Last week I discussed R-Value and what it means, this week I want to expand on that discussion by explaining how R-Value directly translates into your heating and cooling bill. But first we need to understand how heat flows from one place to another. There are three heat transfer mechanisms; Conduction, Convection, and Radiation. All are important in the design of a comfortable home. The three types are individually complicated, with entire books written on each of the three types.
Conduction heat transfer
This week’s blog will focus on the simplest heat transfer mechanism, conduction. Next week, I’ll focus on convection, then radiation the following week. Conduction is the transfer of heat through a solid (also works with liquids and gases) object when one part is warmer than another part as shown in the figure below.
Simplified house example
For the purpose of learning a little about conduction heat transfer, let’s consider an extremely simplified scenario. A 2400-square foot home on two levels, without doors or windows. The house measures 30×40 feet with 8-feet-high ceiling on each level and 1 foot between floors. This gives us an exterior wall surface of 2,380 square feet. But we add another 1,200 square feet for the roof and another 1,200 for the ground floor.
Let’s assume that the inside thermostat is set at 72°F, while the outside temperature is a frosty 0°F. Let’s also assume that there is no wind blowing (a dubious assumption here along the front range).
Heat loss through the walls
Considering only the walls for now, let’s consider how the walls were built. Most homes are built using 2×4 or 2×6 boards with some fiberglass insulation sandwiched between the boards. Typically, on the interior of the wall, there will be a layer of sheetrock. One the exterior side there is some sort of weather cladding, perhaps siding, brick, or stucco. In homes built in the Denver area over the last 30 years, R-14 would be better than average (the reasons for this low R-Value will be detailed in a future blog). If the home was built more than 30 years ago, the R-Value is probably closer to R-9 or even less.
For the newer home, this means that about 12,240 BTUs/hour are escaping through the walls. For the older home, 19,040 BTUs/hour are escaping. (Note: BTU = British Thermal Unit is a unit of heat energy).
Heat loss though the ceiling/roof
For the ceiling/roof assembly, there are a lot of variables; for example, pitched or flat, ventilated or unventilated, truss construction or rafters and joists, and many others. The most common roof type is a ventilated, pitched roof, built using rafters and joists. In this analysis, we’ll assume that the insulation is sitting directly on top of the ceiling sheetrock (very common). It is common for newer homes to have an R-Value of about R-42, while older homes are often closer to R-30.
For the newer home, this means that about 2,057 BTUs/hour are escaping through the ceiling/roof. For the older home, 2,880 BTUs/hour are escaping.
Heat loss through the floor
Finally let’s consider the lower floor assembly. Again, there are a lot of possible designs, such as basement, crawlspace, or slab on grade. For this analysis, we’ll assume that he lower-level floor is built above an unconditioned crawlspace, which is pretty common. For newer homes the R-Value is about R-24, while for older homes it would be closer to R-15. In this scenario, the R-values used are really best case. The insulation easily pulls loose from the floor joists and rodents love to nest in the insulation.
For the newer home, this means that about 3,600 BTUs/hour are escaping through the floor. For the older home, 5,760 BTUs/hour are escaping.
Summing up the walls, floor, and ceiling for the whole house, the newer home would be losing 17,900 BTUs/hour while the older home would be losing 27,680 BTUs/hour.
Replacing lost heat
What would it cost to replace that heat loss? That would depend on the type of heating system the house uses. Many options exist, such as forced air furnaces, burning either natural gas or propane. Another popular option back in the 70ies was electric baseboard heaters. A common option today are natural gas or propane-powered boilers which drive a radiant floor heating system. In recent years various types of heat pumps have become popular. A future blog post will discuss each of these options in greater detail.
For the older home, using electric baseboard heating we’d need to supply approximately 8.1 KWhr/hour of electricity to keep the home warm. At today’s prices, that would be about $1.18 per hour. That adds up after a while.
For the newer home, burning natural gas in boiler (80% efficient), the cost would be significantly less; closer to $0.25 per hour.
Future blogs
It’s important to remember that this analysis is based on a totally unrealistic house, because there were no doors or windows. Window R-Values are often R-3 or less. Additionally, there were no complicated corners, dormers, pop-outs, or other architectural features to complicate things. Finally, I assumed no wind blowing. And an unstated assumption is that there are no air leaks in the house, an unrealistic assumption for most houses. I’ll tackle each of these assumptions in future blogs.
If you have any questions or would like to discuss how Delta Energy Solutions can engineer a solution to meet your goals and save you dollars, please contact us.