Hi Shane,
The cavity in use may be found empirically to have assisted the outer leaf to dry.
However the original function was to help to create an isolated outer skin which would "catch the weather" and prevent water being transmitted to the inner skin.
In other words the outer leaf was always intended to "get wet" and durable materials such as brickwork and concrete were chosen for this purpose, for much the same reason that various metals skins, slates and tiles were selected for roofing materials.
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As the conception of the cavity wall and its construction changed over time, details were developed that relied on structural isolation of the outer leaf so that almost all of the load-bearing of the wall was carried on the inner leaf - how could one transmit a compression load to an outer leaf with only flexible ties? Thus the outer leaf helped with stability issues, as opposed to directly taking loads.
As understanding of insulation factors progressed, cavities were supposed to be "closed" to restrict the movement of air within them to improve insulation. But in an Irish climate this proved unworkable, since it is so wet that
weep holes were needed over every head and at any stepped flashing or cavity tray to allow water throw out towards the outer leaf to "get away".
This was done to avoid saturating the lowest brick on the way causing "salts" to leave their whitish deposit on the surface, disfiguring the elevation. Weep holes that let water out by definition let air and insects in - thus one sees the plastic insect guards in open vertical joints in brickwork and blockwork (the perpend joints).
More recently, timber frame construction uses vent holes both below and Damp Proof Course level and just under the eaves to actually promote ventilation in the cavity. This is not for the benefit of the outer leaf but to ensure water vapour that has migrated through the timber frame construction and condensed on the outer surface of the breather paper (facing into the cavity) can evaporate away.
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To address the risks for increased insulation AND reduced cavity ventilation on outer leafs we need to realize that high quality concrete, brickwork and mortar is not unduly affected by water but is affected by frost.
Drying out a wall depends on two factors - ventilation and heat. An outer leaf is only four inches or 102mm thick, and will dry out in time with a temperature gradient and an occasionally dry outer surface letting evaporation of the water occur, even without a cavity behind it. Once the temperature drops below zero, water in the outer leaf will freeze, as opposed to evaporate.
The question you raise may be less an issue of whether sealing the cavity will restrict the outer leaf in drying out as whether putting more insulation between the warm interior and the potentially freezing exterior will reduce the rate so much that frost could freeze water in the outer skin leading to degradation of the mortar and brickwork - this is known as "flaking" or "spalling". Concrete tends to be less affected than brickwork.
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The current thinking of super-insulating houses in winter climates that are becoming ever colder raises questions about the longevity of mortar, brickwork and render on exposed elevations in wet winter climates where prolonged freezing conditions follow periods of intense and persistent rain. The outer leaf is kept colder for longer because of the insulation and the filling of the cavity means there is little ventilation on that side.
The result could well be an increased incidence of brickwork flaking and spalling and the external render cracking off due to frost action occurring within saturated outer leafs and their coverings.
Regarding your specific query, external insulation protects the structure be definition, but what protects the external render covering the outer leaf? A medium solution may be to insulate internally to the level required by the regulations and apply moisture repellent render to the blocks and paint it regularly with exterior grade paint. This returns to an ancient technique of sealing involving whitewashing walls with lime.
I am monitoring several properties I have completed to see how -
- Full fill in-cavity insulation
- Internally insulated single leaf with rendered blockwork
- Internally insulated cavity walls with rendered blockwork
- solutions are weathering.
Most of these sites are not too exposed and the quality of workmanship and materials was high. All employed traditional detailing with adequate roof overhangs and/or sustantial parapets. To date all forms of construction seem to be holding up find so far. Touch wood.
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.