Estimating the effects of energy conservation construction code
1. Estimating the Effects of Energy
Conservation on Temperature
and Humidity in Buildings
Todd R. Crawford
Bureau of Toxic Substance Assessment
New York State Department of Health
trc05@health.state.ny.us
518-402-7810
2. Where we’re going today
• Energy Conservation Construction Code
• Estimating Heat Energy in Buildings
• Estimating Air Leakage
• Estimating Humidity Levels
• Estimating Thermal Mass
• Estimating Solar Heat Energy
• Review and Conclusions
5. 2010 Energy Conservation
Construction Code (ECCC)
• Chapter 1 General Requirements
• Chapter 2 Definitions
• Chapter 3 Climate Zones, Design Conditions,
Materials, Equipment and Systems
• Chapter 4 Residential Energy Efficiency
• Chapter 5 Commercial Energy Efficiency
• Chapter 6 Referenced Standards
6. ECCC Intent 101.3
• …regulate the design and construction of
buildings for the effective use of energy…
• …use innovative approaches…
• …the improvement of construction
practices, methods, equipment,
materials and techniques shall be
encouraged…
7. ECCC 102 Alternate Materials,
Methods, Designs, Systems
• …this code is not intended to prevent the
use of any material, method of
construction, design or insulating
system…
• …provided it has been approved by the
code enforcement official:
– …meets the intent of the ECCC
– …achieves equivalent or greater energy savings
8. Climate Zones Table 301.1
Zone 6
Zone 5
Zone 4
http://energycode.pnl.gov/EnergyCodeReqs/?state=New%20York
10. R-value
• A measure of the resistance of building
materials and structures to the flow of heat:
• The higher the R-value the better the thermal
insulation.
11. U-value
• The coefficient of transmission of heat through
building materials and systems;
• The lower the U-value the better the thermal
insulation.
– to convert Imperial (Btu. F-1.ft-2.hr-1) to metric units
– multiply U by 5.678 (W.m-2.K-1)
12. Assembly R-value
• A measure of the resistance of assembled
structures to the flow of heat;
• Where i is the individual component of an
assembly (e.g. studs in a wall)
13. Zone 5 House on ECCC 2010
R-38 Ceiling
50 x 30 x 9 feet
Volume = 13500 ft3
R-20 Walls Floor Area = 1500 ft2
Ceiling Area = 1500 ft2
Wall Area = 1440 ft2
R-30 Floor
14. Assembly R-value
U=1/38 %i = 1500/4440
50 x 30 x 9 feet
Volume = 13500 ft3
U = 1/20 Floor Area = 1500 ft2
%i = 1440/4440 Ceiling Area = 1500 ft2
Wall Area = 1440 ft2
U = 1/30 %i = 1500/4440
16. Average Heat Loss in January
(Albany Zone 5)
• Assume Tdiff = 70 – 21 = 49 F
• Interior Design Temperature is 70°F
• Albany January Average Daily Temperature is 21°F
• Estimated Assembly R-value = 27.5
• Assume Insulated Area = 4440 ft2
– no windows, no doors, no thermal bridges…
Average Heat Loss = 7911 BTU/hr
17. ECCC 402.4 Air Leakage (Mandatory)
• The Building Thermal Envelope shall be
durably sealed
• Sealing materials shall allow for differential
expansion and contraction
• Caulk, gasket, weatherstrip
18. ECCC 402.4.1 Building Thermal
Envelope
1. All joints, seams and penetrations.
2. Site-built windows, doors and skylights.
3. Openings between window and door assemblies
4. Utility penetrations.
5. Dropped ceilings or chases adjacent to the thermal envelope.
6. Knee walls.
7. Walls and ceilings separating a garage from conditioned spaces.
8. Behind tubs and showers on exterior walls.
9. Common walls between dwelling units.
10. Attic access openings.
11. Rim joist junctions.
12. Sill plates and headers.
13. Other sources of infiltration.
19. Air Sealing §402.4.2.1
• Blower door test (ASHRAE/ASTM E779)
• Acceptable air leakage is less than 7 ACH at 50
Pascal
• Natural air leakage is estimated by*:
*Air-Tightness of U.S. Dwellings – Sherman and Dickerhoff, 1998
20. Sensible Heat Loss
q = air volume flow (cfm)
= (0.35 x 13500)/60 = 79 cfm
dt = temperature difference ( F) = 49°F
hs = sensible heat (BTU/hr)
=1.08 x (79 x 49) = 4181 BTU/hr
http://www.engineeringtoolbox.com/cooling-heating-equations-d_747.html
21. Latent Heat Loss
q = air volume flow (cfm)
= (0.35 x 13500)/60 = 79 cfm
dwlb = humidity ratio difference
(lb water/lb dry air) = 0.006 – 0.002 = 0.004
hl = latent heat (BTU/hr)
=4840 x (79 x 0.004) = 1529 BTU/hr
http://www.engineeringtoolbox.com/cooling-heating-equations-d_747.html
22. Average Heat Loss (Winter)
• Insulation heat loss = 7911 BTU/hr
• Sensible heat loss = 4181 BTU/hr
• Latent heat loss = 1529 BTU/hr
Average Heat Loss = 13,621 BTU/hr
– Assembly R-value = 27.5
– Indoor Temperature = 70°F
– Average Temperature (Albany in January) = 21°F
– 7 ACH50 = 0.35 ACH
23. Estimated Hot Air Supply
Q = 13,621 BTU/hr
th = 105 F
tr = 70 F
L = 360 cfm
24. Required Outdoor Ventilation Air
• NY Mechanical Code: Table 403.3
– 0.35 Air Changes per Hour (ACH)
• ACH x Bldg Volume = Outdoor Air Supply
• 0.35/hr x 13500 ft3 = 4725 ft3/hr = 79 cfm
Estimated Outdoor Air = 79 cfm
• And from the previous calculation:
Indoor Air Recirculated = 360-79 = 281 cfm
25. Mixing Indoor and Outdoor Air
• xB Humidity ratio of mixed air
• Outdoor Air QA=79 cfm
xA=0.00232 (at 21 F and 100%RH)
• Indoor Air QC=281 cfm
xC=0.00613 (at 70 F and 40%RH)
• Mixed air xB = 0.00537
27. Mixing Indoor and Outdoor Air
• Humidity ratio xB = 0.00537
11% RH in heated air at 105 F
Approximately 35% RH in room at 70°F
• 0.35 ACH decreases RH in room air
Humidity ratio → 0.00232
At 70 F, xB = 0.00232 yields 15% RH
Minimum RH is reached within 24 hours
http://www.humidity-calculator.com/index.php
28. Degradation of Humidity Ratio
0.007
0.006
0.005
Humidity ratio
0.004
0.003
0.002
0.001
0
0 5 10 15 20 25 30
time (hours)
30. Enhancing this Model
• Modeled energy ‘losses’ of a conceptual
building following ECCC
– Table 402.1.1 – insulation requirements
– §402.4.2 – air sealing and insulation
• How does moisture affect our model?
• Is energy “lost” through the building
envelope?
31. Quantity of Moisture Added to the Air
Gallons (liters)
Activity
• An average family of 4 per week
will generate about 14 Cooking 1.7 (6.3)
gallons (53 liters) per Dishwashing 0.8 (3.2)
week through normal Bathing/
0.6 (2.4)
household activities Showering
Clothes
– Keeping the Heat In. 0.5 (1.8)
washing
How Your House Works.
Natural Resources Normal
respiration and
Canada 2009
skin
10 (38)
evaporation
from 4
occupants
32. Estimate Rate of Change of %RH
• 14 gallons per week → 0.695 lbs/hr
• Building Air Volume is 13,500 ft3 at 0.075 lb/ft3
Humidity ratio increases 0.0007 (lbs/lbs) per hour
• No air leakage, RH reaches 100% in 14 hours
• In our Winter model, RH equilibrates around
37% in 19 hours
33. Humidity Ratio with Indoor
Moisture Sources
0.016
0.014
0.012
Humidity ratio
0.01
0.008
0.006
0.004
0.002
0
0 5 10 15 20
time (hours)
34. Moisture and Health
• Ideal situation
• Moisture In = Moisture Out
• Winter situation (Indoor RH% < 20%)
• Relative Humidity stays low
• Skin, eye, sinus irritations and health complaints
• Humid situation (Indoor RH% > 60%)
• Poor air exchange – stuffy, odors, fatigue
• Condensation – visible water damage, mold
35. The Building Envelope
• It is not a perfect barrier
–Permeable to air, moisture, and light
• It gains and loses heat energy
–Thermal Mass
• It gains and loses moisture
–Wetting and Drying
36. Convenient Energy Assumptions
• Buildings are heated and cooled by
convection
• Insulated surfaces have no heat capacity
• All aspects of the building have the same
exterior exposures
37. Heat Energy in Buildings
• Convection – Heating and cooling is supplied
by hot or cold air moving into the building
• Conduction – Heat energy is lost or gained by
conduction through the building frame
• Radiation – Heat energy is lost or gained by
radiation through glazing and onto roofs
38. Heat Capacity and Thermal Mass
• Thermal Mass is the ability of a substance to
hold heat energy ( 402.2.4 Mass walls)
• High Thermal Mass comes from →
– High Heat Capacity
– High Density
– Low Conductivity
– Low Reflectivity (“Albedo”)
• Thermal Mass is NOT the same as Insulation
http://www.yourhome.gov.au/technical/fs49.html
39. Thermal Mass vs. Insulation
Styrofoam Concrete
Specific Heat Capacity
0.3 0.2
(Btu/lbm.°F)
Density
2 150
(lb/ft3)
Heat Conductivity (κ)
0.02 0.5
(Btu/ft.hr.°F)
Reflectivity (Albedo) 0.7 0.4
40. Thermal Mass of a Solid Wall
Area = 270 ft2
Volume = 135 ft3
Concrete wall
20,000 lbs
200,000 Btu
14,000 Btu/hr Jan. Average Temp.
Styrofoam wall Outdoors 21°F
270 lbs
4,000 Btu
550 Btu/hr
Indoors 70°F
42. Solar Exposure Data
• ‘Solar Radiation Data Manual for Buildings’ –
NREL, National Renewable Energy Laboratory
– Solar Radiation
– Climatic Conditions
– Illuminance
http://rredc.nrel.gov/solar/pubs/bluebook/
43.
44. Solar Heat Gain of a South Wall
Area = 270 ft2
Volume = 135 ft3 Jan. Average Solar Rad.
560 Btu/ft2.day
Concrete wall
Insolation Rate:
20,000 lbs
Concrete 10,000 Btu/hr
200,000 Btu
After 9 hours:
Indoors 70°F Wall Temperature +22°F
Jan. Average Temp.
Outdoors 21°F
45. Effect of Thermal Mass on IAQ
Measures used to lower building energy consumption and their cost effectiveness. G.A Floridesa, S.A
Tassoub, S.A Kalogiroua, L.C Wrobel. Applied Energy 73:299-328.
46. Where we’ve been today
• Energy Conservation Construction Code
• Estimating Heat Energy in Buildings
• Estimating Air Leakage
• Estimating Humidity Levels
• Estimating Thermal Mass
• Estimating Solar Heat Energy
• Review and Conclusions
47. Review – ECCC of NY State
• ECCC is intended to achieve the effective use
of energy in buildings
• ECCC applies to new construction and to
addition, alteration or renovation of any
building system or sub system
– Exempts registered historic buildings
• Requires approval by the code enforcement
official
– Must meet Code or [Alternate] meet the intent of
the Code AND achieve equivalent energy savings
48. Review – Heat Energy in Buildings
• Insulation heat loss (Btu/hr)
• Sensible heat (hs)
• Latent heat (hl)
49. Review – Estimating Air Leakage
• ECCC NYS 402.4 Air Leakage (Mandatory)
• 2010 ECCC NYS less than 7 ACH@50 Pascal (7
ACH50)
– 2013 ECCC (Draft) less than 3 ACH50
• Estimating ‘Natural Air Leakage’ from ACH50
– Sherman and Dickerhoff, 1998
50. Review – Estimating Humidity
• An average family of 4 will generate about 14
gallons (53 liters) as water vapor per week
• Winter situation (Indoor RH% < 20%)
• Relative Humidity stays low
• Skin, eye, sinus irritations and health complaints
• Humid situation (Indoor RH% > 60%)
• Poor air exchange – stuffy, odors, fatigue
• Condensation – visible water damage, mold
51. Review – Estimating Thermal Mass
• Thermal Mass is the ability of a substance to
hold heat energy ( 402.2.4 Mass walls)
• High Thermal Mass comes from →
– High Heat Capacity
– High Density
– Low Conductivity
– Low Reflectivity (“Albedo”)
• Thermal Mass is NOT the same as Insulation
52. Review – Estimating Solar Heat
• Estimate Insolation rate from ‘Solar Radiation
Data Manual for Buildings’ – NREL
• Estimate surface albedo
• Estimate surface temperature gain using
specific heat capacities
• Note building aspect dramatically impacts
solar heating
56. References
• NY Energy Conservation Construction Code
– http://publicecodes.citation.com/st/ny/st/b1200v10/index.htm
• LBNL – Residential Building Systems
– http://homes.lbl.gov/
• Building Science Corporation
– http://www.buildingscience.com/
• Conservation Physics
– http://www.conservationphysics.org/
• NREL – Renewable Resource Data Center
– http://rredc.nrel.gov/solar/pubs/bluebook/
57. Quiz
• Which of the following are exempt from the
requirements of the Energy Conservation
Construction Codes?
– New construction of a motel
– Replacing the windows of an apartment building
– Adding roof insulation to the Schuyler Mansion
State Historic Site
– Building a house addition with straw bale walls
58. Questions
• How does the U-value change with increasing
air leakage?
– Increase
– Decrease
– Stays the same
59. Questions
• You receive a complaint of surface stains
around the hot air supply vent during winter.
The problem might be:
– Electrostatic surface charges caused by dry hot air
coming from the vent
– Mold
– Dirty furnace filters
– An unknown source of soot
60. Questions
• Sunlight coming through a clear glass window
provides radiant heat – which of the following
will increase in temperature?
– The interior pane of the window
– The indoor air
– The indoor surfaces illuminated by the sunlight
61. Questions
• A blower door test of a new home shows 6 air
changes per hour at 50 Pascals. Does the
home meet the requirements for 2010 NYS
ECCC?
– Yes, but only if the test was performed by a BPI
certified technician
– Yes, but only if the test method was
ASHRAE/ASTM E779
– No, a visual inspection of the air barriers and
insulation is also required
In order to simplify our discussions about energy in buildings, we will use a conceptual model to describe a building. We will only look at the building envelope and we will assume this building has no windows or doors. The building has four walls, a ceiling and floor. The building envelope is thin and there are holes through which air can leak. This is our conceptual model of the building - it is like a leaky shoe box.
This model describes a building constructed according to the Energy Conservation Construction Code. The building envelope is thicker because it has more insulation than a normal building. There are no holes to prevent air leakage out of the building. This building is similar to a sealed styrofoam box, like a cooler. However, as we shall see when we go through this presentation, constructing a well-insulated, sealed box has consequences on indoor air quality.
This presentation will focus on the building codes in Chapter 4 of the Energy Code – the residential section. We are focusing here because we spend most of our time indoors, and many of the current energy conservation incentives are focused on residential buildings. In this presentation, we will show that the Energy Codes may affect health by affecting the indoor environment – the purpose of this presentation is to help you anticipate and understand how and why the indoor environment changes due to Energy Codes.
The intention of the ECCC is to make people design and construct more energy efficient buildings using better methods, equipment and materials.
Although safety and health may not be compromised when improving energy efficiency of buildings, any practice can be used. The main limitation on new ideas is to show that your innovations meet the intention of the ECCC and that you will actually achieve equivalent or better energy savings than are already outlined in the codes. Also note that you are only required to convince your local code official!
The first step in applying the energy codes is to figure out the standards that apply for your climate zone. New York is divided by county into 3 climate zones – Zone 4 being the warmest and Zone 6 is the coldest.
The most basic requirements of the Energy Conservation Construction Code are to meet the minimum requirements. For example, Table 402.1.1 provides the minimum insulation requirements – here we have selected the code requirements for our conceptual box model of the residential building. You will notice that when you build in colder counties, more insulation is required. We describe the insulation in terms of “R-values” – the higher the R-value, the more effective the insulation is at preventing the loss of heat.
R-value can be described by this formula. The formula shows that when there is a temperature difference between indoors and outdoors, there will be a certain amount of heat lost over the entire insulated surface during a certain period of time. If you look carefully at the formula, you will see that if everything else remains constant, only the amount of heat energy lost affects the R-value – the more heat loss, the lower the R-value – and conversely, the less heat loss the higher the R-value.
The inverse of R-value is U-value. If we rearrange the formula for R-value and put the heat loss term on top, the term “U-value” describes the rate of heat loss per unit area, per degree temperature difference, per unit time. In other words, the U-value describes how quickly heat will be lost through an insulated surface. U-values depend on the units used – most European building standards quote U-values based on metric units, while we use Imperial units in America. You can convert Imperial (American) U-values to metric units by multiplying by 5.678.
For our purposes, we are interested in estimating the total heat loss through the building envelope. That calculation requires estimating the effective R-value of the entire assembly – the “Assembly R-value”. This is estimated by looking at the different areas of the various components, factoring in their U-values, and summing the entire system into the UAssembly value and using that value to calculate the Assembly R-value. Note that again, we are going to simplify this process as much as possible, so we are not going to factor in many important details, like framing studs and corners, which often have significantly different U-values from the rest of the building assembly.
For our purposes, we will estimate the amount of heat loss through the building envelop of our conceptual model residence, which we shall place in Albany County which is in Climate Zone 5. The building has a rectangular 50 foot by 30 foot footprint and is 9 feet high. Thus, we easily calculate the volume, floor and ceiling areas, and the wall area. We also assume the building is insulated according to the minimum standards in Table 402.1.1 – the building has R-38 insulation in the ceiling, R-20 walls and the floor is R-30. In order to estimate the heat loss through this building envelop, we will start by estimating the R-assembly value.
For each component of our building, we will estimate it’s U-value, which is the inverse of the R-value. So the U-value for the ceiling insulation is 1/38, the walls are 1/20, and the floor is 1/30. Each of the components of the envelop has a certain percentage of the total surface area – the ceiling and floor are each 1500/4440 = 0.338, the walls are 1440/4440 = 0.324.
For the purposes of the calculation, it may easier to see this data in a table. The R-value for each component is in column 2. The percent area of each component relative to the entire building envelop is in column 3. The U-value of each component is calculated from the R-value in column 2 and the resulting U-value is given in column 4. The product of U-value times the percent area, i%, is calculated in column 5. The values are sub-totaled in column 5, to give the Uassembly value, which we then take the inverse of to get the R-assembly value. From the calculation, we see that the effective R-value of the building envelop is approximately 27.5. Now that we know the Assembly R-value, we can estimate how much heat is lost through the insulated structure.
In this case, we are going to consider the average winter condition, because we want a picture of the average energy condition of our building. In order to estimate the heat loss through the building envelop, we will use the formula for R-value and will rearrange it to give the heat loss per unit time (BTU/hour). For buildings in New York, we have to assume some things are going to happen during the winter – assume the interior design temperature for heating is 70°F. The exterior temperature is taken from the Average Daily Temperature for Albany in January (from http://www.climate-zone.com/climate/united-states/new-york/albany/), which is in Zone 5. The temperature difference 70 minus 21 (rounded up from 20.6) which gives 49°F. The Assembly R-value is 27.5, and the entire building envelop area is 4440 square feet. Putting these values in to the equation, we estimate that the average heat loss is 7911BTU/hr. As before, we are keeping this simple by not considering the effects of windows and doors, etc. However, heat loss through the building envelope is not the only energy used in residential buildings…
402.4 Air leakage (Mandatory).402.4.1 Building thermal envelope. The building thermal envelope shall be durably sealed to limit infiltration. The sealing methods between dissimilar materials shall allow for differential expansion and contraction. The following shall be caulked, gasketed, weatherstripped or otherwise sealed with an air barrier material, suitable film or solid material: 1. All joints, seams and penetrations. 2. Site-built windows, doors and skylights. 3. Openings between window and door assemblies and their respective jambs and framing. 4. Utility penetrations. 5. Dropped ceilings or chases adjacent to the thermal envelope. 6. Knee walls. 7. Walls and ceilings separating a garage from conditioned spaces. 8. Behind tubs and showers on exterior walls. 9. Common walls between dwelling units. 10. Attic access openings. 11. Rim joist junctions. 12. Sill plates and headers. Foam plastic (spray foam insulation) shall be permitted to be spray applied to a sill plate, header, and rim joists without the thermal barrier as specified in the Residential Code of New York State, Section 314.4 subject to all of the following: a. The maximum thickness of the foam plastic shall be 31/4 inches (83 mm). b. The density of the foam plastic shall be in the range of 0.5 to 2.0 pounds per cubic foot (8 to 32 kg/m3). c. The foam plastic shall have a flame spread index of 25 or less and an accompanying smoke developed index of 450 or less when tested in accordance with ASTM E 84. 13. Other sources of infiltration
402.4 Air leakage (Mandatory).402.4.1 Building thermal envelope. The building thermal envelope shall be durably sealed to limit infiltration. The sealing methods between dissimilar materials shall allow for differential expansion and contraction. The following shall be caulked, gasketed, weatherstripped or otherwise sealed with an air barrier material, suitable film or solid material: 1. All joints, seams and penetrations. 2. Site-built windows, doors and skylights. 3. Openings between window and door assemblies and their respective jambs and framing. 4. Utility penetrations. 5. Dropped ceilings or chases adjacent to the thermal envelope. 6. Knee walls. 7. Walls and ceilings separating a garage from conditioned spaces. 8. Behind tubs and showers on exterior walls. 9. Common walls between dwelling units. 10. Attic access openings. 11. Rim joist junctions. 12. Sill plates and headers. Foam plastic (spray foam insulation) shall be permitted to be spray applied to a sill plate, header, and rim joists without the thermal barrier as specified in the Residential Code of New York State, Section 314.4 subject to all of the following: a. The maximum thickness of the foam plastic shall be 31/4 inches (83 mm). b. The density of the foam plastic shall be in the range of 0.5 to 2.0 pounds per cubic foot (8 to 32 kg/m3). c. The foam plastic shall have a flame spread index of 25 or less and an accompanying smoke developed index of 450 or less when tested in accordance with ASTM E 84. 13. Other sources of infiltration
Most buildings have some air leakage; that is, some air comes into the building and some goes out of the building – generally most of the air comes in at the bottom and exhausts at the top of the building. This air leakage is variable and is influenced by indoor/outdoor temperature differences (warm air rises relative to cold air), and by outdoor winds. A standard measurement of air leakage is the blower door test, where the exterior openings (windows and doors) are closed and the interior doors are opened. A fan or “blower door” is placed in the front door so that it can blow air out of the building – the fan speed is increased until the pressure inside the house is 50 Pascals less than outdoors (50 Pa is equivalent to 1 inch of water or 1/2000 of atmospheric pressure). The air leakage of the house is then estimated under this standard condition and is reported as Air Changes per Hour at 50 Pascals. That is not a normal pressure difference – most indoor/outdoor pressure differences are much lower – and we actually want to know the “Natural Air Leakage” which occurs under more normal conditions. It turns out the Natural Air Leakage is approximately 1/20 of the ACH50, so 7ACH50 gives 0.35 ACH, which by happy coincidence, is the minimum required Air Exchange Rate in Table 405.5.2 of the ECCC.
Given an Air Exchange Rate of 0.35ACH, we can estimate the outdoor air flow volume and hence the ‘Sensible Heat’, or the amount of energy required to change the Air Temperature from 21° to 70°F, or conversely, we are estimating the amount of heat energy lost as warm air leaks out of the building. From this calculation we can see that we need to generate 4181BTU/hr to warm the outdoor air up to room temperature.The 1.08 multiplier is not some mysterious magic number. This number includes the specific heat of air (0.24 Btu per pound per degree F). It takes 0.24 Btu of heat to change the temperature of one pound of air by 1 degree Fahrenheit. The 1.08 also contains the specific density of air (0.075 pounds per cubic foot). The air is measured in cfm yet the specific heat is per pounds of air. The weight per cubic foot of air (0.075 pounds) is needed to convert between the air volume and weight. Also contained in the 1.08 factor is the number of minutes in an hour (60 minutes per hour). This is required to convert between Btu per hour and cubic feet per minute. The factor of 1.08 is the product of the specific heat (0.24 Btu) times the density (0.075 pounds per cubic foot) times the number of minutes per hour (60 minutes).
There is less water vapor in the outdoor airLatent Heat does not change the temperature of the air. The humidity ratio of water vapor to dry air is 0.002 lb/lb at 32°F, and 0.006 lb/lb at 72°F, for an Estimated Latent Heat of 1530 BTU/hr.
These are the main numbers that we need to estimate the Average Heat Loss of this conceptual building. As before, we aren’t estimating the effects of windows, doors, wood framing, etc. This is just energy lost through the insulation and energy used to heat up cold air. We can add our heat energy values together to estimate that the Average Heat Loss of our conceptual building located in Albany in January would be about 14,000 BTU/hr. This is for a residence (BOX) constructed in compliance with the 2010 Energy Conservation Construction Code.
The previous calculation showed that this building will lose an average of 14,000 BTU/hr during the winter. For the sake of much of the Energy Codes, it assumed that buildings are heated with hot air. In order to increase the room temperature, we must supply hot air, usually at a temperature between 100° and 105°F. Therefore, we can estimate the amount of hot air that would be used to maintain the indoor temperature around 70°F is approximately 360 cfm.
We also know that some of the air being supplied must come from outside to maintain our indoor air quality. The outside air supply must be a minimum of 0.35 ACH, which based on our building volume gives 79 cfm. Subtracting the outside air flow from the total air flow, gives the volume of recirculated air.
We calculated the flow rates of recirculated air and outside air because we are interested in one of the main comfort factors of the indoor environment, relative humidity. We can estimate the changes in relative humidity if we know the humidity ratio in the two air flows. In our hypothetical case, the outside air has a humidity ratio of 0.002 and the recirculated air has a humidity ratio of 0.006, which are mixed in the proportion of 79 to 317 respectively. From this calculation we see that as outdoor air is mixed into indoor air the humidity ratio, and hence the relative humidity, decreases.
The psychrometric chart shows the relationships between relative humidity and humidity ratio. The humidity ratio for a particular air stream is constant with respect to temperature. However, the relative humidity decreases with increasing temperature.
As we mix outside air into the indoor air, the humidity ratio decreases until it is the same as the outside air supply. Thus, the relative humidity of the indoor air decreases, which leads to “dry air”. In theory, indoor relative humidity will reach it’s minimum level within 24 hours.
The chart shows the general degradation in humidity ratio (and relative humidity) at 0.35 ACH.
Low RH can lead to apparent degradation of the indoor environment. Dry air moving across a surface will remove absorbed water molecules, which leaves a static charge on the surface. The charged surface attracts dust, dirt and debris which may accumulate into visible deposits and stains. Visible ‘contamination’ is often a precursor to health complaints or it can ‘validate’ claims of health effects. When the relative humidity is low, the moist surfaces of our bodies will dry out leading to itchy skin, dry sinuses, sore throats, etc. These health complaints, when observed alongside visible surface contamination, lead to concerns and usually to complaints about the indoor environment.
Indoor environmental comfort is related to temperature and humidity. As we have seen in this simple model, there will be changes in relative humidity when heating mixtures of indoor and outdoor air. Thus far, we have not considered what happens in an occupied building, where there are sources of moisture. Also, we have kept our model very simple and assumed that all of the energy that passes through the building enclosure is completely exchanged between indoors and outdoors – in other words, the building enclosure does not get heated or cooled during this process. We want a more realistic picture of how energy efficiency affects the indoor environment, so we will now consider factors that are outside the Energy Conservation Construction Codes, starting with indoor sources of moisture.
Some researchers have estimated the quantity of moisture added to indoor air as 14 gallons per week or about 2 gallons per day for a family of four. Notice that the bulk of the moisture vapor comes from the occupants themselves, through respiration and skin evaporation. When you think about the other components of skin evaporation and respiration, you start to realize how important fresh air ventilation is to dilute body odors in indoor air!
Given that a family of four generates about 14 gallons per week of moisture, we can estimate that on average they generate 0.7 lbs/hr. Note that although this is the average, there will be times such as showering, cooking, and sleeping, when the actual rate of moisture generation will be dramatically more (or less) than the average. From the volume of the house and the density of air, we can estimate that the humidity ratio will increase by approximately 0.0007 per hour – for a perfectly sealed building with no air exchange the humidity ratio at 70F will reach saturation in about 14 hours. With 0.35 ACH in our Winter model, the RH will equilibrate around 37% in approximately 19 hours.
We added the humidity ratio curves to our chart to show the changes in humidity ratio which occur without indoor moisture sources (blue line), with indoor moisture sources and without outside air exchange (green line), and with indoor moisture sources and with 0.35 ACH (red line). In real life, the red line will oscillate following the diurnal cycle and following the hot air supply cycle.
Indoor relative humidity is a major factor in comfort and health complaints. One theory of the winter cold and flu season is that low humidity leads to drying of the mucous membranes, which makes us more susceptible to respiratory infections. At the opposite end of the scale, high humidity correlates with condensation and mold growth in indoor spaces. Also, if moisture is accumulating, this generally indicates there is not sufficient fresh air ventilation, which leads to complaints of stuffiness, odors, and often, fatigue.
In engineering, it is convenient to make some assumptions to simplify the calculations. Just as we chose to perform our calculations for a conceptual model rather than an actual building, engineers will first assume that the building envelope does not participate in the energy transformations in a building. However, we know that building envelopes gain and lose heat energy into their thermal mass and that the structure of the envelope is able to conduct heat energy from one part of the building to another. Similarly, we know that the building envelope gains and loses moisture, either through wetting and drying cycles, or freezing and thawing cycles, or both. These considerations are very important for us because they help us to understand and compare the energy efficiency of old construction versus new construction.
In most energy calculations, we assume the building is heated and cooled by convection – that is, a fluid medium transports the heat energy inside an insulated barrier (the building envelope). When the energy is lost through the building envelope, we assume that the structure has no heat capacity – that is, the walls, floors and ceilings do not get warmer or colder. Finally, different aspects of the building have quite different exposures to the environment; for example, there are huge differences in solar heat gain on the north versus the south facing walls of buildings.
There are three forms of heat energy; convection, conduction and radiation. All of these forms of heat energy are important in the indoor environment. We rely on convection to distribute heat energy in indoor air. For most situations, convection provides the sensible heat that we feel as air temperature. Conduction of heat energy through the building frame is very important because that is how we get hot and cold spots, which may cause thermophoresis (or “ghosting”) and condensation. Finally, radiation is important because it can cause high surface temperatures without high air temperatures – thus, a black wall receiving direct solar radiation in winter can have a much higher temperature than the surrounding air temperature.
The building enclosure can hold some heat energy. Consequently, heat energy is not immediately lost through the building enclosure – the building enclosure acts as a buffer, storing some heat energy in the structure. The amount of heat energy that can be held is based on the heat capacity and the mass of the material – collectively called the “thermal mass”. In general, thermal mass is high if the heat capacity of the material is high and the density is high, and the material should have low conductivity and low ‘reflectivity’ (technically, ‘reflectivity’ generally refers to visible light and ‘albedo’ includes infrared and ultraviolet radiation). In other words, the heat energy is held in the mass of the material – the more massive the material the more heat energy that can be stored, and the higher the heat capacity (which is amount of heat energy required to raise the temperature) then the thermal mass is higher. However, we don’t want to conduct the heat away from the building or the room, so we want low conductivity. Finally, objects with low reflectivity are able to radiate and absorb more heat. Notice that these are not the properties which we need for an insulator…
If we compare two materials we can get a picture of their relative thermal masses. In this case, we will compare styrofoam (“expanded polystyrene”) with concrete. This table shows that concrete and styrofoam have similar heat capacities – it takes about 0.2-0.3 Btu to raise the temperature of 1lb of each material by 1°F. However, their densities are very different – concrete is about 75 times denser than styrofoam, so 1ft3 of concrete takes 75 times more Btus to warm up than 1ft3 of styrofoam. Now, the heat is conducted about 25 times faster through concrete than through styrofoam – concrete conducts 0.5 Btu per hour per foot thickness per 1°F temperature difference versus 0.02 for styrofoam. Finally, an unfinished (white) styrofoam surface reflects twice the amount of electromagnetic radiation as a smooth concrete surface.
Consider the thermal mass of a wall made out of concrete versus a wall of styrofoam – using dimensions from the conceptual model (30 feet long by 9 feet tall) and assuming the wall is 6 inches thick (for convenience of calculation), we have an area of 270 square feet and volume of 135 cubic feet. As we saw in the table before, the specific heat capacity of concrete and styrofoam are similar, 0.2 versus 0.3 Btu/lb.°F. However, due to the difference in density, a concrete wall takes much more energy to warm it up – approximately 50 times more energy. Concrete is a worse insulator than styrofoam, transmitting heat energy 25 times faster than styrofoam, but due to the difference in mass, the time required to dissipate all of the stored energy is twice as long for a concrete wall (15 hours) versus a styrofoam wall (7 hours). In other words, the concrete wall will remain warm for a longer period than a styrofoam wall, thus acting as a thermal buffer.If this is a concrete wall, its mass is 20250 lbs, versus 270 lbs for styrofoam.The amount of heat energy stored in this wall under the given situation is… (mass times heat capacity times temperature difference)20250x0.2x51 = 206550 Btu in concrete270x0.3x51 = 4131 Btu in styrofoami.e. 50 times more energy is stored in the concrete wall…(Approximately 13770 Btu in a 270sf wood frame wall – based on COMcheck201)The rate of heat energy loss through the wall is κ.A.ΔT/d.Through a concrete wall the heat energy loss is 0.5x270x51/0.5=13770 Btu/hrThrough the styrofoam wall the heat energy loss is 0.02x270x51/0.5=551 Btu/hrBased on this calculation, we can estimate that a fully warmed concrete wall would lose all of it’s heat energy in 15 hours. A styrofoam wall will lose all of it’s heat energy in 7.5 hours. In other words, if a sufficient mass of concrete is heated then it will lose heat slowly, buffering the heat loss from the building – in contrast, a styrofoam wall can’t hold enough heat energy to buffer the heat loss from the building.
Thus far, we have considered the building to be symmetrical, with all aspects of the building envelop subject to the same temperature conditions. However, we are much more likely to see buildings that have different exposures with respect to sun, wind and precipitation. In particular, we will evaluate the effect of heat energy from sunlight – sometimes called “insolation”. As shown on this diagram, the sun’s path in the sky changes between winter and summer, and during the day. The sun’s path is higher during the summer, and lower during the winter – so different parts of the building receive different amounts of insolation during the year.
The general specifics of solar radiation and insolation are provided by city and state in this reference, which is available online.
A typical entry in the book is shown for Albany, NY. We are interested in reviewing the table, showing the ‘Average Incident Solar Radiation’. Line 1 of the table shows a global average for each month of the year – we are interested in January to consider how our model building might be affected by solar radiation. On average, the incident solar radiation is 560 Btu/ft2.day (±9%). For the most part, this occurs on the Southern aspect of the building.
Let us consider the effect of solar radiation on a south facing concrete wall. The exterior wall is receiving solar energy, which for January in Albany, averages 560 Btu/ft2.day which we will estimate occurs in about 9 hours of daylight during January (http://www.climate-charts.com/Locations/u/US72518003000421.php), of which 40% (albedo=0.4) is reflected from concrete.We estimate this 6 inch concrete wall receives (560)(270)(1-0.4)/9 = 10080 Btu/hr. What is the effect of that solar radiation on the temperature of the concrete? The specific heat of concrete is approximately 0.2 Btu/lb.°F, so we can estimate the temperature change of the wall is (10,000)(9)/(20,000)(0.2) = 22.5°F. Thus, the concrete wall temperature is approximately 22°F warmer than the ambient temperature.
Winter time in Nicosia, Cyprus. Note the line with the x’s for the indoor temperature of a building with 24-inch concrete walls – this shows the effect of the thermal buffering of concrete walls when compared to the line with the black circles showing the ambient outdoor temperatures over 48 hours.MA – Single wall, hollow brick 0.2 m (8 in) and 0.02 m (3/4 in) plaster on each sideMB – Double wall, 0.1 m (4 in) hollow brick, 0.02 m (3/4 in) plaster on each side and a layer of 0.05 (2 in) m polystyrene insulation in betweenMC – steel siding, 0.025 m (1 in) insulation and steel sidingMK – 0.6 m (24 in) heavy-weight concrete walls
In principal, the model that we have considered is similar to assuming that existing housing is like a cardboard box with a loose lid. It is poorly insulated and has a lot of air leaks. As noted in our calculations, we generally assume that these types of buildings are heated by convection – that is, the building is warmed by heating up the air coming into the building and mixing it with conditioned air in the building. Some of the energy spent used to heat that air is lost by conduction through the building envelop and by air leakage. Consequently, we are trying to minimize that energy loss by adopting the Energy Conservation Construction Codes.
The Energy Conservation Construction Codes specify improved insulation and decreased air leakage of new and renovated residential buildings. In effect, we are specifying that the cardboard box must be replaced with a styrofoam cooler, preferably wrapped in plastic to minimize air leaks! Of course, this has serious consequences on the indoor air quality inside the building. We have shown that when outdoor winter air is mixed with indoor air, the relative humidity decreases, leading to visible changes in the environment and perceived health effects – our sinuses dry out, as well as developing respiratory complaints and itchy eyes and skin. On the other hand, we also shown that decreasing the air exchange rate will lead to increased relative humidity in indoor air – this might offset the drying trend during the winter, but during non-heating seasons this will cause increased humidity levels which can lead to water condensation and mold growth.
In most cases, we need to consider that the building envelope is part of the heating and cooling system of the building. The heat energy coming into and leaving a building is buffered by the building envelope. Thus, buildings are more like humidors in that they exchange heat and moisture with the interior environment. The rate of heat and moisture exchange varies depending on the types of building materials and finishes. In this presentation, we looked at the thermal mass of concrete walls as one example of temperature buffering. However, other references also discuss how the building system is affected by moisture buffering and hygrothermal buffering.
The building codes of New York State are available free of charge on-line.Lawrence Berkeley National Laboratory Residential Building Systems group has cutting edge information on indoor air quality, ventilation, infiltration and more.Building Science Corporation is a great resource for readable practical information about all aspects of residential buildings.Conservation Physics introduces some advanced concepts in building science related to hygrothermal characteristics of structural materials.The National Renewable Energy Laboratory is a resource for data and models, particularly for renewable energy.
