Several years ago I found myself a prisoner in my own home. I had signed up for a free quote for replacement windows at a local home and garden show. In my mind, this should have been a 15-minute inspection with a 30-minute quote and discussion at my home. I had no idea I was in for a three-hour ordeal with a pushy salesman who refused to listen. I learned three things that day: never invite a salesman into your home, vinyl windows can never substitute real wood, and triple-pane windows are incredibly expensive.
That day I formed an opinion that triple-pane windows were an incredible, advanced technology that simply had a diminished rate of return due to their high price point. In my defense, my perspective centered around my 80-year old house with partially insulated walls. High performance windows may have only made a small portion of my overall building envelope more efficient.
As an architect, I’ve found building owners and university facility managers feel the same way about triple-pane windows—no one believes the upcharge is worth it. After a few years, I started to wonder if their cost would ever come down enough to prove an acceptable payback. If we want to talk about the true value of triple-pane windows though, first we need to understand how the window system works.
Heat Loss & the Four Battles
Windows must fight heat loss on four different fronts:
Paul Fisette outlines the scientific method of heat loss in his article "Understanding Energy Efficient Windows." Most of our heat loss is via the process known as conduction, the direct transfer of heat through the window. This is the exact same process we use when we heat up a skillet on a kitchen range. In a similar manner, our windows transfer heat directly to the cold exterior (or vice versa). To fight conduction, window manufacturers have developed lower-conductive materials to improve performance. Developments in multiple panes, infill gases, and thermally resistant spacers fight conduction. The window frame material itself can play a big factor in conduction.
Convection also occurs in our windows. Convection movement is commonly described as a draft. This happens when heated indoor air rubs against the cold window glass surface and then drops to the floor. The dropping of the cooled air forces newer warm air to rush to the glass, and thus the cycle repeats. The key to fighting convection centers on raising the surface temperature of the interior pane.
The cold window pane also creates radiant transfer of heat energy waves. Think about a sunny day. You wear a white shirt and your friend wears a black shirt. Soon enough, your friend is hot and you feel fine. That’s because darker colors absorb heat while lighter colors reflect heat. In a similar manner, plain clear glass will absorb interior heat and quickly emit it to the outdoors. Luckily, low-E coatings have been developed to reduce the radiant transfer of these heat energy waves.
The last enemy of window efficiency is air leakage. Air leakage occurs around and through the window unit itself. The type of window (casement, awning, double hung), joining of materials, and overall construction quality all play a factor. Values for air leakage are measured and should be listed in the specifications as cubic feet minute per square foot of window (cfm/ft).
In order to fight the “The Four Battles” of Energy Loss, window manufacturers have developed several options for:
Frame materials and thermal breaks
Glazing tints and coatings
Glazing pane spacers
Though frame material is often the largest differentiator in energy performance, it’s also a tough decision for facility managers and building owners. Energy performance is important, but so is durability and maintenance. In the college housing market, aluminum windows are often chosen. Thermally broken window frames can immensely improve energy performance, but add cost due to the complexity of manufacturing. Some of the better performing window frames include wood (aluminum or vinyl clad), vinyl, and fiberglass. Wood clad windows are often more expensive and the wood can become a maintenance issue. Vinyl is energy efficient and very durable, but is thermally less stable, fades, and can be considered unattractive on the interior. Fiberglass is strong and is the most thermally stable option, matching the coefficient of expansion of the window panes. Fiberglass windows must be painted and they are fairly new to the marketplace.
Glazing technology today is complex. Coatings and tints are designed to block specific wavelengths of energy while allowing for a vast selection of tints, reflectivity, and visible light transmittance. Architects and engineers must pay attention to variables in solar heat gain coefficients (SHGC), u-values (winter nighttime and summer daytime), visible transmittance (VT), light to solar gain (LSG), reflective appearance, and of course, color. Low-E coatings have come a long way since they were first marketed in the mid 1970’s.
Manufacturers began to introduce lower conductive gas infills between the sealed window panes in the 1980’s. An argon filled insulated glass unit (IGU) can improve the center of glass by 0.5R-1R. Higher performance windows are offered with Krypton gas (watch out Superman). Adding infill gasses can have a higher cost than benefit factor, so pay attention.
We all have seen condensation occur on windows, and it typically occurs at the edge of the window pane. The window pane edge is the coldest part of the window because of the thermal bridging that happens at the spacer. In the past, most of these spacers have been aluminum, a highly conductive metal.Spacers are now available in thin stainless steel, plastic, foam, and rubber. Manufacturer’s claim a warm edge spacer can boost the edge temperature by 5 degrees.
Triple-Pane Case Study
Triple-pane windows have been around since the 1980’s. The benefits of an additional pane (and additional air space) include improved u-value and solar heat gain coefficient. Several manufacturer’s claim a triple-pane window can offer 20-30% better efficiency ratings than a double pane unit. Many claim the first cost upgrade can range from 15-30% more. So the big question arises, does the first cost increase in triple pane windows pay back? Every situation is unique, but here is what we found out while comparing window options on one of our own college housing projects.
The Project: College Residence Hall
Scope: New 96 bed, wood-framed residence hall
Location: Donaldson, Indiana
Building SF: 28,900 sf
Operable Window Type: Double Hung
Operable Window Size: 3’-6” wide x 5’-2” tall (18.2 sf)
Quantity of Windows: 51
Exterior Wall R-Value: R-24.3 (U-.041)
In scenario one, the aluminum frame windows carried a premium cost. Many colleges and universities feel this premium is worth the reduction of future maintenance issues and maintenance costs. Aluminum framed windows however, require a sacrifice in energy performance, a reduction of assembly U-value. College housing sleeping units do not typically have large window to wall ratios, though. Our case study’s ratio is 5.7%. This lessens the overall impact of the lower performing windows, creating a minor increase of energy costs amounting to $56 annually.
Scenario two reflects the common decision to save initial first cost by using aluminum clad wood framed windows. Even though they have lower first costs, they are better energy performers than the heavy duty aluminum frame windows. Specifying better glazings, low-e glass, and argon gas minimally increases material costs.
In scenario three, the more expensive triple pane windows require an owner investment of an additional $6,375 over the aluminum clad wood frame windows (scenario two). These high performance windows offer a 31% better U-value and a 70% reduction in SHGC. As mentioned above, the low window to wall ratio lessens the impact the windows can have on the overall building performance. The energy savings amounts to only a difference of $87 annually. This yields over a 70 year payback!
Great advancements have been made in window and glazing technology over the last few decades. New advancements are continually being made and manufacturing costs have come down. Better windows have huge comfort advantages, but first costs and life cycle costs cannot be ignored. Owners and facility managers should lean on architects and engineers to explore the entire building performance and not just individual pieces. After all, every building is unique and every site is different. It’s true we no longer specify single glazed window systems. It appears, at least in the case of college housing, we may not yet be ready for triple glazed window systems.