Masonry Magazine June 1978 Page. 17
TABLE 2
Typical Brick Masonry Cavity Wall
U-Values
| 8-in. Cavity Wall (3-in. Brick both wythes) | U° | 10-in. Cavity Wall (4-in. Brick both wythes) | U° |
|---|---|---|---|
| No fill | 0.40 (2.27) | No fill | 0.37 (2.10) |
| 2-in. Vermiculite fill | 0.18 (1.02) | 2-in. Vermiculite fill | 0.17 (0.97) |
| 2-in. Perlite fill | 0.15 (0.85) | 2-in. Perlite fill | 0.14 (0.79) |
| 2-in. Polystyrene board | 0.08 (0.45) | 2-in. Polystyrene board | 0.08 (0.45) |
| 2-in. Polyurethane board | 0.06 (0.34) | 2-in. Polyurethane board | 0.06 (0.34) |
| 9-in. Cavity Wall (3-in. Brick, 4-in. Lightweight Concrete Block) | U° | 10-in. Cavity Wall (4-in. Brick, 4-in. Lightweight Concrete Block) | U° |
| No fill | 0.28 (1.59) | No fill | 0.27 (1.53) |
| 2-in Vermiculite fill | 0.15 (0.85) | 2-in. Vermiculite fill | 0.15 (0.85) |
| 2-in. Perlite fill | 0.13 (0.74) | 2-in. Perlite fill | 0.13 (0.74) |
| 2-in. Polystyrene board | 0.07 (0.40) | 2-in. Polystyrene board | 0.07 (0.40) |
| 2-in. Polyurethane board | 0.06 (0.34) | 2-in. Polyurethane board | 0.06 (0.34) |
Units are Btu/h ft °F) and W/(m² K).
Vapor Barrier
The effect of cavity wall construction on vapor transmission is not so obvious. The overall vapor permeance remains unchanged compared to a solid wall. However, the dew point location is altered by the increased thermal resistance from the air space, and even more from the insulation, see Technical Notes 7C and 7D. This problem was also investigated during the earlier testing program. The testing program indicated that no vapor barrier is required in insulated cavity walls if the following three conditions are met: (1) each wythe must have a vapor permeance not exceeding three perms (a 4-in. wythe of brick has a perme- ance of about 2.2 perms), (2) average interior relative humidity conditions for heated and air-conditioned buildings should not exceed 50%, and (3) the vapor pressure gradient should be not more than 1 in. (25 mm) of mercury for a period of not more than 30 days.
These conditions are satisfied by most residential, commercial, institutional, and industrial buildings in the continental United States. Laundries, cold storage facilities, indoor swimming pools, hockey rinks, and buildings with high relative humidities are examples of buildings where investigation should be made regarding the use of a vapor barrier.
Thermal Properties
The benefits of reduction of heat transfer through cavity walls is detailed in Technical Notes 21 Revised. However, in some areas of the country, more insulation is required than can be furnished merely by the air space. Studies have shown that the closer to the outside of the wall the insulation is located, the better the thermal performance. One method of obtaining this additional insulation would be to apply furring or insulation, and an interior finish to the wall. This, of course, adds to the cost and defeats one of the most important advantages of the cavity wall-the elimina- tion of the need for furring. Therefore, the installation of insulation in the cavity becomes necessary. Table 2 gives representative U-values for several insulated cavity wall systems.
The mass or weight of the cavity wall, in combination with the insulation, contributes to its excellent perfor- mance. The "M" factor may be effectively used with heavy cavity walls, as outlined in Technical Notes 4B, to account for the mass effect.
Expansion Joints. In an insulated cavity wall, be- cause of the effective thermal isolation of the exterior