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BPI Written Exam - Section 1 Building Science Fundamentals

5. Understand area weighted R-Value

The area weighted R-value is for finding the R-value of a series of materials together as one.  It is most often used when calculating a walls R-value because it considers multiple materials such as drywall, vapor barriers, siding and wall studs, not only the insulation in the walls.  This may be a problem you see on the BPI test, so here is what you need to know.
Terms: 

  1. R-value - the measure of a materials thermal resistance in units of ft²·°F·h/Btu.  Insulation properties are described in terms of R-values.  The higher the R-value, the greater the thermal resistance.
  2. U-value - the overall heat transfer coefficient.  U-values are a measure of conductance a material.  
  • It tell us how much heat conducts through a 1 sq ft area in 1 hr when there is a 1 degree F temperature difference.  
  • The smaller the U-value, the better the object is at reducing heat transfer.  
  • U is the “total” heat transfer through an object such and takes into account convection and radiation.  
  • Window properties are addressed in terms of U-value. 


Relationship between U-value and R-values
U-values are the fraternal twins of R-values, meaning that to find a U-value for a material, simply put 1 over the R-value = U-value. The opposite is true also, to find the R-value of a material given the U-value, take 1 over the U-value = R-value. 
  • 1 / R-value = U-value 
  • 1 / U-value = R-value


The difference between R-value and U-value
  1. R-values can be added together, U-values cannot
Practice Problem:

Find the R-value of this made-up, all-insulation and no stud wall, given the following:

R-values
–Brick = R-0.10 per inch
–Sheet rock = 0.90 per inch
–Batt insulation = 2.6 per inch
–Outdoor air film = R-0.17
–Indoor air film = R-0.68
First create a table of 4 columns of material, R-value per inch, Thickness and Total R-value.

Then you can multiply all the R-values by their thickness to get the total R-value for each material. Before going further, try it on your own an see if your number matches mine.

In homes and buildings, walls are not all just insulation.  We need to account for the studs and other materials as well.
On the test:

What BPI will give you:
  1. All the materials that make up a wall (air film, siding or masonry, insulation, wall studs, etc).
  2. The thickness of all the materials.
  3. The wall area.

What you need to assume:
  1. You will need to account for the wall studs, which are not 100% of the wall area.
  • 16-IOC (inch-on-center) studs account for 20-25% of the wall area (just use 25%) OR 75% of the wall area is an open cavity
  • 24-IOC studs account for 15-20% of the wall area (just use 20%) OR 20% of the wall area is an open cavity


Approach:
  1. Keeping in mind the assumptions above, calculate two sets of R-values, one for the wall cavity (where we just have insulation and no studs), the cavity column, and one for the frame (where we have studs), the frame column)
  2. Make two sets of columns, one for the cavity, one for the frame
  3. List all the materials out on the vertical axis and fill in their R-values
  4. The cavity column will be blank for the wood frame
  5. The frame column will be blank for the insulation type
  6. Add the R-values for both columns together
  7. Convert to U-values by taking 1/R
  8. Multiply the cavity column U-value by 75% or 80% depending if you were given 16 IOC or 24 IOC respectively.  
  9. Multiply the frame column U-value by 25% or 20% depending if you were given 16 IOC or 24 IOC respectively.
  10. Finally, add #8 and #9 U-values up for your total area-weighted U-value.
  11. Convert to R-values by taking 1/U
Here is another sample problem you may see on the BPI test to find the R-value of the wall composition.  Follow the approach above with your 4 columns.  Once you've added the total R-values for all the wall components, remember to multiply the cavity and frame by 75% and 25% respectively.

Next Section

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