Decrement Factor
The fraction of the outdoor temperature swing that still reaches the inside through a building element, expressed as a ratio from 0.0 to 1.0.
What is the decrement factor in building design?
The decrement factor is the fraction of outdoor temperature oscillation that penetrates through a building element to the indoor surface. Expressed as a decimal ratio between 0 and 1, it isolates the AMPLITUDE reduction effect separate from time delay. In summer, when outdoor temperatures swing from cool mornings to hot afternoons, a low decrement factor means the indoor surface temperature rises much less dramatically, preserving occupant comfort without active cooling.
For example, if outdoor air swings 20°C between 15°C (dawn) and 35°C (afternoon), a decrement factor of 0.3 means the interior surface temperature rises only about 6°C. This amplitude damping is the reason a 40 cm solid brick wall keeps you comfortable in a Slovak August while a thin timber-frame wall with poor mass rapidly overheats, regardless of insulation quality.
How does decrement factor relate to thermal lag and thermal mass?
Thermal mass is the material property: the capacity to absorb and store heat energy. Thermal lag is the resulting TIME effect: hours between absorption and release. Decrement factor is the AMPLITUDE effect: how much the swing is attenuated. All three describe the dynamic response of the building envelope, but each answers a different question.
Consider a 30 cm solid brick wall in summer:
- Thermal mass: high (density times specific heat capacity)
- Thermal lag: approximately 10 hours (peak heat delayed past sunset)
- Decrement factor: approximately 0.25 (outdoor 20°C swing becomes 5°C indoors)
Compare a 30 cm timber-frame wall with 20 cm mineral wool insulation and a 5 cm concrete inner leaf:
- Thermal mass: low (mostly insulation, thin concrete)
- Thermal lag: approximately 3 hours
- Decrement factor: approximately 0.75 (outdoor 20°C swing becomes 15°C indoors)
Both can have identical U-values (excellent winter insulation), yet offer opposite summer behaviour. This is why U-value alone is misleading for year-round comfort assessment.
Why is decrement factor invisible in winter but critical in summer?
In winter, steady-state conditions dominate: U-value predicts heat loss well because indoor temperature is stable and outdoor cold is relentless. Decrement factor plays almost no role. In summer, transient (time-varying) conditions take over: outdoor temperature oscillates daily, and the building's ability to resist that oscillation determines whether occupants experience comfort or overheating. This resistance means suppressing amplitude rather than just delaying the onset of heat.
A high-decrement-factor wall (thin timber frame, lightweight, high insulation) reduces heat transmission to near-zero in winter (good U-value) but offers no amplitude damping in summer. The indoor temperature swings almost as much as outdoors. A low-decrement-factor wall (heavy masonry) always transmits some heat but suppresses the daily swing, keeping indoors stable even as outdoor temperatures fluctuate wildly.
How do typical wall constructions compare in decrement factor?
| Wall Type | Typical Composition | Decrement Factor (Qualitative) | Thermal Lag (Hours) | Summer Behaviour |
|---|---|---|---|---|
| Solid brick masonry | 30 cm brick (fired clay) | 0.20–0.30 (very low) | 10–12 | Excellent damping; heat delayed to night; minimal indoor swing |
| Rammed earth or adobe | 35 cm compacted earth/clay | 0.15–0.28 (very low) | 12–14 | Outstanding comfort in dry climates; requires protection in wet regions |
| Concrete block (solid) | 25 cm solid concrete | 0.25–0.35 (low) | 8–10 | Good damping; effective for passive houses with shading |
| Aerated concrete (AAC) | 25 cm AAC plus 5 cm concrete plaster | 0.35–0.45 (medium-low) | 6–8 | Moderate damping; lighter than solid concrete; suitable for hybrid designs |
| Timber-frame with mass layer | 12 cm timber studs, 15 cm mineral wool, 5 cm concrete inner leaf | 0.50–0.65 (medium-high) | 4–6 | Poor amplitude damping; relies on night ventilation and shading to prevent overheating |
| CLT (cross-laminated timber) | 12 cm CLT plus 15 cm insulation plus 5 cm plaster | 0.55–0.75 (high) | 3–5 | Minimal damping; outdoor swings penetrate deeply indoors; requires active shading and ventilation |
| Lightweight insulated sandwich panel | 10 cm insulation core, thin facings | 0.80–0.95 (very high) | 1–2 | No damping; indoor temperature tracks outdoor; risk of daytime and nighttime overheating unless mechanical cooling deployed |
What role does material density and thickness play in decrement factor?
Decrement factor depends on three material properties working in concert: density, specific heat capacity, and thermal conductivity. The interplay is complex. Density increases both heat capacity (better energy storage) and thermal conductivity (faster heat diffusion). This diffusion is essential: heat must move through the material fast enough to reach the inner surface during the outdoor temperature swing.
High density materials like brick and concrete achieve the optimal balance: moderate conductivity (heat penetrates in hours, matching a day cycle) combined with high capacity (energy is absorbed, not just transmitted). Lightweight materials (timber, polystyrene) have low capacity and cannot suppress oscillation. Light but conductive materials (thin metal) transmit heat rapidly with no damping.
Thickness also matters: a thicker layer of the same material has lower decrement factor because the propagating heat wave experiences more damping over distance. Doubling thickness roughly halves the decrement factor qualitatively. A 50 cm brick wall damps better than a 30 cm brick wall of identical material.
How do material combinations affect decrement factor in practice?
| Inner Layer Material | Thickness | Outer Insulation | Expected Decrement Factor Range | Design Implications for Slovakia |
|---|---|---|---|---|
| Solid concrete | 25 cm | 15 cm mineral wool or EPS | 0.20–0.35 | Excellent for passive-house thermal regulation; standard in Bratislava construction; maintains comfort without mechanical cooling |
| Fired-clay brick | 30 cm | 12 cm mineral wool | 0.22–0.32 | Traditional Slovak approach; durable, breathable; good for renovation where exterior walls are accessible |
| Aerated concrete (AAC) | 30 cm | 10 cm EPS or PIR | 0.35–0.50 | Lighter alternative; easier to install; still good damping if blocks are minimum 35 cm and exposed indoors |
| Timber frame (studs) | 12 cm | 20 cm mineral wool plus 5 cm concrete inner | 0.55–0.75 | Modern eco-build trend; requires interior mass exposure (concrete floors) and aggressive shading; consider active cooling budget in southern exposures |
| CLT panels | 12 cm | 12 cm insulation on exterior, plaster finish | 0.60–0.80 | Rapid construction, minimal site labour; high embodied carbon lower than concrete; poor summer performance without mechanical cooling or exceptional shading discipline |
| No inner mass (all insulation) | n/a | 25 cm foam, thin air cavity | 0.85–1.0 | High risk; only viable with mechanical cooling, ERV summer bypass, or geographic isolation from high-temperature days |
Can decrement factor be improved by adding more insulation?
Paradoxically, adding insulation to the exterior does NOT improve decrement factor; it may worsen it. Exterior insulation (EPS, mineral wool) sits outside the thermal mass. Heat absorbed by the outer surface is largely blocked by the insulation and never reaches the inner mass layer to be dampened. The decrement factor remains determined by the inner mass; insulation only improves U-value (steady-state winter performance).
Placing thermal mass on the INTERIOR is far more effective for summer comfort. In a passive-house design with triple-glazed windows and external shading, interior mass (exposed concrete floors, masonry interior walls) damps the heat that does penetrate windows, preventing afternoon overheating. This is why passive-house guidance emphasizes both shading and interior thermal mass, not just external insulation.
What decrement factors are required in passive-house standards?
The Passive House Standard does not explicitly mandate a maximum decrement factor. Instead, it caps heating demand (less than 15 kWh/(m²·year)) and cooling/overheating hours (less than 10% of occupied time above 25°C), which implicitly sets expectations for dynamic performance. In practice, passive-house designs in Slovakia's continental climate typically achieve 0.25–0.45 decrement factors through solid concrete or brick inner structural layers, exposed interior thermal mass, triple-glazing, and external solar shading.
Lightweight construction (CLT, timber frame) can meet passive-house energy targets with excellent insulation and mechanical cooling, but relies on active systems rather than passive amplitude damping. This trades passive robustness for material and operational cost, making it less suitable for Slovakia's hot summers unless mechanical cooling is budgeted and maintained reliably.
Frequently asked questions
- What does a decrement factor of 0.3 mean?
- It means 30 percent of the outdoor temperature swing reaches the indoor surface. If outside temperatures swing from 15°C to 35°C (a 20°C swing), the inner surface experiences only a 6°C swing. A factor of 0.5 or lower provides effective summer damping in residential design.
- How does decrement factor differ from thermal lag?
- Thermal lag is TIME: how many hours the temperature peak is delayed. Decrement factor is AMPLITUDE: how much the swing is reduced. A 30 cm brick wall might delay heat by 8 hours (thermal lag) and reduce the swing to 30% (decrement factor 0.3). Both matter for summer comfort.
- Why do heavyweight walls stay cool in summer while timber frames overheat?
- Heavyweight masonry (brick, concrete) has high density, which increases specific heat capacity and thermal conductivity in the right balance. This yields both long thermal lag and low decrement factor. Timber-frame walls with thin mass layers have short lag and high decrement factor, allowing full outdoor swings to penetrate.
- Can U-value predict decrement factor?
- No. U-value measures steady-state heat flow and says nothing about dynamic behaviour. A 30 cm concrete wall and a 30 cm polyurethane core sandwich can have identical U-values (both 0.15 W/(m²·K)) but vastly different decrement factors. Concrete: 0.2; sandwich: 0.8. Dynamic performance requires material density and thickness, not just insulation level.
- What decrement factors should I aim for in residential construction?
- In passive houses with proper window design and shading, 0.3–0.4 is excellent. 0.4–0.5 is good; solid mass walls deliver this routinely. Above 0.6, summer comfort depends critically on night ventilation and low solar gains. Lightweight construction (0.8+) risks overheating unless combined with active cooling or very cool nights.
- How is decrement factor calculated or measured?
- It is derived from material properties (density, specific heat, thermal conductivity, thickness) through dynamic thermal modeling (EN ISO 52016-1 or ANSYS/Comsol simulation). Laboratory measurement is rare; instead, manufacturers publish values based on construction composition. Always verify for your actual wall buildup, not generic assumptions.