Intelligent air sealing
The best approach: Intelligent membranes
Vapour retarder membranes with a humidity-variable diffusion resistance provide the best protection against condensation water damage to building structures. They become more impermeable to diffusion in winter and protect the insulation against moisture penetration in an ideal manner.
In summer, they can reduce their diffusion resistance very significantly and thus ensure the best possible drying-out conditions.
Operating principle of moisture-variable airtightness membranes
Moisture-variable layers operate on the principle of climate-driven membranes. They respond to the humidity in their environment and adjust their diffusion resistance intelligently to the current requirements.
In winter, the mean ambient humidity of the vapour retarder is approx. 40%. The diffusion direction is from the heated interior to the exterior. The vapour retarder now must have a high resistance to diffusion in order to protect the structure against condensation.
In the summer, the mean ambient humidity of the vapour retarder is more than 80% and the diffusion direction is reversed. Now the vapour retarder must be permeable in order to allow the humidity to evaporate.
Diffusion resistance depending on ambient humidity
- sd value: up to >25 m, in summer <0.25 m
- g value: up to >125 MNs/g, in summer <1.25 MNs/g
- permeance: <0.13 US perms, in summer >13.12 US perms
The values of DB+
- sd value: up to 4 m, in summer <0.4 m
- g value: up to >20 MNs/g, in summer <3 MNs/g
- permeance: 0.82 US perms, in summer 8.2 US perms
For all regions
The greater the spread of the diffusion resistance/permeability from summer to winter, the more protection is provided for constructional elements - even in case of unforeseen moisture penetration.
For the best possible protection against damage to structures, the drying reserve must be higher than the maximum theoretically possible moisture level.
Construction phase: Hydrosafe value (70/1.5 rule)
... or the '70/7.5 rule' using the g value and '70/2.2 rule' using US perms
A vapour retarder should have a hydrosafe sd value of 1.5 m (g value 7.5 m; 2.2 US perms) in order to protect structures against dampness even in the case of the increased relative humidity that can be present during construction work.
The hydrosafe value specifies how well sealed a humidity-variable vapour retarder still is at an average humidity of 70%. Average humidity of 70% will be present if there is 90% indoor air humidity and 50% humidity in the space between the rafters, for example; this level of indoor air humidity can occur when installing screed or plastering walls.
The requirement that sd should be > 1.5 m and < 2.5 m
comes from DIN 68800-2 and is described in further detail by the 70/1.5 rule.
Implications for g value: > 7.5 m and < 12.5 m
vapour permeance: > 2.2 and < 1.3 US perms
Increased moisture should always be allowed to escape from the building as rapidly as possible by opening windows for ventilation. Dryers can accelerate the drying process in the winter. Permanently high relative humidity should be avoided.