Greatly increasing the thickness of house insulation—a technique called super-insulating—is one of the best ways to slash heating bills. In some newly built super- insulated houses—with wall insulation values of R-30 and higher and roof insulation values of R-50 and higher—the savings can be as much as 70 per cent com pared with the cost of heating a similar house with conventional insulation. Super-insulating an older house can yield savings approaching these—by stopping drafts and reinforcing inadequate insulation while improving comfort.
Many features found in new super-insulated houses — thermal blocks between structural parts to interrupt the flow of heat, for example, and special roof sheathing—are impractical for existing homes. But other super-insulating techniques are adaptable to existing houses. The primary technique is to bolster existing insulation with thick extra layers. It’s simple, for instance, to lay extra fiberglass batting in an attic, adding an R value of about 3.33 per inch of thickness. It’s also possible to super-insulate exterior walls.
Before extra insulation can be added, the exterior walls must be thickened to accommodate it. And thickening the walls involves major construction: A wall frame must be built, then covered over with an attractive surfacing material after the new insulation is installed. In almost every case this frame, which can be built of 2-by-4s or 2-by-3s, is best added to the inside of an existing wall. It can rest on the existing floor and can be covered with inexpensive wallboard. Thickening walls on the outside requires a heavy ledger board bolted to the studs to support the new frame, plus expensive exterior siding. However, if your house is due for new siding anyway, superinsulating from the outside may prove feasible.
The major drawback to superinsulating a wall inside is that living space must be sacrificed. A new wall cavity providing room for 6 inches of new insulation will use up 5 square feet of space for every 10 feet of wall thickened; fully superinsulating a two-story, 20-by-40-foot house means the loss of 120 square feet.
Of course, space can be saved by superinsulating only some of the walls. The south wall—warmed by the sun for most of the day—needs superinsulation only in the harshest climates. Or limit your superinsulation to the north wall, or to the north wall and the west or east wall, whichever faces the direction of prevailing winds in your locale.
The major’ physical obstacles to superinsulating walls are windows, doors, electrical boxes and heating outlets. For windows and doors, use 2-by-2 furring strips to extend the jambs back into the rooms; the recesses inside the extended lambs can be covered with wallboard. Electrical cables connected to boxes, forced-air ducts and radiator pipes must be brought into the new room perimeter.
The vapor barrier of a superinsulated wall also requires special consideration:
A superinsulated wall. An inner frame, consisting of studs, a sole plate and a top plate, thickens an exterior wall from the inside to provide space for 6 extra inches of fiberglass insulation, Window and door recesses in the existing wall are extended inward by the frame and by furring strips, used for securing a new vapor barrier, wallboard and trim. An electrical outlet is relocated onto a stud of the new frame, its front flush with the surface of the new wallboard.
It must be better than its counterpart in an ordinary wall. Because less air seeps through a superinsulated wall, less moisture is carried out of the wall cavity. Instead, any moisture that gets inside the wall can condense, matting and ruining the new insulation. The best defense is a continuous sheet of 6-mil polyethylene sheeting, spread over the new insulation. The sheeting will bolster the performance of the standard foil vapor barrier that comes with the new insulation.
If there is an existing vapor barrier, it must be perforated so that moisture can pass through it. Otherwise, even a tiny amount of moisture that penetrated the wall would be permanently trapped between two virtually impervious barriers.
Moving Existing Utilities
Detouring a heating duct. To make room for the sole plate of the new wall frame, move any floor register that is with in 6 inches of the existing wall. First liftoff the register grille. Working from below, cut away a section of the ceiling under the register if necessary, and pull the elbow free from the end of the duct. Cut the duct short and replace the elbow to reroute the air flow to a new opening cut in the floor. Install the grille over the new opening.
Pulling an electrical box forward. If there is enough slack in cables connected to an electrical box, you can avoid having to rewire the wall by simply moving the box from the existing wall to the new inner wall. Turn off the electricity to the box at the house service panel. Hammer a hole through the existing wall surface, and pry out any staples holding the cables near the box. Pull the cables and box loose from the existing wall frame. After securing the new wall frame, remount the box, positioning it so that its front edge will be flush with the new wallboard.
OLD OPENING.
Framing the Inner Wall
1. Adding nailing blocks. If ceiling joists run parallel to the exterior wall and the new wall frame cannot be located directly under a joist, install nailing blocks between joists to secure the top plate of the new frame. First use a pry bar to lever the baseboards and ceiling moldings off the exterior wall and adjacent walls. If the ceiling is covered with wallboard, use a utility knife to cut it away between the exterior wall and the center of the closest joist. If the ceiling is plaster, cut it away with a mason’s chisel and a hammer. Cut nailing blocks from lumber the same size as the joists. Nail the blocks at 24-inch intervals between the joists, with the bottom edges flush.
If the joists run perpendicular to the exterior wall, nailing blocks are not necessary. The top plate of the new wall can be nailed at the points where it crosses the joists.
2 Extending door and window jambs. Nail 2-by-2 furring strips around the door and window openings to provide a nailing surface for wallboard that will be installed later, To install strips around an untrimmed opening, such as the window shown here, set the furring strips exactly ½ inch from the sides and top of the opening to accommodate the wallboard; at the bottom of a window, nail the furring strip flush with the window sill. For openings trimmed with casing, such as the doorway shown here, space the strips 1 inch or more from the outer edge of the trim.
3. Assembling the wall frame. Cut two boards to serve as the new top and sole plates, the same length as the existing wall. On the face of one of the boards, make marks at 16-inch intervals to indicate stud positions. With a helper, hold the marked board against the wall and mark it for additional studs at each of the vertical furring strips framing the doors and windows. Transfer the marks to the other board. Make two cuts in the sole plate to remove the section corresponding to the doorway, and nail the remaining pieces to the floor.
Cut full-length studs to the height of the room minus 3 inches to allow for the combined thickness of the top and sole plates. Set the marked top plate on edge, and nail a full-length stud to it at each mark, skipping the marks above a door or window. To frame around a window, cut two boards the width of the opening. Nail the two horizontally between the studs bounding the opening at the same height as the horizontal furring strips on the existing wall. Nail short studs, called cripple studs, above the header and below the sill at the marked 16-inch intervals.
To frame around a door, nail a horizontal header at ‘the height of the furring strip above the door. Nail cripple studs above the header at the 16-inch marks on the top plate.
4. Erecting the wall frame. With a helper, lift the assembled top plate and studs onto the sole plate. Toenail the bottom ends of the studs to the sole plate. Plumb the frame, and nail the top plate to the ceiling joists or nailing blocks.
Fasten the end studs of the new wall frame to the adjacent walls with toggle bolts if the adjacent walls are hollow where the frame meets them; nail it in place if the frame falls at stud locations.
Installing the New Insulation
1. Perforating the old vapor barrier. Using a power drill fitted with a 1 twist bit, bore air holes through the old wall surface at 1-foot intervals vertically and horizontally. Push the drill bit at least 2 inches into the wall to ensure that it pierces the vapor barrier of the existing insulation.
2. Stapling on new insulation. Wearing gloves, a long-sleeved shirt and a dust mask, fit fiberglass insulation between the studs of the new wall frame, keeping the foil vapor barrier of the material toward you. Staple the flanges of the foil facing to the studs every 6 inches with a staple gun. If necessary, cut the insulation to fit with a long pair of scissors or with a butcher knife; odd-sized pieces and scraps can be butted together to fill all of the spaces around doors and windows. Don’t pack any of the insulation tight; compressing the fibers will greatly decrease the effectiveness of the
3. Adding the second vapor barrier. Fasten a single piece of 6-mu polyethylene over the new insulation, stapling every 6 inches along the studs. At every window and door, slash the plastic from corner to corner in an X pattern; fold the four triangles into the opening, and staple the plastic to the furring.
Cover the stud wall and the recesses of the window and door openings with wallboard.
Trimming the window. At the bottom of the window recess, use a piece of stair-tread lumber to create a finish sill. Cut the tread as long as the window recess is wide and % inch wider than the depth of the recess from the window sash to the new face of the wall. Place the back edge of the tread 1/10 inch from the window sash, and nail through the top of the tread into the framing and furring strip below. Cover the seams between wallboard panels here and elsewhere in the room with joint compound and tape.
Finishing the Wall