Domain 2
Building Science
Air control, vapor control, and drying capacity designed as one assembly.
What this domain covers
Building Science is the physics of the shell: the four control layers — water, air, vapour and heat — and whether each one stays continuous everywhere the building changes direction or material. It covers airtightness as both a design intention and a measured result, the insulation strategy and the control of thermal bridges, the drying direction of every assembly, and the attic and roof approach.
It also sets the architecture for whole-house ventilation and for moisture removal — the decision about what kind of system the house has. How that equipment is sized, commissioned and measured belongs to Indoor Air Quality and Thermal Comfort. Filtration is not handled here.
Why it matters in Central Texas
In an air-conditioned house in a hot-humid climate, moisture in the air is pushing inward for most of the year — the opposite of the direction much conventional detailing assumes. An assembly built to stop that drive on the inside face traps moisture against itself. The Standard requires that assemblies be able to dry, and it treats that requirement as non-negotiable.
A tighter shell also changes what the house needs rather than what it delivers. Once incidental leakage is gone, moisture and anything released indoors are no longer diluted by accident; they are managed deliberately by equipment, or not at all. That is why the envelope decision and the mechanical decision are made together here.
How depth is chosen
Depth is chosen per domain — Essential, Enhanced or Comprehensive — and each domain carries only the rungs it honestly has.
This domain has none. Nearly all of it is concealed once the building is closed in, and what is concealed cannot be revisited without opening the assembly, so every requirement here sits on the floor and is present in every Sage Co. house at every depth. A client may decline a requirement, and that becomes a named exception carried in the Record.
What we document
The Record holds the measured airtightness results with their conditions, equipment and dates — including the diagnostic test run while the air barrier is still reachable, and what that test found and corrected — along with dated photographs of the air barrier, its transitions, the insulation and the roof and attic build-up, and the product data behind each.
An independent third party performs the testing and the inspections. The airtightness figure itself is written into the agreement and the Record and is never published.
Findings in this domain are recorded in the Sage Co. Record. Sage Co. may mark a requirement specified and installed. Only an independent third party can mark it verified, and verification is elected domain by domain — how verification works.
Reviewed by Jake Rangel, September 19, 2026
Related reading
Build this into your house
Sage Co. builds and consults across Austin, San Antonio and the Hill Country. Start with a call and we will walk this domain — and the other ten — against your project.