Lesson 6.1Lesson 6.1 · The Envelope as a System
U-Values and Wall Assemblies
A wall is a stack of resistances that add up — and the whole stack collapses to one number that decides how much heat it lets through.
Five modules of intuition become arithmetic
We've used "U-value" loosely since 5.2 — lower is better, insulation drops it, the roof needs it most. Now make it exact. The U-value is the single most important quantitative property of any wall, roof or floor, and once you can compute it you can compare any construction in any climate on a level field. The idea is simple: a wall is a series of layers, heat passes through every one in turn, and each layer resists the flow. Resistances in series add up — like layers of clothing, where vest + shirt + sweater + coat give the sum of all four. Add plaster, brick, insulation and finish, flip it over, and you have the U-value. The whole of building-fabric thermal physics in one sentence.
Specify every wall by two things, never one: its U-value AND its layer order.
Every layer resists; the resistances add
Every layer of a wall has a thermal resistance R = L/k, where L is its thickness and k its conductivity — and low k means a good insulator. A thick, low-k material gives high resistance; a thin, conductive material gives almost none.
The wall's total resistance is just the sum of all its layers, plus two fixed surface-film resistances — the still air clinging to the inside and outside faces:
Rtotal = Rinside-film + Rlayer1 + ... + Routside-film
The U-value is simply its inverse: U = 1 / R_total, the heat flow per square metre per degree of temperature difference. High total resistance gives a low U-value, which is good. One addition handles any wall and immediately shows which layer is doing the work — almost always the insulation, whose conductivity is ten to a hundred times lower than masonry's.
Why insulation is a different physics, not more wall
Look at what 100 mm of each common material actually resists. Dense concrete (k around 1.8) gives a resistance of roughly 0.06 — almost nothing. A burnt-clay brick (k around 0.8) gives about 0.12 — modest. An AAC block (k around 0.2) reaches 0.50 — genuinely useful. And EPS or mineral-wool insulation (k around 0.035) delivers about 2.9 — an enormous figure.
The lesson is brutal: 100 mm of insulation out-resists nearly two metres of concrete. A thin insulation layer transforms a wall, while doubling the brick barely helps — the conductivities live in different worlds. Insulation isn't "more wall"; it is a different kind of physics dropped into the assembly.
100 mm of insulation beats two metres of concrete. Reach for the insulation, not the trowel.
U-value tells you how much; layer order tells you how it behaves
The U-value tells you how much heat passes. But two walls with an identical U can behave completely differently, because of layer order.
Put the mass inside and the insulation outside, and the mass is coupled to the room: it stabilises the indoor temperature and, in a cold zone, stores solar heat — this is the "internal mass" of selective mass (4.3) and the cold store (5.1). Reverse it — insulation inside, mass outside — and the room heats and cools fast while the mass bakes uselessly outdoors.
So an assembly is specified by both its U-value (total resistance) and its layer order (how the mass behaves). This module's arithmetic meets the climate modules' placement intuition.
Three altitudes on the same idea
Read the band that fits you — or all three.
No maths needed, but the idea is powerful: a wall's ability to keep heat in or out comes mostly from one thin insulation layer, not from sheer thickness of brick or concrete. Adding insulation to walls and roof does far more than thicker masonry — and putting it outside a heavy wall lets the mass steady your indoor temperature.
When a builder or architect proposes a wall, ask for its "U-value." A lower number means lower bills and steadier comfort. Don't be talked into paying for extra brick thickness when a thin insulation layer would do far more for a fraction of the weight and cost.
Compute U = 1/ΣR for every assembly, including the surface films (Rsi ≈ 0.13, Rso ≈ 0.04 for walls) and any air gaps (around 0.15–0.18 for an unventilated cavity). Drive U down with a dedicated insulation layer, not added mass.
Place insulation externally on the mass in composite and cold zones — it couples the mass to the interior and kills thermal bridges — and put it generously on the roof always. Specify by U-value and layer order. Watch the junctions: a great centre-of-wall U is wasted if beams, columns and slab edges bridge straight through it. Then check the result against the ECBC / Eco-Niwas Samhita maxima (6.2).
Each layer's resistance is R = L/k (in m2K/W), and series resistances simply add, so the wall's total is Rtotal = Rsi + sum of layer R + Rso, and U = 1 / Rtotal.
Worked example — a 230 mm brick wall, plastered both sides. Inside film 0.13; plaster 15 mm (k 0.5) = 0.03; brick 230 mm (k 0.8) = 0.29; plaster 0.03; outside film 0.04. R_total = 0.52, so U = 1 / 0.52 ≈ 1.9 W/m2K — a typical bare Indian wall, and a poor one.
Now add 50 mm insulation (k 0.035), R = 0.05 / 0.035 ≈ 1.43. Rtotal = 0.52 + 1.43 = 1.95, so U ≈ 0.51 — a near-fourfold improvement from one thin layer. Double the insulation to 100 mm (R ≈ 2.86): Rtotal ≈ 3.38, U ≈ 0.30 — high-performance.
Note: doubling brick from 115 to 230 mm adds only ≈ 0.14 to Rtotal, so always check whether Rlayer > R of a thicker masonry alternative before you spend on mass. These are centre-of-wall values; real walls also lose heat through thermal bridges at junctions, so the in-situ U is always >= the calculated centre-of-wall U.
“A thick wall is a well-insulated wall — more brick or concrete means better thermal performance.”
Run the method yourself
Build a few walls in the calculator, then do one by hand, before the next lesson.
- 1Start from a bare 9-inch (230 mm) brick wall and read its U. Add 50 mm of insulation — by what factor does U drop?
- 2Build a wall with a U below 0.4 W/m²K. What is the minimum insulation needed, and how much does brick thickness actually matter to the result?
- 3By hand: inside film 0.13, brick 230 mm (k 0.8), 100 mm insulation (k 0.035), outside film 0.04. Work out R_total and U.
- 4Explain in one sentence why two walls with the same U-value can perform differently in a composite climate. (Think layer order and where the mass sits.)
↳ Use the worksheet below to record your answers.
Take it with you
The envelope reduced to one designable number
You can now compute a wall's U-value — but what should you aim for? Not taste: India's Energy Conservation Building Code (ECBC) and the residential Eco-Niwas Samhita set maximum U-values and glazing limits by climate zone, turning this lesson's physics into requirements. Next we decode the code — what it demands, how it varies across the five zones, and how to design a compliant envelope without losing the passive intelligence of the climate strategies.