---
title: "Thermal insulation of walls, doors, windows, roofs"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:thermal-insulation-exposed-walls"
source: https://www.bctengineer.com/q/semester_6/building_technology/thermal-insulation-exposed-walls
site: BCT Engineering
---

# Thermal insulation of walls, doors, windows, roofs

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2075 Bhadra, 8 marks; 2069 Bhadra, 8 marks; 2068 Baisakh (old course), 8 marks (asked 3 times)
- Video: [Super Efficient Exterior Insulation Details with Rockwool (For Architects + Builders)](https://www.youtube.com/watch?v=VwUjkSjllPs) — ASIRI Designs
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/thermal-insulation-exposed-walls
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

What do you understand by thermal comfort / thermal performance of building components? Explain the various methods of thermal insulation for exposed walls, doors, windows and roofs with neat sketches.

## Short answer

Thermal performance is measured by U-value; lower U-value means better insulation. Exposed walls can be insulated with cavity walls, external or internal insulation boards, hollow/AAC blocks, or reflective surfaces. Doors and windows use double glazing, timber/uPVC frames, weather stripping and shading; roofs use terracing, insulation, false ceiling, ventilated double roof, reflective finish or green roof.

## Answer

**Thermal comfort** is a state in which a person feels neither hot nor cold. The **thermal performance** of a building component is its ability to resist heat flow, measured by its **U-value** (transmittance, W/m²°C) or its thermal resistance $R$ (m²°C/W). A low U-value means better performance. For a composite element

$$
U = \frac{1}{R_{si} + \sum \frac{t}{k} + R_{so}}
$$

#### Insulation of exposed walls
1. **Cavity wall**: Two leaves with a 50 mm air gap; the still air resists heat flow. Filling the gap with insulation (glass wool, thermocole, polyurethane foam) improves it further.
2. **Insulating material on the wall**: Boards of thermocole (EPS), cork or mineral wool fixed on the outer face (external insulation, best for using thermal mass) or the inner face.
3. **Hollow blocks / AAC / fly-ash bricks**: Walls built from low-conductivity units.
4. **Thick walls and reflective surfaces**: Light-coloured paint or lime wash reflects solar radiation.

```text
 Out |brick|air gap|insulation|brick| In
```

#### Doors and windows
- Use **double glazing** (two panes with a 6 to 12 mm air space) or low-E coated glass.
- Use timber or uPVC frames instead of metal, because metal conducts heat.
- Fit weather stripping to stop air leakage.
- **Shading**: chhajja, louvres, curtains, and trees on the east and west sides.
- Use insulated, solid-core flush doors.

#### Roofs
The roof receives the most solar heat, especially in the plains.
1. **Terracing / brick-bat coba** on the RCC slab, with 75 mm lime concrete.
2. **Insulation layer**: Thermocole, glass wool or foam concrete on the slab, covered by waterproofing.
3. **False ceiling** with an air space and insulation.
4. **Ventilated double roof**, or a roof over the slab to give shade.
5. **Reflective finish**: white china mosaic or white paint.
6. **Green roof or water spray / pond**: evaporative cooling.

```text
  Waterproofing + tiles
  Mud/lime concrete 75 mm
  Insulation (EPS)
  RCC slab
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A good thermal performance of a component means
   A. A low U-value
   B. A high U-value
   C. A low thermal resistance
   D. A high conductivity
   Answer: A. U measures heat transmittance; low U (high R) means better resistance to heat flow. (Key point: U-value)

2. How does a cavity wall reduce heat flow?
   A. Air in the gap moves quickly to carry heat
   B. The gap makes the wall thicker only
   C. Still air in the 50 mm gap resists heat flow
   D. It reflects all sunlight
   Answer: C. The still air gap resists heat flow, and filling it with insulation improves it further. (Key point: Cavity wall)

3. Which choice is better for windows?
   A. Single glass with metal frame
   B. Double glazing with timber or uPVC frames
   C. Metal frame with no stripping
   D. Large east and west glass
   Answer: B. Double glazing cuts heat flow, and metal frames conduct heat so timber or uPVC is preferred. (Key point: Frames and glazing)

4. Which is a roof insulation method from the answer?
   A. Bare concrete slab
   B. Dark bitumen finish
   C. Thin tin sheet
   D. Insulation layer such as thermocole on the slab under waterproofing
   Answer: D. An insulation layer on the slab, covered by waterproofing, along with reflective finish and false ceiling, reduces roof heat gain. (Key point: Roof insulation)

## Related in Building Technology

- [Thermal performance and building geometry](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-perf-geometry) — Building Technology, chapter 1
- [Purpose and sizing of doors and windows; panelled doors](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adoor-window-purpose-sizing) — Building Technology, chapter 6
- [Achieving thermal comfort, active and passive methods](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aachieve-thermal-comfort-active-passive) — Building Technology, chapter 1
- [Thermal comfort and its classification](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-comfort-classification) — Building Technology, chapter 1
- [General parts of doors and windows](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adoor-window-parts) — Building Technology, chapter 6, asked 2×
- [Human thermal comfort and thermal balance](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ahuman-thermal-comfort-balance) — Building Technology, chapter 1, asked 4×

## Related in other subjects

- [Two-room building: brick plinth, door shutters, plaster](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Abuilding-m-two-room-three-items) — Estimating and Costing, chapter 6
- [Bridge site factors and components with sketches](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-components-sketches) — Transportation Engineering II, chapter 5
- [Classes of insulation by thermal stability](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulation-thermal-classes) — Electric Machine Design, chapter 1, asked 4×
- [Steam plant components and performance improvement](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-components-performance) — Power Plant Equipment, chapter 7