How to Air-Seal an Elevated Floor in a Coastal Home – GreenBuildingAdvisor

Q&A Spotlight

Building an elevated home on pilings on the Carolina coast raises questions concerning moisture infiltration

If you’re building an oceanfront home these days, a common strategy is to elevate the structure on piers to avoid potential damage from storm flooding and, for the moment, stay ahead of rising sea levels caused by climate change. The underside of the floor is just as exposed to heat, humidity, and wind as the walls and roof, making air-sealing and insulation much more complicated than in a typical crawlspace.

That’s what was on “BrockBuilds” mind (we’ll call him Brock) when he posted a question on the GBA forum. He’s about to build a house on the coast in Charleston, S.C. (Climate Zone 3A), that’s elevated on wood pilings. The space under the house is open to the outdoors, enclosed only with louvered panels between the pilings.

This isn’t his first house, so he has most of the envelope details nailed down—ZIP System sheathing on the walls and roof that’s continuously taped. “I’ve done this on several slab-on-grade houses, and it works well to manage air movement,” he says. “Average ACH50 is 0.6 on the past three homes.” The one detail he hasn’t solved is the exposed underside of this elevated floor system.

Air Barrier and Insulation

Brock plans to air-seal and insulate the floor frame. He’s thinking of installing a smart vapor retarder such as INTELLO Plus across the bottom of the floor trusses, seal it to the ZIP sheathing at the walls, and then sheathe the area with 5/8-in. moisture-resistant drywall for a finished garage ceiling. He asks, “Is this a sound way to make the air barrier continuous? Are there better tested details for that specific transition?”

If his air-barrier works, he wants to use fiberglass batts between the trusses. He’s also open to other options;…