Page 1
© John Straube 20011
Air and Vapour Barriers Air and Vapour Barriers --Who needs them, when, and whereWho needs them, when, and where
Dr John F. StraubeDr John F. StraubeAssistant ProfessorAssistant Professor
Dept of Civil Engineering & School of ArchitectureDept of Civil Engineering & School of Architecture
University of WaterlooUniversity of WaterlooOntario, CanadaOntario, Canada
www.civil.uwaterloo.cawww.civil.uwaterloo.ca/beg/[email protected] 519 888 [email protected] 519 888 4015
© John Straube 20012
Overview of PresentationOverview of Presentation
? Why?? Vapor in air? Vapour barriers? Air Barriers? Understanding Driving Forces? Details of air barriers and airflow
© John Straube 20013
Moisture PhasesMoisture Phases
? As for most materials? All four phases
occur commonly in buildings
? AB and VB are interested in vaporform only
GasGas(water vapour,
steam)
frost
sublimateevaporate
condense
freeze
thaw
desorbadsorb
Adsorbed
Liquid(water)
Solid(ice)
© John Straube 20014
Wall Wetting MechanismsWall Wetting Mechanisms
1. Rain Absorptionsplash and drips
3. Built in Moisture3. Built in Moisture
2. Water 2. Water VapourVapouri) Diffusioni) Diffusionii) Convection ii) Convection
(air leaks)(air leaks)
4. Wicking 4. Wicking (from ground)(from ground)
© John Straube 20015
Why use with AB or VBWhy use with AB or VB
? Code requirement• Many codes requires VB
• Few require ABS!
? VB about controlling Moisture• only helps control interstitial condensation
? AB about moisture condensation and• comfort• energy• sound• odour/smoke
© John Straube 20016
Water Vapour PressureWater Vapour Pressure
Page 2
© John Straube 20017
Vapour PressureVapour Pressure
? For water vapour in a container? Higher temperature =
• more energy• higher velocity• harder collisions with wall (higher pressure)
? Greater number of molecules =• more collisions with walls (higher pressure)
• pressure simply another measure for moisture content
© John Straube 20018
Surface must be cold
Cold surface
Warm surface
© John Straube 20019
CondensationCondensation
© John Straube 200110
Water Vapour in AirWater Vapour in Air
? Water vapour exists in all air? Air has a maximum vapour holding capacity
• This capacity changes dramatically with temperature
• When the maximum holding capacity is exceeded, condensation occurs
? These facts are summarized by thepsychrometric chart
© John Straube 200111 © John Straube 200112-10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
Temperature (°C)
0
500
1000
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2000
2500
3000
3500
4000V
ap
ou
r P
res
su
re (
Pa
)
100% RH75% RH50% RH25% RH
Psych Chart: Air Vapour Content vs Temperature
Saturation
50%RH
25%RH
75%
RH
100%
RH
100%RH
Temperature
Air
Moi
stur
e C
onte
nt
= va
pour
pres
sure
(Pa,
in H
g),
hum
idity
rat
io (g
/kg,
gra
ins/
pd)
-10 ºC14 º F
0 ºC32 º F
10 ºC50 º F
20 ºC68 º F
30 ºC86 º F
40 ºC104º F
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© John Straube 200113
Condensation: Cool air contains less vapor
© John Straube 200114
Condensation on Cold Surfaces
Why?• temp drops so RH rises to 100%• molecules slow down and stick to surface
© John Straube 200115-10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
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Vap
ou
r P
ressu
re (
Pa)
100% RH75% RH50% RH25% RH
Indoor Conditions
• 18 - 26°C / 65 – 78°F• 25 to 60%RH(Could be 20 to 70%)
Summer
Winter
-10 ºC14 º F
0 ºC32 º F
10 ºC50 º F
20 ºC68 º F
30 ºC86 º F
40 ºC104º F © John Straube 200116
-10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
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Vap
ou
r P
ressu
re (
Pa)
100% RH75% RH50% RH25% RH
Outdoor ConditionsBurlington, VT•January -10°C / 14°F & 80%•• July 20°C/July 20°C/ 68°F & 75%68°F & 75%
-10 ºC14 º F
0 ºC32 º F
10 ºC50 º F
20 ºC68 º F
30 ºC86 º F
40 ºC104º F
© John Straube 200117-10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
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Vap
ou
r P
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100% RH75% RH50% RH25% RH
Indoor vs Outdoor Conditions
Summer
Winter
VapourPressureDifference
Inward drive can be as big as outward
-10 ºC14 º F
0 ºC32 º F
10 ºC50 º F
20 ºC68 º F
30 ºC86 º F
40 ºC104º F © John Straube 200118
Why use an AB or VB?Why use an AB or VB?
?? Code requirementCode requirement• NBCC -> OBC requires AB/VB if damage can occur• VB only required sometime, AB almost all times
?? VB VB only only helpshelps controlcontrol interstitialinterstitialcondensationcondensation
? AB about interstitial condensation and• comfort• energy• sound• odour/smoke
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© John Straube 200119
Diffusion and ConvectionDiffusion and Convection
?Water vapour moves by
1. vapour diffusionmore to less vapour
2. air convectionmore to less air pressure
vapour is along for the ride
© John Straube 200120
In the beginningIn the beginning
• Find the vapour barrier
© John Straube 200121
Old AssembliesOld Assemblies
? Integral vapour resistance? Massive moisture storage
MasonryMasonry
PlasterPlaster
No No vapourvapourbarrier of coursebarrier of course
© John Straube 200122
Old Framed AssembliesOld Framed Assemblies
? Little to some vapour resistance? Little moisture storage? Little insulation / air leaky = dry!
tt
No No vapourvapourbarrier of coursebarrier of course
© John Straube 200123
Changes . . .Changes . . .
? Increase in wood frame, esp. post WW1? Began to add insulation
• comfort
• wood scarcity, coal
? Moisture problems in cold climatesnoted in 1930’s
? Paint peeling of siding
tt
© John Straube 200124
Dr Frank RowleyDr Frank Rowley
? Professor of Mechanical Engineering at University of Minnesota
? ASH&VE 1932 president? Proponent of using heat flow analogy for vapor
flow in calculations? Conducted full scale house in climate chamber
studies –paid for by insulation companies
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© John Straube 200125
The One US PermThe One US Perm
? “Test house” inside climate chamber
--29 C/29 C/--20 F20 F
21 C/ 70 F40% RH
No VB:No VB:21.5 g/m21.5 g/m22/day /day (0.07 ounce/ft(0.07 ounce/ft22/day) /day)
This research resulted in 1 perm / 60 ng/Pa s m2 or less vapor barriers
ASH&VE Transactions No 44, “Condensation Within Walls” 1938 © John Straube 200126
The Sales Job 1940The Sales Job 1940--19601960
? Paint manufacturers • Stop peeling due to insulation
? aluminum foil manufacturers• Open up a new market• On the recent aluminum foil for cooking:
“in a period of less than 12 months , through national advertising, through local advertising and through point of purchase advertising, a product was created that it will take a lot of promotional work to kill”
© John Straube 200127
Vapour BarriersVapour Barriers
? Vapour barriers in Code: <1 US perm• 1942 first reference
? Canadian codes – as of 1975 ? Vapour retarder approx 2-5 US perm? Measurement Units
• Metric perms ng /(s·m2·Pa)
• US perm grain/(hr·in Hg· ft2)• WVT grams/(sq ft/24 hours)
© John Straube 200128
Air barriers discoveredAir barriers discovered
? Air leakage identified by Neal Hutcheon, 1950: “The rate of condensation is about ten times that at which vapour might be expected to diffuse through …It seems necessary to assume some other mechanism … the leakage of warm moist air outward ..”
? Solution suggested by many: • Why not use the vapour barrier
? Add “sealed” or “continuous” to codes language? Air barriers spelled out in 1985 NBCC
© John Straube 200129
Why poly? Why poly?
? Poly is cheap and easy? Cliff Shirtcliff
• Insulation shortage
• Paper-facings hide insulation
© John Straube 200130
When and whereWhen and where
? Choice of vapour permeance and location of vapor barrier depends on
• Exterior Climate• Interior Climate• Wall Assembly
? Any “rule” that does not consider these factors cannot provide correct guidance
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© John Straube 200131
Some rules: When and WhereSome rules: When and Where
? Old rule: “Place on warm side of enclosure”• summer and winter balance
? If you use a VB on cold side of wall - ventilate!? If extreme temps/RH
• use VB (beware air leakage)• Use lower perm insulation (foam)
? If moderate, use retarder, like paint? Insulated sheathing changes everything? If in doubt - figure it out
© John Straube 200132
Cold
Severe Cold
Hot-Dry
Mixed-Humid
Joe Lstiburek’s Builders Guides
© John Straube 200133
Climate and VBClimate and VB
? Florida is hot and humid• moisture is outdoors
• place VB near outside• allow vapour to flow INWARD
? Winnipeg/Bismark is cold/dry• bitterly cold winters• summers are hot and dry• place VB near inside• allow vapour to flow outward
© John Straube 200134
Climate and VBClimate and VB
? Vancouver/Seattle• moisture is both indoors outdoors
• allow vapour to flow inward and outward• use VR near inside (paint)
? Toronto/Detroit• summers are hot and humid, winters cold• if rain-wetted cladding, inward is important!• allow vapour to flow inward and outward• use VR near inside (paint) or exterior insulation
© John Straube 200135
VB and Wall designVB and Wall design
? Need vapour barriers • low integral vapour resistance
– framed walls with batt
? Rarely need VB:• Integral vapour resistance
– SIPS– spray foam
• Built in VB– concrete
• Kept warm– insulating sheathing
© John Straube 200136
Water Water VapourVapour DiffusionDiffusion
More More VapourVapour
LessLessVapourVapour
Cold weatherCold weather
VapourVapourDiffusionDiffusion
Warm humid weatherWarm humid weather
More More VapourVapour
CondensationCondensation
Cold Surface
Page 7
© John Straube 200137
Vapour DiffusionVapour Diffusion
? Movement from more vapour to less? Slow process? Many materials slow this process
• concrete, brick, stone
? Some stop, or pratically stop it• many plastics (poly), steel, glass
© John Straube 200138
Vapour DiffusionVapour Diffusion
? Movement from more vapour to less? Slow process? Many materials slow this process
• concrete, brick, stone
? Some materials stop, or practically stop it• many plastics (poly), steel, glass
? Some materials resist very little• Housewraps, batt, fibreboard
© John Straube 20013950 F 68 F32 F © John Straube 200140
Air leakage Air leakage vsvs DiffusionDiffusion
? Air leakage is much more critical
? Walls sensitive to diffusion-related condensation will be more sensitive to air leakage
© John Straube 200141 © John Straube 200142-10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
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Vap
ou
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100% RH75% RH50% RH25% RH
Air Leakage vs Diffusion
Summer
Winter
Air LeakageCondensation
Diffusion
50%RH
25%RH
75%
RH
100%
RH
Page 8
© John Straube 200143
Solution to Interstitial CondensationSolution to Interstitial Condensation
? Air leakage – install air barriers and insulated sheathing
? Vapor diffusion – choose materials, insulated sheathing, and/or install vapor barrier
© John Straube 200144
Wall with Insulated SheathingWall with Insulated Sheathing
VapourVapourDiffusionDiffusion
Warm = No Warm = No CondensationCondensation
Sheathing Air Permeable Insulation
© John Straube 200145
VB and Wall designVB and Wall design
? Need vapour barriers • low integral vapour resistance
– framed walls with batt
? Rarely need VB:• Integral vapour resistance
– SIPS– spray foam
• Built in VB– concrete
• Kept warm– insulating sheathing
© John Straube 200146
CalculationsCalculations
? Simple calculations can used to understand the issue
? Computer models are useful for complex situations
© John Straube 200147
Vapour BarriersVapour Barriers
? Vapour barriers in Code: <1 US perm• based on Rowley 1937 • no good science
? Vapour retarder approx 2-5 US perm? Measurement Units
• Metric perms ng /(s·m2·Pa)
• US perm grain/(hr·in Hg· ft2)• WVT grams/(sq ft/24 hours)
© John Straube 200148
LowLow--perm Materialsperm Materials
? 6 mil Poly 0.1 US perm? Vinyl wall paper 0.3 perm? Concrete 1/2 perm for 8” foundation wall? Drywall with a VB paint 0.1 to 1 perm? Brick veneer 1/2 - 2 perms? Extruded foam 1/2 - 1 for 1.5”? Plywood 0.5 to 20 (dry to wet)? Kraft paper 0.3 to 2 (dry and wet)
Page 9
© John Straube 200149
MidMid--perm Materialsperm Materials
? Plywood 0.5 to 20 (dry to wet)? Expanded foam 2.5 - 5 perms for 1 inch? Spray PUR about 2 perms? Drywall with latex paint (2-5 perms)
© John Straube 200150
HighHigh--perm Materialsperm Materials
? Fibreboard over 20? Plywood 0.5 to 20 (dry to wet)? Icynene open cell spray foam 10 - 13? Tyvek other housewraps 20 to 50 perms? Building paper over 10 to 30
© John Straube 200151
VapourVapour Permeance: SheathingPermeance: Sheathing
0
500
1000
1500
2000
2500
0 0.2 0.4 0.6 0.8 1
Relative Humidity
Per
mea
nce
(n
g/P
a s
m2) 11.1 mm OSB
12.7 mm Plywood
Dry
Cu
p
Wet
Cu
p
© John Straube 200152
When and WhereWhen and Where
? Place on warm side of enclosure• summer and winter balance
? If you use a VB on cold side of wall - ventilate!? If extreme temps/RH, use VB? If moderate, use VR? If in doubt - figure it out? Climate is one simple rule
© John Straube 200153
Cold
Severe Cold
Hot-Dry
Mixed-Humid
Joe Lstiburek’s Builders Guides© John Straube 200154
Climate and VBClimate and VB
? Florida is hot and humid• moisture is outdoors
• place VB near outside• allow vapour to flow INWARD
? Winnipeg/Bismark is cold/dry• bitterly cold winters• summers are hot and dry• place VB near inside• allow vapour to flow outward
Page 10
© John Straube 200155
Climate and VBClimate and VB
? Vancouver/Seattle• moisture is both indoors outdoors
• allow vapour to flow inward and outward• use VR near inside (paint)
? Toronto/Detroit• summers are hot and humid, winters cold• if rain-wetted cladding, inward is important!• allow vapour to flow inward and outward• use VR near inside (paint) or exterior insulation
© John Straube 200156
VB and Wall designVB and Wall design
? Need vapour barriers • low integral vapour resistance
– framed walls with batt
? Rarely need VB:• Integral vapour resistance
– SIPS– spray foam
• Built in VB– concrete
• Kept warm– insulating sheathing
© John Straube 200157
Wall w/o Insulated SheathingWall w/o Insulated Sheathing
VapourVapourDiffusionDiffusion
Cold = Cold = CondensationCondensation
Sheathing Air Permeable Insulation
© John Straube 200158
Wall with Insulated SheathingWall with Insulated Sheathing
VapourVapourDiffusionDiffusion
Warm = No Warm = No CondensationCondensation
Sheathing Air Permeable Insulation
© John Straube 200159
DryingDrying
? Vapour barriers stop wetting and drying? Overkill (e.g. poly) can cause problems!? Inward drying is useful in many climates
© John Straube 200160
Vapor flows in both directionsVapor flows in both directions
? Vapour diffuses from more to less? Hence,
• indoors to outdoors in cold weather
• Outdoors to indoors in hot humid weather• From wet materials to in and outdoors
? Locating low permeance materials within assemblies must respect these facts
Page 11
© John Straube 200161
Inward DrivesInward Drives
? Long understood by researchers“When a vapour barrier is used, the wall can loose “When a vapour barrier is used, the wall can loose
moisture only to the outside. In summer, hot sun moisture only to the outside. In summer, hot sun following a rain drives moisture as vapour to the inside following a rain drives moisture as vapour to the inside of the wall, and condensation behind the vapour barrier of the wall, and condensation behind the vapour barrier can occur.” Hutcheon can occur.” Hutcheon 19531953
? Most published work from cold climates• Canada, Sweden, Denmark, Belgium, Germany
? Rarely considered by practioners? “If you don’t know, you don’t see it”
© John Straube 200162
Inward DiffusionInward Diffusion
2.Vapour drives inward
3.Condensation on “cold” surfaces
1. Temperature and solar heating warms
wet material
WettingWettingIf If imimpermeablepermeable
DryingDryingIf permeableIf permeable3.Vapour dries to inside
© John Straube 200163-10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
Temperature (°C)
0
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1500
2000
2500
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Vap
ou
r P
ressu
re (
Pa)
100% RH75% RH50% RH25% RH
Indoor Conditions
• 18 - 26°C / 65 – 78°F• 25 to 60%RH(Could be 20 to 70%)
Summer
Winter
-10 ºC14 º F
0 ºC32 º F
10 ºC50 º F
20 ºC68 º F
30 ºC86 º F
40 ºC104º F © John Straube 200164
© John Straube 200165 © John Straube 200166
VapourVapour Control StrategiesControl Strategies
? Low-perm vapor barriers• limited interior drying, warm weather condensation
? Storage systems• safely store moisture in absorbent materials , e.g.
masonry? Highly permeable walls
• high flow in, high flow out? Insulated sheathing
• shift condensation to exterior, ventilate away
? Integral heat and vapor resistance• e.g., many foams and natural insulating materials
Page 12
© John Straube 200167
Air movementAir movement
? Movement of air (and its heat, moisture, pollutants) from high air pressure to low
? Delivers vapour directly to cold surface
? Air movement causes other problems
© John Straube 200168
ChannelChannel
© John Straube 200169
DiffuseDiffuse
© John Straube 200170
TortuousTortuous
© John Straube 200171
Glass is a perfect VB
© John Straube 200172
Page 13
© John Straube 200173
Airflow Control: WhatAirflow Control: What
? Air flow through enclosure? Air flow within enclosure? Design Goal: Eliminate air flow? Air Flow is driven by Air Pressure Differences
© John Straube 200174
Airflow Control: WhyAirflow Control: Why
? Condensation wetting ? Drafts (= comfort)? Energy ? Pollutant transport - smell, particles, gas? Sound
? NOT just energy!!
© John Straube 200175
Airflow Control: WhereAirflow Control: Where
? Stop airflow = stop most problems ? Can locate anywhere in enclosure? should be protected if possible? multiple layers are good? important in all climates
© John Straube 200176
Air LeakageAir Leakage
? Difficult to predict direction of air pressures and unintentional flow paths
? Damaging airflow direction is:• cold weather inside to outside
• warm weather outside to inside
? Condensation can ONLY occur if both:• air contacts a cold surface, and• air flow is in the direction of more to less vapour
? Many materials stop airflow, most systems do not
© John Straube 200177
Air leakage Air leakage (Convection)(Convection)
More More PressurePressure
LessLessPressurePressure
Cold WeatherCold Weather
VapourVapourDiffusionDiffusion
Warm WeatherWarm Weather
More More PressurePressure
CondensationCondensationif more if more vapourvapour
Cold Surface© John Straube 200178
Air Barrier SystemsAir Barrier Systems
? Air barrier systems are required to stop airflow through enclosure
? ABS can be placed anywhere in the enclosure
? Must be strong enough to take wind gusts
? Air barrier systems must be continuousThey leak at joints, interfaces, penetrations
? multiple air barrier planes are useful for redundancy
Page 14
© John Straube 200179
Air Barrier SystemsAir Barrier Systems
? Continuous• primary need
? Strong• designed for full wind load
? Durable• critical component - repair, replacement
? Stiff• control billowing, pumping
? Air Impermeable • (may be vapour permeable)
© John Straube 200180
Stapled Paper is not an air barrier
© John Straube 200181
Control Air Leakage CondensationControl Air Leakage Condensation
? Stop all airflow? Control driving forces (HVAC/ air pressures)? Control Temperature of
condensing surface? Reduce interior moisture load
(only useful for cold weather)
© John Straube 200182
1. Stop Leaks1. Stop Leaks
“Find the holes and plug them”This requires finicky attention to 3-D
details.
© John Straube 200183Typical Air Leakage Points
© John Straube 200184
Page 15
© John Straube 200185 © John Straube 200186
Air sealing around windows andotheropenings
© John Straube 200187
A small hole in the air barrier
© John Straube 200188
Poly can be (?) an air and vapour barrier
© John Straube 200189 © John Straube 200190Beware: poly is a VB Beware: poly is a VB ---- keep on warm sidekeep on warm side
Page 16
© John Straube 200191
Use drywall, framing members• Seal with sealant, gaskets, etc.•Is stiff, strong•Often easier to get better
Drawing From: Dr. Joe LstiburekBuilders Guide
The Airtight Drywall Approach
© John Straube 200192
© John Straube 200193 © John Straube 200194
ADA Detail ADA Detail WindowWindow
From NRCan Super-E
© John Straube 200195
ADA Detail ADA Detail Rim JoistRim Joist
From NRCan Super-E
© John Straube 200196
2. Control Temperatures2. Control Temperatures
A potent cold-climate strategy for framed enclosure systems
Page 17
© John Straube 200197
Insulated Sheathing Insulated Sheathing
? Increases temperature of first condensation surface in winter• adding R5 to R7 on exterior of R12 batt practically
eliminates possibility of condensation
? Many foam sheathings reduce summer vapourdrives, e.g., they have permeances of M<200
? Some sheathings are vapour impermeable --they reduce drying outward!
? RememberInsulated Sheathing = Moisture Control Strategy
© John Straube 200198
Wall w/o Insulated SheathingWall w/o Insulated Sheathing
Air Air leakageleakage
VapourVapourDiffusionDiffusion
Cold = Cold = CondensationCondensation
© John Straube 200199
Wall with Insulated SheathingWall with Insulated Sheathing
Air Air leakageleakage
VapourVapourDiffusionDiffusion
Warm = no Warm = no condensationcondensation
© John Straube 2001100
3. Control Driving Pressures3. Control Driving Pressures
Wind+stack: understand them.Control HVAC pressures!
Control stack by compartmentalizationReduce interior vapor content
© John Straube 2001101
Driving ForcesDriving Forces
Wind Effect Stack Effect Combustion and Ventilation
© John Straube 2001102
WindWind
? Major driving force? Codes - Extensive data for Structural Design
Loads? Average pressures much lower? Wind Pressure Increases with Height? Exposure Conditions Matter!? Beware Corner and Suction pressures
Page 18
© John Straube 2001103
Wind Speed DistributionWind Speed Distribution
0%
5%
10%
15%
20%
25%
30%
00 22 44 66 88 1010
Wind Speed (m/s)Wind Speed (m/s)
Rel
ativ
e Fr
eque
ncy
Rel
ativ
e Fr
eque
ncy
Avg. 3.6 m/s = 8 mphAvg. 3.6 m/s = 8 mph
1/10 yr cladding: 1 sec/10 yr1/10 yr cladding: 1 sec/10 yr1/30 yr structure: 3 sec/ 30 yr1/30 yr structure: 3 sec/ 30 yr
For Waterloo, CanadaFor Waterloo, Canada
© John Straube 2001104
WindspeedWindspeedVaries with Height and ExposureVaries with Height and Exposure
© John Straube 2001105
Wind speed to PressureWind speed to Pressure? Wind Velocity to Pressure:
• Pstag = 0.647 ? V2 [in Pa and m/s]• Lowrise Velocity @ 10 m= 4 m/s 10 Pa
• Highrise Velocity @ 50 m=8 m/s 40 Pa
VelocityVelocityStagnation PressureStagnation Pressure
© John Straube 2001106
Flow PatternsFlow Patterns
© John Straube 2001107
Pressure Distribution on FacePressure Distribution on Face
PeakSuctions at Sides+
© John Straube 2001108
Wind Pressures / Flow PatternsWind Pressures / Flow Patterns
•• Pressure on windward sidePressure on windward side•• Suction on lee and sidewallsSuction on lee and sidewalls
Page 19
© John Straube 2001109
Wind Pressure DistributionWind Pressure Distribution
Plan View
?? PP
© John Straube 2001110
Stack EffectStack Effect
? Hot air rises? Tall Building in Winter = Heavy Balloon
+
© John Straube 2001111
Stack EffectStack Effect
? “Perfect” Building equally leaky everywhere? Neutral Pressure Plane at mid-height
+
NPPNPP
Cold weather exampleCold weather example
© John Straube 2001112
Stack Pressure DistributionStack Pressure Distribution
Pressures at a Pressures at a specific heightspecific height
Stack EffectStack EffectPressuresPressures
© John Straube 2001113
WindStackRain
© John Straube 2001114
PressurizationPressurization
? More airflow forced into building than sucked out of building = Pressurization
+
Fan
Page 20
© John Straube 2001115
DeDe--PressurizationPressurization
? More airflow forced out of building than forced into building = De-Pressurization
Fan
© John Straube 2001116
Pressure DistributionPressure Distribution
Wind Pressures
Stack Pressure +Stack Pressure +HVACHVAC
PressuresPressures
Combined Combined PressuresPressures
© John Straube 2001117
Control HVAC EffectsControl HVAC Effects
? Can be reduced by controlling fan operation? In many houses, exhaust-only fans
depressurize • less air leaks outward• good for cold weather
? Commercial buildings often pressureize to reduce drafts• don’t do this in cold weather -- plug holes!
© John Straube 2001118
Airflow Within EnclosuresAirflow Within Enclosures
© John Straube 2001119
Pressure DistributionPressure Distribution
Plan View
?? PP
© John Straube 2001120
Pressure DistributionPressure Distribution
Plan View
?? PP
Page 21
© John Straube 2001121
Lateral AirflowsLateral Airflows
Punched Steel Studs
Condensation inside (cold)Condensation backside (hot)
© John Straube 2001122
Internal Internal Stack Effect & InsulationStack Effect & Insulation
Hot air = lightHot air = light
Cold air = heavyCold air = heavy
•• Gaps in batt Gaps in batt insulation on insulation on both sidesboth sides
••Stapled paper!Stapled paper!
Air gapsBatt
Inside
Outside
Common performance problem
© John Straube 2001123
Internal Internal Stack EffectStack Effect
Cold WeatherCold Weather
Hot air = lightHot air = light
Cold air = heavyCold air = heavy
Result: Air Result: Air FlowFlow
•• Gaps in batt Gaps in batt insulation on insulation on both sidesboth sides
•• closed circuitclosed circuit
•• energy costenergy cost
•• cold surfacescold surfaces
© John Straube 2001124
Air movement Air movement (Stack Effect)(Stack Effect)
Cold WeatherCold Weather
Result: Air Result: Air FlowFlow
Hot air = lightHot air = light
Cold air = heavyCold air = heavy
© John Straube 2001125
Air movement Air movement (Stack Effect)(Stack Effect)
More More VapourVapour
Cold WeatherCold Weather
Air FlowAir Flow
Warm WeatherWarm Weather
More More VapourVapour
CondensationCondensationif more if more vapourvapour
Cold Surface© John Straube 2001126
Airflow within enclosuresAirflow within enclosures
? Provide lateral airflow resistance as required? allow easy lateral flow
• batt insulation
• open cell foam
? slow lateral flow• very high-density fibrous insulation• dense packed cellulose
? stop lateral flow• closed cell foam• solid materials
Page 22
© John Straube 2001127
WindwashingWindwashing
? Need some airtightness outside permeableinsulation
? Sealed housewrap, attached building paper? Sheathing sealed with tape
• both OSB and insulated sheathing• high density MI
? High density cavity insulation• some foams, maybe dense cellulose
© John Straube 2001128
Control lateral flowControl lateral flow
Airflow behind cladding causes cooling or "wind washing" of air permeable insulations
Pressure Profile
Suction on sides
Framing members and compartment seperators control air flow
Pressure Profile
Suction on sides
© John Straube 2001129
Using ExteriorUsing ExteriorSheathing as airSheathing as airBarrierBarrier
Not
Not
© John Straube 2001130
How long will the tape last?
© John Straube 2001131 © John Straube 2001132
HVAC InteractionHVAC Interaction
Page 23
© John Straube 2001133
WarmWarmMoistMoistAirAir The Air+ The Air+
Thermal BarrierThermal Barrier
HVAC: The Way it is DesignedHVAC: The Way it is Designed
+
Air Handler
100 100 cfmcfm100 100 cfmcfm
Perfect ductsPerfect ducts
© John Straube 2001134
WarmWarm
MoistMoistAirAir
The Air+ The Air+ Thermal BarrierThermal Barrier
HVAC : The Way it “Works”HVAC : The Way it “Works”
+
Air Handler
100 100 cfmcfm80 80 cfmcfm
20 20 cfmcfm leakageleakage
20 20 cfmcfm
© John Straube 2001135
DepressurizationDepressurization = Air leakage= Air leakageWarm ClimateWarm Climate
More More PressurePressureWarmWarmHumidHumid
Cold Surface
LessLessPressurePressureCoolCool
CondensationCondensationif more if more vapourvapour
© John Straube 2001136
HVAC SolutionsHVAC Solutions
? Hot-humid climates •• pressurizepressurize with conditioned outdoor air
? Cold climates •• depressurizedepressurize by exhausting air
? properly sized transfer grilles (small ? P) ? keep ducts inside! (or seal ducts)? Good air barrier systems for the enclosure
© John Straube 2001137
Internal Moisture LoadsInternal Moisture Loads
? Critically important for cold climate!!Primary load for vapour diffusion and air leakage condensation
? More unknown (!) than exterior? Temperature
• 8 to 76 F (21 to 26 )
? Relative Humidity• 20 to 75% ?
© John Straube 2001138
Air Vapour Content vs Temperature
0
2000
4000
0 20 40 60 80 100
Temperature (F)
Air
Mos
itur
e C
onte
nt =
Vap
our
Pre
ssur
e
Saturation
80%RH
60%RH
Winter
Summer
Air LeakageCondensation
Reduce moisture loads= reduced condensation
Page 24
© John Straube 2001139
Control Interior RH!Control Interior RH!
Cold Climate? Air-to-Air Heat exchanger? Exhaust ventilation
© John Straube 2001140
Interior Air MoistureInterior Air Moisture
Moisture Removal (ventilation, leakage, diffusion?)
Outdoor Air PlusMoisture Production
? Ventilation and Diffusion Remove Moisture
2020--60%RH60%RH
100%RH100%RH
© John Straube 2001141
Sources of Moisture Within Buildings
Source Strength kg per day
People - evaporation per person 0.9 to 1.25 *
Humidifier 2-20+
Hot tub, Whirlpool 2-20+
Firewood, per cord 1-3
Washing floors, counters, etc. 0.2
Dishwashing 0.5*
Cooking for four 0.9 to 2 (3 with gas range)*
Defrosting (frost free) Fridge 0.5*
Typical bathing/washing per person 0.2 to 0.4*
Shower (ea) 0.5
Bath (ea) 0.1+
Uncovered Crawlspace 0.5 / m2
Unvented Gas Appliance (ea) 1
Seasonal Desorption 3-8 depends on the type of construction
Plants/Pets 0.2 - 0.5 (five small plants or one dog)
Total (Typical Family of 4) About 10 , but potential ranges 3 to 40© John Straube 2001142
Internal MoistureInternal Moisture
? Total for Family of 4: 10 to 14 kg/day? CMHC Study Of Detached Homes
• 90%> 3 kg/day and <21 kg/day
? Also drying out of rain wetting
© John Straube 2001143
0%
20%
40%
60%
80%
100%
0 1 2 3 4Air Changes per Hour
Ind
oo
r R
H
5102040
Moisture Production Rate (kg/day)
For Outdoor Conditions of +5 C/80%RH
0.1 ACH
For 775 sf Apartment
© John Straube 2001144
Mechanical VentilationMechanical Ventilation
? Old houses had 3 or more ACH• hence, moisture production rate had little effect on
interior humidity
? New houses have 0.25 to 1 ACH• hence, moisture production dominates interior
humidity
? Mechanical ventilation ensures• proper fresh air supply• reduces energy consumption
• controls interior humidty
Page 25
© John Straube 2001145
Interior MoistureInterior Moisture
? Hot or Humid climates must remove interior moisture by dehumidification / A.C.
? Must size AC for “latent” load? Must still provide guaranteed fresh air? Airtight controls amount of exterior moisture
brought in
© John Straube 2001146
Wetting and DryingWetting and Drying
?? Air leakageAir leakage and DiffusionDiffusion can cause • Wetting AND
• Drying•• DependsDepends on Weather Conditions!
? Vapour barriers and air barriers reduce or slow flow in BOTH directions
?? allall vapourvapour barriers slow inward dryingbarriers slow inward drying?? allall vapourvapour resistant claddings and sheathings slow resistant claddings and sheathings slow
outward dryingoutward drying
© John Straube 2001147
ConclusionsConclusions
? Air barriers and vapour barriers are different• can be combined in same materials
? Vapour barriers control diffusion• use only when needed• place near WARM side only
? Air barriers control air flow• can be placed any where
? Air barriers usually more important• continuity is key!
© John Straube 2001148
Review Air Barrier SystemsReview Air Barrier Systems
? Continuous• primary need
? Strong• designed for full wind load
? Durable• critical component - repair, replacement
? Stiff• control billowing, pumping
? Air Impermeable • (may be vapour permeable)
© John Straube 2001149
ReviewReview
? Air Barrier or Vapour Barrier? When?• Tyvek
• Concrete wall• Steel roof• Poly• Drywall• Peel and stick membrane• Blue Extruded Styrofoam
© John Straube 2001150
ReviewReview
? Where to place the VB in• Miami
• Minnespolis
? Where to place the ABS• Miami• Minneapolis