Welcome to my blog! As I add to this blog, you’ll find answers to your questions about energy, especially answers about your energy. Why do your bills look the way they do? What are you really paying for? How can you reduce the amount of energy you’re using without it costing you more in the long run? You can read a little about me and the goals of this blog in the official “launch” post from August 12, but I want to focus on what you want to know, so feel free to send me questions and comments.
Today’s topic is insulation. That’s pretty basic. With good insulation, your home or building will require less energy for heating and cooling. It works in hot and cold seasons. For decades, the construction industry has standardized on describing building insulation with a number called “R”. Bigger is better, yes, but what does it mean? Mathematically, the rate at which your building loses heat to the environment in the cold season, or gains heat from outside when it’s warm, is calculated by dividing the temperature difference between inside and outside by the R value, then multiplying by the surface area of the wall or ceiling being evaluated. Pretty simple:
Q = (Toutside – Tinside) * A / R, where
Q = heat gain, in BTU/hr
T = temperature, in degrees F
A = the area in ft2
R = the aforementioned R value of the wall or ceiling, in F * hr * ft2 / BTU.
Since R is the denominator, bigger values of R result in smaller values of Q. And if Toutside is less than Tinside, Q is negative, meaning heat “gain” is to the outside, or your building is losing heat.
Of course, it’s more complicated than that. You saw that coming, right? First of all, the temperatures used in the equation are not the air temperatures, they’re the temperatures of the inside and outside surfaces of the walls. Technically, this is the equation of heat conduction, but there are other mechanisms of heat gain and loss, namely, convection and radiation. Convection is the movement of heat by the motion of the air around the walls. On the inside, there is little air motion, mostly driven by fans or the HVAC blower. When the room is still, there is air motion near the wall because of buoyancy – hotter air rises, colder air sinks. But at typical conditions, the convection to/from the wall is fairly slow, so that the wall surface temperature will be a few degrees different from the air temperature. On the outside, the air motion may be quite different – especially when there’s a winter storm going on! The wall surface temperature could be very close to the air temperature. And the surface temperature above the ceiling could be quite hot (or cold), if there is attic space above that isn’t well ventilated.
Secondly, every wall or ceiling is built up of several different materials, each of which have different R-values. So the insulation is only part of the total value of R that needs to be used in calculating the heat gain. Also, there are typically windows – which add their own complexity including the possibility of significant thermal radiation passing through them.
Last, but not least, there are leaks! Cracks around doors and windows, gaps and holes in HVAC ductwork, gaps around wall penetrations where electrical outlets and switches are located, all allow air to pass through them from outside to inside, or vice-versa, air that brings heat with it (another form of convection).
I won’t say too much now about radiation, but I’ve seen outside walls with summer temperatures significantly hotter than the air around them because of radiation heating from sun-heated surfaces like fences or stone coverings nearby.
So you can’t just stack on more insulation and always expect to see a benefit. The benefit may be overwhelmed by some of the other factors. One big potential problem is HVAC ductwork in attic space, outside of the insulation layer. If the attic isn’t well ventilated, the outside surface of these ducts could be exposed to very hot air, say 120 F or 130 F on a hot summer day, which will immediately degrade the performance of your A/C system. Worse than that, if your ducts are 10-20 years old, they could be leaking a large percentage of your A/C airflow out into the attic, doing you no good at all. You could blow in 18 inches of cellulose fiber insulation and not really see a significant benefit.
Having said that, if your house was built 20 or 30 years ago, it’s likely that some of the insulation isn’t doing its job any more. You could benefit from an inspection that detects gaps and thin spots in the insulation of your walls, ceiling and floors. Such an inspection, with a good analysis of the results, will be able to tell you if you can reduce your energy bills by simply refreshing the insulation of your building. SimplEnergy can use thermal imaging with an infrared camera to detect such defects, and then provide an estimate of how much energy and cost you can save by fixing them. Pictures like this will tell exactly where the problems spots are - like the one below, which highlighted a poorly-insulated space where a window was removed when remodeling a bathroom.
Reject that heat transfer! Make sure your “R”s are Robust! And check for other problems so that you get the most out of your dollars spent on improving energy efficiency.
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