A classic way for keeping cold air from leaking into a house, the double-doored entryway is back in fashion as a device for conserving energy. Known in past in carnations as a vestibule or foyer, it’s now called by energy experts an air lock—and when tightly weather-stripped will reduce drafts, a major cause of heat loss, by as much as 15 per cent.
Though credited with substantial savings in the cost of heat, an air lock makes most economic sense when it does double duty as a mud room for a household of children, a pantry for a crowded kitchen, or a convenient coat closet. Purely as an energy-saving device, it may take more than 10 years to pay back the cost of construction; but the bonus of extra space may tip the scales in its favor.
There are many different ways to build an air lock, depending on the floor plan and architecture of your house. The simplest method, in houses with entrance halls, is to wall off a section of the hail and install a second door. It’s almost as easy to enclose a small entrance stoop, which typically consists of a broad concrete slab shielded by an identically sized roof. Permanent walls of wood studs are erected between the roof and the step, and a new exterior door is set into a rough frame in one of the walls. The outside of the stud wall is then covered with siding, the inside with wallboard.
A removable air lock can be built on the same type of entrance stoop, using detachable panels of aluminum and glass. These panels are storm doors, ordered without handles or hinges. Special connecting hardware—metal channels and angles—can be ordered from suppliers to convert the panels into wall sections. In spring, the glass in the panels can be replaced with screens, or the whole enclosure can be dismantled.
Whatever form of air lock- you choose, plan to cover a floor space at least 4 feet square, the minimum area needed for comfortable maneuvering. Local building codes may dictate an even larger floor area. Within this space, the positioning and swinging of the two doors are critical design considerations: The doors should not open into each other, nor should they need to be open simultaneously for a person to pass through the air lock.
To stop drafts, both doors must be weather-stripped. For wooden doors, energy consultants recommend attaching wooden strips that come with a hollow vinyl bead set into the edge. The strips are cut to fit the top and sides of the jamb, then nailed in place. A metal door sweep with a vinyl strip is used to seal the bottom edge of a wooden door. Met al storm doors come with their own weather stripping attached.
Providing light in the confined space of an air lock can be a special problem. Sometimes you can avoid the need to add electric lighting by using glass doors or installing a window in a wall adjacent to outdoor lighting. More likely, however, you will have to wire a lighting fixture into the ceiling of the air lock and a light switch into a nearby wall, tapping an existing outlet for electricity.
Three Approaches to an Air Lock
An inner door in an entrance hail. Sealed off by a new interior door, an existing entrance hail forms an airtight passageway. Jack studs, cripple studs and a header enclose the rough frame for a new prehung door; studs, header and frame are all constructed of 2-by-4s. The door, shimmed into the frame after the new wall is covered with wallboard, is a split-jamb unit, so called because the two halves of the jamb fit together with a tongue-and-groove joint. The casing comes attached to both jamb halves.
A permanent outdoor air lock. Three short stud walls, one fitted with a prehung door, protect an existing entrance door from drafts and provide a convenient storage place for muddy boots and outdoor clothing. The 2-by-4 wall frames—made of vertical studs and horizontal top plates and sole plates—are nailed into the floor of the concrete stoop, to the joists of the overhanging roof and, at the house wall, to the existing studs. Matching pieces of siding, sheathing and wallboard cover the new walls. A glass-paneled door and a window admit natural light into the enclosure during the day, so that electricity is required only at night.
A removable porch enclosure. Aluminum storm-door panels, held by a frame of interlocking aluminum channels, turn a small entry porch into a weatherproof enclosure that can be disassembled and stored. The side panels are set into F-shaped channels (inset), which are screwed into the house wall, the concrete stoop, the porch roof and, at the front, square aluminum posts. The door is installed between a second set of aluminum posts. To prevent drafts, all the panel edges are caulked.