Keeping your home warm in the winter and cool in the summer requires three ingredients: energy, air sealing and insulation. If that sounds complicated, just think about how a cooler works.
We put energy into our homes in the form of heating and air conditioning. Whether that energy stays in our homes depends largely on air sealing and insulation.
Heat flows in three ways: radiation, convection and conduction. Radiation is how the sun heats us or how a radiator heats a space in your home. Convection is moving heat through the air as we do with forced-air furnaces that heat air and then circulate it through the house to keep the us warm. Conduction is the flow of heat through materials in contact with one another.
Heat flows from areas of greater concentration to areas of lesser concentration. When you’re heating your house on a cold winter day, the heat flows from the warm interior of your home to the cold exterior. It flows by conduction through your walls so we use insulation to slow (or resist) the flow of heat. When heat flows through your walls, in both the winter and summer, you’re losing the energy consumed by your heating and cooling systems to make your home comfortable.
Passive + Active Systems
Insulation is called a passive system because it doesn’t have operating costs (it doesn’t need any energy to operate) and it shouldn’t have any maintenance costs. Heating and cooling systems are active systems because they have moving parts with both operating and maintenance costs.
This May Help…
It may help to think of your home as a cooler. The six sides (bottom, four sides and top) of a cooler are like the floor, walls and ceiling of your home. They’re insulated and should be sealed when closed to prevent air leaks through which the interior space could lose energy.
The cooler’s job is to resist the flow of heat so the energy in the contents of the cooler is not lost. If you put ice in the cooler, the energy used to make the ice is slowly lost as the ice melts. The cooler doesn’t care if you put in ice or something warm. Its only job is to resist heat flow. The exterior walls of your house don’t care whether you’re heating or cooling your home. They too need to resist heat flow.
What’s an “R-value”?
The ability of insulation to resist the flow of heat is recorded as an R-value where the “R” stands for “resistance”. A higher R-value means the insulation does a better job preventing heat flow. So, insulation with an R-value of 30 resists heat flow much better than insulation with an R-value of 13. Those values would be written as R-30 and R-13. You’ll typically see R-values stamped or printed on the insulation product or packaging.
The components of an exterior wall and how they’re put together is called a wall assembly. Each component needs to be in the right place for the site where the wall is built. We build walls much differently in the hot, humid south than we do in the northeast or southwest. Insulation is one of the components in an exterior wall assembly.
R-values are additive. If you put two layers of R-13 insulation together, you’ll get an R-value of 26. That’s important for our exterior walls where we need to build up the R-value to meet the requirements of a particular climate. For example, we typically use less insulation in the southeast than would be required in the northeast because the climates are different.
An exterior wall may have many components: wood framing, exterior sheathing, insulation, interior finish, etc. All the components of an exterior wall have some ability to resist heat flow, even if it’s very low. So, we calculate the R-value of all the components in the wall to get the R-value for the entire assembly.
Testing Your Insulation IQ
Now that you have a fundamental understanding of heat flow and insulation, let’s test your assumptions about building materials with these six questions. Some of the answers might surprise you!
1. Which insulates more, an 8″ CMU wall or a single pane of glass? “CMU” stands for “concrete masonry unit” – it’s what some people would call a “cinder block”.
The answer is: An 8″ CMU wall has an R-value of 1.11 and the single pane of glass has an R-value of 0.91. Even though the CMU appears to be much thicker and the glass is transparent, they resist the flow of heat about the same. CMU walls are mostly open on the inside because the units aren’t solid, they have hollow cores.
2. Which insulates more, 4″ of face brick or 1″ of soft wood lumber?
The answer is: A 4″ face brick used to build a brick veneer on a home typically has holes cored in its center. It has an R-value of 0.44 while 1″ of soft wood lumber has a R-value of 1.25. Wood siding on a home may offer better insulation though it requires much more maintenance. Insulation isn’t the only consideration when designing an exterior wall assembly. It may make sense to use a low-maintenance material like brick (if the budget allows) and add insulation elsewhere in the exterior wall.
3. Which insulates more, a 1 3/4″ solid wood door or an insulated metal door?
The answer is: A 1 3/4″ solid wood door has an R-value of 2.17 while a 2″ metal door insulation with a urethane core has an R-value of 15. That’s a big difference when it comes to insulating an opening the size of a door in your exterior wall!
4. Which insulates more, 1″ of fiberglass batt insulation or 1″ of expanded polystyrene rigid foam insulation?
The answer is: Fiberglass batts are often advertised and commonly used to insulate the cavities between the wood studs in an exterior wall assembly. One inch of fiberglass batt has an R-value of 3.7. Expanded polystyrene (EPS) is a rigid foam insulation which you may know under the brand name “styrofoam”. One inch of EPS has an R-value of 4. Should we put the insulation between the studs or use rigid insulation on the outside of the structure?
5. Which insulates more, 1″ of blown-in cellulose insulation or 1″ of open-cell spray foam insulation?
The answer is: Cellulose insulation is recycled paper that’s treated for use as insulation in exterior walls and attics. An inch of blown-in cellulose insulation has an R-value of 3.8 while an inch of open-cell spray foam insulation has an R-value of 3.6. They’re similar in terms of insulation value yet the spray foam offers air sealing properties too. Closed-cell spray foam has an R-value of about 6.25 per inch though it’s more expensive and has other issues to consider when designing the exterior wall assembly.
6. Which insulates more, a single pane of glass or a 1″ air gap between building materials?
The answer is: You already know from the first answer that a single pane of glass has an R-value of 0.91. A 1″ air gap between building materials has an R-value of 1. How can a gap have an R-value? A gap stops conduction: heat flow through materials in contact with one another. Some gaps are built into products like window frames to stop conduction. They’re called “thermal breaks”.
Did any of the answers surprise you? Hopefully these questions and answers have helped you appreciate the importance of designing the exterior walls of a home for the climate in which it’s built. If you’re not comfortable in your home, it’s important to invest in the passive systems like insulation before investing in expensive active systems like furnaces and air conditioners.
There are many, many ways to design exterior walls. Aside from the exterior cladding (brick veneer, wood siding, etc.), there are decisions to be made about structure, amount of insulation, location of insulation in the wall assembly, etc. Your architect should be able to explain your options and their expected results in detail.
Is it possible to have too much of a good thing? It’s important to not over insulate. In the southeastern United States, for example, an R-20 wall is the point at which the return on investment starts to drop off quickly. That’s why it’s important to carefully consider your options – you don’t want to waste money that could be better spent in other areas of your home.
If you’re freezing in the winter and broiling in the summer in your own home, don’t assume that you need insulation. You may have air sealing, leaking HVAC ducts or other issues that need to be addressed in the process of solving your health, comfort and efficiency challenges. Please consider consulting with a professional to figure out your total scope of work (items to be addressed and how to address them) before you get started.
If anyone spots you reading this blog post and asks you what you’re reading, you can tell them you’re reading about thermodynamics.
As always, if you’ve got a comment or question, please share it below. Thanks!
Image by Noah Bolton used under creative commons license.