
bamboo and can be sustainably harvested (If trees are only stripped every 9
years). There is a recycled tire insulation product that can should go under
the cork, especially if there is not a layer of insulating material under
the lightweight concrete:
I agree with only having carpet floors except in the kitchen and baths if
there is not a lightweight concrete slab.
2-1/2² thick exposed/colored concrete floors can be used with the assistance
of an acoustical engineer. The exposed concrete serves as thermal mass,
which holds the warmth in the winter or the cool in the summer with the
opening of windows during the evening while sleeping.

It is important to understand that there are 2 types of sound transmission
through walls and floor/ceiling assemblies: impact, such as footsteps and
air-born sound that sets up vibrations that can be transmitted. I do not
know if there is anything that can prevent a person from hearing someone
hanging a picture on their side of a party wall.

Cast Iron waste lines are silent. Plastic ones are not.

I hope this helps.

In community,
Michael Black, Community Planner/Architect

> Lots of expertise has been shared here about sound-insulating shared walls.
> It would be good to hear more about up-down soundproofing. I have an
> apartment in my house, downstairs, and the sound transmission is
> significant. We used some special sound-reducing ceiling-joist system, but
> it still transmitted a lot. The professionals I consulted said it was a real
> challenge (the up-down), addressed best by MASS. So we added another layer
> of drywall on the ceiling, plus acoustic tile. (Requiring messy retrofitting
> of light fixtures, smoke alarms, etc.) And it still transmits. Fortunately,
> I like opera, as my renter plays thundering piano and sings coloratura at 3
> am! (My bedroom is another floor up, anyway.)
> So, beyond carpeting (which people with allergies might not tolerate well),
> what advice do you wise folks have?
> Also, is there anything that reduces the sound effects of shared plumbing?
> The sound of the neighbors toilets flushing, drains running, etc. In our
> house, I am just used to it, and it doesn't bother me: oh, it's 5 am and S.
> is taking her shower....
> Looking forward to learning more.
> Lynn Nadeau
> RoseWind Cohousing, Port Townsend WA
> where a tiny film of snow has schools closed and kids sliding down hills on
> cafeteria trays- a rarity here these days
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