Effect of different exterior wall insulation schemes on sound control
Hi all,
While reading the recent write-up of an exterior insulation Q&A (https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/making-case-exterior-foam-insulation), I noticed that no one mentioned any benefit impact to exterior sound transmission by increasing cavity insulation or increasing exterior insulation on transmission of exterior sound into the home. Internet CW seems to be that the density of cellulose and rockwool have positive benefits over fiberglass but beyond that, I don’t see a lot of discussion.
So I’m wondering if there is an interest in discussing these questions:
– Do models for STC for different cavity and continuous insulation schemes exist? I haven’t had any luck finding a consolidated listing and most focus on interior partition walls.
– Do the fenestrations overwhelm any benefit of additional insulation depth? (Corollary: what if the windows are triple glazed?)
– If a benefit exists, where does the diminishing returns curve kick in?
– How does double stud wall STC compare to single?
– Is all of this a wash compared to the double drywall green glue approach often used as soundproofing for home theaters etc.? (not exactly a zero impact design choice)
– does the GBA crowd think that is money that could be spent on triple glazing?
Not that GBA need try to ‘sell’ additional insulation but I wonder if some homeowners building custom homes wouldn’t consider the sound benefits of advanced framing, continuous insulation, and 2×6 insulation worth the expense if there was also a exterior noise control benefit?
Anyways, any experiences or data to share?
Cheers,
Keith
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Replies
Keith,
As far as I know, the retrofit programs that try to reduce airport noise in affected neighborhoods emphasize (a) air sealing efforts and (b) upgrading from double-pane windows to triple-pane windows.
I think this is an underrated/undersold aspect of building a PGH or better, with good air sealing, insulation, and windows. Quiet is a large factor in comfort.
No data, but I can say that I was amazed at the exterior sound reduction that we got from just blowing cellulose on top of existing insulation in an attic.
I also think that you are right to point at fenestration. Bad windows let in more noise, and they are expensive to replace later, so building with good windows initially is important.
I will also be interested to see if someone has data on this topic.
P.S. My feeling (again, no data) is that benefits of having cellulose or rockwool plus exterior insulation will far outweigh double drywall and glue. Double drywall is for interior walls after you've taken out all the air transmission paths.
I’m no expert but here’s some ideas:
You can add up the STC numbers for layers of a building assembly to guess STC of the assembly but it doesn’t work well since sound is composed of various frequencies. Different materials transmit a different pattern of frequencies. Layers of dissimilar materials are better at reducing sound transmission of various frequencies than layers of the same material. (Layers of the same material continue to transmit the same frequency spectrum, just less.) The most difficult sound to suppress is low bass frequency thuds, motors. Generally, the heavier the material, the better at reducing sound transmission (but not always true).
For walls:
• Concrete walls (heavy and dense) beat any wood frame wall.
• Double stud walls would be superior to single stud layer 2x6 walls. Alternating studs in partitions, to attach to drywall on only one side of the partition, reduce sound transmission more than all studs attaching to drywall on both sides of the partition.
• Metal resilient channel furring or clips to attach drywall to wood framing studs or joists reduces sound transmission appreciably.
• Wood frame walls filled with quality mineral wool batts or dense-packed cellulose reduce sound transmission much better than lightweight fiberglass. Roxul Safe’n’Sound (fireproof and sound-resistant) and ComfortBatt (claim R-4.2) mineral wool insulation appear to be very similar products of somewhat different densities, targeting sound attenuation or maximum insulating value, respectively. Insulation in partition stud bays will reduce sound transmission between rooms.
• Exterior mineral wool board (e.g., Roxul ComfortBoard) would be far superior to foam or polyiso at suppressing sound transmission.
• Fiber cement siding would reduce sound transmission more than wood, which in turn would help more than vinyl siding.
• Thicker or double layers of drywall add to the mass of the wall and reduce sound transmission. There are new ½” and 5/8” double layer drywall products with an interior layer to reduce sound transmission many times better than double layers of drywall.
• Alternating electrical outlets in different stud bays for adjoining rooms will reduce sound transmission through partitions. Products that wrap electrical boxes in soundproofing materials (or at least caulking holes) can reduce sound transmission through partitions and walls.
• Caulking around pipes in walls can reduce the transmission of sound from the pipe to the wood framing, thereby reducing sound transmission through the framing to drywall.
• Caulking any air gap reduces sound transmission through the air.
Triple pane windows would be superior to double-pane, since there is an additional layer of trapped air and glass. The weakest link appears to be the air gap between the window and rough opening. Layers of caulk between the rough opening air gaps and filling air gaps with foam would seem helpful. Acoustical sealant is used for stopping air infiltration and it works to reduce sound transmission but has an undesirable odor. Shutters, shades and drapes would help reduce sound transmission a little, especially if relatively airtight and heavier.
Audio retailers sometimes sell sound-deadening products and accessories. Rubber feet under audio equipment or legs of furniture could help cut sound transmission. There are specialty products that are more expensive, including mass loaded vinyl (>$1/SQFT) and acoustic panels (>$2/SQFT).
Carpet, padding, floating (not nailed or glued) wood flooring, cork and other underlayment can help reduce noise through floors. Eliminating lights, electrical boxes, plumbing and other gaps in walls and floors and walls can help reduce sound transmission through those gaps.
Solid core doors reduce sound transmission much better than hollow core doors. Insulated steel doors seem better than solid wood doors at reducing noise transmission. Weatherstripping doors and keeping carpet touching the bottom of doors reduces sound compared to leaving air gaps.
From experience:
For soundproofing non-loadbearing interior partitions, I cut a slot through the middle of non-loadbearing 2x4 studs except the top and bottom 20” (where stresses are greatest), to reduce sound transmission. Installed cheap fiberglass batts in the stud cavities, and double layers of drywall on partitions with no doors or windows. Outlets and switch plates were installed in different stud bays for adjoining rooms. Noticed that sound was getting through the partitions mostly at the bottom plate, so filled the bottom plate gap below drywall with caulk. These are cheap improvements that reduce sound transmission appreciably. Yelling and banging on the partition could hardly be heard on the other side. One time my son was crying loudly in one room, and in the adjoining room, we couldn’t hear him until we opened the solid core pine door to the corridor. It’s a shame that hotels and motels don’t pay the small extra price to reduce noise among units.
Links that might be helpful:
Wikipedia: Sound Transmission Class
https://en.wikipedia.org/wiki/Sound_transmission_class
US HUD tables with STC for many wall assemblies:
https://portal.hud.gov/hudportal/documents/huddoc?id=DOC_16419.pdf
Building material STC ratings, and perception of the sound level that’s transmitted:
http://www.audimutesoundproofing.com/how-are-stc-ratings-calculated.aspx
Robert,
That's a really excellent and useful summary.
Evidently double pane windows with disimilar glass are roughly equal to triple pane windows. The multiple inch gap created with double windows (not panes) is far better.
Interesting commentaries.
Thanks for the nice summary of the conventional wisdom Robert. It's handy. The HUD STC pdf you linked above is great for thinking about the different parts and what might provide the most benefit when considering an individual component (single vs double, no fill vs fill, double stud or staggered). However, it kind of can't see the forest for the trees, so to speak. What does a full wall assembly look like for STC? Do fenestrations overwhelm fill and other techniques?
So thanks for the fantastic post and comprehensive post. I guess the whole assembly data and/or whole assembly modeling isn't out there (I can't find it either).
I'm familiar that windows are the typical point of attack for a noise reduction project, especially as relates to airports or railway. Is that because that is the most cost effective option or is that because replacing windows is often the point of least disturbance in the shell of the house? If a larger retrofit is being planned, where is the money best spent?
I guess I'm trying to think about sound like some GBA experts think about efficiency vs solar panels. Should all the money go into windows and forget about unusual techniques for wall sound mitigation or is there sound mitigation benefit to the wall assembly?
You want to keep ratings of the various components (walls, windows, etc) fairly close. For example, 74dB + 80dB = 81 dB and 80db + 80db = 83dB. Ie in this example, reducing to 74dB was largely wasted.
>Do models for STC for different cavity and continuous insulation schemes exist?
Yes, see green glue's website. http://www.greengluecompany.com/test-data
>Should all the money go into windows and forget about unusual techniques for wall sound mitigation or is there sound mitigation benefit to the wall assembly?
See http://www.greengluecompany.com/test-data
Anon:
Thanks for sharing the green glue product STC data. However, those are interior partition wall assemblies, not exterior. They also don't deal with whole assembly performance (i.e. Including windows)
Keith,
Sound acts like water. As water does in a container with a hole in it, sound finds weak spots and can disperse in wave form around corners. So as John pointed out you need to concentrate on the windows and gaps in assemblies, not typically worry as much about the stronger elements, like the solid wall.
If you have a wall assembly with an STC of 50 and a window of say 20, that constitutes half the exterior wall area, the occupants of the room behind won't experience a sound attenuation of 35, They will will likely find something very close to 20. That's why you find lots of STC rated wall assemblies, but none for whole walls.
For exterior you need high mass, brick veneer is the best. Your exterior insulation is going to do nothing compared to 100,000-300,000+ pounds of brick. And yes that is not a typo.
If you have windows, sound will come in through them, end of story.
Just don't buy houses by the airport....
Do any GBA readers remember the article I wrote on the Unity Homes residences assembled on the campus of Dartmouth College? The homes have walls framed with 9.5-inch-deep I-joists used as studs, and the stud bays are insulated with dense-packed cellulose. Windows are triple-glazed.
Last night my wife and I (and our son) were invited to dinner by a professor who lives in one of the homes. He told us that one feature of the homes that he really notices is that they are amazingly soundproof. He lives on Fraternity Row, and the parties get very loud on weekends. He says that everything is perfectly quiet in his living room, but when he opens the front door, the sounds of the parties suddenly assault his ears.
Reports like his are common from owners of superinsulated homes with low levels of air leakage.
It's soundproof until the deep bass arrives via vibration.
Doen't all sound arrive by vibration?