The U value for a traditional block cavity roof system is 0.16 W/m2K, this is very close to the passivhaus standard of between 0.10 - 0.15 W/m2K; however it still does not satisfy it.
When we examine the block cavity system we can discover flaws in its design; due to the levels of insulation there is high heat loss through the structure which is not feasible when trying to achieve the passivhaus standard.
With regards airtightness; quite a high level can be achieved by simply parging the walls and ceiling; however there is the risk of airleakage at junctions where differential movement could occur.
The sketch below is a Passivhaus detail compliments of integrated energy. In this detail thermal bridges and air leakage which are evident in the traditional Irish block cavity roof are eliminated; this provides the structure with the opportunity to reach the passivhaus standard of between 0.10 - 0.15 W/m2K.
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