The 8 Rules of Building Performance

Transcript of Video

Let’s look at the eight rules of building performance and the role that physics plays in the homes we build.

1.     Heat flows from warm to cold
2.     Moisture moves from warm to cold
3.     Moisture moves from more to less
4.     Air out equals air in
5.     Air behaves like fluid
6.     Rain follows gravity
7.     Everything gets wet, let it dry
8.     It all happens at the surfaces and the connections

These rules are nothing more than the laws of physics translated into how they impact our buildings.  These rules deserve a great deal of respect.  Even if we don’t like them or follow them, they still apply no matter where or how we build.

How we follow these rules determines weather our houses will be exposed to structural decay, mold growth, thermal discomfort and high utility bills.  Mold needs three things to grow; organic building materials, such as paper and wood, a temperature range between 40 and 100 degrees and moisture.  Of these three, the only one we can control effectively is moisture.  Understanding and applying the eight rules will reduce the risk of mold growth and structural decay and provide better comfort, safety and durability.  Let’s look at the eight rules one at a time.


Heat flows from warm to cold
In wintertime, the heat inside the house seeks to move outside and in the summer time; heat want’s to move in.  This flow of warm to cold not only affects comfort and energy loss, it also brings moisture and uncontrolled airflow into the walls.  We manage this flow of energy and moisture by; air sealing, properly installing insulation and window selection.

Moisture moves from warm to cold
Rain falls on a brick exterior wall and is absorbed.  The sun comes out, heats it up and drives the water to the interior side of the brick, where it’s cooler.  From there it either drains out or as a vapor, moves out through the wall system.  Air conditioning also pulls the moisture through the wall to the inside.  Here again, the moisture wants to move to where it is cooler – the interior of the wall system.  Temperature differences help drive the movement, so if you don’t design the wall system with the proper materials for your climate, this moisture could condense inside the wall cavity and cause mold growth and decay.  To manage this process, we need to select an exterior weather barrier; sheathing materials, interior finishes that allow the wall to dry along with sheathing materials that together effectively manage the rain and water vapor.

Moisture moves from more to less
This rule primarily applies to the moisture in the form of vapor.  This movement happens through two principal processes – diffusion and airflow.  Diffusion is the movement of moisture in the vapor state caused by either a difference in vapor pressure and/or temperature.  We know that in the summer time, when the humidity outside is high, moisture seeks to move from the outside to the inside, where the air conditioner is working to pull the moisture out of the interior air.  In the wintertime when there’s less moisture in the outdoor air, moisture wants to move from the inside where lifestyles generate more moisture, out to where there is less moisture.  So if moisture moves from more to less, this means the type and permeability of the wall system materials you choose is important.  For example, if you live in a hot, humid climate and you use a vinyl wall covering on the drywall, the moisture being drawn from the outside to the interior of the home can condense on the backside of the vinyl where it can create mold.  Conversely, if you install a low permeable, non-insulating sheathing, in the wintertime, moisture moving from the interior of the home to the outside may condense on the interior side of the sheathing.  This condensation occurs when the moisture reaches its dew point at the cold interface of the exterior sheathing.  This moisture may cause performance problems or decay.

The other significant way moisture moves from more to less, is through airflow.  Moisture can be carried by the flow of air into or out of the home through holes and cracks in the building envelope.  This airflow is caused by a variety of forces including stack effect, the wind and mechanical systems.  Stack effect is created when there is a difference in temperature from the inside to the outside. The cold air leaks into the building through cracks and unsealed openings, as warm air escapes out of the building through similar cracks and unsealed openings in sealing.  When the wind blows against the building, it creates a pressure difference across the building.  On the windward side of the home air flows in, on the leeward side, air is drawn out.  This airflow can transport energy and airflow in either direction.  Fans; both furnace and exhaust fans can create a difference in pressure that can either draw moisture in or allow it to flow out.  So by building a tight envelope and controlling mechanical pressures, we can reduce moisture flow through a building.


Air out equals air in
This is a key principal.  It is not climate related and yet its potentially life threatening.  Very few of us know how much air is exhausted from a clothes dryer or a kitchen exhaust hood.  Do you know?  What we do know is that in physics, for every cubic foot of air we push out of a building, another cubic foot has to come back in to replace it.  When we turn on a kitchen exhaust hood and push air out of the house, an equal amount of air must find a way back inside. This air comes back in through what is called the path of least resistance.  The bigger the hole, the easier the path and the greater the quantity of flow from this area.  The challenge here is that the path of least resistance can often be the flew for the water heater, fireplace or furnace and with this return air can come combustion gases which can produce deadly carbon monoxide.  Other common paths of least resistance include; Soffits & chases, house-to-garage connections, unsealed ductwork in attics and crawlspaces, outlets and switch boxes placed in exterior walls, recessed light fixtures and unsealed foundation drainage systems.  All of these paths and areas can carry unhealthy air.  This intrusion of carbon monoxide, odors and unhealthy air is compounded when consumers choose large exhaust fans and kitchen exhaust hoods without considering a controlled and safe source of makeup air.  To manage this serious problem, consumers and builders need to; select closed combustion appliances, provide a properly sealed building envelope and provide controlled make-up air when large exhaust fans are installed in the home.

Air behaves like a fluid
This means that air and moisture move the same as any fluid by pressure.  Air is a fluid just as water is a fluid, but we don’t manage it like we do water.  We don’t think about transporting it, heating it, pumping it or moving it around, like we do water.  We should be using the same principals of fluid movement for air that we use for water.  Too often, ductwork is installed in crawlspaces, outside walls and attics.  If these ducts are not sealed as tightly as those that carry water, air will escape into the crawlspaces, attics and wall cavities.  In winter, this heated air can easily cause moisture problems when it hits a cold surface and condenses.  It can lead to ice damns and sheathing deterioration, this can lead to mold growth and indoor air quality problems.  We pressure test plumbing vent stacks, but we do not seal and pressure test ducts and yet we install ducts in unconditioned spaces, which can carry unhealthy air into the home.  So by sealing the ductwork, we will prove occupant comfort HVAC system performance and reduce the potential for building failures.  It’s just as important to have leak free ducts, as it is to have leak free plumbing.


Rain follows gravity
The goal is to have the rain spend as little time as possible in contact with the building materials.  To do this, we need to drain the rain.  If we don’t drain the rain and flash our openings properly, water can enter the structure and cause decay, material failure and mold growth.  This means we should install gutters, properly extend downspouts, slope site drainage, manage landscaping and use appropriate foundation waterproofing and drainage systems.  It’s tempting to believe that if we have installed gutters and downspouts, everything is okay.  But remember, rain usually comes with wind.  The wind driven rain passes through all exterior cladding.  It doesn’t matter if it’s wood, vinyl, steel, aluminum, brick, stone, stucco or cement siding.  In climates with heavy rain and high wind, we recommend all cladding be installed over a vented drainage plane or rain screen.

Everything gets wet, so let it dry
There is no stopping water so the key is to build homes that allow the building materials to dry out once they get wet.  For example, at one time, builders kept a ten to twelve inch space between the ground and the exterior cladding.  This concept still holds true today; a minimum of six to eight inches should be maintained between the ground and all materials that can absorb water. However, today, we are often stuck with the idea that homeowners don’t want to see their foundation, they want the siding to go down to the dirt.  When siding or cladding stay in contact with the soil, they will wick water causing paint film failure and degradation.  Capillary action is the wicking of water against gravity.  When wood or concrete materials get wet, the water can move upward, against gravity, within the wood or concrete.  To control this process, the material needs a method that either allows it to dry out or a system that keeps it from getting wet.  If we can’t prevent the material from getting wet, then we need to control the extent at which it wicks.  So by keeping building materials up off the ground or by sealing them, we can prevent capillary action.


It all happens at surfaces and connections
What this means is that each material dictates what is going to happen when they come in contact with the elements and forces in their environment and the adjacent building materials.  Is the material foam, wood or is it masonry?  How does it react to moisture?  Does it absorb water, shed it or drain it?  Does it dry out quickly? How does it react to the environment?  Will the environment alter or degrade the material?  How does it react with adjacent materials?  Most building materials are often picked by price and aesthetics, not by how the surface is going to react to its environment.  We need to choose materials that are compatible with the environment and the materials they’re in connection with if we want the building to last.

All of these rules of building performance apply to the materials we select and the environment we put them in.  One more thing about building performance; this is not a rule but it might as well be – keep clients out of harms way!  Breathing mold spores and breathing carbon monoxide is harms way.  Everybody has to play by these rules of building performance, even your competition.  Your clients expect that you understand and apply these rules when you build their homes.

We’re not trying to build perfect buildings; we’re trying to build more tolerant buildings.  By understanding these rules, we will be able to build buildings that are more tolerant of the elements and forces that impact a building’s durability.  At the same time, keep our clients safe, comfortable and out of harms way.