Retrofitting a house with high performance vinyl windows and patio doors can produce significant energy savings and improved comfort. However, when the right window is installed incorrectly or worse yet, when wrong window or patio door is installed incorrectly, energy savings will be few, structure and furnishings may be damaged, and the occupants' safety may be jeopardized. The energy retrofitter needs to understand more than U-factors and solar heat gain coefficients when choosing and installing a window.

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Planning Your Vinyl Windows and Patio Doors to Meet Building Codes

Well before the patio doors and windows arrive at the site for installation, planning and research should be done to choose a window or patio door that fits the situation. Check the local building department's requirements regarding egress, safety glass, and grade (structural integrity) before obtaining estimates and bids.

Window and Door Egress

Installing retrofit windows and patio doors, especially when the existing window and patio frames are left in place, can significantly reduce the size of the clear opening by as much as 2 inches of height and 3 inches of width. This may prevent emergency egress. That is, occupants may not be able to get out, and rescuers may not be able to get in, if there's a fire.

Most model building codes specify egress requirements for sleeping rooms in the first three stories. They require at least one operable window or exterior door, approved for emergency escape or rescue, to open directly onto a public street, alley, yard, or court. If the emergency escape is a window, it must be operable from the inside without the use of tools. It must provide a minimum clear opening of 5.7 sq.ft. (with a minimum width of 20 inches and height of 24 inches), and the finished sill may be no more than 44 inches above the floor. Patio door glass must be tempered and if a window is less than 18 inches above the floor, its glass must be tempered.

Some jurisdictions have specific egress requirements for retrofit patio doors and windows. Others do not. Know what the local building department requires for egress before replacing a bedroom window.

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Safety Glass

People often install standard glass because that's what was there before. This can be a serious mistake, since codes are constantly being upgraded. Installers who carelessly replace standard glass, like-for-like, could end up defending themselves in court if an occupant is injured by glass shards in a window that should have contained safety glass.

Safety Glass

Safety glass crumbles into relatively safe little chunks resembling hailstones. Most codes require safety glass (almost always tempered glass) to be installed where a window may be subject to human impact. This includes all entrance doors; windows adjacent to a door when the glass is within a 24-inch arc of the closed door (increased from 12 inches in 1994) and the bottom of the window is less than 60 inches above the walking surface; and windows larger than 9 sq.ft. when the bottom is less than 18 inches above the floor, the top is more than 36 inches above the floor, and a walking surface is within 36 inches horizontally of the window.

Consider cost and timing well in advance, because tempered glass costs more and usually requires more lead time at the manufacturer, especially for custom-sized units. High-performance windows with spectrally selective coatings require even longer lead times.

Grade

Even a well-installed window with the best thermal characteristics can leak during a gusty rainstorm if the window is not the proper grade. Grade refers to the structural capability of the window, or its capacity to resist wind loads and to seal against air and water penetration (see "How to Select the Correct Grade of Window"). When the wrong grade of window is installed, the combination of wind pressure and driving rain can force water through the seals between the operable sash, the frame, and through weep holes.

How to Install a Window

Installing a window correctly takes a lot of careful attention. The exact procedure varies with the type of window being removed, how it is removed, and the type of replacement window. However, here are the basic steps for most window retrofits:

Measure

Measure the opening into which the retrofit window will be installed width, height, and depth. Size the replacement window slightly smaller than the opening, allowing clearances recommended by the manufacturer. Vinyl frames require more clearance than those made of wood, aluminum, fiberglass, and composite materials.

Removal of old window

Remove the old window. For wood vertical sliders (single- or double hung windows), remove stops, trim, sash cords, and pulleys. For finned windows, cut away siding to expose the fin so the entire unit can be removed, or cut back the frame as much as possible, leaving the old fins intact.

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Preparation

Prepare the opening. Remove all obstructions, such as loose materials and old caulk. If the opening is severely out of square, fill it in, leveling the bottom sill and plumbing the sides, before installing the new window. Try the replacement window to make sure it fits properly before applying sealant.

Seals

Restore the integrity of the moisture barrier and drip cap. If fins were exposed, seal breaches in the building paper. If the existing metal frame was collapsed to remove it, seal the entire perimeter gap liberally with silicon sealant. Reinstall or seal any damaged flashing. For box frame retrofits, make sure a drip cap is in place to drain away water.

Precaulking

Precaulk the stops, mounting surface, or fins with elastomeric sealant. When installing box frames against existing stops, caulk the surface of the stops against which the window will be placed. For finned windows, install a liberal and continuous bead of caulk either on the fin or on the mounting surface around the entire perimeter.

Bottom support

Install proper bottom support. Metal, wood, and fiberglass frames can be supported with shims spaced per manufacturer's instructions. Vinyl windows need solid, level support across the entire bottom (the sill jamb).

Installation

Install the new window level, plumb, and square. Use a level to plumb the unit. To make sure it is square, (a) adjust until both diagonal measurements are the same or (b) open the slider just past the edge of the sash channel and adjust until the gap is even along its entire length. Anchor the window securely per manufacturer's instructions. Use only corrosion-resistant nails and screws. Predrilling fins and frames is recommended. Anchors must penetrate solid material such as structural framing. Countersink anchors installed through wooden jambs.

Insulation

Insulate gaps between the retrofit frame and the rough opening. Fill the cavity between the retrofit frame and the rough opening with foam backer rod or loosely packed insulation. Non-expanding foam is OK when installed carefully. Never use expanding foam. Weather seal the entire perimeter of the installation with elastomeric caulk. Butt joints and other gaps over 5/8 inch wide need backup material (such as foam backer rod). The seal must be continuous and should be tooled to fill gaps and to remove air pockets and excess material.

Trim

Install exterior and interior finishing trim, also called molding, and caulk It's a good idea to install prefinished weather protected trim using butt joints.