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CONSTRUCTION KNOWLEDGE  >> THERMAL & MOISTURE >>

INSULATION
 

1. What is an R Value?
2. How Does the R Value Relate to the U Value?
3. Does the Hot or the Cold Side Get the Vapor Barrier?
4. What Should I Know about Noise Control?
5. How Does Noise Control Differ from Sound Absorption?
6. What Websites Provide Useful Information about Insulation?
7. What Public Domain Documents are Available for Further Study?
8. Tricks of the Trade & Rules of Thumb for Insulation:

What is an R Value?

The R value is the material's thermal resistance to conduction heat transfer. In simple terms, the R value shows how a material retards heat flow. So 3 1/2" thick piece of wood has an R value of 4, while that same thickness of fiberglass batt insulation has an R value of 13. If we go to 3 1/2" of polyisocyanurate foam, the R value jumps to 23, since that type of foam has the highest R value of common building materials. So the higher the R value, them more the material resists heat flow, that is to say, the higher the R value, the better the insulation value of the material.

It's valuable to remember the basic insulation values per inch for common insulations:

                                                                                             R value per inch of insulation
1.  Fiberglass batt insulation                                                                3.5
2.  Molded expanded polystyrene foam board (white board)                   4
3.  Extruded expanded polystyrene foam board (blue board)                  5
4.  Polyisocyanurate foam board (foil backed board)                              6.5

The following US Department of Energy site http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11620  provides lots of information about R values and insulation products.

All the above concerns the transmission of sound (noise) from one space to another. What about how sound acts within one space?

How Does the R Value Relate to the U Value?

The U value is simply the inverse of the R value. So U = 1/R. Or R = 1/U. The U value often gets used in Building Codes, particularly for the energy conservation analysis. So understand that a 6" fiberglass batt insulation with a R value of 19 has a U value of 1/19 or 0.053.

Does the Hot or the Cold Side Get the Vapor Barrier?

Most folks know that the vapor barrier gets installed to limit moisture transmission. Warm moist air leaking through a wall becomes a problem when it cools down to the dewpoint temperature and condenses (drips) the water out of the air. Water dripping in insulation not only seriously degrades the insulation value but often can start the process of mold formation. So it's important to build in a way to not have condensation in the walls and ceilings.

A common question in construction involves the actual placement of the vapor barrier. Think about batt insulation with Kraft paper backing. Does the Kraft paper get installed facing the inside of the building or the outside? You may be sure you know the answer to that question, but it may be a bit more complicated than you think.

If you work in a colder climate, you'd answer that the Kraft paper vapor barrier definitely goes on the living side of the space (the warm in winter side). And you'd be right, sort of. If you work in a hot and humid climate, you'd answer that the Kraft paper vapor barrier definitely goes on the outside of the space (the warm in summer side). And you'd be right, sort of. The correct answer is that the installation depends on the project location.

In cold climates, the vapor barrier should be on the inside (warm in winter), to stop the moisture from the heated winter air from getting too far into the insulation, where it would get colder and condense. On the other hand, in hot and humid climates, the vapor barrier goes on the outside, since the primary HVAC activity will be air conditioning (cooling). The hot outside air will be stopped at the exterior, so it can't get into the cooler inside space and condense in the wall.

The following site gives some additional information, as well as a map for the USA that delineates the cold and hot/humid climates: http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11810

What Should I Know about Noise Control?

Insulation helps control noise. Think about an apartment building. The sound transmission between the floors and the walls greatly affects how one enjoys the experience of living in the apartment. If you can hear your neighbors every time they talk and walk, you will value the apartment much less than if you have silence. So several measures developed to compare sound transmission and absorption.

The Sound Transmission Class (STC) ratings provide a number that compares the noise levels that pass through different assemblies of walls or ceilings. A wall with wood studs at 16" oc and 5/8" gypsum wallboard on either side has a STC around 37. On the other hand, a double stud wall with 2 layers of gypsum wallboard on either side and batt insulation has a STC of 57. So the single stud wall blocks 37 decibels (dB) of sound transmission while the double stud wall blocks 57 dB. Measuring sound pressures is tricky and depends on the frequencies measured, but the decibel reduction is roughly accurate.

Since the decibel scale is logarithmic, each increase of 10 decibels is a doubling of sound pressure. So the increases in the STC values really improve the walls performance.

In practical terms, you could distinctly hear loud speech through that 37 STC wall, people tend to call walls like that "paper thin". I've built plenty of those STC 57 walls in apartment buildings and the noise complaints about neighbors are rare. Loud speech can't be heard and only quite loud music makes its way through the walls.

Remember when thinking about noise control, that acoustical energy transfers similar to thermal energy. The way a wall works to keep the cold out and the warm in will function similarly for noise control. So, if you were in a house and left the window open all the time, you wouldn't be surprised by high heating bills. The window creates a flanking path for energy transfer. Rather than slowly working through that wall designed to slow thermal transfer, the heat goes right out the window. Flanking paths can ruin the noise control performance of a wall or ceiling assembly as well.

The construction of every wall and ceiling/floor assembly, then, should strive to avoid flanking paths for both noise and heat. The following flanking paths and noise/energy leaks should be considered:

1.  Through ductwork (especially metal ducts rather than fiberglass duct interiors.
2. Open plenums over wall tops.
3. Poor seal at floor to wall edge or wall to ceiling edge.
4. Electrical outlets (don't place back to back through a wall unless you want sound transfer).
5. Cabinet backs not installed over gypsum wall board.
6. Any chase, light, or opening in the wall or ceiling/floor.
7. Louvered doors.

When building walls and ceiling/floor assemblies, always think about thermal and sound transmission. So many call-backs from unhappy customers come from these items. Good Supervisors understand that some extra attention to the details in these areas, often picking up things that aren't clearly shown on architectural drawings, yields a much better project with minimal extra cost or effort.

How Does Noise Control Differ from Sound Absorption?

So the noise control discussed above concerns keeping our spaces free from the noise pollution of surrounding spaces. The other important topic in acoustics involves clearly hearing the sound you're supposed to hear in a space. An auditorium, or even a large meeting room, should allow everyone in the room to clearly hear the speaker's voice. Rooms that are too reverberant, that echo too much, distort the speaker's voice and make the speech difficult to understand.

Walls, ceilings and floors can all absorb sound or reflect sound. Therefore, sound absorption becomes an important measure in construction. The  Noise Reduction Coefficient (NRC) act as the standard measure. An NRC of 0 indicates perfect reflect of sound and an NRC of 1 shows perfect absorption of sound. Some common measures for NRC are:

1. Concrete or painted masonry = .05
2. Carpet = .25
3. Heavy carpet on pad = .50
4. Suspended acoustical ceilings = .50 to .75
5. Fiberglass board 2" thick = 1.00

Some other NRC values for building materials are found at http://www.nrcratings.com/nrc.html

As an interesting aside, acoustical design tends to be a balancing act for most spaces. The less reverberation in a space (which means the more sound absorption on all surfaces) the clearer speech will be heard. On the other hand, more reverberant rooms tend to sound much better for music. Think about the cathedrals in Europe...totally reflective surfaces and wonderfully live musical sound. So for most churches, auditoriums and theaters there must be a balance between speech intelligibility and some reverberation so the room doesn't sound too dead.

What Websites Provide Useful Information about Insulation?

The Johns Mansfield website gives an excellent set of FAQs http://jm.com/insulation/faqs/69.htm as well as lots of product information http://jm.com/insulation/technical_info/2949.htm

The following US Department of Energy site http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11620  provides lots of information about R values and insulation products.

This Wikipedia article about STC ratings http://en.wikipedia.org/wiki/Sound_transmission_class provides more details about noise reduction through walls and ceilings.

What Public Domain Documents are Available for Further Study?

The EPA produced a wonderfully informative paper titled Quieting in the Home back in 1978 that does a great job of explaining basic concepts for noise control and sound absorption in buildings. The pamphlet isn't too detailed or technical, but covers the basics in an accessible way.

Tricks of the Trade & Rules of Thumb about Insulation:

1. The R value of fiberglass batts is 3.5/inch, polystyrene rigid boards is 4 to 5/inch and polyisocyanurate foam is 6.5/inch.
2. The U value is simply the inverse of the R value. So U = 1/R.
3. Check for flanking and leaks for noise and heat at walls and ceilings/floors.