Welcome to the AECB Forums Building Design Cavity Wall Construction

Viewing 4 posts - 1 through 4 (of 4 total)
  • Author
  • #31834

    I have concerns that a fully-filled cavity in a severely exposed location will not address the issue of moisture ingress. I have read lots of threads on this forum and elsewhere which have discussed how best to construct a brick or stone-faced passivhaus. They all seem to end up promoting a block inner skin and brick or stone outer skin with a cavity fully-filled with a compressible insulation.
    My contention is the inner face of the external leaf will get damp (especially with a porous stone on a west-facing exposed location) and this will in turn make the insulation damp. I know it will not get through 300mm of the stuff and affect internal decorations but it will affect the performance of the insulation. Do we know by how much? Is the integrity and stability of the insulation also being compromised by constantly getting a soaking and then drying out? I would be really interested to know if there is any monitoring data which shows how wet the insulation gets and its effect on the physical integrity and thermal performance of the insulation.
    One possible solution I have thought of would be to use an insulated timber frame wrapped in a breathable membrane and then clad it with a stone or brick skin, maintaining a 50mm cavity.
    Your comments would be really welcome.

    Tom Foster

    This is a typical WUFI simulation scenario, and it's shown to be a generally robust solution.

    Yes the outer parts of the insulation get more or less wet, depending on the liquid water absorbtivity of the outer skin material (or treated surface) and on local driving rain (wind direction/strength/duration + rain strength/duration) but it usually dries out seasonally. However under extreme conditions/absorbtivity and/or poor seasonal drying, WUFI may show that the moisture accumulates year by year; then something has to be changed.

    It better be a non-organic, non rotting insulation, as it spends so much of its time above 20%-by-weight moisture content (whether vapour or liquid). Rot won't happen till 27% sustained, unless encouraged by adjacent established rot; 20% is the rot-warning safety point.


    Thank you Tom,
    I will commission a WUFI calculation.

    Tom Foster

    Try me! when you're ready. Can discuss options/variants with you, see how to reduce risks, if any.

Viewing 4 posts - 1 through 4 (of 4 total)
  • You must be logged in to reply to this topic.