The U value for a block cavity wall is 0.21 W/m2K; this does not satisfy the passivhaus standard.
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 walls which is not feasible when trying to achieve the passivhaus standard.
Airtightness is also a very important aspect in preventing heat loss; air leakage must be eliminated. The Irish standard for this is 7 m3/h.m2 at 50 Pa (seven cubic metres of air change per hour for every square metre of floor area when the difference in air pressure between inside and outside is fifty Pascals). This is in comparison with the passivhaus standard of 0.6 m3/h.m2 highlights the difference in the level of attention to detail and high quality workmanship needed in passive construction.
A key point to understand in block cavity construction is that plaster is airtight. This means once the entire surface area of a wall is covered in plaster there will be no air leakage.
Where airleakage problems arise is at junctions, for example where the window board meets the inner leaf. To prevent air leakage at this junction an airtightness membrane could be taped to the window frame and then continued under the window board and down the inner leaf where it would then be parged; using a steel mesh so that the parging (plaster) will grip.
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 construction 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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