Cavity Wall Construction Most residential buildings in the UK are constructed using Cavity Walls Cavity wall shave three main layers. The cavity can be filled with insulation The brick and block layers are joined with wall ties. The main purpose of building a cavity wall is to stop the penetration of moisture to the internal surface of the wall

Partial Fill Cavity Wall Make sure that you can draw and label the parts of a cavity wall. Some of the main features are shown above.

Mortar Bridge It’s very important that the cavity isn’t bridged to lower the chance of moisture getting across the cavity to the inside leaf. Ideally, the cavity should be totally sealed as this would make the thermal and sound insulation better. Sometimes if the wall is exposed to really wet conditions air bricks are laid at the bottom and top of the outer leaf of the cavity to let air pass through the cavity drying out any moisture that has come through the outer leaf.

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Damp Penetration When the cavity hasn’t been kept clean of mortar and Damp Proof Course (D.P.C) hasn’t been used correctly then damp will penetrate and stain the walls like this.

Damp Proof Course (DPC) A damp Proof course prevents the transfer of moisture from the ground into the building. The damp proof course involves the use of a plastic (non-permiable) membrane. The Damp Proof Course (DPC) is 150mm above the ground.

Wall tie spacing Insulation Horizontal spacing of wall ties: 900mm max. Insulation Vertical spacing of wall ties 450mm max’ and staggered. Why 450mm? Because the normal size of fibreglass insulation is 450mm high, it would then fit between the ties. Why staggered? So the strength of the brickwork is spread all over, not in lines.

Wall ties All wall ties should comply with I.S. 268:1986: Metal galvanised wall ties for masonry walls. the use of stainless steel ties is strongly recommended as they’re stronger and last longer. Wall ties hold the two wall sections together. They hold insulation panels in place with clips on.

Insulation retaining clips in use. Wall ties must be long enough to make sure 50mm is laid into the mortar bed of each leaf, i.e. for a 100mm cavity, use a wall tie at least 200mm long. 100mm 50mm 200mm Where cavity insulation batts are used make sure that the tie used suits the insulation and holds it firmly in place. Suitable ties are usually available from the suppliers/ manufacturers of wall insulation batts. Insulation retaining clips in use.

Make sure there is no back fall (i.e. falling inwards) on wall ties. Put ties in as work progresses. Do not push ties into mortar after bricks or blocks are laid as the tie won’t add strength to the inner and outer leaf. Make sure there is no back fall (i.e. falling inwards) on wall ties. Lay the ties in place with the drip (twist) downward. wall tie weep hole cavity tray lintel window or door frame Cavity trays and weep holes are used to help remove moisture from the cavity and prevent it reaching the inside of the buildings. They are often found above wall openings such as doors and windows.

Draw a cross section through a cavity wall and label all the parts. 2. Explain how a cavity wall works.

3. Explain two advantages of cavity wall construction. 4. Explain why a mortar bridge is a problem. 5. What is a damp proof course? 6. Explain how a DPC works?

7. What are wall ties?. 8. Explain clearly two ways that wall ties can be used. 9. Show with a diagram, how wall ties should be spaced and explain why.