I have a project coming up in Charleston, SC (Climate Zone 3A) that’s elevated on wood pilings — the space under the house is open to the outdoors, only enclosed with louvered panels between the pilings. 

The envelope above the floor is settled. Walls and roof are ZIP System sheathing, fully taped, running continuously from the wall sheathing up over the roof deck and down the other side. I’ve done this method on several slab on grade houses and it work well to manage air movement. Average ACH50 is 0.6 on the past three homes.

Where I need input is the underside of the house.

1. Air barrier continuity at the floor plane. My plan is to run a smart vapor retarder (something like INTELLO Plus) across the underside of the floor trusses and tie it into the ZIP sheathing air barrier at the rim/perimeter, so the two systems form one continuous boundary. The membrane will then be covered by 5/8″ MR (moisture-resistant) gypsum board fastened to the underside of the trusses, which serves as the finished garage ceiling. Is this a sound way to make the air barrier continuous? Are there better tested details for that specific transition?

2. Insulation approach under the floor Assuming that air barrier detail works, my plan is fiberglass batts between the floor trusses. Is batt insulation a reasonable choice or should I be looking at another insulation strategy? The next question probably provides a bit more insight into my hesitation with fiberglass.

3. Open-web floor trusses and apparent R-value loss The floor trusses are open-web (not solid-web I-joists), so the batts can’t fully fill the cavity where the diagonal web members are — it looks like we’re losing effective R-value at every web intersection compared to a solid-web joist bay. Is this a real, meaningful loss in practice, and if so, how do people typically address it? Different insulation type, blown-in fill, strapping/furring to add a continuous layer under the trusses, something else? I know foam would bridge nicely across the floor joists. My real concern here is hot air from below reaching the underside of the cool subfloor. 

If I needed to used foam in this instance, would open cell be a viable option or are we concerned with the vapor permeability and the chance for moisture to reach the underside of the subfloor.

Any input on how to sequence and detail these three pieces for an elevated coastal home would be a big help.