I want to talk about windows, insulation, and envelop upgrades but I think the fundamentals of R-value should be discussed first. It will help in understanding how all the parts go together when one understands the importance of R-value.
R-value, is the measurement of thermal resistance used in the building and construction industry. It is also the inverse of U-value. Heat is transferred through conduction, convection and radiation. If your eyes just crossed remembering your high school science days, you’re not alone. This is one of the basics that I teach at the beginning of my building science class, and I repeat at the beginning of my sustainable design class. Let’s discuss what they mean for you, the homeowner.
For most people, R-value is often seen on bags of insulation. It can be found for other building materials such as wood studs, drywall, siding etc, but is not often displayed on the packaging. U-value, thermal transmittance, is usually observed on windows and doors. If you have looked at windows, you have seen the U-value listed because it is required as part of the building industry standards. You may not have known what it meant at the time, but that little number is very important. And contrary to everything else you have ever learned, the smaller the number, the better the window! It is also good to know that U-value is the inverse of R-value. For example, if a window has a U-value of 0.30, its R-value is 3.3. It’s easier to compare the performance of building components when they are listed in the same format. For comparison, the current IECC 2009 for the Northeast requires walls to have a minimum R-value of R-21. So if you look at the window with an R-3.3, and then at the wall with an R-21, you’ll see that the window is a fairly poor performing part of your building envelop. But I digress; there will be more articles all about windows in the future.
R-value gives the building professional an idea the materials ability to resist heat flow. It also works in the opposite direction with the heat entering your home in the summer or primarily cooling climate locations. Every state has a building code, and each building code has a minimum level of R-value necessary to meet the states requirements. In some places in the country they also have additional requirements that have higher performance levels then code. Again, this is a reason why you can’t afford not to hire an architect, and more specifically, one that knows a lot about energy efficiency.
For an energy professional, R-value can translate directly into how many Btu’s your home will use. A Btu (British Thermal Unit) is the amount of energy needed to raise one pound of water one degree F. For building professionals it is the rate at which your home loses heat through the surface (walls, windows, roof, doors) and through air changes (how drafty your home is) The higher the R-value the lower the surface transported heat loss. The building professional will take the R-value, include the air transported heat loss, and tell you approximately how many Btu’s your homes heating system will need to produce to keep you warm this winter.
From here it gets complicated. The air transported heat loss can have an effect on how well the R-value of certain building products perform. The tighter the house becomes the harder it is for standard atmospheric heating systems to work. But the more efficient your home is, the less it will cost you to live in and operate. So the next time you’re concerned about insulation, drafts, and R-value think about hiring an energy professional to help you out, because replacing your windows isn’t the best place to start.