U-value: a guide to structural energy efficiency and thermal insulation

The U-value, or thermal transmittance, states how many watts of heat pass through one square metre of a structure when the temperature difference between inside and outside is one degree. The unit is W/(m²K), and the smaller the number, the better the thermal insulation. In Finland, for example, the reference value for an external wall is 0.17 and for a window 1.0 – so a window loses many times more heat than a wall of the same size. That is why window area and quality affect a house's heat loss more than many assume.

How is the U-value calculated?

The U-value is the reciprocal of the structure's total thermal resistance: U = 1/R. Each layer's thermal resistance is its thickness divided by the material's thermal conductivity, the lambda value. Design values for insulation are typically 0.022–0.043 W/(mK); softwood and log about 0.12 W/(mK). The same thickness of good insulation therefore insulates several times better than wood – but solid timber's heat-storage advantages are recognised separately in the regulations. The structural engineer does the calculation; understanding the principle helps you compare structures at the quotation stage.

For scale: 200 mm of insulation with a conductivity of 0.040 W/(mK) gives a thermal resistance of 5.0 on its own, pushing the U-value towards 0.2. A real calculation includes every layer plus the surface resistances.

U-value requirements 2026: reference values for heated spaces

New-build energy efficiency in Finland is still governed by Ministry of the Environment decree 1010/2017. Reference values for a heated space:

• External wall: 0.17 W/(m²K)
• Log wall, where the log's average structural thickness is at least 180 mm: 0.40 W/(m²K)
• Roof and a floor exposed to outdoor air: 0.09 W/(m²K)
• Floor over a crawl space: 0.17 W/(m²K)
• Building element against the ground: 0.16 W/(m²K)
• Window, door and rooflight: 1.0 W/(m²K)

For semi-heated spaces, such as a heated store or stable, the values are more lenient: external wall 0.26, log wall 0.60, roof 0.14, windows and doors 1.4 W/(m²K). Intended use and temperature level therefore set the requirement level.

Why does the log wall have its own reference value?

Solid log has its own, clearly more lenient reference value of 0.40, because the log wall's heat-storage capacity and moisture behaviour are recognised. A 180 mm log wall meets the requirement in the compensation calculation without additional insulation. The claim that a log house cannot meet today's energy regulations without added insulation is false: the 0.40 reference value, the E-value relief for log buildings and the compensation calculation together make a permit for a genuine solid-log house possible.

On the E-value side, a solid-timber building may additionally exceed the calculated purchased-energy limit by 10–20 per cent depending on building type. The U-value and the E-value thus work together: the envelope's insulation performance is one of the E-value's inputs, and log has its own level in both, recognising the properties of solid timber.

Is the reference value an absolute ceiling?

No. They are the starting level for the heat-loss compensation calculation, not a per-element obligation. An element may be weaker than the reference value if the loss is compensated elsewhere: a better roof, better windows, more efficient heat recovery or a more airtight envelope. Opaque structures also have a fixed maximum – around 0.60 W/(m²K) in a heated space. The designer prepares the compensation calculation for the permit stage; ask to see it, and you will see where your heat losses really come from.

What does the U-value mean in practice?

A low U-value shows up in three ways: a smaller heating bill, an even indoor temperature and less draught by the windows. The U-value only describes heat conduction, though. A poorly sealed house loses heat through its leak points however good the U-values look on paper, so an airtightness test is the natural companion to U-values. The whole matters more than polishing a single structure – the roof's 0.09 already says heat escaping through the roof is the biggest single risk.

When comparing suppliers' structures, look at three things together: the envelope's U-values, the air-leakage rate and ventilation heat recovery. Together they determine heating energy demand, and none can endlessly compensate for the others.

Checklist for comparing quotes

• Ask the supplier for element-by-element U-values: external wall, roof, ground floor, windows and doors.
• Ask which air-leakage rate the E-value was calculated with, and whether an airtightness test is included in the delivery.
• In a log house, confirm the log's structural thickness – at least 180 mm qualifies for the 0.40 reference value.
• Ask to see the compensation calculation before signing.

• The U-value (W/m²K) measures thermal transmittance: the smaller the value, the better the insulation.
• Reference values 2026 (decree 1010/2017): external wall 0.17, roof 0.09, windows and doors 1.0 W/(m²K).
• The log wall has its own reference value of 0.40 – log of at least 180 mm needs no additional insulation.
• Calculation: U = 1/R, a layer's thermal resistance = thickness divided by conductivity (insulation 0.022–0.043, log approx. 0.12 W/mK).
• The reference value is not an absolute ceiling – a weaker element can be compensated elsewhere in the calculation.
• Semi-heated spaces have more lenient values: external wall 0.26, log wall 0.60 W/(m²K).