Hello everyone,
I have a Balex Thermano external rafter insulation installed on my pitched roof (PIR panels with a height of 10cm (4 inches)). According to the installation instructions (as I understand them) and the roofer’s statement, no additional vapor barrier or membrane is required (see [link]). The joints between the panels overlap tongue and groove style and are additionally sealed on the outside (the cold side) with aluminum adhesive tape. On the interior side, I only have a drywall (gypsum board) lining.
This construction has been in place for three years now, and I have not observed any damage or moisture issues.
However, since I recently started researching this topic, I keep coming back to the conclusion that a fully airtight layer towards the interior must be mandatory. Otherwise, condensation should occur in the gaps between the panels underneath the aluminum tape. Even if the joints fit tightly, it is impossible to achieve 100% airtightness. This thought, combined with the fact that I cannot inspect the entire assembly without removing all the interior lining, worries me somewhat. Spot checks at the ridge and eaves look fine for now… but those are only two locations.
Does anyone have experience with this kind of system or construction and could share their insights?
Thank you in advance.
I have a Balex Thermano external rafter insulation installed on my pitched roof (PIR panels with a height of 10cm (4 inches)). According to the installation instructions (as I understand them) and the roofer’s statement, no additional vapor barrier or membrane is required (see [link]). The joints between the panels overlap tongue and groove style and are additionally sealed on the outside (the cold side) with aluminum adhesive tape. On the interior side, I only have a drywall (gypsum board) lining.
This construction has been in place for three years now, and I have not observed any damage or moisture issues.
However, since I recently started researching this topic, I keep coming back to the conclusion that a fully airtight layer towards the interior must be mandatory. Otherwise, condensation should occur in the gaps between the panels underneath the aluminum tape. Even if the joints fit tightly, it is impossible to achieve 100% airtightness. This thought, combined with the fact that I cannot inspect the entire assembly without removing all the interior lining, worries me somewhat. Spot checks at the ridge and eaves look fine for now… but those are only two locations.
Does anyone have experience with this kind of system or construction and could share their insights?
Thank you in advance.
Hi,
the question is justified. 10 cm (4 inches) of Thermano (PIR with aluminum facing) is relatively vapor-tight. If tongue-and-groove joints fit properly and the aluminum tapes are sealed well, the outer layer practically provides the airtightness. The workmanship is crucial.
There is no such thing as 100% airtight. What matters is that no significant convection occurs – condensation happens due to flowing, warm indoor air, not by diffusion alone. If after 3 years there is no moisture, no mold spots, and no musty smell, this indicates a functioning construction. PIR with aluminum usually has an sd-value > 100 m (330 feet), effectively acting as a vapor barrier.
Drywall on the inside is not airtight. If the boards are sealed properly at the top, you do not necessarily need an additional membrane at the bottom. Penetrations (such as spotlights or installations) are more critical.
If you want to be sure: a blower door test with leak detection costs about 500–800 €. Tearing everything open would be excessive.
the question is justified. 10 cm (4 inches) of Thermano (PIR with aluminum facing) is relatively vapor-tight. If tongue-and-groove joints fit properly and the aluminum tapes are sealed well, the outer layer practically provides the airtightness. The workmanship is crucial.
There is no such thing as 100% airtight. What matters is that no significant convection occurs – condensation happens due to flowing, warm indoor air, not by diffusion alone. If after 3 years there is no moisture, no mold spots, and no musty smell, this indicates a functioning construction. PIR with aluminum usually has an sd-value > 100 m (330 feet), effectively acting as a vapor barrier.
Drywall on the inside is not airtight. If the boards are sealed properly at the top, you do not necessarily need an additional membrane at the bottom. Penetrations (such as spotlights or installations) are more critical.
If you want to be sure: a blower door test with leak detection costs about 500–800 €. Tearing everything open would be excessive.
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