Synopsis: Natural builder Ben Graham and custom woodworker Mike Lamp demonstrate how they recently designed and built a 3-1/2-in.-thick R-10 wood door using salvaged cherry lumber. The door’s construction details, product selections, and design choices are explained thoroughly. The second half of the article shows how Graham and Lamp installed the 140-lb. door and integrated it into the home’s building envelope. Exploded drawings show how the door and its two-piece jamb assembly are constructed.

We recently completed a deep-energy retrofit to an 80-year-old Vermont farmhouse. The many improvements we made included new windows, new mechanical systems, and the addition of 4 in. of exterior mineral-wool insulation. Not surprisingly, the project also called for new energy-efficient exterior doors to complement the home’s high-performance building envelope.

For an ultra-efficient house like this one, entry doors are often sourced from Europe, where high-performance doors are more mainstream than in the United States. Unfortunately, most European offerings have a sleek, modern appearance, and our clients wanted something that would fit the style of their farmhouse. With limited options, we set out to build a pair of attractive entry doors that would rival the efficiency of the best-performing units on the market. In addition, we wanted to do so by using common building materials that are readily available.

The 31⁄2-in.-thick R-10 doors that we built are made from site-harvested cherry. They are filled with 2-in. rigid-mineral-wool insulation and include triple-pane low-e glass. We installed the doors in custom-built jambs with a crucial double-weatherstripping detail to help make the entry airtight. Finally, we outfitted each door with the best-built hardware we could find.

Aesthetics and performance drive design

One of the first decisions we had to make was whether to build inset or overlay doors. In the United States, virtually all residential entry doors are inset. The door swings into the frame and lands on a stop. High-performance European doors, which are described as overlay, close onto their frame. Proponents of overlay doors say they seal tighter and are therefore more efficient. However, overlay doors require rabbeted or post-style hinges, which are costly and hard to find in the United States. Rather than order hinges from Europe for several hundred dollars, we decided to build inset doors and add a second layer of weatherstripping to boost their airtightness.

For weatherstripping, we opted to use silicone flipper seals from Research Conservation Technology along the head and side jambs. These soft silicone weatherstrips easily compress to accommodate a wood door’s seasonal movement, and they make the door easy to close. The strips are installed in one saw kerf that aligns with the door’s rabbeted edge and in a second kerf cut into the applied stop. On the bottom of the door, we installed a brushed sweep to seal the door to its bluestone threshold.

Perhaps the most challenging aspect of the doors’ design was finding suitable hardware. For high-performance homes, we encourage all of our clients to select multipoint locks for their doors. The bolts at the top and bottom can help prevent a wood door from warping, increase airtightness by providing even pressure against the door’s weatherstripping, and boost security. Multipoint locks are relatively easy to get through special order, but finding a setup that works with a 3-1⁄2-in.-thick door is not as easy. In fact, the units we found that would fit are distinctly modern in appearance. Given the options, our clients decided to forego the multipoint lock and instead install a high-quality traditional mortise lockset from Rocky Mountain Hardware, which accommodates the door’s thickness and is in tune with the farmhouse style.

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Download the SketchUp model for this exterior door.

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