Sound Proofing - Volume 2

I am a little embarrassed when I now read the previous post on sound proofing. Sometimes you learn the hard way and sound proofing is one of those things we learned about in renovating and restoring 41 Plymouth Street. The specs said we would barely hear loud sounds coming from the other townhouse style unit and that is largely true on the bedroom floors but not always the case on the first floor. On the first floor we missed a few details not in or just assumed by the spec such as sealing up junction boxes and other penetrations in the common wall. I also suspect my contractor at the time did not fully insulate sections of the wall that were hard to access. One of the things I learned is that you just need to be on the site everyday even if that is sometimes not possible. The good news is that the placement of closets all along the common wall on the 2nd floor provided an additional sound buffer that is backing up the spec. At 21 Plymouth we beefed up the spec by using a good old fashioned double wall between the 1st floor units that includes an air space between them. Both parts of the double wall are insulated and we foam sealed the 5/8” sheetrock from both sides and “putty padded” and junction boxes. I think we will do better this time. Thanks to our buyers at 41 for respecting their neighbors and not playing making a peep after 10pm.

Fire Protection / Sound Proofing in Multi-Family Buildings

If you live in an apartment building or attached townhouse – maintaining your privacy from a noise prospective is a big deal.  Even bigger is maintaining a feeling of security that your own unit is protected from fire spreading from another unit in the same building.

The National Building Code requires that multi-family buildings contain a special kind of wall assembly or Fire Protection/Sound Reduction wall where any two units have a common wall or common floor/ceiling. 

In all our new buildings and multi-family renovations we apply a high technical standard to this requirement so that noise does not cross “common walls” and fire does not spread thru a “common wall” for a minimum period of time.   

Please see the chart below that describes the assembly, from Pac International, that we use.  This wall  has a “Sound Transmission Class” or STC rating of 60.   A rating of 60 is quoted as “Superior Soundproofing, most sounds inaudible”.

We also take additional design steps to make these “common walls” even more effective by avoiding ducting, wiring and plumbing in these walls (whenever possible) and designing in “buffer space” on either side of the wall.   An example of these design principles in play at 41 Plymouth is on the 2nd floor where Unit B has a set of closets and the back stairwell as a continuous “buffer” on the Unit B side of the sound/fire wall.  Even very loud noises will not penetrate this setup/design. 

stc 60 .jpg

Continuous Thermal Barrier = Comfortable Home

A continuous thermal barrier is made up of two components:  an air seal that encloses the entire home and a continuous blanket of insulation that reduces the transfer of heat/cold from the outside to the inside.  A continuous thermal barrier has a number of significant benefits:

 -    Keeps the home quieter and warmer in winter and cooler in summer

 -    Reduces the allergens and pollutants entering the home

 -    Enables a smaller, less costly HVAC unit that runs for shorter periods of time

The thermal barrier that is continuous or covers the whole house maximizes the effectiveness of the insulation and reduces the impact of “thermal bridging”.   A “thermal bridge” is a material, such as a wooden wall stud, that provides a pathway for heat or cold to move more easily from the outside to the inside of the house.   

Looking at the attached thermal image photo on the left you can see the wall studs in this home acting as a thermal bridge and allowing cold to enter the house via the wood stud pathway.   This is an illustration of a conventionally framed and insulated home where insulation is only placed in the cavities between the studs.   If wood makes up 25% of a wall the overall effectiveness of the insulation in the wall cavity is reduced as the cold bypasses the insulation by hopping on the wooden “thermal bridge”.

The thermal image on the right shows the same home with a continuous air seal provided by an additional layer of insulation on the exterior of the home.   This additional layer is normally a rigid foam type product that insulates, air seals, reduces the impact of “thermal bridging” and increases the overall effectiveness of the insulation within the wall cavity.   

The externally applied rigid foam also acts as a vapor retarder in much the same way as a house wrap acts as a rain shield or drainage plane between the house sheathing and siding.   The rigid foam will allow some vapor to pass inside to the internal wall or outside to the rain screen.   When using rigid foam as an exterior insulation it is recommended to also use a vapor retarder (not a vapor barrier) between the wall board and the cavity insulation.     In other words, use a vapor retarding material on both sides of the wall to prevent condensation buildup inside the wall.

First Lessons in Building in a Sustainability Way

To enable a well sealed and insulated building envelope you must expose any existing crawl spaces and basement and then insulate/seal those areas.   For a crawl space under a new addition use rigid foam to insulate the sides and under the slab rather than insulating between the floor joists above.  Air flow between existing and new crawl spaces allows warmer air to circulate resulting in warmer floors above.


Next step is to seal those sill plates so that air does not enter the insulated crawl space.   We chose to use a gasket and acoustical sealant - both will remain soft and flexible when dry.





Recycling as much existing building material as possible goes a long way to reducing environmental impact.  Separating out different materials such as concrete and metal enables is the first step.  When I asked about recycling the old HVAC ducting I was told it no longer made financial sense to do that with commodity prices so low.  I was dismayed but figured we would find a way to do it when the time came.  Later I noticed that metal was not making its way into the dumpster but instead was leaving the site piece by piece under the guidance of one of the subs who decided to take on the metal recycling job himself !  I like a guy that does the right thing without being asked! 


New crawl space is insulated and ready for the slab to be poured.

New crawl space is insulated and ready for the slab to be poured.

Concrete removed from rear patio is ready to be moved offsite and recycled as base for a new road beds   

Concrete removed from rear patio is ready to be moved offsite and recycled as base for a new road beds