Vermonters are thinking a lot about energy efficiency these days; we all want to save money on heating costs, add value to our homes, and do our part to protect the environment. One of the best ways to do so is to ensure your home is well-insulated.
If you’ve looked into replacing your windows or insulation, then you’ve probably noticed that manufacturers have different rating systems that tell you how efficient the product is. But before we explore those, it’s essential to understand how insulation works.
Insulation is directly related to heat flow, which operates with three primary mechanisms: conduction, convection, and radiation. Conduction is the way heat moves through materials, such as when a spoon is placed in a hot cup of coffee, conducting heat through its handle to your hand. Convection is how heat circulates through liquids and gases, and it’s why lighter, warmer air rises and colder, denser air sinks. Radiant heat travels in a straight line and heats any solid material in its path that can absorb energy.
Most common insulation materials work by slowing conductive heat flow and, to a lesser extent, convective heat flow. Radiant barriers and reflective insulation systems work by reducing radiant heat gain. To be effective, the reflective surface must face an air space.
Regardless of the mechanism, heat flows from warmer to cooler until there is no longer a temperature difference. In your home, this means that in winter, heat flows directly from all heated living spaces to adjacent unheated attics, garages, basements, and even to the outdoors. Heat flow can also move indirectly through interior ceilings, walls, and floors. The opposite happens in the summer: heat flows from the outdoors to the interior of a cool house.
To maintain a comfortable temperature, your heating system must replace heat lost, and your cooling system must remove heat gained. A properly insulated home provides effective resistance to heat flow, maintaining a stable indoor temperature.
R-value is a measurement of thermal resistance: An insulating material reduces conductive heat flow and prevents heat from transferring from one side of an object to another. The higher the R-value, the greater the insulating effectiveness. The R-value is based on the material, its thickness, and its density. In some insulations, it is also based on temperature, aging, and moisture accumulation. To calculate the R-value of a multi-layered installation, add the R-values of all the individual layers.
As a benchmark, one inch of solid wood has an R-value of 1. In comparison, an inch of blown fiberglass insulation has an R-value of 3.4, and an inch of blown cellulose in an attic has an R-value of 3.7. Spray Foam has an R-value of 7 per inch, making it one of the best insulation options.
The amount of insulation or R-value you’ll need depends on your climate, type of heating and cooling system, and the part of the house you plan to insulate. For a standard home in Vermont, good ratings are R-50 for ceilings, R-21 for walls, and R-30 for floors.
U-value measures thermal transmission rather than thermal resistance. In other words, it determines how effective a material is at preventing heat from transmitting between the inside and the outside of a building. U-value is generally used to rate doors and windows. A good U-value is a low number because it reflects how much heat energy is lost or gained; the lower the U-value, the more energy-efficient the unit is.
A single-pane window has a U-value of 1.0. An old double-pane window has an overall U-value of .50. These kinds of windows were commonly used when building houses in the 80’s, but are no longer acceptable for new construction. The 2015 Residential Building Energy Standard requires double-glazed windows to have an overall U-value of 0.32 or lower. Triple-glazed windows can be as low as 0.13, making them excellent insulators, but unfortunately, these units are costly.