Understanding and optimizing the heat transfer through the walls is important in high performance building design. Using thermal mass and insulation to your advantage with passive design strategies can help reduce the amount of energy that active systems need to use.
It’s important to understand Heat Energy Flows in a building to understand insulation.Insulation primarily is designed to prevent heat transfer from conduction and radiation.
Resistance to conduction is measured by R-value (high thermal resistance = high R-value); Resistance to radiative heat transfer is measured by emissivity (high resistance = low emissivity and high reflectance). Conduction is the dominant factor when materials are touching each other; when there is an air gap between materials, radiation becomes important. Convection usually only becomes an issue when significant air pockets are involved.
Materials used for insulation fall into two broad categories:
(1) Fibrous or cellular products – These resist conduction and can be either inorganic (such as glass, rock wool, slag wool, perlite, or vermiculite) or organic ( such as cotton, synthetic fibers, cork, foamed rubber, or polystyrene).
(2) Metallic or metalized organic reflective membranes – These block radiation heat transfer and must face an air space to be effective.
R-values and Insulation (Conduction)
Below is a table of R-values for some common building products. For a more extensive list, see archtoolbox.
|Material, 1″ (2.5cm) thickness||m2•K/W||ft2•°F•h/BTU|
|Vacuum insulated panel||5.28 – 8.8||R-30 – R-50|
|Polyisocyanurate spray foam||0.76 – 1.46||R-4.3 – R-8.3|
|Polyurethane rigid panel||0.97 – 1.2||R-5.5 – R-6.8|
|Closed-cell polyurethane spray foam||0.97 – 1.14||R-5.5 – R-6.5|
|Extruded polystyrene (XPS), low-density||0.63 – 0.82||R-3.6 – R-4.7|
|Expanded polystyrene (EPS) high-density||0.65 – 0.7||R-3.85 – R-4.2|
|Fiberglass batts||0.55 – 0.76||R-3.1 – R-4.3|
|Cotton batts (Blue Jean insulation)||0.65||R-3.7|
|Open-cell polyurethane spray foam||0.63||R-3.6|
|Cardboard||0.52 – 0.7||R-3 – R-4|
|Rock and slag wool batts||0.52 – 0.68||R-3 – R-3.85|
|Cellulose wet-spray||0.52 – 0.67||R-3 – R-3.8|
|Steel stud (source)||5.3×10-4||R-0.003|
Table of R-values for 1″ thickness of common building materials. From Wikipedia and Klepper, Hahn & Hyatt.
Because R-values are 1 / conductance (U), doubling the thickness of insulation will not cut heat loss in half. Rather, there is an exponential decay of heat flow, where the difference between no insulation and one inch (or one cm) of a particular insulation may save 80% of heat loss, while going from one inch to two inches of that insulation saves an additional 9%, and going from 9 inches to ten inches only saves an additional 1%.
Reduction in heat loss does not follow R-values linearly, but in an inverse logarithmic curve.
Low-Emissivity Insulation (Radiation)
There are many situations where radiative heat transfer is important to avoid–for instance, attics or warehouses where the sun heats the building’s skin excessively. In conditions like this, just a thin sheet of reflective material can make as much difference as adding many inches of conventional insulation. These are usually called “radiant barriers”.
Radiant barriers must have a low emissivity (0.1 or less) and high reflectance (0.9 or more). Thus they are shiny reflective or white materials.
They only reduce radiative heat transfer. Because of this, reflective insulation is only useful on the surface of insulation facing a cavity or the outside air.
Low-emissivity insulation is reflective foil-faced.
Convection and Insulation
Convection through fluids (like air) can also transfer heat. Unwanted convection through the building envelope can cause unwanted heat gains or losses through infiltration (see Infiltration & Moisture). Also, suppressing convection within the materials of the building envelope is often what makes insulation effective.
Convection within the building envelope hurts insulation as well. Still air is an excellent insulator, so good insulation often uses small pockets of air. The main reason that foam insulation is a better insulator than batt insulation is that there is less convection of the air within foam. Aerogel is such a high-performance insulator because it is mostly air, but the micro-scale structure of the aerogel prevents convection of the air held in it.
Fibrous or cellular products prevent conduction by keeping air still (preventing convection). Here’s how:
- Batt insulation traps air in a mat made from a low conductivity solid, such as glass or organic fiber (wool or polyester).
- Open-cell foams trap tiny bubbles of air or other gas in a poor conductor, such as polystyrene or polyurethane. However, gas can still migrate through open-cell foams.
- Closed-cell foams, where gas cannot travel from cell to cell, are the best way to avoid convection.