An Interior Air Barrier Does It Better

Inside-out approach to high performance
Inside-out approach.  aka, backward.

As more buildings strive to achieve airtightness, a common approach we too often see places the air control layer outboard of the insulation layer - often with OSB or plywood - to the exclusion of an inboard air control layer.  On the contrary, if you only have one airtight layer, this is the wrong side - it is a high-performance building turned inside-out.

As airtightness requirements and goals have gotten more stringent:  1.5ACH50, 0.60ACH50 toward 0.30ACH50 and below, designers and builders, finding themselves on the frontiers of airtightness, have gone with what seems to be the most straightforward path to passing the blower door test:  a continuous exterior shell of OSB or plywood.  But this allows conditioned air to readily enter the insulation layer by convective driven air-leaks and diffusion. It also puts a vapor retarder outboard of the insulation and consequently doesn't prevent warm humid air from reaching cold components, increasing the condensation potential, while the exterior retarder unhelpfully restricts the outward drying potential.*

Inside-is-the-right-side placement of OSB for high performance

Instead, let's think about what a high-performance assembly generally wants to be and not just what appears to be the simplest route to passing the blower door test.  To do so we need to acknowledge that, generally speaking, in cold/mixed climates, the exterior wants to be relatively vapor open. The exterior doesn't need this to achieve Passive House airtightness (though tighter on exterior is better) - it just needs to be what we call windtight, to avoid windwashing degradation of the insulating layer.  A better assembly is possible if we concentrate on how to combine, on the interior, the airtight layer and vapor control.

Why an Interior Air Barrier Makes More Sense

Interior air barrier - - photo credit: Naomi Beal
Interior air barrier - photo credit: Naomi Beal

  1. It prevents the conditioned air from entering the insulation layer - keeping the conditioned air within the conditioned space.
  2. It provides better protection against condensation risk - keeping interior humid air away from cold components. (After bulk water intrusion, air leaks and convective movement are the enclosures' biggest liabilities.)
  3. It places the components of this most important control layer in a climate controlled environment - protected from temperature extremes - and ensures maximum longevity.
  4. Leaks are more readily found and easier to repair - as you can typically stand directly next to the air barrier and feel the leaks during a blowerdoor test.
  5. The air control layer can double as a vapor retarding layer - where you want it: inboard of the insulation.

Consequently a mixed/cold climate enclosure from outside to inside generally wants to be:

  1. a protective back-vented rainscreen and vented roof
  2. a vapor-open, windtight, water control layer
  3. fiberous insulation
  4. a vapor variable retarding airtight layer
  5. a protective service cavity with interior finish

Interior airbarrier that is verifiable and repairable.
Interior membrane airbarrier that is verifiable and repairable.

This inboard control layer is best composed of OSB or plywood sheathing (taped with TESCON Vana), or vapor variable membranes like INTELLO Plus or DB+. The fiberous insulation is best as dense-pack, but equal performance can be achieved with batts given the below configurations.

High Performance Assembly with airtight/vapor variable sheathing inboard of  primary insulation layer

In masonry buildings with interior insulation it is best to make the masonry windtight but to put the airtight layer inboard of the insulation.

Masonry retrofit with inboard airtightness.
Masonry retrofit with inboard airtightness.

If using ZIP 1/2" OSB as the outboard wind-tight layer, use INTELLO as the interior air barrier to ensure the assembly is 5x diffusion tighter on the inside than on the outside in winter, yet allows inward drying in the summer/AC season.  See also this blogpost about vented rainscreens.

*OSB dry cup permeability ranges 0.5-0.8, depending on thickness.  Plywood ranges 0.7-1.4, depending on wood type and thickness.  Sources: ASHRAE fundamentals, ASTM E96 dry cup.

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