It is well known that a solid brick wall readily absorbs moisture and transmits heat and cold. A driving rainstorm will often penetrate a 12-inch brick wall so as to dampen the wall paper or spoil the fresco decorations. It is also known that a house with damp walls is unhealthy and a frequent cause of rheumatism; besides this the moisture in the brickwork prevents the mortar, if made of lime, from becoming hard, and is also liable to communicate itself to the woodwork, thereby causing rot.
A building with damp walls will also require the consumption of very much more coal in warming than one with dry walls, as the moisture must be evaporated before the temperature of the walls can be raised.
To overcome these objections to a solid brick wall, particularly in residences and school houses, hollow or vaulted walls have been much used, and are earnestly recommended by various persons.
Theoretically, a hollow wall should prevent the passage of moisture through the wall, and by providing an air space in the wall, make the building much cooler in summer and warmer in winter.
In the actual construction of the walls, however, there are certain difficulties met with, which, to a considerable extent, offset the advantages, so that hollow walls are comparatively little used in this country.
The author believes, however, that their use might be much extended with beneficial results, especially for isolated buildings.
To obtain the full benefit of an air space it should be continuous throughout the wall, and the bond or connection between the two parts of the wall should be of such material and of such a shape that the moisture which penetrates the outer portion cannot be conveyed to the inner portion.
To provide a continuous air space in a wall penetrated by openings is practically impossible, although it may be quite closely approximated.
There are several ways of constructing vaulted walls; these differ principally in the method of bonding and in the thickness of the inner and outer portions of the wall.
Generally, at least one portion of the wall must be made 8 inches thick to sustain the weight of the floors, the other portion being only 4 inches thick. Probably the thicker portion of the wall is more commonly placed on the outside of the wall, but this necessitates extending the floor joist through the air space, and thus to a great extent neutralizing the benefits derived from it. By this method the thicker portion of the wall is still subjected to the injurious effects of the moisture.
For two-story buildings the author recommends that the walls be constructed as shown in Fig. 153. If the wall plate comes above the attic joist the latter may be supported on the 4-inch wall if well built of good hard brick. If the brick are not of very good quality the 8-inch wall should be continued to the upper joist.
When the bricks, mortar and workmanship are of the best quality there is no reason why this construction should not answer for even four or five-story buildings (if used only for dwelling or lodging purposes) by making the inner portion 8 inches thick the full height and increasing the width of the air space to 6 inches.
For warehouses the bearing wall in the lower stories should be increased in thickness.
A hollow wall of a given number of bricks securely bonded is much more stable than a solid wall of the same number of bricks, and will also withstand fire better. It requires much better workmanship, however, than is generally bestowed on solid walls, and the mortar, particularly in the outer portion, must be of the best quality, and preferably of cement.
Nearly all building regulations require that at least the same quantity of brick shall be used in the construction of a hollow wall as would be used if the wall were built solid, and many of them require that both portions of the wall shall be at least 8 inches thick if the wall is used as a bearing wall.
For heavy buildings, with steel floor joist and girders, it is better to build the outer wall of the full thickness that would be required of a single wall, and to make the inner wall only 4 inches in thickness, to serve merely as a furring and to receive the plaster. Where fireproof arches are used for the floors this inner wall might without injury rest on the floor arches.