What's the expected noise level for a basement ADU in old bldg?

Question

I'm closing on a 1910 home that has been completely redone from the studs up, with a 1,000 SQ feet basement ADU. The furnishings are high end, separate entrance, etc and would attract a wealthier tenant. We will live above.

HOWEVER, the main floors are the original fir planks, and in the ADU you can clearly hear conversations, and footfalls.

My Q: What's the expectation in the Seattle basement ADU market for noise levels? We are getting quotes for soundproofing but I'm new to Seattle and don't know what renters expect from a basement ADU. Is this just the norm to hear conversations and people know that when they rent a basement ADU?

Michael Haas · Answer

Grab two 12 packs of green glue and throw an extra layer of drywall up there to be safe - if you think sound is a concern, now is the time to address it. It shouldn't be too expensive of a project if you get a handyman on it, but watch out for the big Seattle contractors, they think every project no matter how small should start at $30,000 and go up from there :/.

Nic Werner · Answer

Thanks everybody for sharing. Returning the share, we met with an acoustical consultant who really does know his stuff, and has his own crew. I'd put him in the "If you want it done right" category, which is why he quoted us $34,000k, and said he'd be able to hit 55+ STC. He knows the assemblies and how to treat the top of the walls as well (if not, will severely diminish the common double gyp-rock approach)

Now, before you shake your head, here is how he would approach to hit 55+ STC. This adds I believe 1.5 inches from ceiling.

Primary issue: Footfalls (just walking around would be hard for someone to focus downstairs)

- 5/8" gyp rock affixed to the subfloor for footfall dampening w/green glue

- Rockwool for high noise/voice attenuation

- Sound channel clips for air separation

- And to go further: double sheetrock with green glue.

- Custom boxes around each of the light cans ($175 each) OR switching to wall sconces

The primary cost is the sheetrocking labor itself, as It's not just the ceiling replacement, it's aligning the new ceiling texture with the walls.

Given there is a stairwell from main floor downstairs, he recommended two solidcore doors, which parts + labor would be $2k each.

What will we end up doing?

I believe the resilient clip channel + gyp-rock + rockwool. will give the most benefit. We don't have the drop space to do both channel and two layers of gyp-rock. We'll be tapping on the floor to see which areas resonate most, and affixing the gyp-rock to those spaces. You don't need 100% coverage: Just like tapping an empty can, you can get the same attenuation by holding with two fingers as you do with your whole hand wrapped around.

As to the doors, we'll wait and see after the approach above.

If someone wants more details on the assemblies, etc, PM or encourage me to add more here :)

Eric Teran · Answer

@Nic Werner noise is measure by STC which stands for sound transmission class. For floor separation between units you want to be around 50 STC. There are various ways to achieve this.

Putting another layer of drywall or sound insulation is not enough. You want to separate the vibration. In addition to the extra layer of drywall and sound insulation there are vibration clips that are installed under the joists and then a metal furring channel is installed running the length of the ceiling. Then the drywall is attached to the furring channel. This separates all the vibration and noise will be minimal.

I did this for my basement (we rent on Airbnb)  in the DC area and have done it for numerous clients. Around here a 1,000 SF ceiling cost around $4k to $8k (drywall, clips, insulation, paint, labor). I did mine around $4k because I acted as the GC, bought the materials, and hired the subs. It was definitely worth it.

One more solution is if you are replacing your flooring on the first floor there are rubber Matta that you can place under your flooring.

Whatever you do try to get the STC rating above 50.

Liz Koser · Answer

It's tough to say without knowing how clear the conversations are. If it is more like you can hear voices in the background, but not understand what is being said or is it you can clearly hear every word? If it is more faint background noise, I'd say most people will be okay with it. Also, is it furnished now? If not, furniture will help with the noise.

In terms of the floor footfall noise, I'd say area rugs and hallway runners make a big difference and could help some with the conversation noise too in terms of absorbing sound. I'd try that before soundproofing. It doesn't need to be soundproof, but I wouldn't want to hear every conversation either. Also, I'm sure you want the privacy too.

Dave Skow · Answer

tough to know ...but  on a  scale of  1 to 10 ...I would  guess it  will be  8+  and probably  annoying for any occupant to contend with

Nic Werner · Answer

Thanks @Dave Skow and @Liz Koser for the quick replies. Clarity-wise, the conversations are absolutely clear to hear - to the point that we'd be concerned for our own privacy. The place is furnished w/staging furniture and rugs, so it is a close approximate.

Our realtor says its a common expectation when renting a basement ADU in an old house, but the common-sense in me says that I wouldn't rent it with that much sound bleed! Footfalls, yes - that's common in old stock like SF as well, but not conversations or a podcast.

Pat L. · Answer

We just finished building a complete 3 bedroom out-law apartment in a basement with 9ft ceilings. They then paid to insulate the ceilings between the 14inch floor joist with a Corning quiet product. BUT with 2 young kids running around upstairs it was definitely a lost cause.

What's the expected noise level for a basement ADU in old bldg?

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