Bianchina: Here's the lowdown on insulation
What is insulation? What’s it made of? How does it work and how is it measured?
Here’s a little background that might help when you undertake your next insulation project.
How heat moves
First of all, it helps to understand the three different ways that heat moves in your home.
• Conduction: This is movement through a material. For example, if you place a cold spoon in a hot bowl of soup, eventually the spoon will get hot through conduction.
• Convection: This is movement through air circulation, with warm (lighter) air rising and cold (denser) air sinking. In a heated room, you’ll feel colder air at the floor and warmer air at the ceiling; that’s convection.
• Radiation: This is the movement of heat in a straight line, from a warm surface to a cold surface. If you stand in front of a campfire on a cold night; the front of you is warm as the fire’s heat moves toward you, while your back is cold as your body heat moves toward the cold night air; that’s radiation.
Two types of insulation are commonly used in the building trades — thermal insulation (for heat) and acoustic insulation (for sound). Here, we’re talking about thermal insulation, which is basically any material that works to stop heat movement and keep the heat where you want it — outside in summer, inside in winter.
Insulation works primarily by stopping heat loss (or summer heat gain) through conduction. In other words, it creates a barrier to stop the heat from moving through the materials of your home.
You also can see that convection plays a role in all this. Since heat naturally rises, attic insulation becomes the most important area to insulate. And your home has convection currents: air flow created as heat moves from warm to cold, as drafts occur, and as wind and other forces create pressure differentials in the home. That’s why wall and floor insulation also are crucial components of a warm, energy-efficient home.
Several good insulation materials are on the market, including fiberglass, cellulose, rock wool, spray foam and foam boards. Your choice of a material is often dependent on where it’s being installed and who’s installing it (you or a professional contractor).
For thermal insulation, the term "R-value" is used to measure and rate the insulation’s efficiency. The R stands for its resistance to conductive heat flow, so the higher the R-value a material has, the more resistant it will be to heat flowing through it, and the better it is as a thermal insulator.
Here are the approximate R-values for some common insulation materials, per inch:
• Fiberglass, blown-in: 2.2–2.9.
• Fiberglass, batts: 2.9-3.8.
• Cellulose, blown-in: 3.1-3.8.
• Rock wool, batts or blown-in: 2.2–3.3.
• Spray foam: 3.6–8.0, depending on type.
• Foam boards (extruded polystyrene): 4.5-5.0.
One of the nice things about the R-value rating system is that for calculation purposes, you can combine different materials. In other words, if you had some old rock wool batts in your attic and you blew some new fiberglass insulation over the top of them, you simply could add the R-value of the old and new insulations and get a new R-value for the entire composite.
When looking at your home’s energy efficiency, insulation professionals will take into account what’s known as thermal bridging, and it’s an important concept for you to be aware of, as well.
Heat movement through conduction will take the path of least resistance. So consider what the inside of your wall looks like. There are studs every 16 inches, with spaces between them. The spaces are filled with insulation, creating a good thermal barrier. But since wood has a low resistance to heat movement, about R-1.3 per inch, heat moves easily through the studs. This is a thermal bridge. The same is true in the attic, where the spaces between the trusses or the ceiling joists are filled with insulation, but trusses or joists allow heat loss.
In an attic, you can consider the use of blown-in insulation rather than batts. Since the blown-in material covers the wood completely, it offers a lot more thermal insulation than batts placed between the wood. In the walls, the problem is more difficult, and the only real solution is to cover the surface of the studs with insulation. The U.S. Energy Department recommends that when new siding is being installed, it’s a good idea to consider adding insulation in the form of sheet foam under the new siding.
Radiant heat movement is different from conductive and convective movement, so in this case radiant barriers are used. A radiant barrier is a thin, very shiny material, resembling a mirror-finish aluminum foil. It’s designed to reflect heat away from a building, and its primary function is to reduce heat gain.
Radiant barriers have no actual R-value of their own, and may be used alone or in combination with thermal insulation materials. They’re typically sold in rolls, and are often installed on the underside of roof framing to re-emit solar radiation in order to keep attics cooler and extend roofing life.
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