How to Flash and Air-Seal a Retrofit Penetration in a Rainscreen – GreenBuildingAdvisor

By Chuck Bickford | August 25, 2025

The advent of rainscreen walls has been a real boon to house performance, and the practice has been around long enough that builders have come up with lots of variations that work. At even its simplest design—sheathing, WRB, furring strips, and siding—a rainscreen wall is still complicated by the layers required to make it function. But what happens when you need to retrofit something into that wall?

That was the question posed by “bobhoperises” (Bob for short) in a Q&A post a while back. He’s interested in a broader answer “that can be used for different penetration types, claddings, and climate zones … to avoid the ‘it depends’ scenario.” But Bob has a specific situation. He’s in climate zone 2 (hot/humid), and the wall in question looks like this: Hardie fiber-cement lap siding, 3/8-in. Coravent rainscreen, Zip System sheathing sealed with liquid flashing, 2×4 bays filled with R-15 mineral wool, and drywall.

He needs to install a 3-in. schedule-80 PVC conduit through the wall, but he doesn’t know how to seal the penetration “given that access to the sheathing is blocked by the cladding.” He writes that he does have access to the interior side of the penetration, but he hasn’t seen any viable strategies for sealing from the interior. “They tend to ignore the possibility of water infiltration by not utilizing any flashing or shingling.”

The topic for this Spotlight is: How does Bob retrofit an air-and water-tight penetration through his rainscreen wall?

The devil is definitely in the details

If Bob was installing the conduit before he had added the rainscreen, he might have used a small block to seal the penetration; there’s some discussion about this method on this recent Q&A question.

Member “maine_tyler,” who writes that he’s…