I have the next build-up with water repellent and foil faced PIR in both sides modelled as vapor retarder (sd=1500).
is it right?
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Evaluating the WC in the PIR I have observed irregular, non-periodic changes in the total water content what makes me think that something can be wrong in the results. any thoughts about this change in trend?
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However there are no larger differences in balances as it can also be recognized in part by the total
water content curve above
The steep vertical increases in water content are obviously not correct. However, in the total balances these problem remain unvisible - this may be because the issue is simply a slightly incorrect distribution of moisture within the building component, which is not apparent either in the total water content or in the surface moisture fluxes.
In any case, you can assume that no moisture accumulation will occur between two layers with an sd value of 1500 m. And ss the total error ist only 0,01 kg or 10 g (0,1 kg/m³ * 0,1 m³) over 10 years, the influence on the neighbour materials remains completely negligable.
In other words, to sum up: You can perform a normal evaluation.
best regards
Daniel
Dr.-Ing. Daniel Zirkelbach, Deputy Head of Department Hygrothermics, IBP Holzkirchen
1- Could the "Assessing the Risk of Interstitial Condensation Runoff" guideline be applied for a foil face PIR insulation which is between an unventilated cavity and concrete?
for the case we have unventilated air layer facing in one side with a brick and the other side with a face foil aluminum PIR insulation, which value should we take? the one from the column Rmet to calculate µ* and lambda*?
Hi Alberto,
regarding your questions:
1) yes, the condensation in air layers part
2) yes, the Rmet values - these should always be used if there is at least one metallic surface connected to the air gap
Christian
thanks, but how can we evaluate the risk of interstitial condensation in a wall which includes face aluminum foil PIR as none of the guidelines include steps to follow for cases with this type of close-cell insulation?.
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as the difference in water content over 10 years is around 25g (above ± 10 - 20 g/year) in the PIR, I have some concerns.
In close-cell insulation no runoff can occur. The water remains in the cells where it forms.
If necessary, an increase in thermal conductivity should be taken into account—as a general guideline, one can assume that for water contents up to 2% by volume or 20 kg/m³, the effect of moisture is already factored into the declared lambda values—only higher values need to be evaluated.
Dr.-Ing. Daniel Zirkelbach, Deputy Head of Department Hygrothermics, IBP Holzkirchen
How is it possible to state that “In closed-cell insulation, no runoff can occur. The water remains in the cells where it forms”?
Correct me if I am wrong, what I undersand is that PIR is a highly moisture-resistant, closed-cell foam, so water cannot easily penetrate or accumulate within its sealed cells. Unlike wood fibre, it does not readily absorb or wick water through the material. If exposed to rain, leaks, or internal vapour, water should mainly remain on the external surface (there are also aluminium foil facings on both sides of the PIR board acting as vapor barrier) and run off, rather than enter the closed cells. Therefore, I do not understand how water could “remain in the cells where it forms.”
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Last edited by Alberto Morales on Thu Sep 17, 2026 11:20 pm -1100, edited 2 times in total.
