They’re resilient, easy to build, and offer higher R-Values than their conventionally framed peers. But did you know that ICF walls also have a unique property that helps them slash the load on heating and cooling systems during significant outdoor temperature swings?
This post will show how by reaching the “steady-state” later than other wall types, ICF walls can maintain indoor temperatures longer without the help of mechanical systems.
What is Steady State?
When a building’s walls get to their steady-state, they operate at their true (i.e., effective) R-Value. At steady-state, the maximum amount of energy is spent to keep the interior space conditioned and this rate of energy consumption remains constant during steady state.
This is why the time it takes a wall to reach its steady-state is so vital.
While a wall is in the process of moving towards steady-state, it consumes significantly less energy over the same time period.
Thus, the longer a wall assembly takes to reach its steady-state, the longer heating and cooling systems can operate at a significantly reduced load. In turn, this load reduction leads to sizable energy savings.
Why ICF Walls Take Longer to Reach the Steady State
Unlike wood- and steel-framed assemblies, which offer a Swiss cheese-style barrier full of thermal bridging and air leaks, ICF assemblies like Element ICF have:
- continuous layers of high R-value insulation on both sides of the concrete core
- an airtight, watertight concrete core
- high thermal mass that’s able to store heat for long periods before releasing it
These properties are key to postponing the walls’ steady state and slashing the load on conditioning systems.
Guarded Hot Box Test
CAN-BEST Laboratories performed a hot box test to see how ICF walls stack up against their conventionally framed peers in reaching the steady state. The test followed the standard ASTM C-1363 “Guarded Hot Box” text procedure and aimed to measure the energy flow across several test samples in the middle of the chamber. One part of the chamber was chilled to -31℉, while the other was heated to +72℉. The test involved the following samples:
- R-20 batt-insulated, wood-framed wall
- R-24 wood-framed wall
- R-20 wood-frames wall, with a layer of R-5 Continuous Insulation (CI)
- Light-gauge steel stud wall with R-5 CI
- R-23.36 ICF wall
Apart from gauging the energy flow, the team also assessed each sample assembly’s time to steady state and R-Value at steady state.
Guarded Hot Box Test Results
We’ve compared each of the traditionally framed samples against an R-23.36 ICF wall. Let’s start with the R-20 batt-insulated, 2×6 wood-framed wall.
R-23.36 ICF Wall vs. R-20 Batt-Insulated Wood-framed Wall
The standard, R-20 2×6 wood-framed assembly reached its steady-state 36 times faster than the ICF wall:
- Time to steady-state, wood-framed wall: 4 hours
- Time to steady-state, ICF wall: 144 hours (6 days)
The wood-framed wall saw its effective R-Value plummet by 26% versus nominal, whereas the ICF assembly retained its nominal R-Value throughout the test:
- Effective R-Value of the wood-framed wall: R-14.8
- Effective R-Value of the ICF wall: R-23
Throughout the test, the ICF wall required 60% less energy to maintain the target temperatures:
- Total energy input, wood-framed wall: 454.3 BTU/hr
- Total energy input, ICF wall: 177.6 BTU/hr
These results were largely repeated with all other samples:
ICF vs. R-24 Wood-framed Wall
- Time to steady state, wood-framed wall: 13 hours
- Time to steady state, ICF wall: 144 hours (6 days)
- Effective R-Value of the wood-framed wall: R-18.7
- Effective R-Value of the ICF wall: R-23
- Total energy input, wood-framed wall: 330 BTU/hr
- Total energy input, ICF wall: 177.6 BTU/hr
ICF vs. R-20 Wood-frames Wall, with R-5 Continuous Insulation (CI)
- Time to steady-state, wood-framed wall: 54 hours
- Time to steady-state, ICF wall: 144 hours (6 days)
- Effective R-Value of wood-framed wall: R-19.4
- Effective R-Value of the ICF wall: R-23
- Total energy input, wood-framed wall: 346 BTU/hr
- Total energy input, ICF wall: 177.6 BTU/hr
Wrapping It Up
The time it takes a wall to reach the steady-state is an essential factor in a building’s energy performance. The longer it takes, the less load is imposed on the building’s conditioning systems.
Thanks to their 2 layers of continuous insulation, airtightness, and high thermal mass, ICF walls like Element ICF take up to 36 times longer to reach the steady-state.
This means heating and cooling systems in ICF buildings can stay inactive for that much longer when outdoor temperatures suddenly change.
