Ever tempted to wear your underwear over your pants? Neither are we! But when it comes to buildings, we’re seeing a lot of common practices that are equally backwards. It turns out that air barriers, like underwear, perform their job much better on the inside. To be more specific, that means inboard of the primary insulation layer.
These days you don’t have to go too far to see buildings that have it all wrong, and put the primary air control layer outboard of the insulation layer, often with OSB or plywood. Most of those projects also go without an air control layer inboard of the insulation. And while best practice is to have both an inboard AND an outboard air barrier - to surround the insulation with airtightness - for goodness sakes, if you only have one airtight layer, outside is the wrong side. It’s literally a high-performance building turned inside-out.
Inside-out approach. aka, backward.
Why Inside-Out? The Easy Way or the Right Way?
As airtightness requirements and goals have gotten more stringent, designers and builders have gone with what at first seems to be the most straightforward path to passing the blower door test: using the sheathing that already wraps the building as the airtight layer.
Building scientists, on the other hand, warn that the “easy way” isn’t necessarily the right way. Even though airtightness may be achieved, conditioned air can readily enter the insulation layer by convection-driven air leaks and diffusion, while leaving sheathing - a vapor retarder - outboard of the insulation (see diagram below). This process increases the condensation risk as warm humid air hits cold exterior surfaces. At the same time, the exterior retarder unhelpfully restricts the outward drying potential.* Hence the “easy way” leaves the wall more susceptible to moisture damages - and therefore less resilient and durable.
Credit: Building Science Corporation
Why an Interior Air Barrier Makes More Sense
Let’s think for a second about what a high performance assembly wants to be, and not just what’s the simplest way to pass the blower door test. Generally speaking, in cold/mixed climates the exterior wants to be relatively vapor open. The exterior doesn't need to be absolutely airtight (though tighter is always better) - it just needs to be what we call windtight, to avoid wind-washing from degrading the insulating layer. The exterior side is the secondary airtight layer.
The primary air barrier layer should be inboard of the insulation - keeping convective air movements away from the insulation and cold components. Better still, that inboard air barrier can now also serve as the vapor control layer, with products such as INTELLO PLUS or DB+ that double as smart vapor retarders and airtight membranes. As a result, by keeping conditioned and moist air out of the insulated enclosure, airtight smart vapor retarders can provide robust protection from moisture damages for the life of the building.
In our series of Smart Enclosure books we integrate this critical thinking about air barriers with overall assembly construction, providing useful tools to help you reach your performance goals.
Diagram of high-performance Smart Enclosure assembly with control layers located for optimal performance.
We can then expand and simplify the reasons - all strong arguments for the primary air control layer to be inboard of the insulation:
Consequently, a mixed/cold climate enclosure from outside to inside generally wants to be:
Airtightness installation for robust resilience and durability.
Finally, note that if using OSB or plywood as the outboard windtight layer, be sure to use INTELLO PLUS as the interior air barrier and vapor control, to ensure the assembly is 5x more diffusion-tight on the inside than on the outside in winter, yet allows inward drying in the summer season when air conditioning exacerbates the vapor drive inward.
*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.