LESSON 3.5 — Thermal Comfort and Building Envelope Performance

A. Standard Map

B. Mechanism in Words

1. Thermal comfort is a psychological state — the mind’s assessment that the body is thermally neutral, neither too hot nor too cold.
2. The body gains heat from metabolism and solar radiation; it loses heat through convection, radiation, evaporation, and conduction. Comfort occurs when these gains and losses are balanced.
3. Building materials slow or accelerate this exchange. High-conductivity materials (metals) transmit heat rapidly; insulating materials resist the transfer.
4. The building envelope modulates the rate at which outdoor conditions reach the interior — thermal mass stores energy and releases it with a time lag; insulation reduces the amplitude of temperature swings.
5. Windows are both the weakest insulating element (lowest thermal resistance) and the primary solar gain aperture — their specification involves a trade-off between daylighting, solar heat gain, and heat loss.
6. Ventilation replaces stale air and removes heat and humidity; natural ventilation uses wind pressure and thermal buoyancy; mechanical ventilation uses fans; hybrid systems combine both.

C. Core Concept Explanations

C1. Thermal Comfort — Definition and Model

Definition (ASHRAE 55): “That condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation.”

The most widely used analytical model for thermal comfort is the PMV-PPD model developed by P.O. Fanger (1970).

• PMV (Predicted Mean Vote): Predicts the average thermal sensation of a large group of people on a 7-point scale from −3 (cold) to +3 (hot); 0 = neutral (comfortable).
• PPD (Predicted Percentage Dissatisfied): The predicted percentage of people dissatisfied with the thermal environment. At PMV = 0, PPD ≈ 5% (even in ideal conditions, ~5% of people are dissatisfied).
• ASHRAE comfort standard: PMV between −0.5 and +0.5 gives PPD < 10% — typically considered acceptable.

C2. Six Factors Affecting Thermal Comfort

Thermal comfort depends on four environmental parameters and two personal parameters:

Environmental (can be measured and controlled by the designer):

Personal (reflect occupant characteristics):

C1b. Adaptive Comfort Model

The adaptive comfort model applies to naturally ventilated and mixed-mode buildings where occupants can open windows, adjust clothing, and use fans. Unlike the fixed PMV band for fully conditioned spaces, acceptable indoor temperature tracks the outdoor running mean temperature.

C3. Comfort Indices

C4. Thermal Properties of Building Elements

Thermal Transmittance (U-value):

U-value measures the rate of heat transfer through a building element per unit area per unit temperature difference.

$$U = frac{1}{R_{total}} quad text{where } R_{total} = R_{si} + R_1 + R_2 + … + R_{so}$$

Thermal Time Lag (φ):

The time delay between peak outdoor temperature and peak indoor temperature due to thermal mass.

$$phi = text{time for temperature wave to propagate through wall}$$

• Lightweight construction: time lag = a few hours (1–4 hours)
• Heavy masonry (300 mm brick): time lag ≈ 8–12 hours
• Earth-sheltered (1 m compacted earth): time lag ≈ 30+ hours

Thermal Diffusivity (α):

A material property combining thermal conductivity (λ), density (ρ), and specific heat capacity (c_p):
$$alpha = frac{lambda}{rho cdot c_p}$$

High diffusivity = heat propagates quickly; Low diffusivity = heat propagates slowly (better for time lag).

C5. Solar Heat Gain Coefficient (SHGC)

SHGC is the fraction of incident solar radiation that passes through a glazing system (directly transmitted + absorbed and re-radiated inward).

ECBC 2017 SHGC limits by climate zone (for conditioned buildings):

C6. Window-to-Wall Ratio (WWR) and ECBC

WWR = Glazed area ÷ Total gross wall area (including glazing), expressed as a fraction or percentage.

ECBC 2017 prescribes maximum WWR for the fenestration of conditioned buildings. Excessive glazing increases both solar heat gain and conductive heat loss/gain, driving up energy use. ECBC sets WWR limits by climate zone.

C7. Thermal Bridges

A thermal bridge is a localised area of relatively high thermal conductance in an otherwise insulating building envelope — a path of low thermal resistance through which heat flows disproportionately.

C8. Ventilation Types and Rates

NBC 2016 Part 8 — Minimum ventilation requirements:

D. Worked Numerical(s)

D1. Parameter Quick Reference

D2. Worked Example 1 — Composite wall U-value (three layers)

Problem: A wall assembly (outside to inside): outer brick 50 mm (λ = 0.84 W/mK); insulation board 75 mm (λ = 0.04 W/mK); inner plasterboard 12.5 mm (λ = 0.16 W/mK). Surface resistances: R_so = 0.04 m²K/W; R_si = 0.13 m²K/W. Calculate U in W/m²K (two decimal places).

Solution:

R_total = 0.040 + 0.0595 + 1.8750 + 0.0781 + 0.130 = 2.1826 m²K/W

U = 1 / R_total = 1 / 2.1826 = 0.46 W/m²K

Answer: U ≈ 0.46 W/m²K — well-insulated wall (insulation layer dominates R_total).

D3. Worked Example 2 — ECBC compliance check (Hot-Dry zone, Jaipur)

Problem: A commercial wall in Jaipur (Hot-Dry) comprises: 20 mm external plaster (λ = 0.80); 200 mm brick (λ = 0.80); 50 mm EPS (λ = 0.035); 12 mm internal plaster (λ = 0.50). R_so = 0.04; R_si = 0.13 m²K/W. ECBC 2017 prescriptive limit for opaque walls in Hot-Dry = U ≤ 0.40 W/m²K. Does the wall comply? If not, find minimum EPS thickness.

Solution:

R_total = 0.040 + 0.025 + 0.250 + 1.429 + 0.024 + 0.130 = 1.898 m²K/W

U = 1 / 1.898 = 0.53 W/m²K → Non-compliant (0.53 > 0.40)

Required R_total = 1 / 0.40 = 2.500 m²K/W

Fixed layers (excluding EPS) = 1.898 − 1.429 = 0.469 m²K/W

Required EPS resistance = 2.500 − 0.469 = 2.031 m²K/W

Required EPS thickness = 2.031 × 0.035 = 0.071 m ≈ 71 mm

Answer: Current U = 0.53 W/m²K (fails). Need ≥ 71 mm EPS for compliance.

D4. Worked Example 3 — Roof U-value (four layers)

Problem: Roof assembly (outside to inside): 10 mm waterproofing (λ = 0.19); 100 mm RCC slab (λ = 1.60); 50 mm XPS insulation (λ = 0.033); 10 mm plaster soffit (λ = 0.72). R_so = 0.04; R_si = 0.13 m²K/W. Calculate U (W/m²K, two decimal places). ECBC Hot-Dry roof limit = 0.33 W/m²K — compliant?

Solution:

R_total = 0.040 + 0.053 + 0.063 + 1.515 + 0.014 + 0.130 = 1.815 m²K/W

U = 1 / 1.815 = 0.55 W/m²K → Non-compliant for Hot-Dry roof (0.55 > 0.33)

Answer: U ≈ 0.55 W/m²K — fails ECBC roof limit; increase insulation thickness or use lower-λ board.

E. Common Confusions

F. Exam Traps

G. Answer-Writing Cues

For U-value questions:

For ECBC compliance numericals:

For SHGC selection:

H. PYQ Linkage Note

I. Mini-Check — Lesson 3.5 (5 Questions)

Q1 (MCQ): The thermal comfort model based on Predicted Mean Vote (PMV) and Predicted Percentage Dissatisfied (PPD) was developed by:
(A) V. Olgyay (B) T.L. Mahoney (C) P.O. Fanger (D) T.R. Oke

A1: (C) P.O. Fanger, 1970 (Thermal Comfort: Analysis and Applications in Environmental Engineering). Olgyay (A) = bioclimatic chart 1963; Mahoney (B) = tabular climate analysis 1975; Oke (D) = Urban Heat Island energy balance 1982.

Q2 (MCQ): Which of the following statements about U-value is correct?
(A) A higher U-value indicates better thermal insulation
(B) U-value is measured in W/mK (thermal conductivity)
(C) U-value is the reciprocal of total thermal resistance; lower is better
(D) U-value and SHGC describe the same thermal property

A2: (C). U = 1 / R_total (W/m²K); lower U = better insulation. (A) reverses the relationship; (B) gives the unit for conductivity (λ), not U-value; (D) SHGC and U-value are independent properties.

Q3 (NAT): A wall has three layers: outer brick (50 mm, λ = 0.84 W/mK); insulation board (75 mm, λ = 0.04 W/mK); inner plasterboard (12.5 mm, λ = 0.16 W/mK). Internal surface resistance R_si = 0.13 m²K/W; external surface resistance R_so = 0.04 m²K/W. Calculate the U-value of the wall (W/m²K, to 2 decimal places).

A3:
– R_brick = 0.050 / 0.84 = 0.0595 m²K/W
– R_insulation = 0.075 / 0.04 = 1.875 m²K/W
– R_plasterboard = 0.0125 / 0.16 = 0.0781 m²K/W
– R_total = 0.13 + 0.0595 + 1.875 + 0.0781 + 0.04 = 2.1826 m²K/W
– U = 1 / 2.1826 = 0.46 W/m²K

Q4 (MCQ): Per ECBC 2017, the maximum permitted Solar Heat Gain Coefficient (SHGC) for fenestration in a hot-dry climate zone (at 40% WWR) is:
(A) 0.40 (B) 0.60 (C) 0.25 (D) No limit applies

A4: (C) 0.25. ECBC 2017 prescribes SHGC ≤ 0.25 for composite, hot-dry, and warm-humid zones. Cold zone has no limit. Temperate zone = 0.40.

Q5 (MCQ): For effective cross-ventilation in a room, openings must be provided on:
(A) The windward wall only (B) The south-facing wall only (C) Both windward and leeward walls (D) The ceiling level only via skylights

A5: (C) Both windward AND leeward walls. Cross-ventilation requires an inlet (windward) and an outlet (leeward) to create airflow across the space. Openings on one side only create single-sided ventilation — air mixing near the opening with limited penetration into the room depth.