Template

Swedish low-energy eco house

Diffusion-open passive-house style wall: cellulose between I-joists, smart vapour retarder and wood fibre board as wind barrier.

Sweden

7 layers · 445 mm

Climate

Indoor temperature 20.0 °C

10 °C 28 °C

Indoor humidity 38 %

20 % 95 %

Set automatically from outdoor climate

Humidity class (EN ISO 13788)

Outdoor temperature -5.0 °C

-30 °C 40 °C

Outdoor humidity 80 %

30 % 100 %

Build-up

Layers from inside to outside

Inside (warm) 7 layers · 445 mm

1 Gypsum fibreboard

Thickness
mm
2 Service layer (wood fibre)

Thickness
mm
3 Smart vapour retarder (sd 2 m)

Thickness
mm
4 Cellulose between I-joists

Thickness
mm
5 Wood fibre wind barrier board

Thickness
mm
6 Ventilation gap ventilated

Thickness
mm
7 Wood panel (träpanel) not effective

Thickness
mm

Outside (cold)

Result

No condensation

The indoor dew point (5.3 °C) is reached in layer 4 “Cellulose between I-joists” – but so little vapour gets there that the vapour pressure stays below saturation.

U-value

0.10 W/m²K

per EN ISO 6946

Indoor dew point

5.3 °C

20.0 °C · 38 % RH

Inner surface

19.4 °C

40 % RH at the inner surface

sd value

3.1 m

effective layers

Interstitial condensation

at 38 % indoors: outdoor temperature below -23.2 °C

at -5.0 °C outdoors: indoor humidity above 86 %

Surface mould risk

at 38 % indoors: outdoor temperature not down to −30 °C

at -5.0 °C outdoors: indoor humidity above 77 %

Cross-section

Temperature and dew point across the build-up – hover for details

Temperature Dew point (linear vapour pressure) Dew point above temperature Condensation (Glaser) Indoor air dew point

Values at layer boundaries

Boundary	x [mm]	sd [m]	θ [°C]	θ_dew [°C]	p_s [Pa]	p [Pa]	RH [%]
Inner surface	0.0	0.00	19.4	5.3	2,248	892	40
1 \| 2	15.0	0.20	19.3	4.7	2,232	856	38
2 \| 3	60.0	0.29	16.3	4.4	1,853	839	45
3 \| 4	60.2	2.3	16.3	-3.2	1,853	466	25
4 \| 5	360.2	2.9	-2.9	-6.5	481	354	74
Outer side	395.2	3.1	-4.8	-7.6	408	321	79

Critical conditions

At which outdoor temperature and indoor humidity does it get critical?

safe Mould risk (surface > 80 %) Interstitial condensation current conditions

Glaser diagram

Vapour pressure must not touch the saturation pressure

Saturation pressure Vapour pressure (Glaser)

How it works

Temperature profile

The temperature drops across each layer in proportion to its thermal resistance R = d/λ. Surface resistances are R_si = 0.25 and R_se = 0.04 m²K/W (0.08 behind a ventilation gap). Layers outside a ventilated air gap are ignored.

Dew point & condensation

The indoor air has a dew point (e.g. 9.3 °C at 20 °C / 50 %). Reaching it inside the build-up alone is not a problem – what matters is how much vapour gets there. The Glaser method compares the vapour pressure with the saturation pressure; where they meet, condensation forms.

Limitations

A steady-state estimate (EN ISO 13788) without capillary transport, moisture storage or thermal bridges. Capillary-active internal insulation is rated conservatively. For a detailed assessment use a hygrothermal simulation (e.g. WUFI).