ASTM E 283 and ASTM E 1424 in conjunction with ASTM C 976 were used to study the effect of airflow on thermal performance of the wall. A typical residential 2 × 4 stud wall was constructed and placed on top of a subfloor, making a 2.44 × 2.74 m (8 by 9 ft) test specimen. This base wall assembly was then covered with two types of XPS sheathing, various housewraps, a 15# felt, and a polyethylene vapor retarder film in 40 different configurations and tested individually per ASTM E 283 and per ASTM C 976. For 24 of the 40 C 976 tests, a differential pressure was induced across the test wall as per and ASTM E 1424. Airflows ranged from undetectable airflow at 0 ∙ Pa ΔP to 1.63 L/s ∙ m² for the base wall assembly alone. Difference in airflow resistance performance between the ASTM E 283 and ASTM E 1424 test methods were noted. Thermal testing results incorporating both ASTM C 976 and ASTM E 1424 for tests 1–28 produced apparent thermal conductances (C-values) in the range of 0.40 W/m² ∙ K for a nondetectable airflow level to 1.81 W/m² ∙ K for an airflow of 1.53 L/s ∙ m² for the base wall assembly alone with a 20-Pa ΔP. The calculated C-value for this base wall assembly was 0.40 W/m² ∙ K. Test results reveal that airflow rates as low as 0.2 L/s ∙ m² could produce a 46% increase in apparent C-value. Similar thermal performance differences were revealed when thicker shiplap XPS sheathing was used. Tests were also conducted using an Air-Tight Drywall configuration showing the effect of “wind washing” on thermal performance. By sealing the gypsum drywall on the base wall assembly tested, the apparent C-value, when exposed to a 12.5 Pa wind pressure, was found to be equivalent to a base wall assembly configuration which allows 0.15 L/s ∙ m² airflow to penetrate completely through.