Can we create a chart of R-value vs. percent heat retention?
user-4053553| Posted inEnergy Efficiency and Durabilityon
I’ll assume such a chart will have to have a specific delta T, so lets say 10C and a time period, lets use 24 hours.
Next question, can this scale, would a 20C delta T be double the heat loss or 48 hours double the heat loss (can it be scaled)?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Alan,
No, it isn't possible to create a chart of R-value vs. percent heat retention.
If there is a difference in temperature (a delta-T) across a building assembly, heat will flow from the warm side of the assembly to the cool side of the assembly. When analyzing the energy use of a building, it's often useful to calculate the rate of heat flow across a building assembly. Such a calculation can be compared to the "miles per hour" shown on a speedometer; both of these are rate calculations.
The formula used to determine the rate of heat flow through floors, roofs, and walls is:
Q = A • U • ΔT.
In other words, the rate of heat flow through a building assembly (in Btu/h) is equal to the area of the assembly (in ft²) times the U-factor (in Btu/ft² • hr • F°) of the assembly times the ΔT (in F°).
You guessed correctly that when you double the delta-T, you also double the rate of heat flow (Q).
If you want a measure the energy required to heat a building for a certain period of time, you would use an energy unit -- for example, BTU -- to quantify the amount of energy you are talking about.
If it takes 1,000 BTU to heat a volume for 24 hours, then (assuming constant unchanging conditions) it would take 2,000 BTU to heat the same volume for 48 hours.
If it takes 1,000 BTU to heat a volume at a delta-T of 10 F° for 24 hours, then it would take 2,000 BTU to heat the same volume at a delta-T of 20 F° for 24 hours.