Innovative Energy Engineering



Air and Vapor Control

Air Infiltration can cause more energy loss than conduction and carries significant amounts of moisture into the perimeter assembly and the interior. Infiltration depends on pressure differences and opening (cracks etc.) sizes. Pressure depends on wind (positive on windward, negative on leeward) and temperatures (stack effect). The HVAC system should be set up to not create large positive or negative pressures.

Infiltration can't compensate for ventilation because:

Assemblies and material should be chosen (and carefully designed, detailed and installed) to meet these maximum standard infiltration rates:

  Maximum Infiltration
Material: 0.02 l/(s-m²)@75 Pa
Assembly: 0.20 l/(s-m²)@75 Pa
Enclosure: 2.00 l/(s-m²)@75 Pa (0.4 cfm/ft² @ 0.3" w.g.)

Estimating Infiltration

In the real world wind and temperature differentials create different pressures than used for above tests. Educated estimates or tests need to be employed for load calculations and energy simulations. Estimating based on air exchanges is not appropriate as it does not account for actual perimeter surface area of a given space. For older and not necessarily sealed buildings an infiltration rate of 0.06 l/(s-m²) (0.12 cfm/ft²) is a good value. Buildings in worse condition or require higher assumed values. Buildings that are very tight due to good design and proper quality management may have infiltration rates of 0.02 l/(s-m²) (0.04 cfm/ft²).

Open doors are assumed to infiltrate at a rate of 0.25 m/s (50 fpm), which needs to be adjusted for the estimated % of time the door is open at the design time. Frequently used doors may be open half of the time during business hours, a side entrance may only be open for a minute every hour. This requires some judgement.

Vapor

Vapor permeability of a material is measured in perm. In order for water that penetrated a material to dry out, the material needs to allow vapor diffusion to at least one side. Vapor barriers on both sides of an assembly or material can trap water.

Perm values are given for a given material thickness (typically 1" etc.). Adding thickness, increases perm-rating (decreases vapor transport):

Enclosure assemblies are classified based on perm-rating. Code considers Class II a vapor retarder. Typically assemblies should be as permeable as possible to allow drying. For example, if Class II is required, avoid Class I.

  Perm Rating SI Perm Rating IP
Class I Vapor Retarder (Impermeable) < 5.7 < 0.1
Class II Vapor Retarder (Vapor Semi-Impermeable) 5.7 - 57 0.0 - 1.0
Class III Vapor Retarder (Vapor Semi-permeable) 57 570 1.0 - 10
Vapor Permeable > 570 > 10

Typical IP perm values of building material are:

Material 1 in. Perm Rating Actual Thickness Actual Perm Rating
Spray Foam Open Cell (Dow Froth Pack) 3.13 3" 1.04
Sprayfoam Closed Cell (Dow CM2045) 2.2 3" 0.73
Extruded Polystyrene (DOW SM) 1.5 3" 0.5
Polyisocyanurate (DOW Thermax Ci) 0.03 3" 0.01
Gypsum 18.75 0.625" 30
Plywood exterior glue 0.175 0.25" 0.7
Plywood interior glue 0.475 0.25" 1.9
OSB 0.625 0.625" 1
Insulation Facing, Kraft     1
Latex Paint, Vapor Retarder   3 mil. 0.45
Polyethylene   2 mil. 0.16
Polyethylene   4 mil. 0.08
Polyethylene   6 mil. 0.06
Aluminum Foil   1 mil. 0.01
Mineral Wool unfaced 120 4" 30
Typical Latex Paint   2 mil. 6
Asphalt Paper     3.2

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