Energy efficiency of wooden houses: U-values, airtightness and passive house timber
A wooden house can be anything from a summer cabin to a certified passive house. What decides its energy use is the U-value of walls, roof and floor, how airtight it is, how few thermal bridges it has, and the windows. Here is how those numbers work for timber.
Geoniks works with wooden houses only — log, timber frame and CLT homes, cabins and villas. We can’t help with concrete, steel, apartment or office buildings.
The U-value in one formula
The U-value says how much heat passes through one square metre of a building part for each degree of temperature difference, in W/(m²·K) — lower is better. For a layered wall it is U = 1 / (Rsi + Σ d/λ + Rse), where d is the thickness of each layer in metres, λ its thermal conductivity and Rsi + Rse ≈ 0.17 m²·K/W the surface resistances of a wall.
Worked examples with softwood at λ = 0.13 W/(m·K):
| Wall | Calculation | U-value, W/(m²·K) |
|---|---|---|
| Solid log, 200 mm | 1 / (0.17 + 0.20/0.13) | ≈ 0.58 |
| Solid log, 300 mm | 1 / (0.17 + 0.30/0.13) | ≈ 0.40 |
| Log 200 mm + 100 mm insulation (λ 0.037) | 1 / (0.17 + 1.54 + 2.70) | ≈ 0.23 |
| Timber frame, 200 mm insulation between studs | insulation and studs in parallel | typically ≈ 0.20–0.25 |
With Geoniks: The Geoniks editor shows the U-value of each wall build-up; change the wall type or thickness and see the effect straight away.
Check my walls in 3D →Airtightness: the hidden half of energy use
Air leaking through joints carries heat and moisture out of the house. Airtightness is measured with a blower-door test as n50: the number of times the indoor air volume is exchanged per hour at a pressure difference of 50 Pa. Wooden houses become very airtight with a continuous airtight layer on the warm side and sealed joints around beams, windows and services. In log houses the corner notches and the joints between logs need gaskets and settling-proof sealing.
Thermal bridges in timber
Because wood itself conducts heat poorly, the thermal bridges of a timber house are smaller than those of concrete or steel frames. The usual weak points are the base of the wall, window installation, the eaves and balcony or deck connections. Installing windows in the insulation layer and running the insulation continuously over the base and eaves deals with most of them.
Passive house in timber
The Passive House standard asks for a space heating demand of at most 15 kWh per square metre per year and airtightness of n50 ≤ 0.6 per hour, with heat-recovery ventilation. Timber frame and CLT are among the most common ways to build passive houses, because thick insulation fits into or onto the timber structure with few thermal bridges. Log houses can reach very low energy levels too, usually with added insulation.
- Walls and roof with low U-values — for passive houses typically around 0.10–0.15 W/(m²·K).
- Triple-glazed windows installed in the insulation layer.
- A continuous airtight layer and a blower-door test before the interior is closed.
- Mechanical ventilation with heat recovery.
With Geoniks: Ask manufacturers that build energy-efficient wooden houses in your country for a price on your design — with the U-values and build-ups you want.
Wooden house prices by country →Frequently asked questions
What is the U-value of a log wall?
For solid softwood (λ ≈ 0.13 W/(m·K)) a 200 mm log wall has a U-value of about 0.58 W/(m²·K) and a 300 mm wall about 0.40. Adding 100 mm of insulation to a 200 mm log wall brings it to about 0.23.
Can a wooden house be a passive house?
Yes. Timber frame and CLT are common passive house systems. The criteria are a heating demand of at most 15 kWh/(m²a), airtightness n50 ≤ 0.6 per hour and heat-recovery ventilation.
Are wooden houses energy efficient?
They can be very efficient: wood conducts heat poorly, so thermal bridges are small, and timber walls take thick insulation. The result depends on the U-values, airtightness and windows of the whole house.
What is n50?
The air change rate at 50 Pa measured by a blower-door test: how many times per hour the indoor air volume leaks out. Lower is more airtight; passive houses need 0.6 or less.