QUESTION: I own a house built with an inner and outer brick wall. There is a three-inch unventilated airspace between the walls with steel ties holding them together. Should this space be ventilated or insulated?
ANSWER: Your brick cavity wall is typical of residential masonry construction. This type of wall has a number of advantages. Because of the cavity, the inner wall resists rain penetration. If rain soaks through the exterior wall, the water runs down to the bottom of the air space and drains out through weep holes left for that purpose.
Cavity walls have 25% greater insulating value than solid walls of the same material and thickness. During construction, insulation can be placed in the cavity for additional protection. However, once the walls are built, it's difficult to put insulation into the air space effectively.
Whether or not there is insulation, the walls must be ventilated to drain moisture which accumulates in the cavity either through rain penetration or condensation. The ventilation is provided by weep holes between the bricks in the bottom course.
These weep holes are basically open joints between the bricks every few feet along the lower course and are formed by omitting the mortar. To avoid dampness in the house, make certain no debris has blocked these holes. You can poke them clear by inserting a narrow-bladed screwdriver but take care not to push it through the flashing that keeps water from the inner wall.
Roof Repair Altered Ventilation of Home
Q: After removing the wood shingles from the roof, filling in the roof deck spaces and recovering the roof with asphalt shingles, mildew has formed on the interior walls. Even though we installed two attic gable vents, the problem persists. The house is still not getting enough ventilation, but how can I correct this?
A: By filling in the roof deck spaces, you altered the ventilation pattern of the house. The attic no longer breathes as easily as it used to. This allows moisture to build up and cause mildew.
Gable vents are helpful but only if they are big enough. The louvers often block out about half of the area of the openings they cover. In fact, if you have a gable vent with outside dimensions of 2-by-2 feet, depending on the type of louvers, and on the insect screen used, you may only have one square foot of effective opening. And, with the roof deck filled in, there may be little or no air circulation between the two gable vents.
An attic should have effective openings that are equivalent to 1-300th of its floor area. A single vent opening into the attic, even though it satisfies this total area requirement, is not usually considered adequate.
The best method for ventilating an attic is to combine soffit vents in the fascia under the eaves with gable vents or, better still, with a number of ridge vents. Ridge vents are capped tubes installed through the roof ridge and vented into the attic, which allows heated air to rise and escape. The insulation should be installed at a lower level, between the attic and floor joints, not the rafters, to allow for air circulation.
For further information on any home problem, write to Popular Mechanics, Readers Service Bureau, 224 W . 57th St . , New York, N . Y . 10019.