Basic Heat Transfer
Heat flows through a roof or exterior wall whenever a temperature difference exists on opposite sides of the structure. The direction of energy flow is always from the warmer side to the cooler side, so the indoor air tends to heat up during the summer and cool down during the winter. Air conditioning and heating systems maintain indoor spaces at a comfortable temperature by removing heat during the summer and adding heating the air during the winter. Insulation in attics, ceilings and exterior walls increases indoor comfort and reduces the amount of work air conditioning and heating systems have to do by reducing heat transfer between indoor spaces and the outdoors.
Three Types of Heat Transfer
Conduction. Conduction is heat transfer through solid materials like concrete, wood and metal. The term R-value indicates the ability of a material—or a roof or wall assembly comprising layers of different materials—to resist heat transfer. A higher R-value means a greater resistance to heat transfer. Insulating materials like fiberglass, cellulose and a variety of plastic materials have high R-values, so they are used to keep heat out of building structures during hot weather and to keep heat inside during cold weather.
Convection. This is heat transfer by air movement. Convective heat transfer increases with air movement and when air is in contact with a surface that is warmer or cooler than the air. As much as 40% of the load on a home or building air conditioning and heating system is due to air movement through cracks, gaps, joints and openings in exterior walls and celings. This air movement is driven by pressure differences between the inside and outside air. Increases in wind speed around a structure increase air movement through walls and ceilings.
Fiberglass and cellulose insulation do not stop pressure-driven air movement because the materials are porous. And even if these materials were not porous, it is impossible to install fiberglass and cellulose perfectly; in other words, so that the material seals the cracks, gaps, joints and openings in the insulated wall or ceiling assembly.
Radiation. Radiant heat transfer occurs when a warmer surface, like a ceiling warmed throughout a hot summer day by heat from the sun striking the roof and flowing into the attic, radiates heat across an open space to cooler surfaces, like the surfaces of furniture and the skin of building occupants. Radiant heat transfer from a ceiling or a west-facing exterior wall (which has been heated by the afternoon sun) is the reason why you can feel uncomfortable in a room during the late afternoon or evening, even though the air temperature in the room should be comfortable.
What You Need to Understand
The R-value of fiberglass and cellulose insulation can only impact 50% to 70% of the load on a home or building air conditioning and heating system because fiberglass and cellulose have no effect on convective and radiant heat transfer. No matter how much you increase the R-value of home or building insulation, you can only impact the conductive heat losses.
Insulation and Diminishing Returns
We learned above that R-value has no effect on the portions of air conditioning and heating load that result from air movement through walls and ceilings or radiant heat transfer from warmer ceilings and walls to cooler interior surfaces during the summer. But there is another problem with increasing R-value in pursuit of greater energy savings: For each extra inch of insulation thickness you add, you get less insulating performance per inch. Insulation thickness and thermal performance have a diminishing returns relationship.
Let us explain.
The concept of R-value was first proposed during the 1940s as an easy to understand number that could be used to quickly communicate the insulating value of a material or a building assembly, such as a wall, roof or ceiling. The R-value of a material is the reciprocal of its thermal conductivity coefficient. For example, the thermal conductivity coefficient of an inch of cellulose is 0.315. Its R-value is 1 ÷ 0.315 = 3.17, or about 3.2.
The higher the R-value, the greater the resistance to heat transfer. When you double the thickness of a material, the R-value also doubles but the thermal conductivity through the new total thickness is cut by half. So let’s say you have a thickness of insulation material with an R-value of 4. The thermal conductivity coefficient would be 1 ÷ 4 = 0.25. If you double the thickness, the R-value becomes 2 × 4 = 8 and the total thermal conductivity becomes 1 ÷ 8 = 0.125. And if we double the thickness again—to an R-value of 16—the total thermal conductivity becomes 1 ÷ 16 = 0.063.
The percentage of heat stopped by the insulation is one minus the thermal conductivity coefficient. So at R-4 we will stop 1 − 0.25 = 75% of the heat transfer. At R-8 we will stop 1 − 0.125 = 87.5% of the heat transfer. And at R-16 we will stop 1 − 6.3% = 93.7% of the heat transfer. As these examples show, we stop less heat per unit thickness every time we increase the R-value. The chart below illustrates this relationship.1
What You Need to Understand
Insulation with an R-value of 8 will stop about 88% of the conductive heat flow through a ceiling. Increasing the R-value to 48—adding six times more insulation thickness—will only increase the thermal performance by about 10%.
Labeled vs. Actual R-value of Fiberglass Batt Insulation
As recently as 2004, the Florida building code required R-19 ceiling insulation. R-19 insulation should stop about 95% of the conductive heat transfer through a ceiling. But during the past few years the ceiling insulation requirement was increased to R-38, an insulation level that should stop about 97% of the conductive heat transfer through a ceiling. You may be asking yourself, given the diminishing returns from increasing insulation thickness, why the Florida building code would have more than doubled the required ceiling insulation thickness to get just 2% more thermal performance. And remember that no matter how much fiberglass or cellulose insulation you use, you will not stop convective heat transfer from air infiltration.
Advances in building science knowledge led to the change in code requirements. Research into the effectiveness of insulation materials, notably at the U.S. Department of Energy’s Oak Ridge National Laboratory, discovered that fiberglass and cellulose insulation do not deliver their theoretical R-values in real world application.
It turns out that, for a variety of reasons, a fiberglass batt labeled as R-19 delivers as little as R-6 in real world application. The reasons include:
- Fiberglass gets most of its insulating value from air pockets in the insulation. The insulation loses R-value when these air pockets are compressed during packaging, shipping and storage, and when the insulation batt is crammed into a smaller space than its designed dimensions.
- The air pockets in fiberglass insulation also shrink over time as the insulation settles under its own weight.
- Researchers found that fiberglass batt insulation loses about a third of its labeled R-value after “typical” installation as a result of gaps and imperfect fit around wires, pipes, electrical fixture openings, and thermal bridging through wall and ceiling framing.
- Wind outside reduces the R-value of fibrous insulation by increasing the air infiltration through the insulation material. This happens because increases in wind speed increase the difference between the outside and indoor air pressures. Just a five mph wind can cut the R-value in half.
The chart above, which is based upon research findings by the U.S. Department of Energy’s Oak Ridge National Laboratory and a peer-reviewed independent research study, shows how fiberglass insulation batts can be affected by packaging, ordinary imperfections in installation fit, and wind. With just a 5 mph wind speed—a typical average wind speed for most of Florida—it is not uncommon for fiberglass batt insulation to lose about two-thirds of its labeled insulation value.
Why Foam Insulation is Better
Unlike fiber insulations (fiberglass and cellulose), the air pockets in foam insulation are completely enclosed. Also, foam insulation expands when installed to fill all joints, cracks and gaps, and to more completely insulate around wires, pipes and electrical fixtures. Enclosed air pockets and a more completely sealed air barrier prevent pressure differences between the outisde and indoor air from forcing air infiltration through the insulation. And finally, foam insulation hardens into a rigid structure shortly after application, so it won’t settle under its own weight. Consequently, foam insulation tends to hold its R-value for the life of the building structure.
- The diminishing returns relationship of insulation thickness and thermal performance is often incorrectly attributed to Fourier’s Law of heat conduction. Fourier’s Law states that the rate of heat transfer through a material is proportional to the temperature difference on opposite sides of the material, multiplied by the thickness. However, the diminishing returns relationship between insulating material thickness and thermal performance is the same regardless of the actual temperature difference across the insulation thickness. The examples above assume that the temperatures on opposite sides of the insulating material do not change, which is exactly the situation we are concerned with when the outside air temperature is hotter or colder than the indoor air temperature and we are using an air conditioning and heating system to maintain a constant indoor air temperature.