Thermal Bridges and Ventilation Losses — Why Your Heat Loss Calculator Underestimates

You enter your wall thickness, insulation type, and window area into an online calculator. The result says 5,500 kWh per year. Your actual heating bill says something closer to 9,000–10,000. Where does the difference come from?

Most heat loss calculators — including simplified versions of ours — only calculate transmission losses: heat escaping through walls, roof, floor, and windows. This is important, but it's only part of the picture. Two major components are usually missing: ventilation losses and thermal bridges.

What most calculators actually calculate

The basic formula is straightforward: Q = A × ΔT × t / R. Take the area of each building element, the temperature difference between inside and outside, the heating season duration, and divide by the thermal resistance. This gives you transmission heat loss— heat flowing through the building envelope by conduction.

For a typical private house in Latvia (120 m², aerated concrete or brick), this calculation gives roughly 5,000–6,000 kWh/year. But this is only about 55–65% of the actual total heat loss.

Ventilation losses — the invisible 30–40%

Every building needs fresh air. In most Latvian private houses, this happens through natural ventilation: air enters through window gaps, trickle vents, or simply opened windows, and exits through exhaust ducts in the kitchen and bathroom. This fresh cold air needs to be heated to room temperature — and that takes energy.

The formula (per EN 12831):

V = heated volume (m³), n = air change rate (h¹), 0.33 = volumetric heat capacity of air (Wh/m³·K), ΔT = temperature difference (°C), t = heating season (hours)

Let's calculate for a typical house:

That's 5,150 kWh— almost as much as the entire transmission loss through all walls, roof, and floor combined. And most calculators don't show it at all.

Thermal bridges — where insulation breaks

A thermal bridge is any place where the insulation layer is interrupted, thinner, or bypassed by a material with higher thermal conductivity. In these spots, heat escapes much faster than through the surrounding wall.

Typical thermal bridges in a private house

• Window reveals— the wall might be 400 mm thick, but at the window frame it narrows to 50–80 mm
• Foundation junction— where the wall meets the foundation slab, insulation often stops or has a gap
• Balcony slabs— a concrete slab penetrating through the insulated wall acts as a direct heat conductor
• Building corners— geometric thermal bridges where the outer surface area is larger than the inner
• Lintels above windows— concrete or steel beams spanning the opening, often poorly insulated
• Roof-to-wall junction— common weak point where the roof structure meets the external wall

How much do they add?

LBN 002-19 specifies ψ_RM = 0.20 W/(m·K) for residential thermal bridges. In simplified calculations per EN ISO 14683, this translates to a surcharge of ΔU = 0.05 W/(m²·K) added to each opaque element's U-value.

For our 120 m² house, thermal bridges typically add 10–15% to transmission heat loss— roughly 600–900 kWh/year. In older uninsulated houses or buildings with many balconies, this can be even higher.

The real picture — an example

Let's put it all together for a 120 m² aerated concrete house in Rīga:

A calculator that only shows transmission losses says 5,500 kWh. Reality is closer to 11,450 kWh— more than double. At €0.08/kWh, that's €440 vs €916 per heating season.

Why this matters especially for private houses

In apartment buildings, ventilation losses are shared across all units and the building structure is relatively compact (less surface per volume). Private houses have a worse situation:

• Higher surface-to-volume ratio— a detached house loses heat through all four walls, roof, and floor. An apartment only through one or two external walls
• More thermal bridges— more corners, more window openings relative to floor area, foundation perimeter exposed on all sides
• Natural ventilation dominates— most private houses in Latvia don't have mechanical ventilation with heat recovery. All warm air exits through exhaust ducts unrecovered
• You're the decision maker— unlike an apartment, you don't need 51% owner vote to improve. You can act on the data immediately

Three steps to reduce real heat loss

Check your house

Formulas based on EN ISO 6946 (thermal resistance), EN 12831 (ventilation heat loss), and EN ISO 14683 (thermal bridges). Thermal bridge surcharge per LBN 002-19. Climate data for Rīga per LBN 003-19 (likumi.lv/309453). Heating tariff €0.08/kWh. This article is for informational purposes only.