Cavity wall – Construction, Advantages & Disadvantages
What is Cavity Wall?
A cavity wall is brick masonry having two walls, called “skins” or “leaves” of masonry separated by an air space and joined together by metal ties at suitable intervals. These walls are generally exterior walls, although sometimes used as interior walls due to good sound.
A cavity wall consists of two walls having an air gap between them about 5cm to 8cm. The Exposed outside wall consists of a 200 mm thick wall and the inner wall is sufficiently thick and strong to carry the imposed loads safely and inner wall thickness restricted to 100mm.
History of Cavity Wall:
Cavity walls construction is not a modern concept, they have been observed in ancient Greek and Roman structures. These types of walls still exist at the Greco Roman town of Pergamum, on the hills overlooking the Turkish town of Bergama, a stone wall of cavity-type construction.
Cavity wall constructions were first built in the United States late in the 19th century and after that interest in and use of cavity walls in this country has increased rapidly. This demand for cavity wall leads to extensive testing to determine cavity wall properties and performance. The main reason behind their popularity is excellent thermal properties, excellent resistance to sound transmission, are superior resistance to rain penetration and high resistance to fire.
Why use Cavity Wall:
When there is a normal external wall is provided in house, as rain flash on its wall catch moisture in it and that transfer toward the inner face of wall which becomes a cause for many defects in the wall. The figure shows the movement of moisture in the house.
Cavity Walls restrict the entry of moisture into your home by mean of air space or cavity provided between the walls.
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Properties of cavity wall:
1) Resistance to moisture penetration:
Single unreinforced 4” masonry is not totally impervious for penetration of moisture. Cavity walls are mainly constructed to work as a moisture detention system. The moisture majorly enters the masonry through airline crack which are available between mortar joints. As moisture enters in the outer wall is restricted to reach up to the inner wall by providing air space between them. Moisture dropdown in air space between the wall and it was taken out weep holes. This process solves the problem of moisture penetration in inner walls.
2) Thermal energy efficiency:
In previous days, an energy conversation is not a big concept in building design. Cavity walls were mainly constructed for their moisture control property. During the mid-1970s, designers became aware of the life cycle cost of buildings so the design of energy-efficient walls was initiated. The cavity became an excellent place to insert insulation, minimizing heat loss and heat gain. Both widths act as a heat reservoir, positively affecting heating and cooling modes. The isolation of the exterior and interior width by the air space allows a large amount of heat to be absorbed and dissipated in the outer width and cavity before reaching the inner width and building interior.
3) Fire resistance:
Results of the ASTM E-119 Fire Resistance Tests and the contents of both the Fire Protection Planning Report (CMIFC) and the Fire Resistance Ratings. A report (AISG) clearly indicates that masonry cavity walls have excellent fire resistance. All types of cavity walls can restrict fire up to 4 hours and some cases more than that also.
4) Structural properties:
Brick masonry load-bearing capacity is satisfactory, yet its structural potential is often overlooked. Three principal factors affecting the overall compressive strength of a wall are the compressive strength of the individual units, the type of mortar, and the quality of workmanship.
Each width in a cavity wall helps resist wind loads by acting as a separate wall. The cross wire of the horizontal joint reinforcement transfer direct tensile and compressive forces from one masonry width to the other. It is also observed that joint reinforcement also responsible transfer of shear, approximately 20 to 30 percent, across the wall cavity.
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Construction of Cavity Walls:
There is no need for a special foundation for the cavity wall; it can be built on a strong concrete base. Two masonry walls are constructed together with the same as normal brickwork constructed, but minimum air space or cavity must be left between them.
The cavity may be poured with plain cement concrete with some slope at the bottom for darning and water enter in the wall. Weep holes are provided in the outer wall at a spacing of a minimum of 1m from each other. Normal standard bricks are used in the inner wall and facing bricks are used for the outer wall.
The leaves or walls are connected by wall ties metal ties, which are generally made of steel and are rustproof. The spacing of horizontal wall ties is maximum 900mm and maximum vertical spacing is 450mm. The metal ties are provided such that they do not carry any moisture from the outer leaf to the inner leaf. Various shapes shaped of wall ties show in below fig.
The stretcher bond is suitable for half brick wall or single brick wall and for one brick thickness or more thickness, English bond or Flemish bonds type constructions are provided. During construction of brick, masonry care should be taken that no fall of mortar in cavity space and if there is some mortar fall, clean it quickly after work is down.
To prevent falling of mortar in the cavity between walls, wooden battens are provided in the cavity with suitable dimensions. These battens are supported on wall ties and whenever the height of the next wall tie location is reached, then the battens are removed using wires or ropes and wall ties are provided.
Two walls are constructed simultaneously. Provide a damp proof course separately for both the wall. For cavity wall indoor and windows, the weep holes are provided above the damp proof course.
Cavity wall position at Foundations: Cavity Extending up to Concrete Bed
- The cavity extends right up to the base of the footing, of the footing, with a damp proof course introduced just below the floor level. This is the more common arrangement.
- However, if brick masonry work is not proper below that GL water will enter through the joints, collect in the cavity and travel through the inner leaf, causing dampness in the flooring. The following type remedies the stated problem.
Position of Cavity at Parapet Level:
- In case of a flat roof with a cavity extended up to parapet wall, the cavity may extend either up to the bottom of coping or up to a level slightly above the flat roof.
- When the cavity extends up to the bottom of coping, D.P.C. course is provided between the bottom of coping and top of the cavity, so that rainwater does not enter the cavity.
- When the cavity is restricted just above the flat roof, one D.P.C. is provided over the top of the cavity.
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Insulation in Cavity Walls:
Wall ties are mostly mild steel thoroughly galvanized or dipped in hot tar and sanded to protect from rust.
- Stainless steel is used now used.
- These are available in various patterns the washer shown is to hold insulation boards in position against the inner leaf.
- These wall ties should be arranged at distance not more than 900 mm horizontally and 450 mm vertically, and should be staggered.
- Ties must be placed at 300 mm vertical intervals at all angles and door and window jambs to increase stability.
Advantages of cavity walls.
Advantages of cavity walls:
- The Cavity wall makes a perfect barrier and there are no chances that moisture can travel from the outer wall to the inner wall.
- As air available in the cavity between the wall is non-conductor of heat and reduces the transmission of heat from the external face to internal one.
- The cavity wall act as a damp barrier, so lead to reduces the cooling cost of the building.
- The construction cost of a 275 mm cavity wall costs less to construct than a 328 mm solid wall.
- It is Cheaper than exterior or interior wall insulation.
- It also maintains existing wall thickness.
- Wall-ties used in a wall may face corrosion problems; cavity insulation makes the outer brick leaf colder, and therefore wetter, which can accelerate rusting of the wall ties. And if the ties hen has to be replaced, there is no satisfactory way of refilling the holes in the insulation, whatever the material.
- Thermal bridging problems.
- The width of the cavity restricts the thickness of the insulation material.
- There are significant numbers of buildings with mortar droppings on ties within the cavity which result in penetrating dampness.
- The cold bridging problem in cavity wall due to the Settlement and saturation of cavity-fill.
- DPC should be provided separately for both walls.
- A cavity in the wall should be properly ventilated or drained by providing weep holes.
- During construction, care should take that no mortar or brick, rubbish, etc fall in the cavity.
- The cavity wall free Vermin’s or mosquitoes between air space.
Keep the cavity clean:
Mortar droppings can gather on wall ties and at the bottom of the cavity creating a bridge for water to pass through to the inner leaf.
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