Heat Transfer Basics
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 indoor air tends to get warmer during the summer and cooler during the winter. Air conditioning and heating systems maintain indoor spaces at a comfortable temperature by removing heat during the summer and adding heat to indoor 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 refers to 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.
Insulation Slows Heat Transfer
Traditional fiber-based insulation materials, like cellulose and the fiberglass batts shown here, reduce heat transfer by slowing the rate of conduction through pockets of air between the insulation fibers. Unfortunately though, because of this porous structure, fiber insulations can’t stop heat transfer caused by air movement, which goes right through the insulating material.
Foam insulations stop conductive heat transfer, but they are much better at stopping the total heat transfer through a wall or roof structure because they also block air movement. Any air pockets within the foam are completely enclosed, air is trapped and can’t go anywhere.
The Key Idea to Understand
The R-value of fiberglass and cellulose insulations 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 heat transfer. No matter how much you increase the R-value of fiberglass or cellulose 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
The Key Idea 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.
A curious reader might wonder why the Florida Building Commission would have doubled the required ceiling insulation thickness to get just 2% more thermal performance! And remember; no matter how much fiberglass or cellulose insulation you use, you will not stop convective heat transfer from air infiltration.
Research findings led to the change in code requirements. Researchers at the U.S. Department of Energy’s Oak Ridge National Laboratory found that fiberglass and cellulose insulation do not deliver their theoretical R-values in real world application. Not even close.
It turns out that, for a variety of reasons, a fiberglass batt labeled as R-19 may deliver as little insulating power as an R-6 batt in real world application. So the building code change built in a safety factor to account for real world performance.
The reasons why fiberglass and cellulose don’t deliver the performance on the label 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 collapse over time as the insulation settles under its own weight.
- Fiberglass insulation batts installed between wall framing studs and ceiling joists do not stop heat transfer through the more conductive wood or metal structures. The phenomenon of heat bypassing insulation and flowing through wall framing studs and ceiling joists is called thermal bridging. Framing studs and ceiling joists account for about 10% of wall and ceiling surface area, respectively. (Spray foam typically covers the top chords of roof trusses, on the underside of the roof sheathing, cutting off thermal bridging pathways.)
- 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.