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BPI Written Exam - Section 1 Building Science Fundamentals

7. Understand Driving Forces (Including Natural and Mechanical: Pressure, Temperature, Moisture Differential

Terms: 

1. Pressure - pressure moves from high to low. Think of a pop can.
2. Temperature - heat moves from hot to cold. Think of a stove.
3. Moisture - water moves from wet to dry. Think paper towel

This is one of my favorite sections because it is so easy to memorize but it has deep implications and can make you look really good in front of a homeowner when explaining why their home gets so dusty, room so hot or floor so cold.  The terms above will definitely be on the BPI test, so I would commit them to memory.

Natural forces include:

1. Wind - wind forces can really drive air leakage in a home, just has house pressures from AHUs (air handler units).  A homeowner may really notice their homes' air leakage if their city is windy relatively often and give you valuable insights during the interview as well as touching on their hot spots.
2. Outside temperature - BPI has specific standards for each climate zone.  Depending on where you are you will have to deal with humidity, moisture and mold.
   
3. Rain - during the walk through of the exterior of a home, you should be inspecting for proper drainage, that everything slopes away from the home to prevent mold and moisture build up.  Same thing with the roof line. 
4. Outside moisture - water moves from wet to dry, so any stagnation of water will eventually enter the house and begin to rot the interior walls.
5. The stack effect - the stack effect is a natural process that occurs in homes and buildings where colder air enters around the floor or basement of a home and naturally moves up and exits through the top of the home.  It is most pronounced in 3 story homes.  How to do reduce or stop the stack effect?  By air sealing (see paragraph below on holes and pressures).  If you find a home with a crawlspace and original wood floors and the house is leaky, you can educate the homeowner on the stack effect as your case for air sealing. Just imagine what kind of air they are breathing in the house, it's basically the crawl space air including the rodents, feces, insects and possibly mold!

Mechanical forces include:

1. The heating and cooling system - if you don't live in a windy climate, the AHU is the next biggest driver of air leakage, either from 1) just the whole house being pressurized or depressurized; 2) from specific room pressures being too high or too low; 3) the ductwork, the AHU really pushes a lot of pressure through the ductwork so any holes in the ductwork tend to be more exaggerated 
   
2. Exhaust fans (on a tight home or if they are large scale)
3. Attic fans (on a tight home)
4. Inside moisture (not really mechanical but you need to know it) - this is where you need to be aware if you come across a tight home with no exhaust fans or way to remove moisture to the outside.  Interior moisture left alone or constantly present can create mold growth.
   

You really need to have these topics in this whole section in the front of your mind the entire time you are performing an energy audit.  It is too easy for novice energy auditors to concentrate just on the motions, running a blower door test, inspecting the attic for open wall chases and misalignments, walking through the house with an IR camera and think they are doing a good audit.  Without understanding these concepts and their applications, an auditor is just putting lipstick on pig.

What drives air leakage?

The answer is a pressure difference and a hole and you need both of them to get air leakage, if you stop one, you stop the other.  

The pressure difference comes from MECHANICAL FORCES like the AHU (air handler unit) or exhaust fans or attic fans if the home is very tight or NATURAL FORCES like wind or the stack effect.

The AHU creates a lot of pressure when it kicks on, enough pressure is generated to blow air through all the ductwork up 3 stories or across the house 30 feet.  That's why if you are doing a time-and materials contract always start sealing the ductwork connections closest to the AHU, because it is under the highest pressure.  Leakage closer to the AHU under 100 Pa of pressure is much more important than leakage 30 feet downstream under 10 Pa of pressure.  The room pressure should also be considered because opening and closing doors is essentially like putting a huge dampener on the ductwork to that room if the room pressure is high.  High room pressures can contribute to air leakage, dust, allergens and make the HVAC system work harder.  What is a high room pressure?  You may notice that doors close by themselves when the HVAC system kicks on, that is because it has a high room pressure.  By relieving the room's pressure, you reduce the amount of air leaking, dust and increase circulation back to the return.

An exhaust fan may not be a blower door, but on a tight home, they can definitely have an adverse effect. A tight home is much more sensitive to doors closing and small fans running than a leaky home because it creates a vacuum effect.  This is bad because outside air can come into the home through existing leakage or worse carbon monoxide can enter from an attached garage or backdraft down a water heater or furnace flue and enter the home.  You may not have to worry about smaller exhaust fans, but when you go to newer homes that have commercial range hoods suitable for Iron Chef, you really need to be aware. 

I heard a new Energy Star home was built so tight, that the toilet flushed when the front door was closed... I don't know if that is true, but it is a funny story.  Attic fans have been shown to cause the same adverse effects where the attic fan will cause gas appliances to backdraft into the home.  Now that is scary.

A hole is any connection to the outside or attic space from leaky windows and doors, drywall gaps around exhaust fans, registers and canned lights, and electrical and plumbing penetrations.

So to stop air leakage you can either air seal the home or pressure balance the home.  Since usually the AHU is going to create some negative or positive pressure every time it runs, air sealing is usually easier (excluding room pressures).

Pressure moves from high to low

Examples are:

1. Your AHU, the air closest to the blower is under the highest pressure (therefore the leaks closet to the AHU are the highest priority).  
2. Room pressures, when you shut interior doors, you are essentially putting a dampener on that duct line unless there is a way for the pressure or air to escape back to the return by either a transfer grille, jump duct or return duct.
3. During an energy auditors combustion safety tests, the home gets put under the most negative pressure possible (aka worst case) for each space where a gas appliance is located (the "CAZ").  This usually means turning on the all exhaust fans, AHU and dryer to create a negative pressure in the house in attempt to backdraft the appliances so we can catch a potential H&S hazard before it actually happens.
4. Dominant duct leakage test can tell us if there is more leakage on the supply side (negative number) or the return side (positive number) of a duct system.
5. The outside wind forces can put out homes under a positive pressure on one side, and a negative pressure on the opposite side.

Temperature moves from hot to cold

Examples are: 

1. The heat from 150 degree attics in Phoenix entering through uninsulated kneewalls
2. A room that has direct western exposure with lots of windows will get hot in the afternoon
3. Our body heat is being removed off our skin into the atmosphere, that is why we get cold

Moisture moves from wet to dry

Examples are

1. Water adsorbing through the drywall from a damp soil area from a hose leak
2. Steam from a hot shower adsorbing through the walls to cause dry rot

Next Section

1. 1a. Basic terms and definitions
   
   1. Understand airflow in buildings / ducts: CFM, CFM50, CFM25, ACHn, ACH50, FPM
      
   2. Understand equipment efficiencies: AFUE, SSE, SEER, EER, HSPF
   3. Understand power and energy: watts, BTU/hr, ton of refrigeration  watt-hours, BTU, therm, decatherm
   4. Understand effective leakage area
      
   5. Understand area weighted R-Value
      
   6. Understand baseload / seasonal energy use
   7. Understand driving forces (including natural and mechanical: Pressure, temperature, moisture differential
   8. Understand behavior of radiation: emissivity, reflectivity, absorbtivity
   9. Understand thermal resistance / transmittance: R and U Values; including conversions
   10. Understand latent / Sensible heat: evaporation, condensation / specific heat, heat capacity
       
   11. Understand total equivalent length
       
   12. Understand basics of dehumidification / Humidification as well as measurement equipment
       
   13. Understand and convert Pressure units: Inches of Water Column (iwc), Pascal (Pa)
       
   14. Understand, identify thermal bridges
       
   15. Understand pressure boundary 
       
   16. Understand/define stack effect 
       
   17. Understand and define exfiltration and infiltration 
       
   18. Natural / mechanical ventilation 
       
   19. Understand net free area 
       
   20. Understand input / output capacity 
       
   21. Understand peak electrical demand 
       
   22. Understand permeability and perm rating 
       
   23. Understand standby loss 
       
   24. IAQ (indoor air quality): moisture, CO, dust

1b. Principals of energy, air & moisture

1. Thermodynamics: conduction, convection, radiation, ΔT including air movement due to temperature gradients
   
2. Factors that affect insulation performance: density, installation, moisture
   
3. House pressurization/depressurization by various forces
   
4. Heat gain / loss: internal, solar, heat transmission, air leakage 
   
5. Power and energy: BTU content of fuels, capacity of combustion appliances and electrical appliances 
   
6. Moisture transport mechanisms: bulk water, air leakage, diffusion, capillary action 
   
7. Identify areas of highest relative humidity 
   
8. Principles of combustion: combustion analysis, CO 

1c. Combustion science

1. Combustion analysis: oxygen, flue-gas temperature, carbon monoxide 
   
2. Carbon Monoxide (CO) testing of combustion appliances 
   
3. Basics of: Combustion appliance venting, draft, and combustion air including identification of proper sizing/vent tables 
   
4. Understand combustion safety issues: Combustion air, draft, worst case / baseline depressurization, spillage, backdrafting, unvented combustion appliances 
   

Quick Links

1. BPI Written Exam
2. BPI Field Exam
3. Blower Door Test 
4. BPI Standards

The main topics of the BPI BA exam are listed out below.  Click on a link that interests you, or you need some brushing up on to learn more on each subject.

1. Building Science Fundamentals
1a. Basic terms and definitions
1b. Principals of energy, air & moisture
1c. Combustion science

2. Buildings and Their Systems
2a. Building components
2b. Conservation strategies
2c. Comprehensive building assessment process
2d. Design considerations

3. Measurement and Verification of Building Performance
3a. Applied diagnostics and troubleshooting

4. BPI National Standards and Project Specifications
4a. Comprehensive building assessment

5. Analyzing Buildings Systems
5a. Comprehensive building assessment
5b. Appliances and lighting

6. Conduct and communications
6a. Conservation strategies