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Quieting Drainpipes

Plastic pipes are cheaper than cast iron, but they're also noisier

Ask This Old House Crew
Photo by Matt Kalinowski

I'm installing a new bathroom on the second floor and have to install a new waste stack. I've been told that cast iron drainpipes are a lot less noisy when water runs through them, but the plumbers I've talked to prefer to use plastic piping, and their bids for using plastic are much lower than their bids for the same work with cast iron. I want quiet pipes that don't cost a lot. What are my choices?
— Sue, Minneapolis, MN


Richard Trethewey replies: Cast iron drain- pipe is worth every penny in terms of its quietness, but I'm not surprised that your plumbers don't want to mess around with it. Plastic piping, whether it's PVC or ABS, is far less expensive, easier to handle, easier to cut, and easier to join. It's also noisier, no doubt about it.
While there's no way to entirely eliminate those annoying sloshing sounds, there are a few ways to reduce them. One thing to look at is where the pipes will be routed; it's always better to go through a closet wall instead of a living room wall, if you have the choice.
You can also reduce noise transmission by insulating around the pipe with sheets of sound-
absorbing foams ( or with open-cell spray foam insulation ( Batts of fiberglass insulation aren't dense enough to be good at soundproofing.
Another option is to mix drain materials, the way we did at the TV project in Billerica, Massachusetts. There, the drains were all plastic except for a vertical length of cast iron that ran beside the dining room wall. Tom Silva then had foam insulation sprayed all around it, just to make sure that pipe would never be heard.


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