Abstract: A portable testing apparatus to detect air leaks across large sealed gaps comprising an elastomeric, air impervious sheet sized and shaped to overlap a gap between an opening in a structure and an insert, a vacuum plenum extending through and sealing with said sheet, means for attaching the plenum to a vacuum source, channels, with openings therein for uniformly distributing and drawing a vacuum between a sheet and the structure, extending from the plenum along the inside surface of said sheet. The testing apparatus also comprising means for detecting the passage of air through the sealed gap when a vacuum is drawn therethrough.