Cavity wall construction

A cavity wall is a wall made of two separate leaves (skins) of masonry, usually brick or block, separated by a continuous air space (cavity) of 50 to 100 mm and tied together by metal ties.

Construction

• The outer leaf (103 or 115 mm) resists the weather; the inner leaf (100 to 230 mm) carries the load.
• Cavity width: 50 to 75 mm (normally not more than 100 mm).
• Wall ties: Galvanised or stainless steel ties (vertical twist, butterfly or fish-tail type) placed at 900 mm horizontally and 450 mm vertically, staggered, sloping down towards the outer leaf, with a drip.
• DPC: Placed at the base, at openings and at the top of the wall, with a flashing (cavity tray), so that water in the cavity is thrown out; weep holes at 1 m centres in the outer leaf at the bottom.
• The cavity is kept clean, and it is closed at the eaves, openings and top by a ring beam or a solid closer.
• Insulation can be fitted in the cavity.

 Horizontal section:           Vertical section:
 |outer|  cavity  |inner|        roof slab
 |_____|<-50mm->|_____|         |  |   |  |
   tie ------------ tie         |  |   |  |
                                ==DPC====(weep hole)
                              plinth

 Elevation (outer leaf, x = wall tie):
 |____|____|____|____|____|____|
   x          x          x        <- ties 900 mm apart
 |__|____|____|____|____|____|_|
        x          x          x   <- next row 450 mm higher, staggered
 |____|____|____|____|____|____|
 ======= DPC ===== o weep hole o =

Merits

1. Prevents rain penetration and dampness.
2. Good thermal insulation (cool in summer, warm in winter).
3. Good sound insulation.
4. Reduces efflorescence on the inner face, and good facing bricks are needed only for the outer leaf.
5. Lighter than a solid wall of the same overall thickness, with adequate stability.

Demerits

1. Costlier than a solid wall, by about 20 %.
2. Needs skilled work and supervision; mortar droppings can bridge the cavity.
3. Takes more time to build.
4. Ties can transfer moisture if badly placed.
5. Needs extra DPC and details at openings.