217. Even a solid brick wall readily absorbs moisture and also transmits heat and cold. A driving rainstorm of several days duration will often penetrate a 12 or 16 inch wall, and so dampen the brick as to affect the wall paper or plastering, or spoil fresco decorations. The moisture from the brickwork prevents the mortar, if of lime, from becoming hard, and may communicate dampness to the woodwork, causing rot. Damp walls in a building necessitate the consumption of much more fuel to warm the rooms than when the walls are dry, because the moisture must be evaporated before the temperature of the rooms can be raised.

To overcome this difficulty, several methods are resorted to; the one most in use consists in furring the outside brick with furring strips (see Carpentry), and lath and plaster on these. The danger from fire spreading through these strips, especially in hospitals, schoolhouses, and isolated private residences, has caused many excellent authorities to recommend the use of hollow brick walls.

218. Hollow walls are intended to keep moisture from passing through, and, on account of the air space provided, keep the building much cooler in summer, and warmer in winter. Difficulties in construction are met with, however, that largely offset their advantages, so that hollow walls are seldom used in this country. There is no doubt but that their use might be much extended, with good results, more especially for isolated buildings. The objections to hollow walls are that more ground area is required, and the cost of construction is increased.

The air space should be continuous throughout the wall in order to obtain the full benefit of the space. It is also well to have the bond, or connection between the two parts of the wall, of such shape and material, that any moisture penetrating the outer portion of the wall cannot penetrate the inner part. Brick bonding neutralizes some of the benefit gained, by permitting the passage of moisture through the wall where it is bonded.

It is practically impossible when a wall is penetrated by openings, to provide a continuous air space, though it may be closely approximated.

219. Fig. 85 shows one form of hollow wall with an 8-inch outer wall a; a 2-inch air space b, and a 4-inch inner wall c. This wall is bonded every sixth course in height, and every 8 inches in length, as shown at d. The header bricks e that join the bond d, are three-quarter bats, and the bond bricks have 2 inches bearing on the front wall.

Hollow Walls 87

Fig. 85.

220. Fig. 86 shows a 10-inch wall that has a 4-inch wall a; then the 2-inch air space b, and the inner 4-inch wall c. The bond brick are cut at an angle as shown at d, and where they miter in the front wall the front brick are also cut, as at e. The 2-inch space left in the rear wall c, where the bond brick occurs is filled with a closer f; this closer is a quarter bat.

Hollow Walls 88

Fig. 86.

221. A method of bonding which retains the full benefit of the air space, is by means of metal ties, as shown in Fig. 87. Here a is the 4-inch outer wall; d, the air space; c, the inner 4-inch wall; and d, d, the metal ties. The ties are known as Morse ties, and are made of steel wire galvanized, and from 7 to 16 inches long. Other forms of ties are shown at (e), (f), and (g); these are made of iron and can be procured of any blacksmith. The form shown at (g) is probably the best, as the lugs on the end of the tie fit between the courses of brick. All metal ties should be dipped in hot asphalt to keep them from rusting.

Hollow Walls 89

Fig. 87.

When any of the metal ties d, (r), or (f) are used, they should be spaced every 24 inches in every fourth course. The tie (g), being stronger, need be used in every eighth course only.

222. Fig. 88 shows the construction of a hollow wall around a window opening; a shows the outside 4-inch wall; b, the 2-inch air space; c, c, the 12-inch cellar and the 8-inch first-story inner walls; d is the bonding brick alongside the window opening; e, the window sill; f, the window lintel; g, the brick arch over the window opening; h, the asphalt covering on top of the arch, to prevent any moisture from passing through the hollow space into the window opening; j, the steel wire bonders that bond the inner and outer walls together; kt the second-story floor-beams; l, the first-story beams; and m, a pipe to drain any moisture that might collect in the hollow wall.

Hollow Walls 90

Fig. 88.