Air Barriers & Vapour Barriers. Water Vapour Pressure. The 4 phases of Water. Water Vapour in Air. Water Vapour Pressure

Transcription

1 Building Science Air & Barriers Air Barriers & Barriers Air Barriers Control Air Leakage Heat (for comfort & energy considerations) Smoke & odours Sound Moisture Barriers Control Diffusion Moisture The 4 phases of Water Water Pressure Liquid (water) thaw freeze Solid (ice) Air Barriers & Barriers only deal with the Phase evaporate condense Gas (water vapour, steam) adsorb frost desorb sublimate Adsorbed Water Pressure Water in Air 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 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 the psychrometric chart BalancedSolutions.com 1

2 Psych Chart: Air Content vs Temperature Air Moisture Content = vapour pressure (Pa, in Hg), humidity ratio (g/kg, grains/pd) Temperature Saturation 100%RH 75%RH 50%RH 25%RH Pressure (Pa) 100% RH 75% RH 50% RH 25% RH -10 ºC 14 º F 100%RH 0 ºC 32 º F 10 ºC 50 º F Tem per at ur e ( C) 20 ºC 68 º F 30 ºC 86 º F ºC 104º F Methods to get Condensation Condensation: Cool air contains less vapor Cool air increase RH Heat air decrease RH Indoor Conditions C / F 25 to 60%RH (Could be 20 to 70%) Summer Pressure (Pa) Outdoor Conditions: Waterloo January -7 C / 80% July 20 C / 75% Winnipeg January 19 C / 80% July 20 C / 70% Outdoor Conditions Pressure (Pa) Winter % RH 75% RH 50% RH 0 25% RH -10.0ºC ºC ºC ºC ºC ºC 14 º F 32 º F 50 º F Tem per at ur e ( C) 68 º F 86 º F 104º F 100% RH 75% RH 50% RH 25% RH -10 ºC 14 º F 0 ºC 32 º F 10 ºC 50 º F Tem per at ur e ( C) 20 ºC 68 º F 30 ºC 86 º F 0 40 ºC 104º F BalancedSolutions.com 2

3 Indoor vs Outdoor Conditions Water Transport Pressure Difference Summer Winter Pressure (Pa) Diffusion (like heat conduction) more to less vapour Air Convection (like heat convection) more to less air pressure vapour is along for the ride 100% RH 75% RH 50% RH 25% RH -10 ºC 14 º F 0 ºC 32 º F 10 ºC 50 º F Tem per at ur e ( C) 20 ºC 68 º F 30 ºC 86 º F 0 40 ºC 104º F Diffusion Water Diffusion Movement from more vapour to less Slow process Many materials slow this process concrete, brick, stone Some stop, or practically stop it many plastics (poly), steel, glass May cause condensation (but not usually) More Warm humid weather Less Condensation Diffusion Cold Surface Cold weather More 32 F 50 F 68 F BalancedSolutions.com 3

4 Retarders retarders are needed to control vapour diffusion Don t need be continuous small tears and openings OK Usually placed near the warm side of the wall or roof -- near the inside in our climates Semi-permeable barriers allow more design flexibility (and more drying) BalancedSolutions.com 4

5 Barriers barriers in Code: <1 US perm based on Rowley 1937 no good science retarder approx 2-5 US perm Measurement Units Metric perms ng /(s m 2 Pa) US perm grain/(hr in Hg ft 2 ) WVT grams/(sq ft/24 hours) Retarders Inner permeance usually should be less than outer in our climate unvented metal cladding, use polyethylene inside Walls with insulated sheathing do not need as much inner resistance e.g., paint works well in this case Too low inner permeance resists drying, promotes summer condensation e.g., polyethylene is often too much Low-perm Materials (<1/60) Mid-perm Materials (1-10) 6 mil Poly 0.1 US perm (5.7 metric) Vinyl wall paper 0.3 perm (17.1) 8 Conc. Fdn. wall 1/2 perm (28.5) Drywall with a VB paint 0.1 to 1 perm (5.7-57) 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) ( ) Plywood 0.5 to 20 (dry to wet) ( ) Expanded polystyrene foam perms for 1 inch ( ) Spray PUR about 2 perms (120) Drywall with latex paint (2-5 perms) ( ) High-perm Materials (>10/600) Fibreboard over 20 (>1100) Plywood 0.5 to 20 (dry to wet) ( ) Icynene open cell spray foam ( ) Tyvek other housewraps 20 to 50 perms ( ) Building paper over 5 to 20 ( ) Permeance (ng/pa s m2) Permeance: Sheathing mm OSB 12.7 mm Plywood Dry Cup Relative Humidity Wet Cup BalancedSolutions.com 5

6 In the beginning Old Assemblies Find the vapour barrier Integral vapour resistance Massive moisture storage Masonry No vapour barrier of course Plaster Old Framed Assemblies Changes... Little to some vapour resistance Little moisture storage Little insulation / air leaky = dry! No vapour barrier of course t Increase in wood frame, esp. post WW1 Began to add insulation comfort wood scarcity, coal Moisture problems in cold climates noted in 1930 s Paint peeling of siding t Dr Frank Rowley The One US Perm 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 This research resulted in 1 perm / 60 ng/pa s m 2 or less vapor barriers Test house inside climate chamber 21 C/ 70 F 40% RH -29 C/-20 F No VB: 21.5 g/m 2 /day (0.07 ounce/ft 2 /day) ASH&VE Transactions No 44, Condensation Within Walls 1938 BalancedSolutions.com 6

7 Air barriers discovered Air leakage identified by : 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.. Neal Hutcheon, 1950 Solution suggested by many: Why not use the vapour barrier Add sealed or continuous to codes language Air barriers spelled out in 1985 NBCC When 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 When 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 (retarder, like paint) Insulated sheathing changes everything If in doubt - figure it out Climate is one guide Severe Cold Cold Mixed-Humid Hot-Dry Climate 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 Vancouver/Seattle Climate and VB 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 BalancedSolutions.com 7

8 VB and Wall design Likely 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 OK for cool climates if the ratio of exterior R value to interior is high. Calculate. Good for hothumid climates that rarely get cold NEVER for cold climates Works everywhere without calculation Wall w/o Insulated Sheathing Wall with Insulated Sheathing Sheathing Permeable Insulation Insulated Sheathing Permeable Insulation Cold = Condensation Diffusion Warm = No Condensation Diffusion BalancedSolutions.com 8

9 Drying Inward Diffusion barriers stop wetting and drying Overkill (e.g. poly) can cause problems! Inward drying is useful in many climates 1. Temperature and solar heating warms wet material 2. drives inward Drying If permeable 3. dries to inside Wetting If impermeable 3.Condensation on cold surfaces Wet materials C % RH VERY high vapour pressure Summer Winter Pressure (Pa) 100% RH 75% RH 50% RH 25% RH Tem per at ur e ( C) -10 ºC 14 º F 0 ºC 32 º F 10 ºC 50 º F 20 ºC 68 º F 30 ºC 86 º F 40 ºC 104º F Warm climates BalancedSolutions.com 9

10 Cool climates Summary for Barriers flows from more to less It goes both ways (In-Out & Out-In) Barriers do not need to be continuous Don t always need a separate vapour barrier many walls have integral resistance Be careful to allow drying Barrier = Drying Retarder Back to Air Barriers There must be a continuous, durable, strong and stiff assembly of materials that is defined as the plane of air tightness in all buildings with conditioned space Remember Barriers Control Diffusion Why? 1. Moisture wetting and drying Air Barriers Control Air Leakage Why? Six reasons. Heat (for 1. comfort & 2. energy considerations) 3. Smoke & 4. odours 5. Moisture 6. Sound Why Air Barriers for Moisture? Air leakage moves moist air (vapour) through the assembly to locations where it can condense Two air leakage concerns for moisture: 1. Through Wall Air Barrier 2. Wind Washing Condensation: Cool air contains less vapor Cool air increase RH Heat air decrease RH BalancedSolutions.com 10

11 Air leakage vs Diffusion Air leakage Air leakage is much more critical Walls sensitive to diffusion-related condensation will be more sensitive to air leakage Diffusion Air Leakage vs Diffusion 1. Air leakage occurs at higher outdoor temp than diffusion 2. Air leakage results in a much greater rate of condensation Air Leakage Condensation Summer Pressure (Pa) Winter % RH 75% RH 50% RH 25% RH Diffusion Tem per at ur e ( C) Surface must be cold for air leakage condensation to occur Cold surface Glass is a perfect VB but tiny cracks Warm surface BalancedSolutions.com 11

12 Air Leakage & Condensation 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 Controlling Air Leakage Condensation 1. Stop all airflow 2. Control driving forces (HVAC/ air pressures) 3. Control Temperature of condensing surface 4. Reduce interior moisture load More Pressure Air leakage (Convection) Warm Weather Less Pressure Condensation if T < Dewpoint Air Leakage Cold Surface Cold Weather More Pressure Airflow Control: Where Stop airflow = stop many problems = Air Barrier Can locate anywhere in enclosure Should be protected if possible Multiple layers are good Important in all climates Air Barriers are Systems (not materials) 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 continuous They leak at joints, interfaces, penetrations multiple air barrier planes are useful for redundancy BalancedSolutions.com 12

13 Air Barrier System Requirements Continuous primary need Strong designed for full wind load Durable critical component - repair, replacement Stiff control billowing, pumping Air Impermeable (may be vapour permeable) 1. Stopping Leaks Find the holes and plug them This requires finicky attention to 3-D details. Big Air Leakage Points Typical Air Leakage Points Big Air Leakage Points A wonderful air barrier system if joints sealed too. Why was a poly sheet added to the interior? BalancedSolutions.com 13

14 Air sealing around windows and other openings A small hole in the air barrier Poly can be (?) an air and vapour barrier Beware: poly is a VB -- keep on warm side BalancedSolutions.com 14

15 The Airtight Drywall Approach Use drywall, framing members Seal with sealant, gaskets, etc. Is stiff, strong Often easier to get better Drawing From: Dr. Joe Lstiburek Builders Guide Cracks and Openings 2. Controlling Temperatures A potent cold-weather strategy for framed enclosure systems 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 vapour drives, e.g., they have permeances of M<200 Some sheathings are vapour impermeable -- they reduce drying outward! Remember Insulated Sheathing = Moisture Control Strategy BalancedSolutions.com 15

16 Wall w/o Insulated Sheathing Wall with Insulated Sheathing Crack Air leakage Crack Air leakage Cold = Condensation Diffusion Warm = no condensation Diffusion Air permeable insulation Air permeable insulation Insulated Sheathing Fibreboard A high density cellulose R2.5/inch not air or water tight EPS, XPS, PIC may be airtight if closed cell (not EPS) control wind washing water tolerant vapour barriers to vapour retarders Semi-rigid MFI not airtight but a drainage layer R values of 4 to 4.4/inch vapour permeable May Exterior Sheathing as air Barrier Not Not How long will the tape last? BalancedSolutions.com 16

17 3. Controlling Driving Pressures Wind+stack: understand them. Control HVAC pressures! Cold weather suck Hot-humid climates blow Driving Forces a. Wind Wind Effect Stack Effect Combustion and Ventilation 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 Values Windspeed Peak loads are high (over 1000 Pa) Average pressures much lower (<50 Pa) Wind Pressure Increases with Height low-rise average pressure about 5 Pa twenty story building about 40 Pa BalancedSolutions.com 17

18 Wind Pressures / Flow Patterns Pressure on windward side Suction on lee and sidewalls b. Stack Effect Hot air rises Tall Building in Winter = Heavy Balloon + b. Stack Effect Stack Pressure Distribution Perfect Building equally leaky everywhere Neutral Pressure Plane at mid-height + Pressures are height dependent (i.e. different at each specific height) Stack Effect Pressures NPP Wind Stack Rain Pressurization More airflow forced into building than sucked out of building = Pressurization + Fan BalancedSolutions.com 18

19 De-Pressurization Pressure Distribution More airflow forced out of building than forced into building = De-Pressurization Combined Pressures Stack Pressure + HVAC Pressures Wind Pressures Fan Control Driving Forces (Pressures) HVAC 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 pressurize to reduce drafts don t do this in cold weather -- plug holes! Control Stack effect by horizontal compartmentalisation Controlling Interior Moisture Loads Most difficult to predict Ventilate to remove excess moisture Internal 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%? 100% RH 75% RH 50% RH 25% RH Air Leakage vs Diffusion Reduce moisture loads = reduced condensation Air Leakage Condensation at High Interior Moisture Summer Winter Condensation at Low Interior Moisture Tem per at ur e ( C) Pressure (Pa) BalancedSolutions.com 19

20 Cold Climate Air-to-Air Heat exchanger Exhaust ventilation Control Interior RH! The Moisture Balance for Interior Air Wetting = interior moisture production added to outside air conditions Safe Storage Capacity 100% RH Drying = moisture removed by ventilation, leakage, diffusion 20-60% RH Sources of Moisture Within Buildings Source Strength kg per day People - evaporation per person 0.9 to 1.25 * Humidifier Hot tub, Whirlpool 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 / m 2 Unvented Gas Appliance (ea) 1 Seasonal Desorption 3-8 depends on the type of construction Plants/Pets (five small plants or one dog) Total (Typical Family of 4) About 10, but potential ranges 3 to 40 Internal 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 construction moisture Indoor RH For Outdoor Conditions of +5 C/80% RH 100% 80% 60% 40% 20% 0% 0.1 ACH For 775 sf Apartment Moisture Production Rate (kg/day) Air Changes per Hour Mechanical 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 BalancedSolutions.com 20

21 Air Barrier Systems Summary Air Barrier Systems Summary Air barrier systems are required to stop uncontrolled airflow ABS can be placed anywhere in the enclosure Must be strong enough to take wind gusts Must be continuous Must be durable Should be stiff enough not flap around Air barrier systems must be continuous They leak at joints, interfaces, penetrations Hence The air permeance of the materials is less important than continuity of the system Air permeance should be low say less than about 0.2 lps/m Pa, usually better Summary Conclusions Air leakage and Diffusion can cause Wetting AND Drying Depends on Weather Conditions! barriers and air barriers reduce or slow flow in BOTH directions all vapour barriers slow inward drying all vapour resistant claddings and sheathings slow outward drying Air barriers and vapour barriers are different can be combined in same materials 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! Website University of Waterloo Building Engineering Group BalancedSolutions.com 21