R-value rates the insulation. Effective R-value rates the wall.
The gap between the two is larger than most people expect, and it is the number that actually decides your heating bill.
Two different numbers
R-value is the resistance to heat transfer through the insulation material alone. It is a simple way to rate a product, but it does not account for convection, meaning air movement through and around the insulation, and it does not account for radiation, meaning heat lost through walls, roof and windows. It describes the batt, not the building.
Effective R-value is the resistance to heat transfer taking into account all of the wall's material components, as well as convective heat loss and radiant heat loss. It describes the wall as built. This is the number to think about when comparing insulation for a new home.
Once insulation is installed in a wall system, its rated R-value becomes largely irrelevant. What matters from that point is what the assembly does.
The same insulation, three different results
Take R-20 fibreglass insulation. On the packaging it is R-20. Installed in a wood framed wall, its effective R-value is about R-16. Installed in a steel studded wall, it can fall as low as R-7. The insulation did not change. The wall around it did.
The reason is thermal bridging. Every framing member running from the warm side of the wall to the cold side is a path for heat to bypass the insulation entirely. Wood conducts heat reasonably well. Steel conducts it far better, which is why a steel studded wall loses so much of the rated value.
| Assembly | Rated | Effective | Why |
|---|---|---|---|
| Fibreglass batt, on its own | R-20 | R-20 | No framing, no bridging. This is the number on the bag. |
| In a wood framed wall | R-20 | about R-16 | Studs bridge from inside to outside, and wood conducts heat. |
| In a steel studded wall | R-20 | as low as R-7 | Steel conducts heat far better than wood, so each stud is a much stronger bridge. |
Always ask what framing factor a figure assumes
Framing factor is the percentage of a wall made up of framing rather than insulation. More framing means more thermal bridges and a lower effective R-value, so any credible effective R-value figure states the framing factor it was measured at. A number quoted without one cannot be compared with anything.
Typical residential walls fall somewhere between 14% and 25% framing depending on window and door openings, corners and structural requirements. The same wall assembly will measure noticeably differently at each end of that range.
Measured performance, Pacific SmartWall®
Independently measured by RDH Building Science Inc. at a 14% framing factor. At 23% framing the same assemblies measure R-18.2, R-23.3 and R-29.0.
The difference comes from routering the insulation into the framing rather than stuffing it between, which removes the bridges. See the full test data and run your own numbers.
R-value translates directly to money
The higher the effective R-value of your envelope, the less you pay to heat the house and to keep it heated, every year for as long as you own it. Insulation is a one-time cost. Heating is a recurring one. That asymmetry is the whole argument for getting the envelope right at the start.
Working within a budget, effective R-value should be a major consideration in planning a new home rather than a detail settled late. Wall depth is one of the few decisions that is straightforward to make early and expensive to change afterwards.
R-value questions
R-value is the resistance to heat transfer through the insulation material alone. It does not account for convection, meaning air movement through and around the insulation, or for radiation, meaning heat lost through walls, roof and windows. Effective R-value is the resistance to heat transfer taking into account all of the wall's material components as well as convective and radiant heat loss. Effective R-value is the number that matters when comparing wall systems.
Its rated R-value becomes largely irrelevant, because what you now have is a wall system rather than a batt of insulation. R-20 fibreglass installed in a wood framed wall gives an effective R-value of about R-16. Installed in a steel studded wall it can fall as low as R-7, because steel conducts heat far better than wood and every stud becomes a bridge.
Every framing member running from the warm side to the cold side of a wall is a path for heat to bypass the insulation. This is called thermal bridging, and it is why the framing factor, the percentage of the wall made up of framing rather than insulation, appears alongside any credible effective R-value figure. A wall tested at 14% framing will measure better than the same wall at 23% framing.
RDH Building Science measured effective R-21.1 for a 2x6, R-27.1 for a 2x8 and R-33.9 for a 2x10, all at a 14% framing factor. At 23% framing the same walls measure R-18.2, R-23.3 and R-29.0. A conventional 2x6 wall with fibreglass batt measures roughly R-13 effective.
R-value translates directly to money. The higher the effective R-value of the building envelope, the less you pay to heat the house and keep it heated, every year for as long as you own it. Because insulation is a one-time cost and heating is a recurring one, the arithmetic favours getting the envelope right at the start rather than improving it later.
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