"Should we replace the windows?"

It's a question many contractors are asked at the start of a building renovation. Windows are often the main aesthetic feature of a building and clients justifiably wonder if replacing them would add value to the property and save energy and operating costs at the same time.

Well, depending on the age and type of window, repairing them may be a better option. In many circumstances, window repair has been proven to boost windows to good-as-new performance and costs far less than installing new windows.

The main question: Will the aesthetic boost of installing brand new windows be worth the additional cost or would repairing them make more sense financially?

In the author's 30-year experience retrofitting building envelopes, cost ratios between replacement and repair consistently range from 12:1 to 8:1. Factors that influence that ratio include the current state of the existing windows and the scope of work needed to upgrade windows to good-as-new performance.

These numbers are well supported by a Canada Mortgage & Housing Corporation (CMHC), Research Report conducted in the late 90s (CMHC Project No: 96-81 & RD-0102-A, Desnoyers Mercure Inc., Patenaude-Chiovitte Inc.) that describes the results of a four-building window repair study. The researchers found that installing new, high quality weatherstripping, foam and sealants around the existing aluminum slider windows reduced air leakage by 54 to 83 percent, and achieved an air-leakage rating comparable to new windows.

Retrofit has an especially high success rate with aluminum slider doors and windows. Although popular for their low cost and easy installation, this type of window and door is not energy efficient by today's standards.

What's the problem?

Much of the inefficiency results from the poor performance of sliders in terms of air leakage and deterioration of weatherstripping. Experience has shown that this leakage is one of the most common problems affecting building envelopes and their long-term durability.

Window performance problems result from three closely related issues: air infiltration, condensation and ice build-up.

Condensation and frost formation on exterior sashes is the result of warm, moist indoor air exfiltrating through interior sashes. It condenses (or freezes) on the inside face of the exterior sashes before it has the opportunity to escape to the exterior. Stack effect exacerbates the problem at the top of the building, especially in winter. Making interior sashes more airtight will reduce condensation formation on the inside face of exterior sashes, thanks to the net positive pressure of the inside of the building envelope compared with the outside.

Testing

Luckily, these types of problems are easily and cost-effectively addressed in a retrofit. The windows should be tested for pre-retrofit air leakage levels using portable test apparatus, including an exhaust blower, a control valve, flow meters, a differential manometer and a test chamber made of polyethylene film with retaining bars attached to the interior side of the window frame. The methodology is outlined in the ASTM E-783 test Standard Method for Field Measurement of Air Leakage Through Installed Exterior Windows and Doors. Testing records the amount of air leakage across a specimen window at a test pressure differential of 75 Pa representing a wind speed of 40 kph (25 mph).

Materials

Window retrofit materials should be of the highest quality possible-it makes no sense to use cheap products as a replacement for failed materials. These materials should include:

V-strip (large and small "V")

Pile weatherstripping (with fin)

Corner seal/dust plugs

Siliconized acrylic latex sealant, suitable for interior/exterior caulking and a back-bedding glazing compound on a variety of surfaces

Installation

The window area should be clean and free of debris. This may require wire brushing, grinding, sandblasting or solvent washing. When applying sealant, tooling is recommended immediately after application to insure firm, intimate contact with the joint interface. Joint backing should be installed when possible to control the depth of the sealant bead (either via backer rods or polyethylene tape). Weatherstripping and seals should be installed strictly in accordance with the manufacturer's guidelines, ensuring that materials are plumb, square and level whenever possible. Weatherstripping and sealing should be installed so that a watertight, airtight seal is formed. This may require replacement of the wheels in the window tracks so that the window can continue to slide freely, while ensuring a tight seal. Post-retrofit testing can be used to confirm this has been accomplished.

To save or to spend?

The biggest cost in a window retrofit is the installation itself. In a window replacement, the installation is only a small portion of the overall cost. Of course, a retrofit is not ideal in all situations. It may not be possible to restore older windows to a standard comparable to new windows in terms of air, vapor or water leakage, and in this case, replacement is a better option. For heritage buildings and historic sites, where replacement is not allowed, window repair can still make a significant contribution to improved building performance (see sidebar).

However, the cost savings available to building managers from the repair option can be substantial. The simple payback, or return on investment, for window repair (which is, in the author's experience, often less than five years) in terms of energy savings will often be much shorter than window replacement. When you consider a savings difference of hundreds of dollars per window, it can really add up. Repairing windows may not be as glamorous as replacing them, but it can be a smarter option.

About the Author

Tony Woods is president of Canam Building Envelope Specialists Inc. of Mississauga, Ontario, a company with a 25-year history of solving building envelope problems. During his career he has served as president of the Ontario Building Envelope Council and on more than 10 Standards Committees dealing with air leakage control, air barriers, ventilation, etc. 

The steel sash operable windows on the Kenson Apartments in downtown Toronto were old, drafty and warped from the passing of time. Huge gaps and irregular surfaces also made them very difficult to seal. 

The only solution, says Martin Vuksinic, Field Project Manager for Canam Building Envelope Specialists, was to retrofit all 355 windows using cast-in-place silicone gaskets.

Vuksinic's team applied a bead of silicone to the sash, followed by a 2 mil clear plastic strip to allow the window to shut, compressing the silicone. Once set, the team opened the window, removed the plastic and trimmed the excess silicone for a neat appearance.

"This method completely air seals the window, improving occupant comfort and energy efficiency," says Vuksinic. "It also improves the sound insulation, which in downtown Toronto can be of great benefit."

Vuksinic says his team has used the silicone gasket window retrofit method successfully on other heritage buildings including Trinity College and the North Wing of the Ontario Legislature at Queen's Park.