Thermal Comfort

What are the six factors that determine thermal comfort?

Thermal comfort is not the air temperature shown on a thermostat. It is the product of six interacting factors: four environmental and two personal. Four are fixed by the building design and operation: air temperature, mean radiant temperature (the average surface temperature of surrounding surfaces), air velocity (air movement), and humidity. Two depend on the occupant: clothing insulation (measured in clo units) and metabolic rate (measured in met units, where 1 met = 100 W/m2 of body surface area).

All six must be considered together. A room at 22°C with single-glazed windows, low humidity, still air, appropriate clothing, and normal activity can feel uncomfortably cold. The same room at 20°C with triple-glazed windows, moderate humidity, gentle air circulation, and identical clothing and activity can feel comfortably neutral. The difference is mean radiant temperature and air velocity, not the air temperature alone.

Which factor explains most real comfort complaints in well-built homes?

Mean radiant temperature. In winter, a person sitting next to a single-glazed window or poorly insulated exterior wall loses heat by radiation to that cold surface. The body radiates longwave energy toward the cold glass or wall, and because the surface temperature is low (often 10–15°C below room air temperature), the radiant heat loss is significant. The occupant feels chilled despite adequate air temperature and cranks the thermostat upward, paying higher energy bills without gaining comfort.

This is why thermal insulation, especially window upgrades and envelope sealing, raises comfort at a lower air temperature. Triple-glazed windows with low-emissivity coatings keep the innermost glass surface warm, reducing radiant heat loss and eliminating the cold-surface effect. A well-insulated exterior wall does the same. The practical payoff is stark: the same occupant in the same room, clothed identically, with the thermostat set 2–3°C lower, feels equally comfortable because mean radiant temperature is higher.

In summer, the mirror image occurs. A poorly shaded roof or south-facing facade radiates heat back into the room at night and early morning, creating discomfort even with air conditioning running and windows closed. This is why external shading is critical in continental climates like Slovakia's.

How do the six comfort factors compare in practical importance?

Factor	Type	Typical Range	Why It Matters
Air temperature	Environmental	18–26°C	Primary comfort driver, but alone insufficient; must pair with mean radiant temperature
Mean radiant temperature	Environmental	18–26°C ideally; 15–28°C acceptable	Dominates winter discomfort; within 3–4°C of air temperature in most spaces
Air velocity	Environmental	<0.2 m/s at head height in winter; up to 0.6 m/s in summer	Low velocity prevents draught; moderate velocity aids summer cooling
Relative humidity	Environmental	30–60%	Outside range causes discomfort; <20% dries skin, >70% feels sticky and risks condensation
Clothing insulation (clo)	Personal	0.5–1.5 clo typical indoor; 1 clo ≈ light jacket	Occupants adapt by changing clothes; affects comfort expectation by ±3°C
Metabolic rate (met)	Personal	1 met (seated office work) to 5 met (vigorous exercise)	Higher activity generates more internal heat; raises comfort temperature by up to 5°C

What is the PMV/PPD model and why is it limited?

The PMV/PPD Comfort Index is a standardized model for predicting comfort. PMV (Predicted Mean Vote) is a numerical scale ranging from -3 (too cold) to +3 (too hot), with 0 representing thermal neutrality. It is based on operative temperature, which combines the effects of air temperature and mean radiant temperature. PPD (Predicted Percentage Dissatisfied) translates a PMV value into the expected percentage of occupants who will report discomfort.

Standards such as ISO 7730 and ASHRAE 55 recommend a PMV between -0.5 and 0.5 (nearly neutral) to keep PPD below 10 percent, meaning at least 90 percent of occupants are satisfied. Some standards allow a slightly wider range of -0.7 to 0.7 in existing buildings, accepting up to 15 percent dissatisfaction.

The honest limitation: even at optimum conditions (PMV = 0), approximately 5 percent of occupants will still be dissatisfied. This baseline dissatisfaction exists because individuals differ in metabolism, clothing choice, local air exposure, and thermal history. A designer cannot achieve "comfort for everyone" by targeting PMV alone. The model is a practical guide, not a guarantee.

Standard	Building Type	PMV Range	Max PPD (%)
ISO 7730	New buildings	-0.5 to 0.5	<10%
ISO 7730	Existing buildings	-0.7 to 0.7	<15%
ASHRAE 55	All buildings	-0.5 to 0.5	<10%
ISO 7730	Hard limit (extreme)	-2 to 2	<20%

What are local discomfort effects and why do buildings with good overall comfort still generate complaints?

Local discomfort effects are complaints about specific body parts, even when whole-body thermal balance is neutral. They are common in well-designed buildings.

Main types: draught (unwanted air movement), vertical temperature stratification (cold ankles, warm head), cold floors (below 16–18°C), and radiant asymmetry (radiative heat loss to cold surfaces). A passive house can still fail if a bedroom has an unshaded window (radiant asymmetry), or poor perimeter insulation (cold floor), or ventilation creates draught at head height.

How does the adaptive comfort model differ from the PMV/PPD model?

The adaptive comfort model, supported by field studies in naturally ventilated buildings, recognizes that occupants adapt to their environment. In summer, people in naturally ventilated buildings expect higher temperatures and feel comfortable at ranges where an air-conditioned building would be too warm.

The model correlates acceptable indoor temperature to outdoor temperature. In cool months, occupants may feel comfortable at 21–22°C. In warm months, the same occupants may feel comfortable at 26–27°C. This flexibility reflects adaptation and seasonal expectation. The model applies to naturally ventilated and hybrid buildings, not fully air-conditioned ones, explaining why Slovak family homes with operable windows tolerate wider ranges.

Why is summer overheating now as significant as winter cold in Slovakia?

Slovakia's continental climate brings cold winters and increasingly hot summers. Better insulation for winter can trap heat in summer if the building lacks adequate shading, ventilation, or thermal mass. Summer overheating in a passive house can occur if the designer neglects solar gains through south-facing glazing or roof heat absorption.

Insulation itself does not create heat; the solution is to design passive cooling: external shading on south and west windows, night ventilation when outdoor temperature drops below indoor, and thermal mass (concrete floors, masonry walls) to buffer temperature swings. Summer comfort in a well-insulated house requires as much attention as winter comfort.

What do Slovak building norms require for thermal comfort?

Slovakia applies STN 73 0540, which sets minimum U-values for the building envelope. Compliance improves thermal comfort by raising mean radiant temperature and reducing cold surface effects. However, the standard does not prescribe PMV or PPD targets, nor does it address local discomfort effects or summer overheating. Meeting U-value limits is necessary but insufficient; a builder can still create discomfort if windows are unshaded in summer, if ventilation is inadequate, or if details like thermal bridges and cold floors are neglected.