How to Flash the Gap Between a Chimney and Exterior Wall – GreenBuildingAdvisor

Q&A Spotlight

A Boston homeowner plans to add exterior insulation and new siding but first needs the best way to waterproof the gap between a brick chimney and the wall

Compared to products such as vapor-permeable WRBs, energy-recovery ventilators, and solar panels, flashing seems almost dirt simple. At its most basic, it’s simply a bent piece of metal or plastic designed to divert water away from the joint between two exterior surfaces. In the fight against water intrusion, it’s as important as the roof or windows. (Try living in a house without it and see what happens.)

Flashing is a weatherproofing detail that’s been used for centuries and is now even celebrated with its own day of recognition–the International Flashing Awareness Day. (This year it’s on Wednesday, August 26, 2026). GBA’s editor Randy Williams even wrote an article to cover the new holiday. But despite years of documented experience, installing flashing can still be challenging, especially for the uninitiated.

Enter “PeterPablo22” (we’ll call him Peter). He posted a question on the GBA forum to find out the best way to flash the joint between his chimney and the exterior wall. He’s renovating the exterior of his 1960 Cape-style home in Boston (climate zone 6A). He had torn off the cedar shingles, the ½-in. fiberboard substrate, and the tar paper to expose the plank sheathing. In its place, he plans to apply Blueskin WRB, followed by 2-in. Timber HP TimberBoard wood fiber insulation, a ¾-in. rainscreen, and LP SmartSide siding. He’s looking for advice on how to flash the gap between the brick chimney and the exterior wall.

His initial approach is to cover the gap with Siga Fentrim tape: “My idea is to tape the brick to the Blueskin WRB with the SIGA Fentrim and then apply a sealant between the siding and the chimney after I install my LP SmartSide.”

Because the gap between the brick and the siding is small, he’s not…