Cloverleaf, currently under construction in Truro, Massachusetts, is a mixed-income housing development with townhomes that are EnergyStar and HERS 45, and a multi-family building that is Passive House certified. While constructing the townhomes, which are sequenced ahead of the Passive House building, we’ve learned the importance of installation precision for the envelope, most notably with underslab insulation. The slab on grade is a key part of the envelope, and thermal insulation continuity with airtight detailing is essential to achieving Passive House certification. To ensure airtightness at the transitions at thickened slabs, a miter saw is kept on site specifically to cut 45-degree angles in the rigid foam insulation at these joints. Though the process requires extra time and care, the improved air sealing and thermal continuity are critical. Moving forward, we consider this a best practice for our Passive House projects and should show insulation with mitered joints at thickened slabs and haunches.