1- If I follow the guideline on “Assessing the Risk of Interstitial Condensation Runoff,” what minimum RC value should I consider? I ask because I do not have a water- and vapour-tight surface in my assembly, and I do not consider it would be appropriate to consider the concrete layer as one (correct me if I am wrong, I have seen projects that concrete layer is used as a airtightness layer reaching lower ach in blower door test with this strategy).
In your case, I would be cautious about using the "50 g/m² without adjacent insulation" limit.
The runoff guideline was developed primarily for condensation occurring on water- and vapour-tight surfaces such as membranes, vapour retarders, metal sheets, etc., where liquid water can collect and run off.
If condensation is predicted at the interface between the foil facing of the PIR and the cavity, then the foil facer itself behaves as a water- and vapour-tight surface. The concrete behind the cavity would not normally be the relevant surface for the runoff assessment, even though concrete can act as an airtight layer in practice.
Therefore, I would evaluate the condensate amount at the foil facer surface. For a smooth foil surface, the base value would generally be the minimum retention capacity without any surface surcharge. The inclination-related surcharge can then be added according to the guideline if applicable
2- Also, if there is no risk of interstitial condensation runoff, does that necessarily mean that the assembly will not experience moisture-related issues, even if the RH in the unventilated air cavity and at the outer side of the insulation remains above 80%, as shown in the image?
The runoff assessment only answers the question:
Is enough condensate forming at an interface that liquid water can run down or drip?
It does not assess:
- elevated equilibrium moisture contents,
- thermal performance reduction,
- durability of adjacent materials.
Therefore, a construction can satisfy the runoff criterion and still experience moisture-related problems if RH remains high for long periods.
For example:
- 85-95% RH for extended periods may be problematic for some materials.
- Metallic components may be exposed to increased corrosion risk.
- Organic contaminants or dust deposits may support mould growth if suitable substrates are present.
So the runoff criterion should be regarded as only one assessment criterion among several.
3- One other point I am concerned about is the area I have highlighted in the image. The aluminum carrier rails have a protective oxide layer, but can corrosion still occur under certain environmental conditions, particularly if the RH remains high for extended periods as showed in the WUFI film? Can interstitial condensation in the outer part of the external insulation to move inwards through this aluminum carrier rails? as If yes, how to prevent it as I do not have any breather membrane.
Pure aluminium is generally highly resistant to corrosion because of its naturally occurring oxide layer.
However, corrosion can still occur under certain conditions, for example:
- persistent surface wetting,
- trapped moisture in joints,
- galvanic contact with dissimilar metals,
- crevice conditions with limited drying.
From a WUFI perspective, consistently high RH alone does not automatically imply corrosion. The more relevant question is whether actual liquid water forms on the rail surfaces and remains there for significant periods.
Regarding moisture transport along the rails:
Yes, aluminium rails can locally act as thermal bridges. Their lower temperature may increase the likelihood of local condensation compared with the surrounding insulation.
However, WUFI 1D cannot realistically represent:
- moisture migration along the rail profile,
- local condensate accumulation at connections.
A 2D or 3D analysis would be required if these effects are considered critical.
The condensation occurring in the outer insulation does not simply "move inward through the aluminium" by diffusion. The more likely mechanism would be local surface condensation on cooler metal components.
4- If the air cavity is ventilated, should I keep considering the sd value, long wave and short wave properties of the aluminum panel? As the effective hygrothermal behaviour is governed by the open joints + ventilated cavity, not by diffusion through the aluminum (the aluminum itself is essentially vapour-impermeable)
Yes, but their importance changes.
For an aluminium cladding panel with an adequately ventilated cavity:
- the sd-value of the aluminium becomes largely irrelevant, because the moisture exchange is dominated by ventilation through the cavity and joints rather than diffusion through the metal itself;
- the long-wave emissivity remains important because it influences radiative heat exchange across the cavity;
- the solar absorptivity (short-wave properties) remains important because it determines how much solar energy is absorbed by the cladding and therefore influences cavity temperatures.
So in the simulation I would use these values.
Christian
