Design and Retrofit of Building Envelope

CHAPTER 1

Introduction

1.1 OVERVIEW

The information presented in this book on building envelope is about concepts, processes and ideas and it looks at the way parts of the envelope work. What the engineer needs for problem solving of building envelope sick syndrome is innovation and reliability. This book is not to offer finite design solutions for building envelopes, but a text where numbers appear to explain performance. Judgment and familiarity of limits of solutions are essential for decision-making in the choice of the system, materials and details of the building envelope.

Wisdom in Building Engineering

In dealing with an interdisciplinary subject such as building envelope systems, it is helpful to remember that:

1.You cannot teach a man anything, you can only help him to find it by himself.

2.When you can measure what you are speaking about, and express it in numbers, you know something about this subject.

1.2 BUILDING ENVELOPE DEFINITION

The building envelope is the enclosure, barrier, and separator of the outdoor environment and the desired indoor environment. It is also the protector from the undesired outdoor threats of danger. It consists of three components: roof, walls (above & below grades) and slab-on-grade, see Fig. 1.1.

Figure 1.1 Components of the Building Envelope

1.3 HUMAN REQUIREMENTS OF BUILDING ENVELOPE

The following are human requirements of the building envelope :

•Sensitivity to thermal changes and air movements

•Sensitivity to vibrations and noise

•Problems of vision, within a building and looking out from a building

•Social aspects and location

•Safety

1.4 BARRIER REQUIRMENTS

The following are the requirements of the building envelope as a barrier between the exterior conditions and the desirable interior environment (Table 1.1):

•Durability

•Strength and Safety

•Rigidity and Control of drift

•Control of heat flow (using insulation)

•Control of air flow (using air barrier)

•Control of vapor flow (using vapor barrier)

•Control of rain penetration (using cavity and rain screen walls)

•Control of solar and other radiation

•Control of sound transmission

•Control of fire protection

•Aesthetically pleasing

•Economical requirements

Table 1.1 Interior/Exterior Barrier Factors

1.5 COMMON PROBLEMS IN BUILDING ENVELOPE

Sick building envelope syndrome is common in modern buildings. Figure 1.2 shows problems encountered in residential buildings with brick veneer cladding in cold climate in North America. Most Common problems are:

1.Moisture / leakage due to water penetration through the building envelope; often creates condensation on the surface or concealed within multi-wythe exterior walls

2.Efflorescence –A form of staining

3.Cracking

4.Interior Staining, and

5.Drafts

Note that moisture penetration through the interior wythe of exterior cavity walls can cause interior staining, mold/mildew that result in poor indoor air quality.

Figure 1.2 Problems in residential buildings with brick veneer in North America

Many of these problems can be avoided during the design stage by adequate integrated design and proper detailing. For retrofitting of existing masonry building envelope refer to chapter 10.

1.6 ENERGY REQUIREMENT FOR BUILDING ENVELOPE

Figure 1.3 shows an energy model for the building envelope. The energy required to maintain desired indoor environment (such as temperature, relative humidity and air flow) is a function of the outside environmental conditions.

Figure 1.3 Energy model for building envelope

1.7 CLASSIFICATION OF EXTERIOR WALLS

Exterior walls in building envelopes can be classified as:

•Permeable: Such as brick veneer

•Impermeable: Such as glass curtain wall

•Single-Wythe: Stone, brick, block (masonry/concrete), or glass/metal framing

•Multi-Wythe: such as cavity walls

Exterior walls can also be classified based on mass:

Heavy Mass Construction -Figure 1.4 shows components of a cavity wall, which is an example of heavy mass construction. This wall type is most common in the north east of the US. Figures 1.5 through 1.9 show examples of institutional buildings with exterior masonry walls and brick veneer suitable to provide energy efficient and sustainable buildings with adequate resistance to rain penetration. The following three types are common for heavy mass construction:

a.Brick veneer (skin) with metal stud backup

b.Brick veneer with block backup

c.Stone cladding (skin) with block backup

Figure 1.4 Components of a cavity wall

Light Mass Construction -Light mass construction includes:

a.Glass curtain wall

b.Metal Curtain Walls

c.Wood Framing

d.Vinyl Siding

1.8 TYPES OF BUILDING ENVELOPES

There are varieties of types of building envelopes depending on building plan, use and location. In the northeast US, most of the walls are cavity walls because of the high demand for thermal resistance and water penetration resistance.

A variety of materials for building envelopes are used. The following are typical examples:

•Glass curtain wall ( Fig. 1.5) with its most obvious properties of light reflection and transmittance

•Brick veneer( Fig. 1.6) , with its rich colors and textures offers attractive and durable material for building envelopes, particularly in the north-east of the US.

•Glazed brick veneer, as shown in (Fig. 1.7), offers attractive and durable cladding of buildings.

•Glazed concrete blocks, as shown in Fig. 1.8, offer different patterns and colors.

•Loadbearing stone masonry ( Fig. 1.9) walls go back to the beginning of the 20 th Century with massive gravity wall construction

•Stone cladding (Fig. 1.10) is popular for high-rise buildings. Surfaces are polished and, therefore, impermeable. In addition, this system has high resistance to rain penetration.

•Stucco (Fig. 1.11) is commonly used for housing and residential construction. Metal lath is used to provide resistance to shrinkage cracking.

•Drivette system & Exterior Foam Insulation System (EFIS): Heavy-duty stucco with a rough durable surface (Fig. 1.12)

•Glass block is an attractive veneer that provides adequate lighting. Note the confining effect of the steel frame (Fig. 1.13)

•Wood cladding; note the flashing at the window top (Fig. 1.14).

Combination of the above systems can be accommodated in one building, if so desired

Figure 1.5 Glass curtain wall

Figure 1.6 Brick veneer

Figure 1.7 Glazed brick veneer

Figure 1.8 Glazed block veneer

Figure 1.9 Loadbearing stone masonry

Figure 1.10 Contemporary stone cladding

Figure 1.11 Stucco veneer

Figure 1.12 EFIS envelope

Figure 1.13 Glass block

Figure 1.14 Wood Cladding

1.9 GLASS CURTAIN WALL CONFIGURATION

A glass curtain wall system typically comprises clear and opaque elements that are critical to adequate performance. The clear component is predominately glazing while the opaque areas may consist of metal panels and louvers, stone ( Fig. 1.5) and concrete panels.

Glass is the most important component of the curtain wall system. It provides the occupants’ vision to the exterior and introduces daylight to the interior surface. Glass is characterized by type, color, size and strength. There are three types of glass :

1-Monolithic-comprises of a single thickness ranging from 0.25 in. (6 mm) to 0.5 in. (12 mm).

2-Insulating units -comprises of two glazing assembly each has two glass lites separated by an air space. Spacers with primary and secondary seal are provided between the exterior and the interior assemblies. For more details, refer to Section 5.14.

3-Laminated glass-consists of multiple layers of monolithic glass lites assembled with a laminate interlayer membrane between the lites.

1.10 COMPONENTS OF MASONRY CAVITY WALL

Figure 1.15 shows a typical exterior masonry wall. The following are the wall’s components:

1.Clay or concrete brick veneer. Typically, 4 in. (100 mm) thick

2.Air space [1.5-2.0 in. (38-51 mm)]-to drain water from rain penetration

3.Air barrier-to control air flow

4.Board insulation-to control thermal flow

5.Vapor barrier-to prevent concealed condensation

6.CMU backup wall-to support veneer and resist lateral loads

Figure 1.15 Exterior cavity masonry wall with brick veneer

1.11 DESIGN CONSIDERATIONS

Many design considerations have to be accounted for to achieve high performance and sustainable building envelope. There considerations are:

1. Architectural

•Building layout and shape

•Efficiency

•Fenestration (windows for lighting) -also consider area, distribution and orientation

•Detailing

•Movement joints

2. Structural

•Load determination

•Load distribution

•Calculation of internal forces

•Resistance/Material

•Serviceability

•Strength

•Relative in-plane moments between the structural frame/wall and the skin/cladding

3. Environmental

•Thermal

•Moisture/Air

•Sound

•Fire

4. Mechanical

•Heat

•Ventilation

•Air-Conditioning

1.12 HIGH PERFORMANCE BUILDING ENVELOPE DESIGN

Building should provide the following features for high performance:

•Structural soundness

•Functionality

•Initial low cost

•Low maintenance cost

•Aesthetically pleasing

•Durability / longevity

A more recent design method called Performance-Based Design calls for specifying expected performance based building type, importance of the building and accepted level of risk.

1.13 COMPONENTS OF BUILDING SKIN

Figure 1.16 shows the following components of a typical brick veneer system:

1.Material: brick, block, concrete, stone, glass, wood, stucco

2.Windows: type, size, distribution, orientation

3.Movement Joints: horizontal and vertical joints to accommodate thermal and moisture expansion (refer to chapter 3)