Exterior Walls

Walls can best be described by the components which form their main structural elements. Exterior walls most commonly found in residential construction therefore, are wood frame, masonry, or wood frame with masonry veneer facing.

A bearing wall is one which supports loads from above, such as a floor, a ceiling or a roof. A non-bearing wall has no load resting on it. Interior non-bearing walls are more appropriately called partitions. Not all exterior walls are bearing walls, since floor or roof construction does not necessarily rest on all exterior walls of a building. Instead, load usually rests on some walls, while the remainder are of the same construction, but serve only as exterior enclosure.

WOOD FRAME:

Frame exterior walls consist of 2” x 4” or 2” x 6” vertical stud members placed at 16 inches or 24 inches on centers, set upon a base “sole plate” of matching dimensions. The sole plate attaches to the concrete or wood floor assembly acting both as a means of alignment of the studs as well as a fastening member. See (Figures 12, 18, and 22). Tops of the studs should be capped with two members, again called plates, for alignment and to form a resting surface for overhead construction. Open spaces between the studs must be insulated to improve the resistance of the wall to heat loss and heat gain. See the detailed discussion on INSULATION.

Exterior faces of the studs should receive a sheathing which is a rigid covering of plywood, ¾” wood boards, or rigid insulation board, to which a layer of building paper and the exterior finish material is applied. See (Figures 34, 35 and 36). Exterior finish materials include wood or metal siding, plywood siding, stucco, masonry, etc. Interior finish materials such as gypsum board, wood paneling, lath and plaster, etc. are attached to the interior surfaces of the studs.

(CAUTION) In cold climates, a vapor resistive material, called a vapor barrier, should be installed on the interior of the wall framing prior to the interior finish materials, to prevent condensation of moisture within the wall.

ill. 34: DIAGONAL WOOD SHEATHING ON WOOD FRAMED EXTERIOR WALL

ill. 35: PLYWOOD SHEATHING ON EXTERIOR WALLS

ill. 36: FIBERBOARD SHEATHING ON EXTERIOR WALLS. Corner or diagonal bracing shall be provided back of fiberboard sheathing except as noted below.

Two studs are recommended in lieu of 2”x4”, because of the increased insulation thickness which can and sh9uld be installed.

Two (2) inch horizontal blocking the full width of the studs should be installed at approximately the vertical mid-point of stud walls for fire-stopping.

Stud walls must be braced in both directions to prevent their collapse from wind and other forces. Floors, roofs, and interior cross partitions provide the necessary bracing for forces which occur against the broad faces of the wall. However, to provide stiffness to resist collapse from the forces which occur along the long thin line of the wall, studs must have plywood sheathing (an excellent stiffener) or wood or metal diagonal bracing. Diagonal bracing members are cut into the faces of the studs, and extend from a high point at each corner, diagonally down, across and fastened to, several studs, terminating at and being fastened to the sill plate at the foundation. See (Figures 22 and 36).

(CAUTION) There is a cost savings trend in the warmer regions of the country where stucco is often used as the exterior finish material, for wood frame houses to be constructed using rigid expanded plastic board (such as Styrofoam), approx. 1 inch in thickness, as the exterior sheathing instead of a more dense or solid material. Wire “chicken mesh” is fastened over the plastic board, and the stucco then applied. While the foam plastic material does provide some slight additional insulating value, it has no nail or fastener holding capabilities, plus is very soft, easily broken and punctured. It would not take much force or effort, in the our opinion, to accidentally (or deliberately) punch an opening thru a wall of this make-up. Security to unauthorized entry is therefore minimal.

Wood frame construction is very susceptible to damage from insects such as termites, water infiltration or leaks, and of course, is combustible. Wood can be specially treated to resist all of these destructive factors, but such treatment greatly increases the cost of the material, and so is not normally done.

MASONRY:

Masonry walls are constructed of brick or concrete block, the minimum allowable thickness for single story heights being usually 8 inches. Masonry may require vertical re-bar reinforcement depending on codes and location. If so, the bars are embedded in cement grout poured into the hollow cores of the masonry. All masonry should have horizontal reinforcement located in every 2nd or 3rd bed joints. See (ill. 11).

(CAUTION) Masonry is not a good insulating material, so in all climates some additional materials should be added to gain additional insulating qualities. A common method in use is to furr—or build out—the interior surfaces of the masonry with wood or metal strips which are fastened to the masonry, between which is installed insulation, and to the faces of which the interior finish is attached. The thicker the furring and insulation, the better the insulating value of the wall. Increasingly common today is to simply construct the equivalent of a 2”x3”, 2”x4” or 2”x6” non-bearing stud wall alongside and against the interior face of the masonry, as interior furring, with full-depth insulation placed between the studs. See (ill. 37).

In order to provide a proper support surface and nailing capabilities for floor or roof members which will rest on the masonry wall, a 2 inch thick treated wood member, or plate, is installed on top of the wall, anchored by steel anchor bolts exactly as described for masonry foundation walls in, under the discussion on ANCHORAGE. See (Figures 12 and 38). If the first floor is wood frame construction, flashing should be installed between the wood construction and the masonry wall, similar to that described for masonry veneer construction. See (ill. 31).

As stated earlier, while masonry has a very low insulation “R” value, it nevertheless has a very high ability to slowly absorb significant amounts of heat; conversely, it's very slow to release that absorbed heat and cool down. These thermal storage and thermal lag characteristics can be used to advantage to help reduce dependency on mechanical heating and /or cooling systems. One way is to attach the insulation and a protective finish to the exterior face of masonry exterior walls, leaving interior faces exposed. By so doing, the masonry is not subject to wide swings of exterior heat build-up or heat loss, while at the same time its thermal lag aspect tends to even out and stabilize interior temperatures.

ill. 37: FURRING and INSULATION ON INS IDE FACE OF WALL; PLAN VIEW SHOWING FULL STUD FURRING

ill. 38: ROOF and CEILING CONSTRUCTION ATTACHMENT TO MASONRY BEARING WALL

In houses designed to take advantage of solar heat, masonry partitions and walls can be strategically located so as to be heated by direct sunlight, with the subsequent slow release of that heat being a major factor in the heating of the interior spaces after the sun has set. Much research and experimentation on this phenomenon has been done in the southwestern United States.

WOOD FRAME WITH MASONRY VENEER:

The structural load-carrying portion of this assembly is a wood framed stud wall exactly as described earlier. The differences in the veneer wall are that the exterior finish is a layer of brick, block or other masonry which is non-structural and non- bearing, but is attached to the back-up stud wall for its support. The sheathing material on the outside face of studs must be of plywood or wood boards which have nail-holding capability, so that the veneer can be periodically fastened—or tied—to the back-up wall. This is necessary because the veneer is relatively thin (usually 4 inches or less in thickness), and does not have the stability to stand alone without back-up support. The usual method of attachment is by galvanized metal strips called wait ties, which are installed in the joints of the masonry as it's being laid up. The wall ties are then bent up and nailed to the sheathing, preferably also into the studs. Because water can, and will, permeate thru most masonry veneer, there should be an air space separation between it and the wood sheathing; and , flashing plus weep holes should be provided at the base sill. See (ill. 31).

INSULATION:

Heat creates molecular activity in materials; the higher the amount of heat, the higher the activity. Molecules of highest activity transmit part of their energy to those of lower activity. Therefore, heat always flows from the hottest materials to the colder. This is the principle of thermal conduction.

Each material has a certain amount of resistance to the conduction—or flow—of heat thru it; some, such as metals, are very low in resistance, and some such as insulating materials have very high resistance to heat flow.

The resistance to heat flow for a 1 inch thick one square foot section of any material is expressed as the coefficient ‘k’. ‘k’ = the rate of heat flow (BTU’s) per degree of temperature, per hour, per square foot, per inch of thickness.

The total thermal resistance of one square foot of a particular material of a fixed thickness is called its Resistivity ‘R’. ‘R’ = the resistance to heat flow (BTU’S) per degree, per hour, per square foot for the particular thickness. The greater the ‘R’ number of a material, the greater the resistance to heat flow, and therefore the greater the insulation value.

The summation of all the individual ‘R’ factors for all materials in a wall, floor or roof equals the total thermal Resistance of the assembly.

‘R’ wall = R1 + R2 + R3 + … + Rn

The overall expression of heat transmission thru walls, roofs, floors, windows, etc. is expressed as ‘U’. The ‘U’ value is equal to One (1) divided by the sum of all the individual ‘R’ values of the materials in the assembly.

Therefore, U = 1 / (R1 + R2 + R3 + … + Rn)

Since ‘U’ is the reciprocal of ‘R’, the lower the number for ‘U’ the better the insulation value of the assembly. Thus, a wall having a ‘U’ 0.1 is much better at resisting the flow of heat thru it than one with ‘U’ = 1.0, by a factor of 10. The opposite would be true for the total Resistivity ‘R’ of the wall.

As the above would suggest, 6 inches of a certain kind of insulation has greater resistance ‘R’ to heat flow than 4 inches of the same material. As proof of this, following are ‘R’ values listed in 1987 by the Certaineed Corp. for various thicknesses of their Fiber Glass Building Insulation Blankets:

There are several sets of energy standards in effect for the country, which set minimum levels of total resistivity ‘R’ for residential floors, sidewalls and ceiling. The recommendations vary depending on the location or ‘Zone’ of the country in which the residence is located. In some areas, heat loss from the building due to cold exterior temperatures and wind chill factors is the main determinant; while, in other areas the need to cool or air-condition because of the generally higher prevailing outdoor temperatures controls; and , in still others, it's a combination of both. In general today the absolute minimum ‘R’ values in the most mild Zones are as follows:

All other Zones increase from these values. There are many types of materials used to make insulating products. All have different ‘k’ values per inch, and therefore different ‘R’ values for various thicknesses. Some are made in flexible blanket form such as the familiar fiberglass insulations, 0 are of pre-formed rigid board material which includes Styrofoam, urethane, wood fiber, etc. Still others can be custom 0 such as spray-on urethane. Each insulation is made for a specific set of purposes and applications, which should be investigated before use.

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