Exposure Zones, Cavity Wall Insulation and Residual Cavities
Building with cavity walls is a common construction method within the UK, however within certain parts of the country there are limitations and regulations governing the construction of cavity walls.
These may lead to an increase in the thickness of the wall, encroaching on the space envelope of the building. This blog post looks at some of the factors that must be taken into consideration when constructing a cavity wall focusing on the risk of damp penetration.
Cavity walls are usually built with a residual cavity of 50mm between two layers of bricks, or between bricks and blockwork. To increase the thermal performance of the wall, to meet the levels required by Building Regulations/Standards, it is usual to either partially (while maintaining the 50mm cavity) or fully fill the cavity with insulation. Other insulation methods are available, but we are only looking at cavity wall insulation in this post.
The cavity wall acts as a barrier against penetrating dampness; the outer face works as a first means of defence against rain penetration. Any rain or damp that manages to penetrate the outer layer of brick should run down the inside of the bricks to controlled exit points, rather than crossing the cavity to the inner layer of bricks and through into the building.
Adding insulation to the cavity should not, in most circumstances, compromise the wall’s resistance to dampness. Adding the insulation to the cavity makes for a warmer, more comfortable living environment within the room, without decreasing the space available.
Full Fill Cavity
In certain areas of the country, the risk from driving rain may mean that full fill cavity wall insulation, like mineral wool or expanded polystyrene beads, may not be used to fill a cavity, as there may be an increased risk of rain penetration, although this does depend on the type of cladding used.
The Government’s Energy Saving Trust website states that cavity wall insulation may not suitable in properties which are exposed to severe risk from the amount of wind driven rain. Basically, in this situation, any damp or rain that penetrates the outer layer of bricks may be carried across the cavity by the insulation, through to the inner layer of bricks/blocks, and appearing as damp on the inner wall.
The UK has four different levels of exposure zones which indicate the approximate amount of wind driven rain which the building may be subject to, however if the building is in a sheltered location then it may be considered to be in a zone which is one lower than that indicated by the map of exposure zones.
For the areas in zone 4, which have a severe risk of exposure from penetrating rain, where full fill cavity wall insulation may not be suitable, then partial fill cavity wall insulation may still be used. This gives an air gap between the outer layer of bricks and the insulation which will prevent the water penetration. Guidance provided by NHBC, states that there should be a 75mm residual cavity rather than the usual 50mm cavity in properties within this zone.
Kingspan Insulation in Cavity Walls
If using Kingspan Kooltherm K8 Cavity Board or Thermawall TW50 the NHBC has stated that the residual cavity width only needs to be 50mm in all exposures zones, including areas of very severe exposure to wind driven rain (exposure zone 4), where the outer leaf is fair faced masonry with tool flush joints. This is because the Kingspan boards resist moisture penetration. We are one of the few companies which can offer this thinner solution with a residual cavity of only 50mm. This, when combined with our premium performance insulation results in a thinner wall which still meets the required Building Regulations/Standards.
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