Where Does the Airtightness Layer Go?
Getting layering right in New Zealand buildings for summer and winter.
Following on from the previous article, one of the ingredients in modern buildings is the creation of a deliberately airtight shell to conserve energy (warm or cool).
We often hear about attempts to achieve airtightness and vapour control on the outside ‘like they do in the USA’. This is a misinterpretation of their common building methodology (bearing in mind there are radically differing climates across the USA resulting in different layering methodologies). The airtightness / vapour control layer is still toward the inside face of the insulation. In their typical timber frame construction, the insulation layers are commonly outside the framing line, so when you employ an airtightness layer it too is outside the framing line, but still on the inside of the insulation line.
Airtightness is always achieved toward the warm side of the insulation, and the combined reasons for this are simple:
- Air should be prevented from moving into and through insulation
- In winter conditions in New Zealand the dew point temperature is close to the centre of the insulation
- The dew point temperature cannot be magically moved to the outside by increasing the thickness or by changing the type of insulation
- Relative humidity increases as temperature decreases
- High humidity (above 80%) exposes materials to conditions supporting mould growth
- Interstitial condensation (liquid water) risk increases when moisture is allowed to move beyond the dew point temperature in a wall or roof structure
We define the control of air and moisture as being ‘airtightness’, and the control of wind and rain as being ‘weathertightness’. Airtightness and weathertightness are different animals and must be dealt with on the side of the construction they are attacking. The ferocity of these animals change depending on where in the country you are — cold humid, warm humid etc. The fight is a dynamic one and is reflected in the material choices made.
Therefore, an airtight layer on the warm side of the insulation
- Prevents moisture flow by convection
- Aids in the performance of the insulation itself by keeping the air molecules still
- Aids in the performance of the insulation itself by keeping the insulation material dry
- Eliminates the immediate and continuous need to replace energy leaking via infiltration/exfiltration pathways
- Aids in maintenance of the internal surface temperature above the dew point temperature, thereby avoiding condensation forming on internal surfaces
- The vapour control element of INTELLO also prevents moisture flow into the structure through diffusion during winter conditions while allowing moisture out in summer conditions
Known performance vs guesswork
There is no place for guesswork. That should be clear from the previous article. Using hygrothermal analysis is a very cheap way of predicting building enclosure and materials performance, and is much cheaper than remediation work. There are some simple construction methods we employ with the INTELLO vapour control membrane that do not need further hygrothermal analysis to know they are ‘safe’ ways of building for a large majority of our climates in New Zealand.
The results of predicting hygrothermal performance before you build are dry durable buildings, and dry durable occupants.
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