Presented by: Jennifer Doyle, P.E., RRC, LEED AP

Transcription

1 Presented by: Jennifer Doyle, P.E., RRC, LEED AP

2 Moisture Gas: Molecules are spread out "Vapor" suspended in air forms a mixture, present everywhere Liquid: Molecules are closer Rain Condensation Pipes Solid: Molecules are closest Ice Snow Freeze-Thaw

3 HOW MOISTURE ENTERS A BUILDING: Air Vapor Diffusion Fluid OF THESE THREE, AIR MOVEMENT ACCOUNTS FOR MORE THAN 98% OF ALL WATER VAPOR MOVEMENT IN BUILDING CAVITIES.

4 Moving from High to Low Images copyright Building Science Corporation, June 2003 Example: Hot-Humid Climate with Air Conditioning

5 Vapor Drive Images courtesy Whole Building Design Guide,

6 Measuring Vapor Transmission: Perms Scientific Definition: The number of grains of water vapor passing through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury (1" Hg =.49 psi) at 73.4 F (23 C). Real World Definition: A measure of the amount of "holes" in a material that allow vapor through it. Tyvek: 23 perms Aluminum Foil: 0.1 perm Any material with a perm rating of less than 1.0 is considered a vapor retarder.

7 Vapor Retarders vs. Barriers Vapor Retarder (ASHRAE): The element that is designed and installed in an assembly to retard the movement of water by vapor. Class (ASTM E 96) Perm Rating (perms) Proper Term (ASHRAE) Examples I < 0.1 perm or less Impermeable Roof membranes, Polyethylene Film, Glass, Aluminum Foil, Sheet Metal, Foil Faced Sheathing II > 0.1 perm perm Semi-impermeable Oil-Based Paints, Vinyl Wall Coverings, Unfaced Extruded Polystyrene (greater than 1 ), OSB Sheathing III > 1.0 perm - 10 perm Semi-permeable Plywood/OSB, Bituminous Kraft Paper, Traditional Hard-Coat Stucco, Unfaced EPS, Unfaced XPS (less than 1 ), Fiber Faced ISO, #30 Felt, Most Latex Paints > 10 perms Permeable Unpainted Gypsum and Plaster, Unfaced Fiberglass Batts, Some Latex Paints, #15 Felt, Asphalt Fiberboard Sheathing, Housewraps

8 Vapor Retarders vs. Barriers Vapor Retarder (ASHRAE): The element that is designed and installed in an assembly to retard the movement of water by vapor. Vapor Barrier: A Class I vapor retarder. Breathable Material: Could be a Class III vapor retarder semi-permeable. Adaptive Vapor Retarders: Becomes more permeable to water vapor at higher humidity levels, pores open/close in film (typically very thin sheet)

9 Where to Use a Vapor Retarder? International Building Code requires a vapor barrier in northern climate zones 5, 6, and 7 Under slab in all zones What about Zones 1-4?

10 The Debate The Department of Energy bases their recommendation on Heating Degree Days Over 2,200 (cold climate): YES, on interior side 1,900-2,200 (mixed climate): NO Under 1,900 (hot climate): YES, on exterior side or NO HOWEVER... Heating Degree Days only refer to temperatures, and tell us nothing about humidity. When is vapor a problem?

11 Humidity Relative humidity (RH): Percentage of the amount of moisture in the air vs. the maximum amount the air can contain at that temperature. Directly related to TEMPERATURE. Example: Summer Morning Air at 75 degrees F with 90% RH SUN WARMS THE AIR Summer Afternoon Air at 95 degrees F now RH has dropped to 45% - same amount of vapor, more thermal energy

12 Humidity ASHRAE Design Interior Conditions: MOST Buildings

13 Humidity Other Interior Conditions: Interior Pools Data Centers Gymnasiums Seasonal

14 When Vapor is a Problem VAPOR IS ONLY A PROBLEM IF IT CONDENSES REPEATEDLY IN THE WRONG LOCATION. Condensation Within the Building Envelope Requires: 1. Vapor Drive 2. Cold Condensing Surface If you do not have both factors above, condensation will likely not occur. Condensation becomes a Problem When: 1. It is unplanned 2. It does not dry out 3. It occurs at materials that are vulnerable to moisture If you do not have all conditions above, condensation will likely not lead to noticeable issues (rust, rot, mold).

15 Condensation Occurs when air loses the energy needed to keep water in a vapor state without energy water molecules are attracted to each other and become liquid, at the DEW POINT TEMPERATURE. T pt = Temperature at a point in the assembly R ext = Sum of R values from the pt to the exterior R tot = Sum of all R values T diff = Absolute value of temperature difference across assembly T out = Exterior Ambient Temperature Images courtesy Whole Building Design Guide,

16 Breathability Allow drying of the wall, roof, slab assembly in one direction A Good Rule-of-Thumb: Put the vapor barrier on the WARM side of the insulation. All materials on the COLD side of the insulation should have a permeability at least 5x greater than those on the WARM side.

17 Forgiveness Allow drying of the wall, roof, slab assembly in one direction Images courtesy Whole Building Design Guide,

18 Vapor Retarders and the Envelope: Below Grade Required in ALL Zones at: Under Slab Crawl Spaces Basements

19 Vapor Retarders and the Envelope: Below Grade Required in ALL Zones at: Under Slab Crawl Spaces Basements

20 Vapor Retarders and the Envelope: Below Grade Required in ALL Zones at: Under Slab Crawl Spaces Basements

21 Vapor Retarders and the Envelope: Walls Very Cold: Minneapolis Minnesota Images and notes courtesy of BIA tech note 47: Condensation Prevention and Control

22 Vapor Retarders and the Envelope: Walls Very Cold: Chicago, IL Air flow retarders and vapor diffusion retarders are installed on the interior of building assemblies, and building assemblies are allowed to dry to the exterior by installing permeable sheathings towards the exterior. Images copyright Building Science Corporation, June 2003

23 Vapor Retarders and the Envelope: Walls Mixed-Humid: Tupelo, Mississippi Building assemblies need to be protected from getting wet from both the interior and exterior, and be allowed to dry to either the exterior or interior. 3 Strategies: Cold/Hot Climate assembly + HVAC + forgiveness Flow-Through Middle of Wall Images copyright Building Science Corporation, June 2003

24 Vapor Retarders and the Envelope: Walls Hot/Humid: San Antonio, TX Images and notes courtesy of BIA tech note 47: Condensation Prevention and Control

25 Vapor Retarders and the Envelope: Walls Hot-Humid: Charleston, South Carolina Air flow and vapor diffusion retarders are installed on the exterior of building assemblies, and building assemblies are allowed to dry towards the interior. Images copyright Building Science Corporation, June 2003

26 Vapor Retarders and the Envelope: Roofs ROOFSARE VAPOR RETARDERS WHEN DO YOU NEED ANOTHER? ZONES: Very Cold: YES Mixed/Humid: NO Mixed/Dry: NO Hot/Humid: NO Hot/Dry: NO CONS: No Drying Leak Detection Entrapped Moisture

27 Other Considerations MEMBRANE PERFORMANCE: Track Record Fastener Penetrations Modified ASTM D1970

28 Summary of Recommendations 1. PLAN AHEAD. CLIMATE will vapor move inward, outward, or both? ASSEMBLY where will dew point occur? DRYING - Allow this to happen in at least one direction 2. RULE OF THUMB. Materials on the COLD side of the insulation at least 5x permeability of those on the WARM side. 2. CHECK THAT INTERIOR DESIGN CONDITIONS DO NOT PROMOTE VAPOR TRANSMISSION Interior relative humidity 40% or less in winter Ventilate enclosures meeting ASHRAE Standard 62.2 or If a special use area, use a vapor retarder OR dehumidify.

29 Questions?