R-value has become a modern insulation fairy tale.
It is a number that has been repeated so often that it has achieved an almost unquestioned status. Yet on its own, the R-value is a largely meaningless figure when used to describe how insulation actually performs in real buildings.
Why R-Value Alone Doesn’t Define Insulation Performance
It is impossible to define the effectiveness of an insulation system using a single number. To do so ignores critical real-world factors such as air movement, moisture, installation quality, and long-term performance.
This short video explains why R-values alone don’t reflect real-world insulation performance.
Why the R-Value Myth Persists
The continued reliance on R-values largely favours fibre insulation products. Under laboratory conditions, fibre insulation can achieve respectable R-values. However, those conditions bear little resemblance to the environment inside most buildings.
Consider what happens when insulation is exposed to:
- wind
- moisture
- air movement
- real temperature swings
In these situations, the effective R-value of fibre insulation can drop dramatically, in some cases approaching zero. Solid insulation materials, by contrast, are far less affected by these conditions.
One Number Cannot Describe Performance
No rational person would buy a property knowing only one of its dimensions.
If someone offered land for sale and described it only as “a seven,” you would immediately ask:
- Seven acres?
- Seven square metres?
- Seven miles square?
- Seven stories?
Without context, the number is meaningless. The same applies to insulation.
Yet building codes mandate R-values of 20, 30, or 40 without addressing how insulation performs once installed in a real structure.
Laboratory Testing vs Real-World Conditions
R-values are determined using an ASTM (American Society for Testing and Materials) test. While this test was intended to provide meaningful comparisons, it contains inherent bias.
The test favours fibre insulation products such as:
- fibreglass
- rock wool
- cellulose
It does not account for:
- air movement
- wind pressure
- water vapour
- moisture saturation
Fibreglass insulation may achieve an R-value of approximately 3.5 per inch in a laboratory. However, that value assumes zero wind and zero moisture. These conditions simply do not exist in real buildings.
Even small amounts of moisture can reduce a fibre insulation’s R-value by 50 percent or more.
Air Movement: The Hidden Energy Thief
Most buildings leak air.
Even when doors and windows are closed, the average home has air leaks equivalent to the size of an open door. This air movement carries heat with it, undermining insulation performance.
To demonstrate this, hold a lit candle near an electrical outlet on an external wall during windy conditions. The flame will flicker or extinguish entirely.
Fibre insulation allows air to:
- pass through it horizontally
- circulate vertically within wall and ceiling cavities
These internal convection currents continuously move warm air upward, allowing heat to escape unnoticed.
Vapour Barriers and Moisture Traps
TFibre insulation relies heavily on correct vapour barrier placement. Incorrect installation can lead to severe moisture problems.
In some cases, vapour barriers are installed on both sides of the insulation. This traps moisture inside the wall cavity, where it accumulates over time. The result can be:
- saturated insulation
- structural damage
- mould and mildew growth
Fibre insulation requires ventilation on at least one side to function effectively. Ventilation, however, introduces air movement, which further reduces insulation performance.
Solid Insulation Systems
Solid insulation materials include:
- expanded polystyrene (EPS)
- cork
- foam glass
- polyisocyanurate and polyurethane board stock
These materials perform better than fibre insulation in the presence of wind or moisture. However, most solid insulation boards suffer from a critical flaw: air infiltration around the edges.
Unless board stock insulation is fully bonded and sealed, air can move behind it, rendering it far less effective than its R-value suggests.
Why Spray-In-Place Polyurethane Is Different
Spray-in-place polyurethane insulation is unique among solid insulation systems.
When properly applied, it:
- expands in place
- bonds directly to substrates
- fills cracks, gaps, and voids
- creates a continuous air seal
This seal is critical. In many buildings, heat loss is driven more by air leakage than by conductive heat transfer.
R-values become irrelevant if air can bypass the insulation.
Case Studies and Real-World Performance
Throughout the 1970s, many homes and commercial buildings were insulated with relatively thin layers of spray-in-place polyurethane foam. Despite having lower nominal R-values than fibre insulation, these buildings consistently demonstrated:
- dramatically reduced heating and cooling costs
- improved comfort
- superior long-term performance
In multiple documented freezer and cold storage projects, spray-in-place polyurethane insulation reduced energy demand so significantly that:
- fewer compressors were required
- operating costs were cut by more than half
- system performance exceeded engineering expectations
These results were repeatable and consistent across projects.
While some may refer to this as anecdotal evidence, it reflects real-world outcomes observed over decades.
Heat Loss vs Surface Temperature Control
Insulation serves two distinct purposes:
- Reducing heat loss
- Controlling interior surface temperature
Heat loss can often be controlled with relatively modest insulation thickness. Surface temperature control, however, frequently requires additional insulation to prevent condensation and moisture-related issues.
This distinction is critical in applications such as:
- cold storage
- underground housing
- high-humidity environments
In many cases, increasing insulation thickness has little impact on heat loss but a profound effect on surface temperature stability.
Thermal Mass and Heat Sinks
Materials such as concrete, brick, earth, and water act as heat sinks. When insulated correctly, these materials can significantly stabilise interior temperatures through a property known as thermal diffusivity.
Spray-in-place polyurethane insulation applied to heat sinks can effectively double the apparent R-value by moderating temperature swings over time.
This is why structures such as adobe buildings and monolithic domes perform exceptionally well despite relatively low nominal R-values.
The Bottom Line on R-Values
R-values can be useful indicators, but only when:
- installation conditions are defined
- air movement is controlled
- moisture exposure is considered
Without these factors, R-value comparisons are misleading.
One inch of properly installed spray-in-place polyurethane insulation can outperform multiple inches of fibre insulation under real-world conditions.
R-value tables should include equivalent performance under actual installation scenarios. Until they do, R-values will remain an incomplete and often misleading metric.
Final Thoughts
Insulation performance cannot be reduced to a single number.
Effective insulation requires:
- air sealing
- moisture control
- correct installation
- long-term stability
When these factors are addressed, insulation systems perform as intended. When they are ignored, even high R-values fail to deliver real comfort or energy efficiency.
Adapted from
The Polyurethane Foam Book by David B. South