The thermal efficiency of a building is improved by installing a weather block and vent member across the space between the ends of joists resting on a plate having between them an insulation blanket having a vapor barrier adjacent a ceiling on the bottom of the joists. The member blocks the flow of air towards the end of the vapor barrier and the ceiling and sometimes down past the plate in a wall inside covering and down pass the inside covering and the vapor barrier on the blanket insulation between the wall studs, and redirects it upwards along the rafters. It also blocks the flow of air across the plate, to eliminate the Bernoulli Effect thereat which was operative to suck the out the air between the wall-stud insulation vapor barrier and the wall interior covering. The weather block and vent is field adapted to the parameters of the building and is factory scored for easy field adaptation and so that it can be shipped flat for transportation economies.