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    • I’m in the midst of remodeling a townhome. Replacing the old flooring with a new engineered wood floor is the next step (ha).

      I’m going the rounds with my contractor and subcontractor about acoustic underlayment. Since a major part of the renovation is finishing a basement, I want to make sure there isn’t that annoying sound of “footfall” (I’m learning new terms) on the wood floor upstairs coming down through the floor and ceiling into the basement living area.

      The sub doesn’t want to mess with installing an underlayment (more work), so he has forwarded the spec sheets on two adhesives that he wants to use. The marketing language says they are, “sound-reduction” adhesives <eye roll>.

      I have now spent way too much time researching NRC (noise-reduction coefficient), STC (sound transmission class), IIC (impact sound transmission), and different floor structures. :/

      This is a long-shot, but I wonder if there is anyone out in cake-land who has any practical experience with this stuff? If not, well, then, at least I’ve made my mark by adding a few new topics and making Chris try to figure out where this crazy post belongs. Hahaha. :)


    • I love talking about this stuff! Great questions with no straightforward answers :)

      I’ve done a lot of research on this when building my house. You really have to ask yourself what it’s worth for a good STC/IIC rating. The price goes up exponentially with STC/IIC ratings.

      IMHO the Sound Proofing Company’s guide on sound proofing floors (and walls) is the one of the best pieces of literature out there on the subject. Professionally speaking, traditional flooring contractors are not experts on soundproofing, so I would take their advice with a grain of salt. You can probably educate yourself better than anyone with literature online. The high end AV installers who build home theaters are the experts. If you want a profession opinion, it might be worth consulting them for an opinion, but it's not rocket science to install good sound proofing materials. A decent flooring contractor can do this, though they might drag their feet because it's something out of the normal scope of work. The key follow the directions exactly. For example, too much or too little green glue can yield poor results. Methodically measuring out the glue is important. Easy task, but it's even easier to take a short cuts.

      I'm assuming you're not replacing the joists. Wouldn't ever recommend it, extremely expensive. So an assembly like this would be wonderful in terms of sound isolation, but expensive both in terms of labor and materials. I have green glue in my house, and it really does wonders.

      What do you currently have in mind for your soundproofing subfloor assembly?

    • Brian Strong

      What about flipping the solution around and thinking about what can be done on the ceiling of the basement? I'm sure there are sound deadening insulations that can help. There is also a product called QuietRock that can be used instead of sheetrock that may help.


      I'm no expert, but I love this type of stuff. This guy Matt Risinger has a great YouTube channel about construction and soundproofing is a topic that regularly comes up.

    • Wow. Great information and valuable links! Thanks, Kevin! I am forwarding this stuff to my contractor.

      The floor was originally carpeted—we have pulled up the carpet and pad. The tile setter sub has his tile cutting equip sitting in the subfloor just outside the master bath, and, of course, that machine uses water to mitigate the dust, but that has also created a puddle on the subflooring, which probably means warpage. Arg. I am just waiting to see how my contracotor deals with that issue. (There is such a precarious balance to maintain as a single female in this situation—say too much, and I’m labeled an “impossible bitch,” don’t say enough, and contractors think I’m too stupid to know the difference between quality and slipshod work. Ugh.)

      I already made a stand re: sound-muffling insulation around the bathroom pipes. (Nothing worse than hearing a toilet flush from all corners of the house!) Even though I had told them several times to double-up on the wall insulation, they blew me off. Then they had to tear out sheetrock to get to the pipes and wrap them after a test of the upstairs tub drain echoed loudly through the basement bathroom. Yeah, they already know I’m serious when I keep telling them I want to mitigate sound.

      Hopefully, my contractor will pass this information along to the flooring sub and they’ll work out an acceptable solution without me having to make another stand. :/

    • Good thinking, Brian. I’ve pondered that angle, too. In this case, trying to figure out a way to soundproof the ceiling is a little more problematic because the ceiling is already lower than normal (because it’s in a basement). Given the fact that this space will probably be used mostly by my kids who live all over the world (and hence, when they come to visit, tend to stay for awhile), and the fact that all four of them stand between 6’ and 6’6” there isn’t any extra headroom. So... back to looking at the upstairs flooring. :)

    • Is the basement ceiling already drywalled? If not, Quiet rock 5/8" would be a pretty decent solution.

      And if it's drywalled already, replacing drywall is an expensive task. But if you could spare about 3/4" off the ceiling height, quiet rock + a layer of green glue below your current drywall could do the trick. That is bringing in another trade / project to the job so could be costly.

    • Yep. Ceiling is already drywalled. :/ I did tell the contractor twice to please double-up on the ceiling insulation. However, I didn’t actually see what was installed. (In a previous build, I snuck into the house during a weekend and installed additional insulation myself, but the timing was not right to do that on this project.)

      I forwarded that info you posted above to my contractor and then the discussion took place about adding a layer of plywood over the underlayment to the subfloor. While he thought the sub-contractor would probably prefer to install the hardwood on a layer of plywood rather than on a rubber underlayment, he himself is nervous that an additional layer of plywood would mess up the transitions at the stairway leading to the basement, as well as all the exterior door thresholds. Meh. He probably has a point.

      Keeping my fingers crossed that tomorrow a final decision will be made, and floor installation will stay on schedule for next week...

    • Yeah, you want to be careful with stairs, they are a major failure point on residential inspections. At least here in San Jose California, they will fail you if you have a stair even 1/4" higher than the max height of 7 3/4". That solution 3 adds 2" above your existing subfloor and below your finished floor, so it would most likely require raising the stairs <2" and reconciling the difference at the bottom.

      In terms of insulation, it makes nearly no perceivable difference in sounds transmission. For example, in walls it's been found that you go from a 37 to a 40 STC when you go from 1/2" gypsum with no insulation to 5/8" gypsum with insulation. A <5 STC increase is not noticeable. It's a myth that insulation does a lot. Before insulation is covered by drywall, it's really really quiet in the construction site because it's directly absorbing sound waves. Once you have drywall screwed to both sides (or floor / drywall between floors), it becomes a big drum transferring sound through the studs from room to room.

      That's why it's so important to completely separate materials, so big sheets of plywood or gypsum can move freely on each side, which let's them absorb sound and transfer it to heat instead of passing it through to the other side. Not well versed in the physics of it, but I think that's the theory of it. Please correct me if I'm wrong.

      With that said, I still insulated between my interior floors and walls because insulation is extremely cheap, and there are no real downsides besides cost. It doesn't take up anymore room and it's hidden. STC per dollar, it's probably on the cheaper side of all the solutions.