Chapter 6: Insulation and Passive Design

The Most Important Investment

If you take away one message from this entire book, it’s this: insulation is the highest-ROI investment you can make for an autonomous home. Every euro spent on insulation reduces the cost and complexity of every other system — heating, solar, batteries, wood supply.

The Impact of Insulation

Insulation paid for itself, then generates savings for 30–50 years.

Heat Loss Breakdown

Understanding where heat escapes guides investment priorities:

Typical Heat Loss Distribution (Uninsulated House)

Target U-Values for Autonomous Homes

The U-value is thermal transmittance: lower = better insulation. Units are W/(m²·K).

Insulation Materials

Comparison Table

Choosing the Right Material

• New build: Wood fiber or straw bale for eco-build; mineral wool for budget
• Exterior wall renovation: EPS or mineral wool (ETICS system)
• Interior wall retrofit: PIR (thin) or mineral wool (budget)
• Attic/roof: Blown cellulose (quick, cheap) or mineral wool batts
• Floor: XPS (moisture-resistant) under slab

The Thermal Envelope

The “thermal envelope” is the continuous insulated boundary around your living space. Breaks in this envelope (thermal bridges) can account for 10–30% of heat loss.

Common Thermal Bridges

Key principle: Insulation must be continuous — like a sleeping bag with no gaps. Any break lets heat pour out.

Airtightness

Even well-insulated houses lose heat through air leaks. In an old house, air infiltration accounts for 20–25% of heat loss.

Measuring Airtightness

Airtightness is measured by a blower door test (depressurization test), expressed as n₅₀ (air changes per hour at 50 Pa pressure difference):

Where Air Leaks

• Around windows and doors (30–40%)
• Electrical outlets on exterior walls (15–20%)
• Pipe and cable penetrations (10–15%)
• Attic hatches and access panels (10–15%)
• Wall-floor-ceiling junctions (10–20%)

Sealing Solutions

Blower door test cost: €300–600. Well worth it to identify leaks.

Passive Solar Design

Passive solar design uses building orientation and materials to capture, store, and distribute solar heat — no mechanical systems required.

Principles

1. South-facing glazing: Large windows on the south facade capture winter sun
2. Thermal mass: Heavy materials (concrete, stone, tile) store daytime heat and release it at night
3. Overhang design: Roof overhangs or awnings shade south windows in summer (when sun is high) but allow sun in winter (when sun is low)
4. Minimize north glazing: North-facing windows lose heat with no solar gain

Glazing Ratios

Solar Gain Calculation

On a clear winter day in central France, south-facing windows receive:

• December/January: ~2–3 kWh/m² of glass per day
• March/October: ~3–4 kWh/m² per day

For 10 m² of south-facing triple glazing (g-value 0.5): \(\text{Solar gain} = 10 \times 2.5 \times 0.5 = 12.5 \text{ kWh/day}\)

This is significant — 12.5 kWh of free heat is equivalent to burning ~7 kg of wood in a stove.

Thermal Mass Sizing

Water is the best thermal mass per volume — some passive solar homes use water walls or tanks behind south-facing glazing.

A 15 cm thick concrete floor slab (120 m²): \(E = 120 \times 0.15 \times 2{,}300 \times 0.88 \times 5 / 3{,}600 = 50.6 \text{ kWh (for 5°C swing)}\)

This stores enough heat for an entire night in mild weather.

Ventilation: The Insulation Partner

A well-sealed house requires mechanical ventilation to maintain air quality. This is not optional — it prevents moisture buildup, mold, and CO₂ accumulation.

Ventilation Systems

Heat Recovery Impact

A double-flow VMC with 85% heat recovery on a 120 m² well-insulated house:

Payback for double-flow VMC: 6–15 years depending on heating source.

Earth Sheltering and Bioclimatic Design

Earth-Sheltered Construction

Building partially or fully underground leverages the earth’s stable temperature (10–14°C year-round in France at 2 m depth):

Cost premium: 20–40% over conventional construction, mainly for waterproofing and structural reinforcement.

Earth Tubes (Canadian Wells / Puits Canadien)

Bury 30–50 m of 200 mm pipe at 1.5–2 m depth. Air drawn through the tube is:

• Pre-heated in winter (from -5°C to +5–10°C)
• Pre-cooled in summer (from 35°C to 18–22°C)

Renovation Priority Order

If renovating for autonomy, this is the optimal investment order:

Note: Do steps 1–3 before investing in a new heating system. The reduced heat loss often means you can install a smaller, cheaper heating system.

The Passive House Standard

The gold standard for energy-efficient buildings:

A 120 m² passive house needs only 1,800 kWh/year for heating — achievable with:

• A small 3 kW wood stove (burned occasionally)
• Or a tiny heat pump
• Or even just the heat from cooking, appliances, and body heat + solar gains

Passive house cost premium: 10–20% over standard construction, but heating costs approach zero.

📊 Quick Reference — Insulation Investment:

Every step halves the heating need — and therefore halves the wood, solar panels, and batteries required.

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