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1

An understanding of the building envelope is important for real estate professionals for multiple reasons:


1. To properly inspect and analyze the market appeal of a commercial building you need to understand its basic construction and perform-ance goals.


2. The observed state of the building envelope offers clues about the overall condition of the building and can be an indicator of possible deferred maintenance and the need for major repairs. These symptoms may be evidence of larger problems, for instance:
 


The building envelope is the combination of elements that separate the building’s exterior and interior environments. This is also commonly referred to as the building enclosure by architects and engineers. For the purpose of this text, the two should be considered synonymous.
 
− Conspicuous water staining on the ceiling of upper floor levels may indicate a progressive failure of the roof system, which could be very costly to repair.


− Poor sealing around window glazing may indicate serious issues with moisture infiltration within the building envelope cavities.


− The upper corners of the wall cladding near the roof line and cornices may show signs of deterioration.


− Exterior doors and windows that are dificult to open or close may indicate excessive differential settlement.


− Poor interior air quality may compromise tenant comfort, leading to leasing issues.


3. The effectiveness of the building envelope is also an important factor in determining the mechanical and electrical system requirements and overall energy eficiency of the building. As the cost of energy for heating, cooling, and ventilation continues to increase, commercial building tenants and owner-occupiers are paying more attention to the Operations and Maintenance (O&M) expenses. A poorly designed or maintained building envelope will likely translate into extra-ordinary O&M expenses – a symptom of a non-competitive building that will impact market value.

An understanding of the building envelope is important for real estate professionals for multiple reasons:


1. To properly inspect and analyze the market appeal of a commercial building you need to understand its basic construction and perform-ance goals.


2. The observed state of the building envelope offers clues about the overall condition of the building and can be an indicator of possible deferred maintenance and the need for major repairs. These symptoms may be evidence of larger problems, for instance:
 


The building envelope is the combination of elements that separate the building’s exterior and interior environments. This is also commonly referred to as the building enclosure by architects and engineers. For the purpose of this text, the two should be considered synonymous.
 
− Conspicuous water staining on the ceiling of upper floor levels may indicate a progressive failure of the roof system, which could be very costly to repair.


− Poor sealing around window glazing may indicate serious issues with moisture infiltration within the building envelope cavities.


− The upper corners of the wall cladding near the roof line and cornices may show signs of deterioration.


− Exterior doors and windows that are dificult to open or close may indicate excessive differential settlement.


− Poor interior air quality may compromise tenant comfort, leading to leasing issues.


3. The effectiveness of the building envelope is also an important factor in determining the mechanical and electrical system requirements and overall energy eficiency of the building. As the cost of energy for heating, cooling, and ventilation continues to increase, commercial building tenants and owner-occupiers are paying more attention to the Operations and Maintenance (O&M) expenses. A poorly designed or maintained building envelope will likely translate into extra-ordinary O&M expenses – a symptom of a non-competitive building that will impact market value.

2

BUILDING ENVELOPE PERFORMANCE GOALS

 

The main purpose of the building envelope is to provide protection from the elements, such as temperature fluctuations, solar radiation, air pressure, wind, and humidity.

 

The building envelope is designed to resist transfer of water, air, water vapour, and sound from exterior to interior environments. In order of mention, this resistance is provided by the cladding, air barrier, vapour barrier, and insulation.

 

The envelope must also transmit adequate levels of light to the interior through proper placement of windows. Finally, the envelope contributes to the building’s aesthetics, forming much of its external appearance. The architect’s goal in designing the envelope system is to achieve all of these goals at a reasonable cost.

BUILDING ENVELOPE PERFORMANCE GOALS

 

The main purpose of the building envelope is to provide protection from the elements, such as temperature fluctuations, solar radiation, air pressure, wind, and humidity.

 

The building envelope is designed to resist transfer of water, air, water vapour, and sound from exterior to interior environments. In order of mention, this resistance is provided by the cladding, air barrier, vapour barrier, and insulation.

 

The envelope must also transmit adequate levels of light to the interior through proper placement of windows. Finally, the envelope contributes to the building’s aesthetics, forming much of its external appearance. The architect’s goal in designing the envelope system is to achieve all of these goals at a reasonable cost.

3

BUILDING ENVELOPE PERFORMANCE GOALS

 

The location and mass of buildings, structures, and other activities (e.g., vehicle trafic) in urban and suburban settings will also impact the internal environment of a building. Some examples of the urban or built environ-

ment impacting the building envelope are as follows:

• Smoke or emissions from industrial activities

• Heat sinks associated with the concentration of large buildings in a downtown core area

• Wind patterns created by buildings of different height, mass, orien-tation influencing the wind speed and direction of wind impacting a building envelope

• Solar shading from adjacent structures

• Pollution and noise from heavy vehicle traffic

BUILDING ENVELOPE PERFORMANCE GOALS

 

The location and mass of buildings, structures, and other activities (e.g., vehicle trafic) in urban and suburban settings will also impact the internal environment of a building. Some examples of the urban or built environment impacting the building envelope are as follows:

• Smoke or emissions from industrial activities

• Heat sinks associated with the concentration of large buildings in a downtown core area

• Wind patterns created by buildings of different height, mass, orien-tation influencing the wind speed and direction of wind impacting a building envelope

• Solar shading from adjacent structures

• Pollution and noise from heavy vehicle traffic

4

BUILDING ENVELOPE PERFORMANCE GOALS

 

In addition to resisting external natural and other local influences, a building envelope must also satisfy safety needs, aesthetics, and archi-tectural-design goals. As well, the serviceability of the building envelope must be considered at the design stage.

 

For example, access to windows for cleaning, to cladding for painting, to the roof for repairs, and so on, should be a consideration at the design stage to avoid future costly renovations. These factors play a large role in the cost of the building; as serviceability requirements decrease, so do building envelope costs. Durability of the building envelope is important because it can be expensive to repair or replace roofs and cladding, and future costs can be optimized with intelligent forecasts of lifecycle costs for the building envelope.

BUILDING ENVELOPE PERFORMANCE GOALS

 

In addition to resisting external natural and other local influences, a building envelope must also satisfy safety needs, aesthetics, and archi-tectural-design goals. As well, the serviceability of the building envelope must be considered at the design stage.

 

For example, access to windows for cleaning, to cladding for painting, to the roof for repairs, and so on, should be a consideration at the design stage to avoid future costly renovations. These factors play a large role in the cost of the building; as serviceability requirements decrease, so do building envelope costs. Durability of the building envelope is important because it can be expensive to repair or replace roofs and cladding, and future costs can be optimized with intelligent forecasts of lifecycle costs for the building envelope.

5

Thermal Performance

In order to maintain a certain temperature within a building there is usually a requirement to expend energy either to cool or heat the interior air. The amount of energy required to accomplish this will reflect the buildings overall energy eficiency. There are many sources of energy loss. The flow of heat is one, while air loss is another factor. Other areas of energy loss are related to the eficiency of the heating and cooling systems which contain a multitude of electric components such as motors and pumps.

Thermal Performance

In order to maintain a certain temperature within a building there is usually a requirement to expend energy either to cool or heat the interior air. The amount of energy required to accomplish this will reflect the buildings overall energy eficiency. There are many sources of energy loss. The flow of heat is one, while air loss is another factor. Other areas of energy loss are related to the eficiency of the heating and cooling systems which contain a multitude of electric components such as motors and pumps.

6

Regulating heat flow is dependent on the outside temperature.

Generally humans are comfortable when the interior environment is 20 degrees Celsius. If the exterior temperature is cooler, the goal is to prevent heat from flowing out. If the exterior is warmer, the goal is to prevent heat from flowing in. Newton’s law of cooling states that the rate of heat flow is proportional to the difference in temperature. Thus, the more extreme the outside temperature is the greater resistance to heat flow is required.

There are numerous ways that heat can flow through the building enclosure. Heat travels by:

 

conduction, between two touching solids such as a pot and an element on the stove;

 

radiant flow, heat waves flowing off hot objects to cooler zones, such as soup left out to cool; and

 

convection, hot liquid or gas rising to be replaced by cooler liquid or gas, such as with boiling water.

Regulating heat flow is dependent on the outside temperature.

Generally humans are comfortable when the interior environment is 20 degrees Celsius. If the exterior temperature is cooler, the goal is to prevent heat from flowing out. If the exterior is warmer, the goal is to prevent heat from flowing in. Newton’s law of cooling states that the rate of heat flow is proportional to the difference in temperature. Thus, the more extreme the outside temperature is the greater resistance to heat flow is required.

There are numerous ways that heat can flow through the building enclosure. Heat travels by:

 

conduction, between two touching solids such as a pot and an element on the stove;

 

radiant flow, heat waves flowing off hot objects to cooler zones, such as soup left out to cool; and

 

convection, hot liquid or gas rising to be replaced by cooler liquid or gas, such as with boiling water.

7

Thermal Performance

 

Conduction allows heat from the building to flow through a continuous path of wall materials that connect the indoors to the outdoors. Consider a wall framed from 2×6 studs: the interior surface will normally be gypsum wallboards screwed to the wall studs; the exterior has plywood sheathing nailed to the stud; and then the exterior finishes such as siding or stucco are fastened to the sheathing.

 

While there is a cavity between the studs, the studs themselves create a “thermal bridge” connecting the interior and exterior environments.

 

Within the same 2×6 wall assembly, radiant heat flow can occur across the cavity between the studs. Without insulation, heat can radiate from the warm side to the cold side. Just like the way you can feel the heat radiating away from the hot element on a stove. Similarly, occupants can lose heat by radiant flow such as when standing next to a single-paned window on a cold winter day.

 

Continuing with the 2×6 wall assembly example, convection will also occur within the cavity between the studs. Again, if left uninsulated, convection currents will develop because the air adjacent to the warm side will rise while the air on the cold side will drop. Thus the air will follow a circular path, as air rises past the warm side it will pick up heat and as it drops down past the cold side it will lose heat.

Thermal Performance

 

Conduction allows heat from the building to flow through a continuous path of wall materials that connect the indoors to the outdoors. Consider a wall framed from 2×6 studs: the interior surface will normally be gypsum wallboards screwed to the wall studs; the exterior has plywood sheathing nailed to the stud; and then the exterior finishes such as siding or stucco are fastened to the sheathing.

 

While there is a cavity between the studs, the studs themselves create a “thermal bridge” connecting the interior and exterior environments.

 

Within the same 2×6 wall assembly, radiant heat flow can occur across the cavity between the studs. Without insulation, heat can radiate from the warm side to the cold side. Just like the way you can feel the heat radiating away from the hot element on a stove. Similarly, occupants can lose heat by radiant flow such as when standing next to a single-paned window on a cold winter day.

 

Continuing with the 2×6 wall assembly example, convection will also occur within the cavity between the studs. Again, if left uninsulated, convection currents will develop because the air adjacent to the warm side will rise while the air on the cold side will drop. Thus the air will follow a circular path, as air rises past the warm side it will pick up heat and as it drops down past the cold side it will lose heat.

8

Thermal Performance

 

Thermal conductivity is the term that describes the ability of the building enclosure to moderate heat flow. It can also be thought of as measuring the building envelope’s ability to protect the occupants from daily and annual fluctuations in outside air temperature.

 

Thermal conductivity is described in two ways, as follows:

 

The heat transfer coeficient of building envelope materials is referred to as the U-value. The U-value is an indicator of how well a material conducts heat. It is measured as the amount of energy lost over a given area for a given temperature difference, described in Watts per square metre per Kelvin. A high U-value means the material is a good conductor and a poor insulator. A low U-value means the material is a poor conductor and a good insulator.

 

The overall thermal resistance of the building envelope components is referred to as the R-value. The R-value is simply the mathematical inverse of the U-value. It is the area and temperature difference that cause a given amount of energy loss – measured in Kelvin square metre per Watts. The higher the R-value, the greater the thermal resistance of a material or the overall building wall system. In Canadian climates, the recommended R-value for commercial construction wall assemblies ranges from 18 to 31, depending on the region (National Energy Code 2011). For example, in Edmonton (climate zone 7A), the minimum above grade wall insulation

Thermal Performance

 

Thermal conductivity is the term that describes the ability of the building enclosure to moderate heat flow. It can also be thought of as measuring the building envelope’s ability to protect the occupants from daily and annual fluctuations in outside air temperature.

 

Thermal conductivity is described in two ways, as follows:

 

The heat transfer coeficient of building envelope materials is referred to as the U-value. The U-value is an indicator of how well a material conducts heat. It is measured as the amount of energy lost over a given area for a given temperature difference, described in Watts per square metre per Kelvin. A high U-value means the material is a good conductor and a poor insulator. A low U-value means the material is a poor conductor and a good insulator.

 

The overall thermal resistance of the building envelope components is referred to as the R-value. The R-value is simply the mathematical inverse of the U-value. It is the area and temperature difference that cause a given amount of energy loss – measured in Kelvin square metre per Watts. The higher the R-value, the greater the thermal resistance of a material or the overall building wall system. In Canadian climates, the recommended R-value for commercial construction wall assemblies ranges from 18 to 31, depending on the region (National Energy Code 2011). For example, in Edmonton (climate zone 7A), the minimum above grade wall insulation

9

Thermal Performance

 

Within the building industry it is common to refer to windows in terms of their U-value while walls and roofs are referred to in terms of their R-value. In reality these values reflect the same thermal performance properties, just expressed differently. An R-Value of 27 is a U-Value of 0.037 (the inverse of the R-value, U = 1/R).

Adequate insulation and an airtight building envelope can reduce heat loss in winter and heat gain in summer. Insulation may consist of mineral or glass fibre batts, various types of rigid insulation such as polystyrene and polyurethane, or formed in place insulation such as polyurethane.

 

Air should also be prevented from passing between the inside and outside of the building by proper functioning of the air barrier, and air velocity within the building should be regulated so that occupants are not exposed to drafts. Poor thermal control of a building will waste conditioned air, allow excess noise penetration, and can allow water vapour to enter and condense within the walls.

 

One of the primary functions of any wall assembly is to regulate the air temperature within the building. In general the temperature inside is different than the temperature outside. The thermal gradient is the gradual change in the temperature within the wall that results from one side being warmer and one side being cooler. For example, in winter the outside of the wall is cold and the inside is warm – the temperature gradient illustrates how this temperature change occurs within the wall.

The thermal gradient for a wall could be graphically plotted. This will show the temperature at any given point within the wall. This can be used to determine the exact location of the dew point within the wall. The dew point in the wall reflects the temperature at which the water vapour in the air condenses into water; this will also be dependent on the relative humidity of the air within the wall. This is critical in determining the appropriate location of the vapour barrier, in order to stop condensation from forming within the wall cavity.

 

Plotting the thermal gradient and dew point                  the air condenses into water; this will also be dependent on the relative

Thermal Performance

 

Within the building industry it is common to refer to windows in terms of their U-value while walls and roofs are referred to in terms of their R-value. In reality these values reflect the same thermal performance properties, just expressed differently. An R-Value of 27 is a U-Value of 0.037 (the inverse of the R-value, U = 1/R).

Adequate insulation and an airtight building envelope can reduce heat loss in winter and heat gain in summer. Insulation may consist of mineral or glass fibre batts, various types of rigid insulation such as polystyrene and polyurethane, or formed in place insulation such as polyurethane.

 

Air should also be prevented from passing between the inside and outside of the building by proper functioning of the air barrier, and air velocity within the building should be regulated so that occupants are not exposed to drafts. Poor thermal control of a building will waste conditioned air, allow excess noise penetration, and can allow water vapour to enter and condense within the walls.

 

One of the primary functions of any wall assembly is to regulate the air temperature within the building. In general the temperature inside is different than the temperature outside. The thermal gradient is the gradual change in the temperature within the wall that results from one side being warmer and one side being cooler. For example, in winter the outside of the wall is cold and the inside is warm – the temperature gradient illustrates how this temperature change occurs within the wall.

The thermal gradient for a wall could be graphically plotted. This will show the temperature at any given point within the wall. This can be used to determine the exact location of the dew point within the wall. The dew point in the wall reflects the temperature at which the water vapour in the air condenses into water; this will also be dependent on the relative humidity of the air within the wall. This is critical in determining the appropriate location of the vapour barrier, in order to stop condensation from forming within the wall cavity.

 

Plotting the thermal gradient and dew point                  the air condenses into water; this will also be dependent on the relative

10

Thermal Performance

 

The vapour barrier should be located on the “warm side” of the dew point. A vapour barrier, as its name implies, prevents the movement of vapour and is typically comprised of a polyethylene sheet. In a typical residential wood frame wall assembly, the vapour barrier should be attached to the wood studs on the interior side (under the drywall finish).

 

Thus, warm moisture laden air is prevented from entering the wall cavity where it may be cooled suficiently to form condensation within the batt insulation.

 

This is akin to condensation that forms on the bathroom mirror after a shower – the warm, humid air from the shower condenses when it touches the cool surface of the mirror.

Thermal Performance

 

The vapour barrier should be located on the “warm side” of the dew point. A vapour barrier, as its name implies, prevents the movement of vapour and is typically comprised of a polyethylene sheet. In a typical residential wood frame wall assembly, the vapour barrier should be attached to the wood studs on the interior side (under the drywall finish).

 

Thus, warm moisture laden air is prevented from entering the wall cavity where it may be cooled suficiently to form condensation within the batt insulation.

 

This is akin to condensation that forms on the bathroom mirror after a shower – the warm, humid air from the shower condenses when it touches the cool surface of the mirror.

11

Thermal Performance

 

Openings in the building envelope create a number of building design challenges. Windows and doors may decrease the cover area of thermal insulation and reduce the fire resistance and noise reduction capability of the building envelope. However, modern windows and doors create fewer of these problems than in the past, with higher-performance, lower-maintenance materials and more advanced installation techniques. For example, modern glazing is normally multi-paned and can include reflective coatings to mitigate heat loss or gain.

Thermal Performance

 

Openings in the building envelope create a number of building design challenges. Windows and doors may decrease the cover area of thermal insulation and reduce the fire resistance and noise reduction capability of the building envelope. However, modern windows and doors create fewer of these problems than in the past, with higher-performance, lower-maintenance materials and more advanced installation techniques. For example, modern glazing is normally multi-paned and can include reflective coatings to mitigate heat loss or gain.

12

Thermal Performance

 

Thermal mass: building components that absorb excess heat and radiate it back to the space when the external heat source has been removed. All parts of the building contribute to its thermal mass. However, more dense and massive components such as concrete walls and floor slabs will have greater thermal mass effects. For example a south-facing concrete wall will absorb heat during the day, reducing the heat build-up inside the building. In the evening, when the air cools down, the concrete wall will release the stored energy as radiant heat.
 

Thermal Performance

 

Thermal mass: building components that absorb excess heat and radiate it back to the space when the external heat source has been removed. All parts of the building contribute to its thermal mass. However, more dense and massive components such as concrete walls and floor slabs will have greater thermal mass effects. For example a south-facing concrete wall will absorb heat during the day, reducing the heat build-up inside the building. In the evening, when the air cools down, the concrete wall will release the stored energy as radiant heat.

 

13

Thermal bridging: building components with higher thermal conductivity that conduct heat more rapidly through an insulated building assembly, such as a steel stud in an insulated stud wall. Insulation reduces the effects of thermal bridging.

 

However, thermal bridging cannot be entirely avoided. For example, insulation must be fastened to the structure using metal screws or nails; these fasteners will act as thermal bridges due their high thermal conductivity.

Thermal bridging: building components with higher thermal conductivity that conduct heat more rapidly through an insulated building assembly, such as a steel stud in an insulated stud wall. Insulation reduces the effects of thermal bridging.

 

However, thermal bridging cannot be entirely avoided. For example, insulation must be fastened to the structure using metal screws or nails; these fasteners will act as thermal bridges due their high thermal conductivity.

14

Radiant barriers: low emissivity glazing (“Low-E” glass) is a coated glass that reduces the amount of heat transference through a window, keeping the heat out in the summer and restricting loss of interior heat in winter.

Radiant barriers: low emissivity glazing (“Low-E” glass) is a coated glass that reduces the amount of heat transference through a window, keeping the heat out in the summer and restricting loss of interior heat in winter.

15

Convective transfer resistance: techniques to limit the loss of heat by convection, or the natural tendency for warm air to rise and cool air to settle.

 

This can be accomplished by limiting the space between panes in double-glazed windows. If this gap is too large, the air between the sheets of glass may start to move in a loop between the two sheets, creating a convective cell.

 

This movement increases heat transfer between the cold pane and the warm pane and results in heat loss. A convection oven works using this principle: a fan moves hot air in a loop through the oven and increases heat transfer to the food, which reduces cooking time.

Convective transfer resistance: techniques to limit the loss of heat by convection, or the natural tendency for warm air to rise and cool air to settle.

 

This can be accomplished by limiting the space between panes in double-glazed windows. If this gap is too large, the air between the sheets of glass may start to move in a loop between the two sheets, creating a convective cell.

 

This movement increases heat transfer between the cold pane and the warm pane and results in heat loss. A convection oven works using this principle: a fan moves hot air in a loop through the oven and increases heat transfer to the food, which reduces cooking time.

16


Moisture Resistance

Moisture intrusion can result in a range of problems including corrosion of metal studs, metal ties and cladding, staining, rot in wood-framing and plywood, and reduced effectiveness of insulation. Water infiltration may eventually lead to human health issues related to mould and poor interior air quality.

 


There are three factors that can influence the water tightness of a building: the presence of water; an opening in the envelope; and a driving force. It is impossible to keep the outside of a building absolutely dry, therefore the presence of water must always be assumed.

 

Minimizing the number of openings in a building envelope may be possible, but this will usually be in conflict with architectural design requirements that will normally prevail.


Moisture Resistance

Moisture intrusion can result in a range of problems including corrosion of metal studs, metal ties and cladding, staining, rot in wood-framing and plywood, and reduced effectiveness of insulation. Water infiltration may eventually lead to human health issues related to mould and poor interior air quality.

 


There are three factors that can influence the water tightness of a building: the presence of water; an opening in the envelope; and a driving force. It is impossible to keep the outside of a building absolutely dry, therefore the presence of water must always be assumed.

 

Minimizing the number of openings in a building envelope may be possible, but this will usually be in conflict with architectural design requirements that will normally prevail.

17

Moisture Resistance

 

During the design and construction of the building envelope, many forces must be eliminated or neutralized to contribute to a water tight structure. These forces include:

 

Gravity: water flows “downhill”; e.g., off the roof down the wall.

Momentum: once in motion, water has momentum that requires a stopping force in order to bring it to rest; e.g., rain running down a sloped roof may have suficient momentum to be driven under a flashing. Wind-driven rain is another example of this effect.

 

Surface tension: a tension force that builds up on the surface of a liquid (e.g., how insects can “walk” on the surface of the water) – surface tension can “pull” water through small openings in a building envelope.

 

Capillary action: water will flow into very small holes or tubes, often in defiance of gravity: e.g., water in contact with the base of a concrete wall can be “wicked” up the wall.

 

Vapour diffusion: vapour exerts a pressure similar to air pressure called “vapour pressure” which is proportional to absolute humidity and temperature. Like air pressure, vapour pressure tends to equalize; i.e., there is a flow of vapour from a region of high humidity to low humidity.

Air pressure: Water vapour is carried by the air in which it is contained. Outside air can be drawn into the building when the exterior air pressure is higher than the interior air pressure. This process also works in reverse, where inside air can be drawn through the walls to the outside. If air leaks through the building envelope and cools due to the “thermal gradient”, the dew point will be reached and some of the vapour will condense into water within the envelope.

 

 

Moisture Resistance

 

During the design and construction of the building envelope, many forces must be eliminated or neutralized to contribute to a water tight structure. These forces include:

 

Gravity: water flows “downhill”; e.g., off the roof down the wall.

Momentum: once in motion, water has momentum that requires a stopping force in order to bring it to rest; e.g., rain running down a sloped roof may have suficient momentum to be driven under a flashing. Wind-driven rain is another example of this effect.

 

Surface tension: a tension force that builds up on the surface of a liquid (e.g., how insects can “walk” on the surface of the water) – surface tension can “pull” water through small openings in a building envelope.

 

Capillary action: water will flow into very small holes or tubes, often in defiance of gravity: e.g., water in contact with the base of a concrete wall can be “wicked” up the wall.

 

Vapour diffusion: vapour exerts a pressure similar to air pressure called “vapour pressure” which is proportional to absolute humidity and temperature. Like air pressure, vapour pressure tends to equalize; i.e., there is a flow of vapour from a region of high humidity to low humidity.

Air pressure: Water vapour is carried by the air in which it is contained. Outside air can be drawn into the building when the exterior air pressure is higher than the interior air pressure. This process also works in reverse, where inside air can be drawn through the walls to the outside. If air leaks through the building envelope and cools due to the “thermal gradient”, the dew point will be reached and some of the vapour will condense into water within the envelope.

18

One of the primary functions of any wall assembly is to regulate the air temperature within the building. In general the temperature inside is different than the temperature outside.

 

The thermal gradient is the gradual change in the temperature within the wall that results from one side being warmer and one side being cooler. For example, in winter the outside of the wall is cold and the inside is warm – the temperature gradient illustrates how this temperature change occurs within the wall.

 

The thermal gradient for a wall could be graphically plotted. This will show the temperature at any given point within the wall. This can be used to determine the exact location of the dew point within the wall.

 

The dew point in the wall reflects the temperature at which the water vapour in the air condenses into water; this will also be dependent on the relative humidity of the air within the wall. This is critical in determining the appropriate location of the vapour barrier, in order to stop condensation from forming within the wall cavity.

One of the primary functions of any wall assembly is to regulate the air temperature within the building. In general the temperature inside is different than the temperature outside.

 

The thermal gradient is the gradual change in the temperature within the wall that results from one side being warmer and one side being cooler. For example, in winter the outside of the wall is cold and the inside is warm – the temperature gradient illustrates how this temperature change occurs within the wall.

 

The thermal gradient for a wall could be graphically plotted. This will show the temperature at any given point within the wall. This can be used to determine the exact location of the dew point within the wall.

 

The dew point in the wall reflects the temperature at which the water vapour in the air condenses into water; this will also be dependent on the relative humidity of the air within the wall. This is critical in determining the appropriate location of the vapour barrier, in order to stop condensation from forming within the wall cavity.

19

Thermal Performance

 

The vapour barrier should be located on the “warm side” of the dew point. A vapour barrier, as its name implies, prevents the movement of vapour and is typically comprised of a polyethylene sheet. In a typical resi-dential wood frame wall assembly, the vapour barrier should be attached to the wood studs on the interior side (under the drywall finish). Thus, warm moisture laden air is prevented from entering the wall cavity where it may be cooled suficiently to form condensation within the batt insulation. This is akin to condensation that forms on the bathroom mirror after a shower – the warm, humid air from the shower condenses when it touches the cool surface of the mirror.

Thermal Performance

 

The vapour barrier should be located on the “warm side” of the dew point. A vapour barrier, as its name implies, prevents the movement of vapour and is typically comprised of a polyethylene sheet. In a typical resi-dential wood frame wall assembly, the vapour barrier should be attached to the wood studs on the interior side (under the drywall finish). Thus, warm moisture laden air is prevented from entering the wall cavity where it may be cooled suficiently to form condensation within the batt insulation. This is akin to condensation that forms on the bathroom mirror after a shower – the warm, humid air from the shower condenses when it touches the cool surface of the mirror.

20

There are three general approaches to controlling the entry of external moisture into a building: mass-wall systems, face-sealed systems, and pressure equalized wall systems.

 

Mass-wall systems are the historic approach that relies on the thickness of the wall to shed or deflect most of the moisture from rain and absorb the remaining surface moisture. Multi-wythe masonry walls in historic brick buildings work in this manner. Care must be taken when retrofitting such buildings so as not to disrupt the natural ability for the masonry to absorb moisture and dry out. Exterior paint and interior finishes can be very detrimental to such systems as moisture can get trapped or driven back into the building.

 

Face-sealed systems have limited application in rainy climates. They rely on waterproof exterior cladding with well-sealed joints. Such systems require a high degree of ongoing maintenance because sealed joints will inevitably fail, establishing clear pathways for moisture to enter the wall cavity. In BC’s Lower Mainland, the numerous buildings constructed in the 80s and 90s had face-sealed wall assemblies that ultimately failed, leading to the well-docu-mented “leaky condo crisis”.

 

Another problem with face-sealed systems in modern insulated buildings is that they are susceptible to changes in exterior versus interior wall temperatures. CMHC reports that the main problem with these systems is thermal movement and cracking, with failures typically occurring at the joints. Related problems are the deterioration of sealants with the impact of constant humidity, freezing and thawing, and direct sunlight. Due to the nature of the wall system, moisture entering the wall cavity will often become trapped, amplifying the impacts.

 

Pressure equalized wall systems are the norm in most modern building enclosures. Such walls are sometimes referred to as rain-screen wall systems. These systems are very robust in terms of keeping water out of the building. The exterior cladding is separated from the main structural wall with a cavity that is pressure equalized with the outside pressure. Water that manages to migrate past the outer cladding surface can drain away, provided the cavity is wide enough to create a capillary break so that water cannot wick across. Without this cavity, water on the exterior wall surface can get drawn into the wall because air pressure inside buildings is often lower than the air pressure outside.

There are three general approaches to controlling the entry of external moisture into a building: mass-wall systems, face-sealed systems, and pressure equalized wall systems.

 

Mass-wall systems are the historic approach that relies on the thickness of the wall to shed or deflect most of the moisture from rain and absorb the remaining surface moisture. Multi-wythe masonry walls in historic brick buildings work in this manner. Care must be taken when retrofitting such buildings so as not to disrupt the natural ability for the masonry to absorb moisture and dry out. Exterior paint and interior finishes can be very detrimental to such systems as moisture can get trapped or driven back into the building.

 

Face-sealed systems have limited application in rainy climates. They rely on waterproof exterior cladding with well-sealed joints. Such systems require a high degree of ongoing maintenance because sealed joints will inevitably fail, establishing clear pathways for moisture to enter the wall cavity. In BC’s Lower Mainland, the numerous buildings constructed in the 80s and 90s had face-sealed wall assemblies that ultimately failed, leading to the well-docu-mented “leaky condo crisis”.

 

Another problem with face-sealed systems in modern insulated buildings is that they are susceptible to changes in exterior versus interior wall temperatures. CMHC reports that the main problem with these systems is thermal movement and cracking, with failures typically occurring at the joints. Related problems are the deterioration of sealants with the impact of constant humidity, freezing and thawing, and direct sunlight. Due to the nature of the wall system, moisture entering the wall cavity will often become trapped, amplifying the impacts.

 

Pressure equalized wall systems are the norm in most modern building enclosures. Such walls are sometimes referred to as rain-screen wall systems. These systems are very robust in terms of keeping water out of the building. The exterior cladding is separated from the main structural wall with a cavity that is pressure equalized with the outside pressure. Water that manages to migrate past the outer cladding surface can drain away, provided the cavity is wide enough to create a capillary break so that water cannot wick across. Without this cavity, water on the exterior wall surface can get drawn into the wall because air pressure inside buildings is often lower than the air pressure outside.

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The Effects of Air Pressure, Humidity, and Wind

 

Multiple external environmental factors interact with the building envelope to influence the temperature and humidity of the interior space, and consequently occupant comfort. As well, building occupants and mechanical systems also generate heat and humidity. These internal influences also need to be taken into account when designing the appropriate envelope system.

 


Humidity impacts buildings in the form of water vapour and condensed water vapour in the form of rain and snow.

 

An effective building envelope should function to deflect rain, allow moisture to drain away from the building, and allow buildings to dry after exposure to humidity and moisture.

 

Since the impact of humidity is closely related to air pressure and wind, it is useful to think of these three factors as a combined environmental influence impacting the building envelope.

 

The specialized study of the interaction of these forces on buildings is referred to as building physics. To gain a basic understanding of building physics one must begin by examining the influence of air pressure, humidity, and wind separately.

The Effects of Air Pressure, Humidity, and Wind

 

Multiple external environmental factors interact with the building envelope to influence the temperature and humidity of the interior space, and consequently occupant comfort. As well, building occupants and mechanical systems also generate heat and humidity. These internal influences also need to be taken into account when designing the appropriate envelope system.


Humidity impacts buildings in the form of water vapour and condensed water vapour in the form of rain and snow. 

 

An effective building envelope should function to deflect rain, allow moisture to drain away from the building, and allow buildings to dry after exposure to humidity and moisture. 

 

Since the impact of humidity is closely related to air pressure and wind, it is useful to think of these three factors as a combined environmental influence impacting the building envelope. 

 

The specialized study of the interaction of these forces on buildings is referred to as building physics. To gain a basic understanding of building physics one must begin by examining the influence of air pressure, humidity, and wind separately.

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The Effects of Air Pressure, Humidity, and Wind

 

Air pressure is always present within and outside the building envelope. To understand the impact of air pressure on buildings we need some basics in building physics. The air pressure inside a building will typically be different than the outside atmospheric air pressure, also known as the barometric pressure. The outside barometric pressure will depend on the altitude (air is less dense at higher elevations), the time of day, and impact of the sun on warming air, clouds, and wind. While air pressure is constantly changing (rising or falling) outside the building, the goal of the building designers is often to maintain a constant air pressure within the building to control air circulation, cooling, and heating within a narrow set of parameters for thermal comfort.
A perfectly sealed building, essentially a spaceship, would not be affected by changes in barometric pressure. The reality is that all buildings, no matter how eficient and modern, permit air leakage in and out. Common points of air leakages in interior and external surfaces of wall systems are around windows, doors, vent openings, unions between wall panels, and apertures required for mechanical and electrical systems. Gaps in vapour membranes between the finished interior of a building envelope and the insulation are the main source of air leakage from interior space. Roof systems with poorly or incorrectly installed and sealed flashing are other points of air leakage.

 

The Effects of Air Pressure, Humidity, and Wind

 

Air pressure is always present within and outside the building envelope. To understand the impact of air pressure on buildings we need some basics in building physics. The air pressure inside a building will typically be different than the outside atmospheric air pressure, also known as the barometric pressure. The outside barometric pressure will depend on the altitude (air is less dense at higher elevations), the time of day, and impact of the sun on warming air, clouds, and wind. While air pressure is constantly changing (rising or falling) outside the building, the goal of the building designers is often to maintain a constant air pressure within the building to control air circulation, cooling, and heating within a narrow set of parameters for thermal comfort.
A perfectly sealed building, essentially a spaceship, would not be affected by changes in barometric pressure. The reality is that all buildings, no matter how eficient and modern, permit air leakage in and out. Common points of air leakages in interior and external surfaces of wall systems are around windows, doors, vent openings, unions between wall panels, and apertures required for mechanical and electrical systems. Gaps in vapour membranes between the finished interior of a building envelope and the insulation are the main source of air leakage from interior space. Roof systems with poorly or incorrectly installed and sealed flashing are other points of air leakage.

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