This type of clay is imaginatively titled, Brick Clay. Read all about how to use a vertical DPC here and how they work. It is at this point that there is least insulation and cold temperatures can travel through the walls.Ī vertical DPC inserted between the two walls will stop any damp from getting in, but the cold spot inside can still lead to condensation forming on the wall. The second diagram above shows, looking from above, how a cavity wall is returned to close the cavity when meeting a door or window opening. This makes a section of the internal wall cold and even damp allowing the warm air in the room to condense in this “cold spot”.īecause the wall is covered by very porous plaster this damp cold spot can be home to any number of mould spores which will soon show as a dark mouldy patch which, no matter how hard you scrub, will not go away. This moisture can soak into the brickwork above the DPC and, via the mortar in the cavity, soak into the internal wall. The top of the heap was above the height of the DPC.Īs a consequence of these two very regular occurrences, ground water can rise up into the soil heap which is probably already damp anyway. At the same time, while the garden was being dug, a heap of spoil was left against the wall. The mortar has bridged the cavity allowing moisture and cold temperatures to pass between the skins of the cavity wall. This has obviously gone unnoticed and when the insulation has been added to the cavity it cannot get down to the required DPC level. First a lump of mortar has fallen from the trowel into the cavity. The top diagram above shows what can happen when two very common things occur during the construction of a cavity wall. This can be particularly noticeable at the sides of window and doors (reveals) where the cavity is closed to allow the fixing of the frame. This is usually the result of a transfer of cold temperature from the outside wall to the inside via what is known as a bridged cavity. We have had many questions over the years about damp and cold spots on walls in an otherwise warm room. Even a dropped trowel of mortar can collect on a wall tie and transfer moisture and cold temperatures across the cavity. It is important when building cavity walls to keep the cavity free from debris at all times. The cavity also, in almost all cases, is used to insulate the internal wall against heat loss from inside. The cavity, in a cavity wall, is there to prevent moisture from traveling from the outside skin to the inside skin. The wall ties are often used to hold sheets on cavity insulation in place and the type and thickness of this insulation will be dictated by the Building Control officer. The ties should always slope towards the outer skin very slightly to stop any moisture in the cavity being able to travel towards the inner wall. If the cavity is between 76 and 100mm wide the ties should be positioned at maximum intervals of 750mm horizontally and 450mm vertically. If the cavity is between 50 and 75mm wide the ties should be placed at a maximum spacing of 900mm horizontally and 450mm vertically. The two skins of a cavity wall are held together by wall ties built into the mortar bed of the bricks and blocks. The actual construction of a cavity wall can vary but in all cases (Building Regulations Approved Document A) the leaves of a cavity wall must be a minimum of 90mm thick with a minimum 50mm cavity in between. You may also find our Cavity Walls Project useful. The Rules for Building Cavity Walls and Key Things to Look Out For
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