Note: This Instructional Module information comes from our Training Manual. The complete Training Manual can be ordered from our Program and comes with a video, transparency masters, module publications, and many other educational resources.

Module Learning Objectives

• Understand that the home works as a system
• Identify building science strategies for a particular climate
• Learn about the roles ventilation, moisture, and humidity play in indoor air quality problems in the home

Notes to the Program Leader:

This module contains workshop information for two separate program presentations: Basic Building Science for Cold Climates and Basic Building Science for Hot and Mixed Climates. Each presentation is a suggested teaching plan for a workshop of about 30 to 45 minutes. Each presentation is designed to provide building science strategies for particular climate concerns in your area of the country. Information from both presentations, though, might apply and be useful for your workshop. The Module Workshop Evaluation (at the end of the Basic Building Science for Hot and Mixed Climates presentation) can be used for either presentation.

Basic Building Science for Cold Climates

Script for Transparency #1

Building Science: Keeping Elements in Balance

Building science is the study of how buildings function under various environmental conditions. Building scientists study how heat is generated or lost in a house and how to make houses more comfortable and healthy. Most important, building scientists have learned that buildings work as a system of inter-related elements.

This graphic shows a mobile with various elements of a building. When we try to achieve a healthy and safe building with optimal indoor air quality, the control of heat, air, and moisture are the most important building science elements to focus on (along with health and safety).

These five elements are counterbalanced in this image by the envelope of the building and its mechanical systems. The envelope is the actual shell of the building, which includes the structural elements and the insulation.

Occupants are primarily concerned with the bottom portion of the mobile: durability, comfort, and affordability. Occupants affect the durability and comfort of the house by the way they operate its mechanical systems.

Script for Transparency #2

The Building as a System

Building science includes several important concepts. First, heat flows from hot to cold. The use of insulation does not control heat flow. Second, heat itself does not rise; warm air rises. This is an important factor in air leakage. Cool air is brought into lower areas of a house as warm air leaks out the top.

Air leakage is also a comfort problem because it dries out the house. Warm, moist air leaking out the top of the house is replaced by cool, dry air from outside. If a house is too leaky, or the rate of leakage is too high, the house will be exceedingly dry and unhealthy.

Third, there are three methods of heat transfer: conduction (the molecule to molecule transfer of heat that occurs when objects are in contact with each other), convection (the movement of heat by a fluid), and radiation (the transfer of heat by a hot object without the need of a transfer device or contact, as occurs when the sun heats the earth or when a wood stove heats a space).

Fourth, insulation traps air or other gases, which slows down heat loss. Fifth, building homes "airtight" is a major way to prevent heat loss. A common misunderstanding is that you can build a building too tight. This is not true! You can only underventilate it.

Script for Transparency #3

Three Methods of Heat Transfer

Shown here are the three methods of heat transfer. Conduction is illustrated by the heating of a pan on an element of a stove top. The pan is in direct contact with the heating element and is heated by conduction from that heating element. The two are in contact molecule to molecule and heat is conducted from the stove top heating element to the pan.

Convection is illustrated by showing the movement of air in a wall where the temperature on one side of the wall is warm and the temperature on the other side is cold. If there is no insulation in the wall cavity, the air is warmed and rises on the warm side and falls and cools on the cold side, setting up a convection "loop" or "cell" that accelerates heat transfer.

Radiation is shown here by the heat transfer in all directions from the wood stove to the surrounding space. This is a good way to illustrate that indeed heat doesn’t rise, but rather is directed outward from a radiant source in all directions. Radiation does not require a medium such as air to be transferred -- it is the major way the sun heats the earth.

Script for Transparency #4

Energy Flows in Housing

This diagram shows the typical energy flow for a house built around 1980. The left side shows energy inputs. The passive solar energy input is mostly a function of design and window placement. Purchased energy inputs come from lights, appliances, people, and space heating.

The right side of this overhead shows the typical percentage of heat loss through various building elements. Only 10% to 15% of the energy loss of the house is attributable to the attic/roof because attics and roofs are typically some of the better insulated areas of the house. Roofs can often be insulated without increasing the size of the attic space and additional insulation can be easily installed. It is more difficult to add insulation to walls; however, walls account for only 15% to 20% of the typical heating loss of a house.

The basement and windows/doors, combine to account for up to half the total heat loss of a house. Insulating basements and crawlspaces helps reduce heat loss. Advances in window and door technology over the last decade have improved the thermal performance of houses.

Air leakage is perhaps the most important and least evident source of heat loss. It can account for as much as a third of your home’s heat loss. Controlling air leakage can significantly improve the energy use of your building. However, as your house becomes more airtight, you must have a ventilation system to provide adequate air exchange.

Script for Transparency #5

More Building Science Concepts

There are a few more points that are important to understand about how buildings work. First, air flows from high pressure to low. Generally, this means that the cooler portions of the building such as the crawl space or basement have a lower pressure and the warmer portions of a building near the ceiling or attic have a higher pressure. This means that buildings generally pull air in at the bottom and drive air out at the top.

Second, the dew point is a function of temperature and relative humidity. When the air at a particular temperature is at 100% relative humidity, it is said to be saturated. This is called the dew point -- the temperature at which water vapor condenses to liquid. In a home during winter, moist air cools as it nears a window and therefore condenses on the window.

Third, as air is heated it absorbs moisture from the interior space and carries this moisture as it exits the building through air leakage. If the moisture condenses in the building envelope before it exits the building, the accumulating liquid can degrade indoor air quality by fostering the growth of biological pollutants such as mold and mildew.

Finally, a vapor retarder should be placed on the warm side of the building envelope to prevent the transport of water vapor to a place in the wall or ceiling where it could condense and cause biological indoor air quality problems.

Script for Transparency #6

Relative Humidity

The question often arises as to what is the best relative humidity (RH) range for human health and comfort as well as for preventing moisture damage in the building. Building scientists agree that maintaining an indoor RH in the range of 35% to 55% is optimum.

High humidity in a house can lead to microbiological problems. Certain bacteria grow better when the moisture level is high. Molds and fungi thrive in a high moisture environment. In addition, moisture damage can lead to structural failure from disintegrating gypsum wallboard or rotting wood.

Low humidity (less than 30% for long periods of time) is just as unhealthy as high humidity, but for different reasons. Most of the human body’s defenses against upper respiratory illness depend on moist mucous membranes. Low RH levels in a building keep the mucous membranes too dry, which impairs their ability to transport disease microbes out of the body.

Script for Transparency #7

Relative Humidity and Pollutants

This chart summarizes research that has been conducted on what happens at different levels of relative humidity (RH). Note that humidity levels from 0% to 100% are shown on the horizontal line. If we go down the list on the vertical line, we can see that bacteria populations are high at low levels of RH, drop off at 30% RH, then begin to increase again at 60% RH.

Levels of viruses are high at low RH levels till about 50% RH, then begin to increase again at 70% RH. Fungi and dust mites need moisture to survive. We begin to see fungal growth at 60% RH and mites at 50% RH.

Research on respiratory infections and RH is limited, but suggests that more infections occur at lower levels of RH. Allergy problems and asthma occur at both low and high RH levels. Chemical interactions (here mainly formaldehyde emission) begin at about 30% RH and increase as RH increases. Ozone is a respiratory irritant that is produced by some appliances. Its levels are highest at low levels of RH.

The main feature of this chart is that there is an optimum zone of RH between 40% and 60%, where the presence of all these pollutants is minimized. However, be aware that in very cold weather, moisture levels in the home over about 50% RH can cause condensation problems, leading to the growth of mold and mildew.

Note to the Program Leader: A good resource for extremely cold climates is the "Alaska Housing Manual," Alaska Housing Finance Corporation, 520 E. 34th Avenue, Anchorage, AK 99501. In very cold climates, relative humidity above 50% cannot be tolerated by commonly available window technologies. Therefore, the authors of the "Alaska Housing Manual" have shifted the optimum zone of relative humidity by 10% to the left from what is shown here. Consult the "Alaska Housing Manual" for more information.

Script for Transparency #8

Dew Point

This overhead illustrates the importance of the dew point. Water vapor is created by heating a pan of water on the stove, with the stream of vapor moving toward the wall.

The left side of this overhead shows that, without a vapor retarder on the warm side of the wall, water vapor will enter the wall freely. During most of the year, the wall cavity will be colder than the interior wall, which will cause water to condense inside the wall.

The right side of this overhead shows how the moisture is prevented from entering the wall when a vapor retarder is placed on the inside of the wall.

Script for Transparency #9

Dew Point and Windows

Because windows are the most poorly insulated element of a building, and have the lowest R-value of insulation, they are often the first surfaces on which condensation occurs.

This chart shows the dew point for three types of windows. When the temperature is 70^o F inside and 0^o F outside, condensation will occur on the interior surface of a single-glazed window when the indoor relative humidity level is only 15%; on a double-glazed window at 42%; and on a low-emissivity window at 65%. By increasing the energy efficiency of the window, condensation is less likely because the interior glass surface does not get as cold.

Script for Transparency #10

Air Leakage is the Primary Moisture Transport Mechanism

This building science concept is demonstrated by showing all of the holes and penetrations that normally occur in buildings. In construction or retrofit, attention must be given to sealing these holes to get control of air leakage. Without this control, no other efforts to improve the indoor air quality could succeed because ventilation will be ineffective if air leakage is too great.

Script for Transparency #11

Leaky Buildings

In the past, people relied on air leakage to provide ventilation. However, leaky buildings do not guarantee good indoor air quality. Particularly in a cold climate, comfort and health problems result if the building is too leaky. A building leaking air during the coldest part of the winter will cause continuous drying of the indoor air and can lead to increased upper respiratory disease and discomfort.

Leaky buildings also enhance the transport of outdoor pollutants and help them enter the building. Leaky buildings allow for more insect entry as well.

High heating and cooling costs result if the building is too leaky, since the heated air leaks out so quickly.

Script for Transparency #12

In-House Moisture Generation

In 1990, the University of Illinois published a study of various activities of a family of four that contributed to in-house moisture generation. A family of four, just by breathing, can produce ½ pint of liquid water per hour, which is distributed as vapor in the air inside a house. Additionally, ½ pint can be produced by a shower or bath. Mopping a kitchen floor is a big source of moisture and can produce as much as 4½ pints in the house.

It doesn’t take much water evaporating in a building to raise its relative humidity to an unhealthy level. Four to 6 pints of water can raise the relative humidity of a 1,000 square foot house from 5% to 60%.

Script for Transparency #13

Biological Growth

Biological problems in buildings require three basic conditions. First, there must be moisture of 20% by weight on most wood products (including the paper backing on gypsum wallboard), and the temperature range must be between about 40^o F and 100^o F.

Second, there must be fungal spores or bacteria available to initiate the growth of biologicals in the building. Fungal spores or bacteria are fairly widespread and almost always present in the air.

Third, there must be a nutrient source. Materials made of wood provide food for biological growth.

To prevent biological problems, particularly fungal growth, it is essential to keep the materials in the frame of the building dry.

Note: Information for this section compiled with permission from Alaska Housing Manual, published by the Alaska Housing Finance Corporation, 520 E. 34th Avenue, Anchorage, Alaska 99501, authored by Todd Hoener, Phil Loudon, Sue Mitchell, Mike Musick, Randy Nicklas, and John Woodward. Third edition published in January 1998. Transparency #7 copyright © Theodore D. Sterling and Associates, Ltd., Vancouver, B.C. All other diagrams copyright © Alaska Housing Finance Corporation.

Prepared by:
Richard D. Seifert
Professor/Housing and Energy Specialist
Alaska Cooperative Extension Service
October 1999

Basic Building Science for Hot and Mixed Climates

Script for Transparency #1

Building Science for Hot and Mixed Climates

Air infiltration rates in closed houses are greatly reduced in warm weather because:

• The stack effect is reversed (cool air is stable). Stack effect occurs when heated air escapes from upper levels of the home and is replaced in lower levels of the home by drawn-in, outdoor air.
• Wind speeds are lower than in winter.

The net effect is that there is much less air infiltration into our houses in summer than in winter. Since more homes have central air conditioning now, many houses are closed up all summer.

Script for Transparency #2

Air Changes

If there is very little outdoor air coming into our homes, indoor air quality can deteriorate. Stale air is not adequately flushed out and more moisture and other pollutants stay inside the structure.

ASHRAE Standard 90.2 recommends houses have a minimum of 0.35 air changes per hour of outdoor air to provide acceptable indoor air quality. Houses are often closed in the summer for security reasons with no mechanical ventilation provided.

Script for Transparency #3

Building Pressurization Due to Return Duct Leakage

A poor A/C duct system can cause many indoor air quality problems. Return duct leaks will draw in air from the space around the return ducts, which may include high levels of radon, moisture, odors, or other pollutants. This will not only impact A/C operating costs, but can result in mold growth within the warm and humid return duct system where it will not be visible to the homeowner.

Script for Transparency #4

House Depressurization Due to Supply Duct Leakage

A common problem in southern houses is when the indoor unit of an air conditioner or heat pump is located in a central hall closet with a single return and supply ducts routed through the attic. If there are leaks in the supply ducts, the entire house will become depressurized, with outdoor air leaking through all available openings.

This situation can draw in radon from underfloor slabs or crawlspaces, as well as prevent natural draft appliances from venting properly. A similar situation results when supply ducts are located in a crawlspace.

Script for Transparency #5

Excessive Moisture

Excessive moisture is a major cause of indoor air quality problems in warm climates. Moisture can be caused by high outdoor moisture brought in with ventilation air, poor dehumidification by oversized cooling units, high Sensible Heat Ratio (SHR) ratings on cooling equipment, unvented combustion heating systems in mild weather, or wet basements or crawlspaces that connect to the living space.

Script for Transparency #6

High Indoor Humidity and Indoor Air Quality Problems

While there are a number of health and indoor air quality problems that are caused by low humidity levels, the primary indoor air quality problem caused by high humidity levels results from the growth of mold. Many people exhibit allergy-like symptoms when exposed to high levels of mold spores. These symptoms are usually temporary and go away after leaving the space.

Script for Transparency #7

Other Causes of Moisture Problems

Leaky ductwork is not the only way that outdoor air with excessive moisture can enter a house. Many homes have a variety of exhaust appliances, such as clothes dryers, range hoods, and bath vents. Even power roof vents can depressurize the living space if there is an inadequate vent area in the attic.

High moisture levels can be made even worse when there are cool surfaces present, which permit mold to grow. Cool surfaces can result from cold walls below grade, or even from poor air circulation behind furniture or cabinets against outside walls.

Script for Transparency #8

Cooling-Dominated Region in the United States

The United States can be divided into regions where moisture problems must be handled differently due to relative heating and cooling design conditions. The Gulf Coast and Lower Atlantic states have quite low heating requirements and very high cooling needs, accompanied by high outdoor humidity levels. For this cooling-dominated region, air conditioning must be used to keep the indoor humidity acceptably low when the outdoor humidity level is very high.

Script for Transparency #9

Building Pressurization for Cooling-Dominated Areas

Just as the vapor barrier should be on the warm (more humid) side of the structure in heating climates, so too should it be in cooling-dominated climates. In this case, that puts the vapor barrier on the outside of the wall’s insulation.

Impermeable, vinyl wallpaper should not be used in such climates as it acts as a vapor barrier and can cause condensation behind the paper. Because air leakage can penetrate walls, the building should be pressurized if possible to prevent the humid outdoor air from penetrating the outside walls.

Script for Transparency #10

Building Pressurization for Mixed Weather Climates

Much of the U.S. has roughly equal heating and cooling requirements. For these regions, the house should be designed to account for cold outdoor temperatures as well as hot and humid outdoor conditions.

In these areas, the vapor barrier should normally be placed on the inside of the walls. However, the wall ought to be capable of drying to the exterior should moisture condense inside the wall during humid summer conditions. If possible, pressurize the building in summer and depressurize in winter so only dry air penetrates the walls.

Script for Transparency #11

Ventilation

Ventilation of attics and crawlspaces is still needed primarily for moisture problems, even in cooling-dominated climates. Powered roof vents are common in the South, but often cannot be justified based on energy savings when typical attic insulation is present.

Crawlspace vents that close in cool weather are not needed if the water pipes are properly insulated. Such vents are not desirable if there is any source of water going into the crawlspace, such as from a spring or surface drainage. High humidity air from the crawlspace can be drawn into the living area due to depressurization from venting devices.

Script for Transparency #12

Nighttime Cooling

Using natural ventilation of houses for nighttime cooling should be done with some measure of caution. The nighttime low temperature is usually near the dew point of the air. If the air allowed into the house has a higher moisture content than the design indoor condition, the use of the air conditioner the next day may not be able to properly remove all the excess moisture from the indoor materials.

The dew point at standard room conditions is about 58^o F. Sensible cooling of the air can compensate somewhat for higher humidities. However, windows should probably not be opened for nighttime cooling if the temperature is not expected to get below about 65^o F.

Script for Transparency #13

Basements

Basements are a potential source of indoor air quality problems due to the possibility of radon, as well as high relative humidities present at the cool, below-grade walls. The humidity problem can be made worse by having carpeting or other materials on the floor or walls.

Letting outdoor air come into an unconditioned basement allows the basement humidity level to rise so mold can grow on the cool walls and floor, or in materials that cover them. To keep basement walls warmer during humid summer months and prevent mold growth, insulation should be applied to either the inside or the outside of below-grade walls. Pressurizing the basement with a small amount of air from the living space will help prevent the infiltration of both outdoor air and radon.

To download an IAQ PowerPoint Presentation