Innovative Energy Engineering

Air and Vapor Control

Air Infiltration can cause more energy loss than conduction. It also is hard to predict in load calculations and the designer needs to apply large safety factors, which may lead to expensive over-sizing. Air also carries water vapor and can bring significantly more moisture into assemblies 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 real world wind and temperature differentials create different pressures than used for above tests. To estimate infiltration for load calculations and energy simulations we need educated guesses and judge based on overall condition of the building or based on actually performed infiltration tests. Older and not necessarily sealed buildings an infiltration rate of 0.06 l/(s-m²) (0.12 cfm/ft²) is a good value. For buildings in worse condition or with known high infiltration rates higher values have to be assumed. 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). Estimating based on air exchanges (i.e. 0.6 ACH) is not appropriate as it does not account for actual perimeter surface area of a given space. Imagine a given room with all walls being perimeter or just one wall and one sees the flaws in ACH methods. With relatively good R-values of walls and roofs, infiltration load is dominant. This is made worse by the fact that in cooling season most of infiltration load is latent (vapor). This causes building damage, discomfort and requires more de-humidification capabilities of HVAC systems.


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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