Home Heat Loss Diagram: Where Your Money Is Escaping

Your home is constantly losing heat. Through the roof, walls, windows, floor, and every gap and crack in the building fabric, warmth flows from inside to outside. Understanding where that heat goes — and how much it costs — is essential for making smart decisions about insulation, heating, and whether a heat pump is right for your home.

This visual guide quantifies heat loss from every part of a typical UK home, shows how insulation changes the picture, and explains the financial impact on both gas boiler and heat pump running costs. The data comes from the Energy Saving Trust, building physics research, and real UK home assessments.

The Heat Loss Overview

In a typical uninsulated UK home (pre-1980s construction, no retrofit insulation), heat escapes through five main routes:

Percentages for an uninsulated detached home. Costs based on £1,100 annual gas heating bill. Source: Energy Saving Trust.

Through the Roof: 25% of Heat Loss

Heat rises, and in an uninsulated home, up to 25% escapes through the roof. Loft insulation is the single most cost-effective improvement available.

Loft insulation to 270mm is required for BUS grant eligibility (if recommended on your EPC). It is also frequently funded through ECO4 for eligible households. The payback period is typically 1-3 years — one of the best investments in home energy efficiency.

Through the Walls: 35% of Heat Loss

Walls represent the largest heat loss route in most homes. The solution depends on your wall type:

• Cavity walls (post-1930s): Cavity wall insulation costs £500-£1,500 and reduces wall heat loss by 60-70%. Often ECO4 funded. Quick installation (half a day).
• Solid walls (pre-1930s): External or internal wall insulation costs £5,000-£15,000 but reduces wall heat loss by 50-60%. More disruptive and expensive, but very effective.

For heat pump installations, wall insulation is particularly valuable because it reduces heat demand, allowing a smaller heat pump and lower running costs.

Through the Windows: 10% of Heat Loss

Windows are thin points in the building envelope. Their contribution to total heat loss depends on glazing type:

• Single glazing: Loses approximately 5.8 W/m²K — very poor
• Double glazing (standard): Loses approximately 2.8 W/m²K — reasonable
• Double glazing (A-rated): Loses approximately 1.4 W/m²K — good
• Triple glazing: Loses approximately 0.8 W/m²K — excellent

In a well-insulated home with good loft and wall insulation, windows become the weakest link — accounting for 30-35% of remaining heat loss.

Through the Floor: 15% of Heat Loss

Floors are often overlooked but contribute 10-15% of heat loss. Suspended timber floors can be insulated from below (£400-£800 DIY, £1,000-£2,000 professional). Solid concrete floors are harder to insulate but lose less heat than suspended floors. Rugs and carpet provide modest improvement on any floor type.

Through Draughts: 15% of Heat Loss

Draughts — uncontrolled air leakage through gaps around windows, doors, letterboxes, pipework holes, and floorboard gaps — account for approximately 15% of heat loss. Draught-proofing is the cheapest insulation measure (£100-£300 for a full house) and provides immediate comfort improvement alongside energy savings.

The Insulated Home: A Different Picture

When a home has good loft insulation, cavity wall insulation, double glazing, and draught-proofing, the total heat demand drops by 40-60% and the proportions shift dramatically:

In an insulated home, windows and ventilation become the dominant heat loss pathways. This is where MVHR systems and high-performance glazing deliver the next level of efficiency improvement.

Why Heat Loss Matters Especially for Heat Pumps

Heat loss matters for any heating system, but it matters even more for heat pumps for two specific reasons:

1. Heat demand determines heat pump size and cost. A well-insulated home needs a smaller heat pump (7-8kW vs 10-12kW), which costs less to buy and install. The cost difference can be £1,000-£2,000.

2. Lower heat demand enables lower flow temperatures. A well-insulated home can be heated with 35-40°C flow temperature instead of 45-50°C. Lower flow temperatures mean higher COP (efficiency), which means lower running costs. The difference can be 15-25% on annual electricity bills.

Investing in insulation before or alongside a heat pump installation delivers a double benefit: the insulation saves energy directly, and it also makes the heat pump work more efficiently. Combining insulation with a heat pump and solar panels creates the most cost-effective home energy system available.

Fix Priority Order: Best Return on Investment

Frequently Asked Questions

Where does most heat escape?

In uninsulated homes: walls (35%), roof (25%). In insulated homes: windows (30-35%) and ventilation (20-25%). Total heat loss drops 40-60% with good insulation.

How much does poor insulation cost me?

£350-£500/year extra on a typical gas heating bill. With a heat pump, poor insulation also reduces efficiency, adding further costs.

What is the cheapest improvement?

Draught-proofing (£100-£300) with 1-2 year payback, followed by loft insulation (£300-£500) with 1-3 year payback. Both are excellent value.

Should I insulate before getting a heat pump?

Yes — it reduces heat pump size needed, enables lower flow temperatures, and improves efficiency. The BUS grant requires EPC-recommended insulation.

How do I find out where my home loses heat?

Your EPC shows insulation levels. A heat pump installer conducts a full heat loss survey. Thermal imaging provides visual evidence of problem areas.

Do windows really matter?

In well-insulated homes, windows are the biggest heat loss point (30-35%). Upgrading from single to double glazing saves 10-15% of total heat loss.