A Little Building Science

A Little Building Science
Understanding temperature and humidity
Nigel Isaacs, Principal Scientist
Why worry about moisture?
►2010 House Condition survey
– 40% NZ homes internal moisture problems
– Many cold & damp, with 10% “very damp”
►Consequences
– Material deterioration, Mould & Mildew
– Poor living environment
– Occupant health issues
►NZBC – some relevant clauses
–
–
–
–
Clause E3 Internal Moisture
Clause E2 External Moisture
Clause G4 Ventilation
Clause H1 Energy Efficiency
2
Understanding our environment
►Buildings (& energy) modify our environment
►Hygrothermal
– warm, cool, humidity
►Visual
– light, glare, romance
►Acoustic
– speech, noise, warnings, music
►Indoor Air Quality (IAQ)
– breathing clean, fresh air, ventilation
►Activities
– keeping clean, doing accounts, talking, sleeping …
3
Temperature & Humidity
►Hygrothermal
– ‘hygro’ = wetness, fluid
– ‘thermal’ = heat
►Air is a fluid
– But we know it is not ‘pure’
►So what is it made from?
4
"Air" of Today
►‘Air’ contains
– Oxygen (discovered 1780s)
– Water (discovered 1800s)
►Why does the water matter?
– SWEAT – evaporates to cool the body
– Dogs pant, humans sweat
– But what about machines & materials …
5
Quiz Question 1
►What happens when you heat a fluid?
Answer:
►It gets lighter
►Hot air rises
Hot
Cold
►Cold air sinks
6
What do these have in common?
7
Experiment Time
►Cold & Boiling water in two jugs
►Why is water on the outside of cold jug?
– Wipe it – what is left behind?
– Taste it – does it taste good?
►Why does steam rise from the hot water?
– DO NOT put finger in it
– Hold finger above it – what does it feel like?
►If the water is warm, what my finger feel?
– Mix a little cold & hot water in an empty glass
– Put finger in, pull it out, hold in air.
– What does finger ‘feel’ like? (not just wet!)
8
Why is the cold water jug wet?
►Dew has formed
– Moisture from the air has condensed
– NOT pure or clean – picked up smells, dirt
►‘Dew Point’ temperature
►Can we predict the dew point temperature?
– Of course
9
Why does steam rise from hot water?
►Steam above hot water, hot food
►Why ?
– Too much water for the air (saturation)
– BUT only close to the water source
►Gradually steam (water) mixes with air
– No longer too much
►Cold breath in winter
10
Why warm water but cold finger?
►Dip finger – thin layer of water
–
–
–
–
Finger at blood temperature (37°C, 98.7°F)
Water evaporates
Evaporating takes energy
Removing energy = cool
►Process also works other way around
–
–
–
–
Cool steam
Gives up the energy
‘latent heat of evaporation’
Condensing Boilers
11
So what ?
►What has happened
– Wet cold-water jug = to much water for air (dew)
– Steam above hot water = too much water for air
– Evaporating water = cool finger
►How to link these together?
12
Understanding water & air
►Willis Carrier
– 1902: Invented air conditioning for a printing works
– Why?
• Paper changed size
• Colours misaligned
– A/c for people came later
►Air conditioning is costly
– How to minimise costs?
Key question:
– How much water?
Answers
Rivoli Theater in New York City, 1925
– Hygrometric Chart (1906)
www.carrier.com
– Psychrometric Formulae (1911)
13
Water in the air
►Glaisher (1847) (English)
– Wet & Dry Bulb Temperatures
Cotton
wick
Wet bulb
Dry bulb
Water
Thermometer
‘bulb’
Whirling hygrometer
14
Psychrometric Chart
20
25
C
o
ulb
Te
mp
era
tur
e
(W
BT
)
18
80%
60%
50%
16
20
15
14
40%
13
12
15
W
et
B
17
11
30%
10
9
10
8
20%
7
6
5
5
0
Relative
Humidity
(RH)
4
10%
3
2
1
Humidity Ratio (grams water / kg dry air)
19
0
0
5
10
15
20
25
30
35
Dry Bulb Temperature (DBT) oC
15
Humidity – 2 types
►Measures of the amount of water in the air
– Both shown on psychrometric chart
►Relative Humidity (RH)
– Proportion of water in the air compared to how
much the air can hold at that temperature
– % (per cent)
►Absolute Humidity (or Humidity Ratio)
– Amount of water in the air
– Grams of water per kilogram of dry air (gm/kg)
16
At home with the Jones …
►Bedroom
– 2.4 m high
– 4 m long
– 3 m wide
2.4 m x 4 m x 3 m = 29 m3
2.4 m
3m
4m
►In the morning, why are the windows wet?
►Lets take the whirling hygrometer visiting
17
Whirling about ...
►Whirl (spin) around until reading stable
– 1-2 minutes
►Wet bulb 16.5°C
►Dry bulb 21°C
18
Using Psychrometric Chart
19
C
BT
)°
(W
ulb
Te
mp
era
tur
e
W
et
B
20
25
Humidity Ratio (grams water / kg dry air)
Dry bulb 21 oC
Wet bulb 16.5 oC (70% RH)
Dry air = 1.2 kg/m3
Volume = 29 m3
1.2 kg/m3 x 29 m3 x 11 gm/kg
≈ 0.4 litre of water
Max
= 1.2 x 29 x 16 = 0.6 litre
= 0.2 litre more
any more is TOO MUCH
18
80%
60%
50%
+ Occupant moisture
20
17
16
15
14
40%
13
12
Room moisture
15
11
30%
10
9
10
8
20%
7
6
5
5
Relative
Humidity
(RH)
0
4
10%
3
2
1
0
0
5
10
15
20
25
30
Dry Bulb Temperature (DBT) °C
35
19
Why so little condensation?
►2 people in bedroom add about 1.6 litres
– 0.1 l/hr x 8 hr x 2 people = 1.6 litres overnight
– Thin layer of dew – annoying, but not that much
– Where does the rest of the water go?
►Storage of water
– Natural materials absorb and release moisture
– Temperature drops: moisture absorbed
– Temperature rises: moisture released into air
►Removal of water
– ‘air’ the room: open windows, doors
20
Why does cheese dry out in the fridge?
20
25
C
BT
)°
oC
– Wet bulb 0
– Dry bulb 4 oC
►Fridge is (very) dry
►Room
– 18 oC
– 63% RH
– 8 gm/kg air
►Cheddar
18
80%
60%
50%
17
16
20
15
14
40%
13
12
15
W
et
B
– 50% RH
– 2 gm/kg air
ulb
Te
mp
era
tur
e
(W
►Measurements:
Humidity Ratio (grams water / kg dry air)
19
11
30%
Cheese (Room) moisture
10
20%
10
9
8
7
6
5
5
0
4
10%
Fridge moisture
3
2
1
0
0
– ~39% moisture
– Will dry out in room air
5
10
15
20
25
30
35
Dry Bulb Temperature (DBT) °C
21
Why do air conditioners drip?
20
25
►Sydney design temperatures:
18
80%
– Wet bulb 23 oC
– Dry bulb 29 oC
50%
17
16
Outside
moisture
20
►Inside temp. 19°C, 80% RH
►Absolute humidity reduced15
Outside 16 gm/kg
Inside 12 gm/kg
1.2 kg/m³ x 29 m³
0
x 4 gm/kg
– 1 Air change / hr
= 0.14 litre
– 5 ACH
= 0.7 litre
60%
40%
Inside moisture
15
14
13
12
11
30%
10
9
10
8
20%
7
6
5
5
4
10%
3
2
1
0
0
5
10
15
20
25
30
35
Dry Bulb Temperature (DBT) °C
22
Humidity Ratio (grams water / kg dry air)
19
Quiz Questions 2 & 3
►How do you reduce humidity?
Answers
►Heat air up
– Hot air can hold more water
►Cool the water
– Water ‘falls’ out
– Temp. below dew point
23
What about the steam?
►Heat ‘energy’ throws water into air
– Too much water, too fast
– Small volume of air becomes 100% RH
– IF air < 100% RH, air absorbs steam
– Increases the overall RH of the air
►But water doesn’t have to be boiling
– Steamy breath in winter
►Water moving to air not always seen
24
House Moisture Loads - Estimates
Source
Moisture
Un-vented clothes dryer
12 litre/day
Clothes washing
2 litre/day
Un-vented cooking
2.2 litre/day
One shower
200 ml/day
Dishwashing
500 ml/day
Person – at rest
100 ml/hr
Person – active
300 ml/hr
Unflued gas heater (2kW for 2 hr)
600 ml/2 hr
Also plants, fish tanks, drying towels, …
Source: BRANZVENT handbook
Estimates only – huge ranges
25
Quiz Question 4
►Why does double glazing reduce
condensation?
Answer
►It keeps the temperature of the inside
glass warmer than the dew point
►Dew forms on cool surfaces
26
Single or Double Glaze?
► Glass
– Good conductor
– Heat quickly escapes
► Air
– Reasonable insulator
– Traps heat
► Single glass
– Cools quickly – DEW
► Double glass (air gap)
– Cools slower
Photo: John Burgess, BRANZ
27
Why does the window fog up?
20
25
C
o
Humidity Ratio (grams water / kg dry air)
19
ulb
Te
mp
era
tur
e
(W
BT
)
18
80%
50%
17
16
20
15
14
40%
13
12
Room moisture
15
W
et
B
60%
11
30%
10
9
10
Window surface
20%
8
7
6
5
5
0
4
10%
3
2
1
0
0
5
10
15
20
25
30
Dry Bulb Temperature (DBT) °C
35
28
Body in a bed
40°C
Under sheet
temperature
► Bedtime
35°C
Temperature
30°C
– To bed at 0 hours
– Gets up at 8 hours
25°C
20°C
Room temperature
15°C
► Night variations
10°C
5°C
0°C
0
4
8
12
16
20
24
► Day time
80%
Room RH
– Bed gives up moisture
– Attempts to follow room
70%
60%
Relative Humidity
– Turning over
– Movement around bed
50%
► Bed RH rises & falls
40%
Under-sheet RH
– Obvious, but not expected
30%
20%
10%
0%
0
4
8
12
16
20
24
Time of Day
Data: Malcolm Cunningham, BRANZ
29
Warm up to reduce RH
►Just change the temperature
20
25
– No extra water load
Humidity Ratio (grams water / kg dry air)
19
18
80%
►Room not too hot
60%
50%
17
16
20
15
– 20 °C = 75% RH
14
40%
►Cooler overnight
12
15
11
– 15 °C = 100% RH
30%
10
9
10
►Warmer next day
– 25 °C = 55% RH
13
8
20%
7
6
5
5
0
4
10%
3
2
1
0
0
5
10
15
20
25
30
Dry Bulb Temperature (DBT) °C
35
30
Why cold & damp?
20
25
19
Add moisture
Increase RH 80%
18
60%
50%
17
16
20
15
14
Reduce temperature Increase temperature
40%
13
Increase RH
Reduce RH 12
15
11
30%
10
9
10
8
20%
7
6
5
5
0
4
10%
3
2
1
Humidity Ratio (grams water / kg dry air)
Fabric & materials
absorb & release
moisture
0
0
5
10
15
20
25
30
Dry Bulb Temperature (DBT) °C
35
31
Bioclimatic Chart - guidance
►Comfort varies:
– culture
– individual
– physical condition
– psychological expectation
– not precise ( ± little)
►Americans (sedentary, clothed)
– Temperature:
• 20 °C to 26 °C
– Humidity:
• 20% to 80% Relative Humidity (RH)
Milne & Givoni "Architectural Design Based On Climate“
in "Energy Conservation Through Building Design" (Watson 1979)
32
Achieving Comfort
19
ulb
Te
mp
era
tur
e
(W
BT
)
o
C
20
17
16
20
15
14
40%
13
12
15
W
et
B
18
11
30%
10
9
10
8
20%
7
6
5
5
0
Relative
Humidity
(RH)
4
10%
3
2
1
Humidity Ratio (grams water / kg dry air)
Sensible = Temperature changed, not absolute humidity
25
Dehumidify = Remove water
Humidify = Add water
80%
60% 50%
0
0
5
10
15
20
25
30
Dry Bulb Temperature (DBT) °C
35
33
Moving the comfort zone
►Bioclimatic chart based on
– Sedentary (seated)
– 1 standard unit of clothing
– White American
►Moving or working makes us feel warmer
►More clothes trap heat & we feel warmer
►No similar experiment for NZ
34
Clothing – a form of Insulation
Clothing Ensemble
Insulation
(Clo)
Nude
0
Shorts
Tropical
(shorts, short-sleeve shirt, socks, sandals)
Men's light summer
(long trouser, short-sleeve shirt)
Men's Business suit
(plus cotton underwear, long shirt, woollen socks)
Women's indoor ensemble
(skirt, long blouse, jumper, underwear, stockings)
Men's heavy suit
(above + woollen coat)
0.1
0.3 - 0.4
0.5
1
0.7 - 0.9
2.0 - 2.5
35
Clothing (Clo)
20
25
ulb
Te
mp
era
tur
e
(W
BT
)°
C
19
18
80%
60%
50%
16
20
15
14
40%
13
12
15
W
et
B
17
11
30%
10
9
10
8
20%
7
6
5
5
0
Clo 0
Clo 0.6
Clo 2.4
4
10%
3
2
Clo 0.9
1
Humidity Ratio (grams water / kg dry air)
More clothes, the higher
the comfort temperature
0
0
5
10
15
20
25
30
Dry Bulb Temperature (DBT) °C
35
36
Activity - a comfort modifier
Activity
Sleeping
Reclining
Sitting
Standing, relaxed
Walking
level, 3.2 km/hr
level, 4.8 km/hr
level, 6.4 km/hr
+15 deg, 3.2 km/hr
House Cleaning
Typing
Gymnastics
Dancing
Hand sawing
Heavy work (eg. steel forming)
Metabolic Rate
(met)
0.71
0.81
1
1.21
2
2.6
3.81
4.6
2.00 – 3.41
1.21 – 1.40
3.02 – 4.02
2.41 – 4.41
4.00 – 4.83
3.52 – 4.52
37
Activity (met)
20
25
ulb
Te
mp
era
tur
e
(W
BT
)°
C
19
18
80%
60%
50%
16
20
15
14
40%
13
12
15
W
et
B
17
11
30%
10
9
10
8
20%
7
6
5
5
0
4
10%
1 met
3
2
3 met
1
Humidity Ratio (grams water / kg dry air)
More activity, the lower
the comfort temperature
0
5 0met
5
10
15
20
25
30
Dry Bulb Temperature (DBT) °C
35
38
Quiz Question 5
►Why does the speaker feel warmer than
the audience?
Answer
►Moving about – physical work
– BUT not every one is happy at the same
temperature & RH
– Expect about 95% people to be happy
– ‘PMV’ – Predicted Mean Vote
39
"St. Jerome In His Study" (c1475)
Antonello da Messina
40
Where does corrugated iron corrode?
Dew – air cannot hold all moisture
Cooled by night sky
-> Condenses on underside
8 am, 30 November, Nepal
-> Corrodes from underneath
41
Machapuchare, Nepal. 6,993 m
Evening Patterns – what’s going on?
Garage – cold inside
Shop – warm inside
Thermal bridge – timber structure
- timber better insulator (warmer)
- less condensation
6.45 pm, 6 August, Wellington
- less mould
42
My house is damp – what do I do?
1. Extract moisture at source
–
Extract fan
2. Stop moisture coming into house
–
Sub-floor moisture control
3. Ventilation
–
Windows, passive ventilation
3. Keep warm – passive
–
Insulation
5. Keep warm – active
–
Heat
► Other actions ...
43
Eco house for Barbie (& Ken?) 2011
•
Winner AIA Architect Barbie® Dream House™
•
•
•
•
•
•
Solar panels installed on the roof
Green house for Barbie’s love of nature
Landscaped rooftop & efficient irrigation system
Operable shading devices using perforated screens
Bamboo flooring & Zero VOC paint throughout
Locally sourced & manufactured materials &
furnishings
Landscaped garden for Barbie’s lovely pets
Low flow toilet + sink fixtures & Energy Star
appliances
Minimum site disturbance
Decidedly NO parking garage & driveways
Signature Pink Scooter is her vehicle of choice
Minimum site disturbance
•
•
•
•
•
•
2
2
• 455
m house + 186
m open
deck
“Barbie’s dream house is the
quintessential
Malibu
beach
house;
it is modern, functional, spacious, fun and most of all sustainable”
http://ase.org/efficiencynews/architects-design-eco-friendly-home-barbie
www.aia.org/aiaucmp/groups/aia/documents/pdf/aiab090126.pdf
44

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