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BPI Written Exam - Section 1 Building Science Fundamentals

15. Understand Pressure Boundary

There are two important boundaries to know for the BPI exam and during a career in home performance. The first is a thermal boundary and the second is a pressure boundary.  The thermal boundary is boundary between the inside and the outside of the house as defined with insulation and an air barrier.  You need both (insulation + air barrier) 100% in contact with each other for a proper thermal boundary.  A lot of energy audits and home performance upgrades involves finding and correctly aligning these two together... more on this in the Thermal Boundary section.

The second term, pressure boundary, has to do with the connectivity of the house to the:

  1. Outside
  2. Attic
  3. Garage
  4. Other rooms inside the house

An important point to remember is that two things drive air leakage.

  1. A hole
  2. A pressure difference (created from the stack effect wind, chimneys, exhaust fans or the AHU)

Take either of those two things away and you have solved your air leakage problem.  In many cases, it is impossible to eliminate all the possible causes of a pressure difference so air sealing the holes and gaps is our only option.  That's why finding where our pressure boundary is, is so important.

The connections (aka holes in the wall) happen in more obvious places like canned lights, top plate leakage, missing top plates, electrical penetrations and also in not so obvious places like duct chases, basements, crawlspaces, behind stair chases, behind air handler closets or wall cavities that can only be found by using pressure diagnostics.  Imagine a two story, 4000 sq ft house with both AHU in the attic, a bonus room above the garage and soffits and arches over every room.  By using pressure mapping we can discover without even going into the attic...

  1. If the garage attic is connected to the 1st floor and 2nd floor cavity
  2. If there is an open duct chase
  3. If a 1st floor archway is connected by a wall cavity to a 2nd floor bedroom
  4. How well connected (the leakage severity) the house / basement is to the attic
  5. How well connected the ductwork is to the outside
  6. Which room has the most leakage
  7. Pin point the area where drywall should be cut open to air seal a cavity (depends on the savings vs cost)
  8. If there is a correct thermal boundary (insulation up against drywall/ air barrier)
  9. How big the holes are
  10. How much CFM reduction we can expect by sealing each hole

We can measure how well connected each of these is with pressure mapping. 

There are several ways to do pressure mapping depending on the questions you have for each home.  Some tests should be done on every house, some only if you have an attached garage for example, but here is a run down of the different pressure mapping tests.  All these test require a manometer and most require the use of a blower door depressurized / pressurized to 50 Pa.

Rules

If you know the pressure reading of one zone, you automatically know the reading of the corresponding zone.  This is because we always have the blower door running at 50 Pa.  It is easier for me to write this out in a formula, where:

Pressure A + Pressure B = 50 Pa
Pressure A is the zone that we choose to get a pressure pan reading from, whether it be an electrical outlet, a small hole we drill in a soffit, the ductwork after we tape all the registers shut and insert our tube or by removing the attic hatch (pressurize so you don't suck insulation)

When measuring pressure boundaries, a zone can be any room, attic, crawlspace, outside or basement.  We always measure a zone with respect to (WRT) another zone.  For example we measure the room pressures of each room with respect to the outside.

Zonal / zone pressure mapping

This tells us if the zone we are measuring is more inside or outside. Zones can be taken out canned lights, electrical outlets, entire rooms or by drilling a small hole in a soffit or wall cavity and sticking our tube in.

0 - 10 Pa = minimal leakage
11 - 39 Pa = confused area. Yes this is a large range but the higher you go, the more leaks or bigger the connect is to the outside
40 - 50 Pa = outside

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