Building the warm door

It's hard to complain about the performance of modern, pre-hung metal exterior doors. They keep out weather like a submarine hatch but often fail to excite me. This kind of dissatisfaction is why I began making high-performance exterior wooden doors back in 1990. The oldest are now in their 13th year of service and continue to work as well as the day I hung them. My recipe for cozy, site-built, exterior doors includes three parts: a weather-sealed exterior skin of 3/4-inch thick vertically-laid tongue and groove lumber, a 1-inch thick inner structural frame that surrounds an insulating foam core, and another 3/4-inch thick indoor layer of tongue and groove lumber similar to the exterior face. The top, bottom and sides are capped with a 1-inch thick border of grooved wood that goes around the perimeter, both for good looks and to keep rainwater out from between internal laminations. Whether you build it yourself or have a professional do the job, real wood exterior doors are practical, effective and beautiful.

I recommend an inner frame made of wood that measures a full 1-inch thick. Standard 5/4 decking stock, available at every building supply outlet, is idea. It matches the standard thickness of rigid foam insulation you'll install later for warmth. The main thing to remember about the door's inner frame is that it must be strong. The tongue and groove indoor and outdoor layers won't add much resistance to sagging, so rigidity depends largely on the frame. The easiest, super-strong way of connecting the top, bottom and sides of the inner frame is with lap joints, screwed and glued together with a weatherproof adhesive. Moisture shouldn't ever get this far, but if it does, at least you don't have to worry about the glue letting go. Regular carpenters glues go soft after extended contact with moisture, so you'll need to use something else. Either type II PVA glue or polyurethane adhesive works best. Look for names like Titebond II, Weathertite or any brand of liquid polyurethane glue.

The outdoor skin keeps driving rains out of the inner core, yet it must also be free to expand and contract seasonally. The 3/4-inch by 3-inch wide tongue and groove boards I use for this purpose do a good job, but only if the joints between neighbouring pieces are sealed with caulking during assembly. The best product for this job -- polyurethane caulking -- wasn't available when I made my first doors, but I certainly do recommend it now. It's strong, sticks like crazy, remains flexible and takes paint. No other high performance caulking boasts all these claims.

Choose the best boards for the exterior face of your door, with the knottier pieces reserved for the inside, where it's always dry. While you work building the outer door face, lay a bead of caulking down the groove of each board before nestling it with its neighbour. Each board should also be glued and nailed to that part of the underlying frame that it crosses, using stainless steel finishing nails driven through the tongue of each board, diagonally. With the outer skin complete, cut pieces of 1-inch thick rigid foam board to fit within the spaces of the inner frame, leaving a 1/2-inch gap all around. Secure the sections of foam to the back of the outer skin with glue, to keep it still in preparation for air-sealing the rigid sheets to the door frame with expanding polyurethane foam.

If you count your time (or pay for someone else's), building a wooden warm door probably comes out more expensive than a ready-hung metal one. In fact, I can guarantee it. That's why there's a place for easy-install doors of all kinds. And though I'm glad they're there, I'm also glad there's an opportunity to exercise the craftsmanship that led me to the trade in the first place.