How Much Heat Does an Old Window Lose — and What LBN 002-19 Requires (U ≤ 1.10)

Latvia's building standard LBN 002-19 sets a maximum heat transmittance coefficient for windows and balcony doors of U ≤ 1.10 W/(m²·K) for residential buildings and non-residential buildings, and 1.30 for production buildings. This article shows what that number means in kilowatt-hours, using four typical window types.

What the standard says about windows

• Table 3 of LBN 002-19, row “Windows and balcony doors”: 1.10 for residential houses, homes for the elderly, hospitals and kindergartens; 1.10 for non-residential buildings; 1.30 for production buildings (W/(m²·K)).
• The calculated U-value of an individual structure element shall not exceed the Table 3 maximum (paragraph 11).
• The window value is the calculated value for a whole standard window with the frame making up 30% of the total area, calculated under EN ISO 10077-1 — not the glass-only figure.
• The standard sets out the procedures for the energy-efficiency design of building envelopes for heated buildings to be newly erected, rebuilt and renewed (scope, paragraph 1.1 of the English translation).

Source: LBN 002-19, English translation on likumi.lv (checked 14 October 2026). This article does not determine whether a specific window replacement in an existing apartment falls under the standard.

Heat lost through one square metre of window

The calculator uses four typical window types. The table gives the modelled transmission loss per square metre over a Rīga heating season (indoor 21 °C, outdoor 1.1 °C, 4,608 hours), next to the LBN 002-19 limit:

U-values are typical figures based on EN 673 and manufacturer data used as defaults in this calculator, not the value of a specific product. Heat loss = U × ΔT (19.9 K) × 4,608 h per m². A double-glazed unit at the typical U of 1.4 is above the 1.10 limit.

A 602 series apartment: 8 m² of windows

The calculator's default 602 series panel apartment has 8 m² of old double-glazed windows (U 2.8) and 40 m² of wall. Same apartment, same climate, only the window type changed:

Going from the default old double glazing to windows exactly at the 1.10 limit is a difference of roughly 1,250 kWh per season in this model — about 61% less heat through the windows. For comparison, one square metre of this apartment's wall loses about 147 kWh per season, while one square metre of old double glazing loses about 257 kWh: the window loses roughly 1.7 times as much per square metre as the wall around it.

Euro figures use the Rīgas Siltums tariff published from 1 October 2026 (€80.96/MWh excl. VAT; see our tariff article). The winter tariff in Rīga is not yet known, so the kWh column is the more reliable one.

What this does not tell you

• It counts transmission through the glazed element only. Solar gains through the glass and air leakage around the frame are not included.
• The U-values are typical figures, not the value of a specific product on the market.
• It does not cover what a replacement costs or how long it takes to pay back — no sourced price data is used here.
• It does not determine whether a particular window replacement in your building has to meet LBN 002-19.

Check your own windows

Heat-loss figures are calculated with this calculator's own EN ISO 6946 methodology and Rīga climate defaults (LBN 003-19). This article is for informational purposes only — actual results depend on your building's condition, windows and heating system.