I’ve recently worked on revising the BuildingGreen Guide to Insulation Products and Practices, so I’ve been steeped in all sort of insulation materials, including some oddball products. One of those is vacuum insulation -- the principle of a Thermos bottle. In theory, vacuum insulation is a great idea. To understand why, it helps to know a bit about heat flow.
How a vacuum slows
down heat transfer
There are three modes of heat transfer: conduction, convection, and radiation, and if we remove most of the air molecules from a space -- as occurs when we draw a vacuum -- we largely eliminate the first two of those heat transfer mechanisms.
Conduction is the flow of heat from molecule to molecule. It’s the reason a cast iron skillet handle heats up, but thermal conduction also occurs across a layer of air, as kinetic is transferred from one air molecule to the one next to it. If we remove most of those air molecules by creating a vacuum, there will far less conductive heat flow.
Convection is the transfer of heat by moving molecules from one place to another. Warm air rises, and these convection currents carry heat -- for example, this is the primary means that heat is delivered to a room from baseboard convectors (often called radiators). In a vacuum there are far fewer air molecules so convection of heat nearly stops.
Only radiant heat flow occurs to a significant extent in a vacuum, because radiation is not dependent on air molecules. That’s why low-emissivity surfaces are so important in vacuum panels. The Stanley Thermos bottle has a very shiny, low-emissivity (low-e), inner surface that helps to control radiant heat transfer; the same sort of low-e surface is included in various vacuum insulation panels.
The net result is that an inch-thick vacuum insulation panel can provide a center-of-panel insulating value of R-25 or even more -- compared with at most R-7 for standard rigid foam insulation.
The "hardness" of a vacuum
The key property of a vacuum is it’s pressure or how "hard" it is. We often measure that with Torr units. One Torr is exactly 1/760th of a standard atmosphere (1.3 x 10-3 atm), or approximately 1 mm of mercury. With a very hard vacuum, more of the air molecules are sucked out, resulting a greater negative pressure. The walls of a typical Stanley Thermos bottle contain a relatively hard vacuum of 10-6 Torr. With such a hard vacuum, that Thermos bottle can keep coffee hot all day. By comparison, the vacuum in a flat vacuum insulation panel is typically only 1/1000th as strong (10-3 Torr).
The harder the vacuum, the more difficult it is to maintain it. Thermos bottles are very carefully designed with a cylindrical design for optimal strength. With flat panels, it’s very hard to achieve comparable strength and maintain such a hard vacuum -- particularly at the edges.
Using vacuums to insulate more than our coffee
If vacuums work so well to keep our coffee hot all day, why not use them to insulate our houses? Vacuum insulation panels have been used to insulate some high-tech demonstration homes, such as entrants in the Solar Decathlon student design competition in recent years, but high cost makes them impractical for real buildings. There’s also the problem that puncturing that vacuum insulation panel will significantly reduce it’s insulating performance. (I can imagine the disappointment of spending thousands to insulate a home with vacuum insulation panels and then hearing a hiss while hanging a painting!)
However, these vacuum insulation panels (sometimes called VIPs) could make a great deal of sense in certain value-added products like refrigerators, freezers, water heaters, and entry doors. Whirlpool actually used a VIP that Owens Corning produced for a while (the Aura panel) in a high-efficiency refrigerator in the mid-1990s, but then dropped both the refrigerator and the use of VIPs.
But I believe the benefits of R-25 or more in a one-inch-thick panel are significant enough -- especially as we try to get more usable volume in refrigerators without growing the exterior dimensions -- to warrant the embrace of vacuum insulation. These could also be a great solution for exterior doors that are notoriously poorly insulated.
A new VIP on the market
There are a number of VIPs on the market. Most have a rigid, porous substrate surrounded by an impermeable metal skin. The latest product to come along is made by Dow Corning (no relation to Owens Corning). This panel, not yet widely available, is one inch thick and has a center-of-panel insulating value of R-39 and a "unit R-value" (accounting for the edges) of R-30, according to the company.
The Dow Corning product has a core made of fumed silica cake, a remarkable "microporous" material that provides R-8 per inch even without a vacuum. This material allows a very high insulating value even with a softer vacuum. The core is reinforced with silicon carbide and polyester fibers for structural support, and it is encased in an inner layer of polyethylene and an outer layer of polyethylene, polyester, and aluminum. The panels are vacuum-sealed, and the edges are heat-sealed.
According to an Environmental Building News article, these Dow Corning panels should cost $10-12 per square foot. At that cost, I believe VIPs can be very practical for those appliance and exterior door applications noted above.
Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to firstname.lastname@example.org.