How do we create a virtual air-tight assembly in a building? The quick answer is tape and perseverance. Below the slab we install a 15 mil vapor barrier that is taped and flashed at all penetrations. This barrier will be the first step towards sealing the building as well as removing any risk of radon from the building. (more on this in a future post) From here, this layer is attached to the exterior wall sheathing and then the sheathing is taped at every possible seam. Now we have a underfloor and wall system acting as one unit. The bottom of the roof or ceiling plane then gets taped similar to the walls. All lighting penetrations, vents or access panels must be treated with detail to pull this off. The windows and doors are then installed and air sealed with tape or wet flashing. Once this is complete, we conduct a blower door test to scientifically measure the actual air changes per hour or ACH. Our goal is .6 ACH.