Imagine this scene: it's February, you turn on the heating and — with perfect timing — your living room decides to imitate the North Pole. You, proud owner of bills as swollen as a balloon, wonder: “But where does all that heat go?” Don’t worry: you don’t need to dress like a polar explorer to find out. With a bit of observation, a few simple (even DIY) tests, and a pinch of logic, you can figure out where your house is losing heat, how much this loss is costing you, and which interventions — from immediate to radical — are most effective.
This article is practical, in-depth, and a bit sarcastic: I’ll explain causes, tests, concrete calculations, intervention priorities, and how to talk to the installer without sounding like a fool. Grab a cup of tea (hot, hopefully) and let’s proceed.
1. First of all: why heat loss is a serious matter (and not just because it makes your bill infuriating)
Heat loss = more energy consumed = higher bills + greater environmental impact. But it’s also:
Reduced comfort (that feeling your sofa is a block of ice),
Possible condensation and mold problems,
Oversized heating system to compensate for losses.
Understanding where and how you lose heat means first and foremost saving money and living better.
2. The most common points of heat loss (the list of culprits)
Old or poorly sealed fixtures (windows, French doors): often the main cause.
Uninsulated walls or with thermal bridges: single walls, uninsulated formworks.
Roof and attic: vertical loss that can exceed 25% of the total.
Floor above garage or basement: surfaces in contact with unheated spaces.
Utility boxes and walkways, pipe connections: small holes, big problems.
Blinds/roller shutters with uninsulated boxes: often ignored boxes become cold chimneys.
Worn frames and seals, drafts under doors.
Poorly designed MVHR or air vents: ventilation is necessary, but if not balanced it loses heat.
3. Quick tests (DIY, no expensive tools)
These tests tell you where to look; they don’t replace a professional survey, but are very useful.
Paper sheet test for drafts
Turn on the heater at home, bring a light sheet of paper or an extinguished flame close to window sashes, frames, and air vents. If the paper is drawn in or the flame bends, you have a draft.
Closed door test (simple but effective)
With the front door closed, drop a paper ball or tissue near the perimeter: if air comes in, seal it. It’s basic but often works on thresholds.
Listen to the house
In the dark and silence, run your hand near window joints: your skin senses air currents your eyes can’t see.
Cold water glass test
Place a glass with cold water near a wall (at night). If droplets or condensation form quickly, the wall is cold and there may be a risk of condensation/mold.
Wall-vs-hand test
Run your hand over the wall: if it’s colder than the others, it’s a point of heat loss. Not scientific, but often revealing.
4. Professional tests (when to call a technician)
If you want numbers and intervention priorities, call a professional for:
Infrared thermography
A thermal camera (in winter, with the system on and sufficient difference between inside and outside) clearly shows thermal bridges, losses through glass and frames, poorly insulated areas. It’s the fastest way to see the escaping heat.
Blower door test (airtightness test)
A fan-mounted door creates a vacuum in the house and measures unintentional air changes (air changes per hour — ACH). It helps to understand how much the house loses due to air infiltration and provides a quantitative value for design.
Thermography + complete energy audit
In addition to visual data, a technician can perform loss calculations, suggest targeted interventions, and estimate savings and payback times.
5. Understanding the numbers: a practical example (step-by-step calculations)
Let’s take a simple example to understand how much heat loss through a window can cost (yes, I know: you love numbers).
Simplified situation:
Window: glass surface = 2 m².
Thermal transmittance coefficient of the glass (U) = 2.8 W/m²K (single glass or old double glazing with high U).
Temperature difference between inside and outside (ΔT) in winter = 20 K (e.g. 20 °C inside, 0 °C outside).
Heating operation hours per day = 10 hours.
Days of heavy use per month = 30 days.
Hypothetical electricity/gas price per kWh = 0.25 €/kWh (use your rate for accuracy).
Calculation of power lost (W) through the window:
U × A × ΔT = 2.8 × 2 × 20.
Digit-by-digit calculation: 2.8 × 2 = 5.6. 5.6 × 20 = 112 W.
So the window loses 112 W (that is, 0.112 kW) continuously with that ΔT.
Energy lost in a day (kWh):
2. Power (kW) × hours of operation = 0.112 kW × 10 h = 1.12 kWh/day.
Calculation: 0.112 × 10 = 1.12.
Energy lost in a month (30 days):
3. 1.12 kWh/day × 30 days = 33.6 kWh/month.
Calculation: 1.12 × 30 = 33.6.
Monthly cost of that window:
4. 33.6 kWh × 0.25 €/kWh = 8.40 €/month.
Calculation: 33.6 × 0.25 = 8.4.
So, a single window in these conditions can cost you about 8.40 € per month (and beware: if you have 6-8 windows the bill goes up). This is a simplified example: in a real house there are many other variables, but it gives you the idea.
6. Intervention priorities: where to act first
Not everything is worth the same: to maximize comfort and savings, follow this priority:
Eliminate drafts (seals, door thresholds) — low-cost intervention, immediate payback.
Insulate the attic (if not already done) — excellent cost/benefit ratio.
Replace or improve fixtures (low-emissivity glass, thermally broken frames) — medium investments, big effect.
Insulate external walls (external insulation or from the inside) — structural intervention but very effective.
Insulate floor above unheated space and floors if necessary.
System interventions: thermostatic control, thermostatic valves, zoning, metering — improve the efficiency of what you already have.
Advanced solutions: replace boiler with heat pump, integrate photovoltaic + batteries — consider after reducing losses.
7. Practical interventions and solutions — from the simplest to the most structural
Quick and inexpensive actions (often DIY)
- Seal drafts with adhesive seals or silicone.
- Replace door sweeps.
- Heavy curtains or temporary insulating panels for the night.
- Thermal films for glass (temporary solution for old glass).
- Medium-cost improvements
- Replacement of window seals and frame adjustment.
- Double glazing or low-emissivity glass.
- Attic insulation with panels or blown-in insulation.
- Insulation of roller shutter boxes.
- Structural interventions (higher costs, strong results)
- External or internal thermal insulation.
- Complete replacement of fixtures with high-performance frames (low U).
- Insulation of floor and floors.
- Thermal design of the building (audit + targeted interventions).
8. The question of costs and payback: a couple of realistic considerations
Low-cost interventions (seals, cheap attic insulation) often pay back in 1–3 years.
New fixtures: payback in 5–15 years depending on climate zone, energy rates, and incentives.
Thermal insulation: high initial cost, but significant savings, payback often in 10–20 years; tax incentives can greatly reduce payback time.
Practical advice: ask for multiple quotes, compare the cost per m² insulated, and consider tax or local incentives (if available).
9. Common mistakes (and how not to make them)
Insulating without fixing drafts: you improve the walls but leave the windows broken? Makes little sense.
Poor ventilation sizing: insulation without ventilation leads to mold.
Choosing cheap and poorly installed materials: installation matters as much as the material.
Patchy work: fragmented interventions may not give the desired results; follow a plan.
10. Bonus: how to talk to the installer (useful phrases, not dumb ones)
- “Can you show me the thermal design data and heat loss calculations?” — yes, it’s your right.
- “Did you consider thermal bridges and roller shutter boxes?” — often ignored.
- “Could you include thermography or blower door in the quote?” — very useful for prioritizing.
- “What is the estimated payback time for this intervention?” — we want numbers, not promises.
- “What materials will you use and what is their certification?” — avoid those who answer vaguely.
11. Quick checklist (print and take with you when evaluating the house)
- Check for drafts at windows and doors (paper/smoke test).
- Inspect roller shutter boxes and blinds.
- Check attic insulation (presence and thickness).
- Check walls for cold spots and mold.
- Evaluate frames: single glass? double? seals?
- Check floor above unheated spaces.
- Consider thermography or blower door if you want quantitative data.
- Request quotes with estimated savings calculation and payback times.
- Check available incentives and deductions.
12. Don’t wear your down jacket forever (unless it’s fashionable)
Understanding if your house is losing heat is the first step to spending less, living better, and avoiding your sofa becoming the coldest place on earth. Start with the simple tests — the ones you can do right away — and then move on to professional analyses if you want numbers and priorities. Solutions exist, and often don’t require miracles: just attention, planning, and good craftsmen.
