Continuous Insulation and Doors – GreenBuildingAdvisor

By Leah Karmaker | August 28, 2025

Details for window installation in thick walls abound. Doors, not so much. Door thresholds, even less. In this article, we focus on the one detail we’ve had a hard time finding and figuring out when building walls with continuous exterior insulation: the door threshold.

The purpose of a threshold

Door thresholds need to serve several functions. They have to keep water and air out, hold up to substantial use, and minimize thermal bridging, all while being fully and easily operable. In projects with exterior continuous insulation, maintaining and protecting the exterior insulation, providing structural support, thermally breaking the sill, and specifying a durable sill extender are often challenges.

Our details and assemblies

We typically use inswing, insulated fiberglass doors with thermally broken thresholds (product recommendations to be covered later in this article). Let’s talk about our foundation assembly briefly: We often use ICF’s for our foundation walls and call for the inner layer of insulation to be cut down, creating a condition where the slab extends across the foundation wall and abuts the outer layer of foundation insulation. When installing doors over a slab with exterior insulation, it is possible to maintain the outer layer of the foundation insulation depending on the requirements of the sill extender. The foam may need to be cut down to accept a treated wood buck for support, as shown in the detail. In either case, the foam or the buck is covered in self-adhering flashing that folds up the door jambs and laps over the WRB covering the foundation insulation (check out this primer on flashing door openings for more information). The door threshold can now be installed over the flashing, set in sealant to air-seal the door to the slab. Lastly, metal flashing is installed to the underside of the sill to…