Window Retrofit Technologies

A team from the Center for Energy and Environment, and the University of Minnesota’s Center for Sustainable Building Research received a Conservation Applied Research and Development (CARD) grant to study the benefits of window retrofits, specifically the cost effectiveness and energy savings of window panels and applied films in a climate such as Minnesota’s. 

Window retrofits are an important tool in energy savings, as there are over 300 million square feet of windows in Minnesota as estimated by the final report. Windows are the source of 34% of overall energy use in commercial buildings, and 30% in residential buildings. New windows cost between $30 and $90 per square foot, while window retrofits can cost as low as $3 to $7 per square foot and often achieve the same energy savings as a window replacement. 

In Minnesota, there is less energy saving potential for window retrofits compared to warmer regions of the United States, as 95% of Minnesota’s windows are double glazed or better, compared to the national average of only 60%.

Nevertheless, there is potential to save 12 trillion Btu from window retrofits in Minnesota, which represents 1.6% of Minnesota’s annual residential and commercial energy use. Furthermore, 26% of the savings potential, 3.4 trillion Btu, can be achieved with retrofits that pay back in 15 years or less. While 15 years is a longer payback, it is cost effective due to a window’s 35 year average lifespan. In addition window retrofits have many non-energy benefits, including the reductions in ultraviolet radiation, noise, drafts off of existing windows, and infiltration.

To determine the savings potential for the five retrofit technologies studied, climate-specific energy modeling was used to simulate the energy impact of the retrofits on residential houses, and commercial buildings. Over 4,000 total simulations, on 16 different reference buildings were performed in order to evaluate the general impact of changing the insulation level of the window, climate zone, and building type on total building energy. The climates of both southern and northern Minnesota were modeled separately.

The average saves for the five modeled technologies, weighted by the window area of all building types in Minnesota, were best for the low-e window panels, 39 kbtu/sq ft, followed closely by the clear panels, 31 kbtu/sq ft (Table 1). The placement of the panels (inside or outside of the existing window) did not affect the savings potential, nor did choice of glazing (glass or plastic). Window panels were most effective at saving energy because they add an extra layer of air to the window assembly. 

Table 1:

Savings potential for five window retrofit technologies studied

Technology	Savings Potential (kBtu/ft2)
Window panels with a low-e coating	39
Clear window panels	31
Moderate solar heat gain (0.46), low-e applied film	13
Low solar heat gain (0.30), low-e applied film	-7
Tinted window film	-13

The film with moderate solar heat gain provided savings in most buildings, with a weighted average savings of 13 kBtu/ft2. In contrast, the film with low solar heat gain coefficient (SHGC) led to an energy penalty in most cases, due to the reduction in passive solar heating during the winter months; the weighted average increase in energy use was 7 kBtu/ft2 of building window area.

Tinted films, without a low-e coating, were also evaluated as they are often promoted as energy saving products. While they are effective in hot, sunny climates where solar heating causes increased cooling loads, in Minnesota the tinted films block solar heating in winter, leading to increased heating requirements. Overall, a building with tinted film applied to all the windows used 13 kBtu/ft2 more than the same building with clear windows. They are not recommended as a window retrofit to save energy in Minnesota.

The cost of window retrofits is shown in Figure 1. Self-installation reduces the cost but is not available with all products (in particular the best performing window film can only by purchased from a professional installer). The cost of retrofits is about one-quarter of the cost of replacement windows, so they are a good short-term (10-20 years) option for homeowners wishing to save energy and improve comfort while deferring the cost and inconvenience of window replacement.

Figure 1:

Cost of Window Retrofits

The study found that because of do-it-yourself installation (i.e. no labor cost), the economics of retrofits were much better for residential houses than commercial buildings (where professional installation is required and generally doubles the cost). Climate also has a big impact on the overall savings with colder Minnesota regions showing higher savings. In Duluth, energy savings for houses are about 27% higher than in Minneapolis-St Paul.

The cost of a whole house retrofit ranges from $1,200 for self-installed clear window panels to $1,500 for professionally installed moderate SHGC window film to $2,500 for professionally installed low-e window panels. Table 2 shows that this leads to paybacks from 11 years to over 24 years with natural gas heat, and as short as 4 years with electric resistance heating or delivered fuels.

Table 2:

Payback of Residential Window Retrofit Project in Climate Zone 7 (from RESFEN)

Fuel	Market Cost	Unit Cost ($/100)	Annual Savings for a Zone 7 home with low-e window panel retrofit	Payback of $1,200 investment (years)	Payback of $2,500 investments (years)
Natural Gas	$0.85/therm	$ 0.85	$108	11	24
Electricity	$0.11/kWh	$ 3.25	$319	4	8
Propane	$2.62/gal	$ 2.85	$282	4	9
Fuel Oil	$3.83/gal	$ 2.76	$273	4	9

Three of the studied window retrofit products are good candidates for utility Conservation Improvement Programs, primarily in residential homes, because they will save energy for many years following their installation, without requiring any human interaction beyond installation. Current paybacks are long, and modest utility rebate programs are suggested primarily to inform consumers that these products are effective. Full details of the study are available in the final report, Window Retrofit Technologies: Increased Energy Efficiency without Replacement.