The Job of Insulation Is to Slow Heat Flow
Heat always moves from warm to cold
Most places in North America aren't a comfortable 67°F to 78°F. To stay comfortable, we heat and cool our houses. Insulation slows the flow of heat into or out of a house.
While heat can move three ways (conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., convection, and radiation) it rarely moves in only one of them. Except in outer space, all three mechanisms are working together in varying degrees.
Different insulation materials have different success at slowing these heat-flow mechanisms.
Heat flows through materials by conduction
Conduction is the flow of heat energy by direct contact, through a single material or through materials that are touching.
Substances that conduct heat readily are called conductors, while substances that don't conduct heat readily are called insulators. Metal is a good conductor; foam is a good insulator. Wood falls somewhere in between.
Convection is the movement of air (or another fluid) in response to heat
When air or a liquid is heated, it expands, and therefore becomes less dense, so it rises. The rising warm air displaces cooler air, which sinks. When the motion is constant, it’s called a convective loop.
Woodstoves and windows cause convective loops by heating or cooling (respectively) the air closest to them.
Even in homes with airtight walls and ceilings, convective loops can feel like a cool draft and be uncomfortable to the people in the room.
Convective loops can occur inside poorly insulated wall cavities, too, degrading the performance of the insulation.
Radiation heats objects, not air
Radiation is the transfer of heat by electromagnetic waves that travel through a vacuum (like space) or air.
Radiation cannot pass through a solid object like plywood roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . When the sun shines on asphalt shingles, heat is transferred to the plywood sheathing by conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow.. After the plywood has been warmed by conduction, it can radiate heat into the attic.
Radiant barriers are materials (for example, aluminum foil) with a low-emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces. (low-e) surface. Although radiant barriers have a few applications in residential construction—they are sometimes integrated with roof sheathing—they are rarely cost-effective when compared to conventional insulation options.
Thicker is better
In cold weather, a puffy parka holds in your body heat. Insulation does the same thing for a house. The thicker the insulation, the better it works to reduce heat flow from the inside of a home to the outside during winter, and from outside to inside during summer.
The thermal barrier of a home should consist of a continuous layer of insulation on all sides—including the lowest floor, the exterior walls, and the ceiling or roof.
Doubling the thickness of insulation will double the insulation's R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. , cutting heat loss in half. Each time the insulation layer is doubled in thickness, this rule applies. The energy saved per year by doubling insulation from R-10 to R-20, however, will be considerably more than the energy saved by doubling insulation from R-20 to R-40, because of the law of diminishing returns. In some cases, like an attic, it’s worth piling on more insulation because there is plenty of room. It’s much more expensive to add that much insulation to exterior walls.
It takes more than just insulation to slow heat
Stopping air leaks is just as important as — maybe more important than — adding insulation. Unless builders prevent air from leaking through walls and ceilings, insulation alone won't do much good. Not only are drafts uncomfortable, but air moving through insulated cavities can cut the efficiency of the insulation by as much as 50%.
Some insulation types make good air barriers, and some don’t. In all cases, it’s best to keep the insulation tight to the air barrier.
THERMAL BRIDGING IS CONDUCTION IN ACTION
When there is no insulation in a roof or wall, the framing is the most insulated part of the assembly. It has the highest R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. . Softwood lumber has an R-value of 1.25 per inch, so a 2x6 stud has an R-value of almost 7. As soon as you put insulation between the studs or rafters above R-7, however, the framing becomes the weak thermal link. If the framing cavities are filled with closed-cell spray foam insulation, the insulation has an R-value of about 36. At that point, the studs or rafters become a glaring weakness in the design.
Building scientists call this phenomenon "thermal bridging" because the studs or rafters bridge the space between inside and outside the thermal envelope.
If you look for it, thermal bridging can sometimes be seen from either inside or outside. Inside, it can cause a problem called ghosting, or cold stripes behind the drywall during winter. These cold stripes can encourage condensation that leads to the accumulation of dust particles on the drywall; eventually, visible vertical stripes may form. Outside, you can see the effect of thermal bridging in snow-melt patterns on roofs and drying patterns on walls.
A continuous layer of rigid foam installed on the inside or outside of a wall or roof drastically reduces thermal bridging through the framing.
R-VALUE MEASURES HOW WELL INSULATION WORKS
Heat flows from hot to cold; it can’t be stopped, but it can be slowed
If we measure the rate at which heat flows through a building material or building assembly—for example, a wall or a roof—we can calculate a number (the R-value) to indicate its insulating ability. The higher a material’s R-value, the better the material is at resisting heat flow through conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., convection, and radiation (outlined above). Insulation manufacturers report R-values determined by tests following ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. standards (for example, ASTM C518).
Common insulation types and their R-values
Residential insulation materials have R-values that range from about 3 to 7 per inch. The amount of insulation installed in any given building assembly depends on the climate, the part of the house being insulated, the project budget, and local code requirements.
- Batts and blankets: R-3.1 to R-4.1 per in.
- Blown-in and loose-fill insulation: R-2.6 to R-4.2 per in.
- Rigid foam: R-3.6 to R-6.8 per in.
- Closed-cell spray foam: R-6 to R-6.8 per in.
- Open-cell spray foam: R-3.5 to R-3.6 per in.
Green homes go beyond code minimum
The U.S. Department of Energy has developed a list of recommended insulation levels for different climate zones. The climate zones are represented on the map (click to enlarge).
For more information on climate zones, see All About Climate Zones.
Houses heated by natural gas, fuel oil, or an electric heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. should use the R-values set out by the DOEUnited States Department of Energy. and listed below as a base. Because electric heat is relatively expensive, houses with electric resistance heat need more insulation than shown in the table below.
In some parts of the country, minimum code requirements for insulation already (or may soon) exceed these DOE recommendations. For example, the 2009 International Residential Code requires cold-climate builders to include a minimum of R-20 wall insulation and R-15 basement wall insulation.
DOE recommended R-values for various parts of a house
|Zone||Attic||Wall||Floor||Slab edge||Basement wall (framing cavity insulation)||Basement wall (continuous rigid insulation)|
|1||R-30 to R-49||R-13 to R-15||R-13||R-4||R-11||R-10|
|2-3||R-30 to R-60||R-13 to R-15||R-13 to R-25||R-8||R-11||R-10|
|4||R-38 to R-60||R-16 to R-21||R-25 to R-30||R-8||R-11||R-4|
|5||R-38 to R-60||R-16 to R-27||R-25 to R-30||R-8||R-11 to R-19||R-10 to R-15|
|6-8||R-49 to R-60||R-18 to R-27||R-25 to R-30||R-8||R-11 to R-19||R-10 to R-15|
In any case, green builders almost always exceed minimum code requirements for insulation thickness. Many energy consultants, including Betsy Pettit and Joseph Lstiburek, now recommend that cold-climate homes include R-60 ceilings, R-40 above-grade walls, R-20 basement walls, and R-10 basement slabs.
Some builders go further; for example, an Illinois home designed to meet the rigorous German PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard is insulated to nearly R-60 on every side—even under the slab.
AIR AND MOISTURE ARE PART OF THE PICTURE
Insulation can't work in a wind tunnel
No matter what type of insulation you choose, it will perform poorly if installed in a house that is riddled with air leaks. Because many types of insulation (like loose fill and batts) work by trapping air, leaky walls, roofs, and floors mean poor thermal performance. For this reason, building scientists are fanatical about air-sealing. To get the most out of batts and blown insulation, every house needs an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. adjacent to or contiguous with the insulation layer.
Some types of insulation are fairly effective at stopping air infiltration. For example, when rigid foam is used as wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , it can be an effective barrier, as long as the seams are taped. Spray polyurethane foam creates a very effective air barrier.
But neither rigid foam nor spray foam addresses air leaks at the seams where different components meet, such as under the bottom plates of walls. An air barrier is only effective if all of these seams and intersections are addressed with gaskets, glues, or sealants.
Of all available insulation materials, fiberglass batts are the most permeable to air leakage—so permeable that fiberglass is used to make furnace air filters. Because it doesn’t restrict air flow, fiberglass is often singled out and derided for its poor performance.
In fact, much of the criticism of fiberglass insulation is unwarranted. As long as fiberglass is installed in a house with an adequate air barrier, it will perform well. Fiberglass performs best when installed in a framing cavity (for example, a stud bay or joist bay) with an air barrier on all six sides.
Installation details for high-quality fiberglass batts have been incorporated into the insulation installation guidelines established by the home raters from the Residential Energy Services Network (RESNET).
For every location in a house, there are always several ways to create an effective air barrier. However, not all methods are equally easy to achieve. In many locations, including rim-joist areas, spray polyurethane foam is so much faster than alternative methods that its use has become almost universal among builders of high-performance homes.
Moisture can piggyback on air
There’s another benefit to stopping air: less moisture in roofs and walls. That's because most moisture problems in walls and roofs are caused by moisture transported by air. Vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. is a much smaller problem.
Moisture can accumulate in a wall or ceiling when warm, humid interior air leaks through cracks in the shell. When this exfiltrating air encounters a cold surface—for example, OSB wall sheathing—the moisture in the air can condense into liquid and puddle in the wall cavity. The same thing can happen in summer, when warm, humid outdoor air leaks through cracks in the wall. If the home is air-conditioned, the moisture in this infiltrating air can condense when it reaches any cool surface—drywall, ductwork, etc. The best way to limit this type of moisture migration is to install an effective air barrier. If air isn’t leaking through cracks in a home’s walls and ceilings, the problem is nipped in the bud.
A Buyer's Guide to Insulation
Environmental Building News:
Insulation: Thermal Performance Is Just the Beginning
Oak Ridge National Lab:
ORNL Whole Wall R-Value Calculator gauges the energy implications of type of framing, spacing, and thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. .
Energy Star HomesA U.S. Environmental Protection Agency (EPA) program to promote the construction of new homes that are at least 15% more energy-efficient than homes that minimally comply with the 2004 International Residential Code. Energy Star Home requirements vary by climate.:
Thermal Bypass Checklist — Air-barrier requirements for fiberglass-batt jobs are also included.
- Ty Keltner, Cold Climate Housing Research Center
- Dan Morrison
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