How To

Reduce Heating & Cooling Costs

 

On beautiful days your open windows bring gentle breezes, sights, and sounds into your home. But on harsh days with blazing heat or bone-chilling cold you want protection, and a closed window can’t provide that alone. Adding energy-efficient window treatments can be the most effective investment you make to control your utility bill. And while you’re doing what’s practical, Hunter Douglas can help you do what’s stylish, adding long-lasting value to your home.

40% of the heat that escapes from your home leaves through the windows. And 50% of the solar heat that enters a room comes in through—you guessed it—the windows. Obviously, energy efficient treatments can translate into considerable utility bill savings, especially if your windows are on a cold north exposure in the winter or a sun-saturated west wall in the summer.

Understanding R-value

Building materials are assigned an R-value according to the product’s ability to resist heat movement. The higher the R-value the better it insulates your home. Most windows have an R-value of 0.9 to 3.0 and can be responsible for 40 to 70 percent of heat or cold transfer for an entire home. The heat loss or gain at your home’s windows is impacted by the following:

1. Window properties. Window frames are typically metal, wood or vinyl. The R-value of a metal frame can be 5 to 20 percent lower than a wood or vinyl frame window. A single layer of uncovered window glass has an R- value of 1.16. Advances in glass coatings and assembly methods are improving the energy performance of new windows, but a window’s age, quality of construction, and the condition of sashes, weather stripping and caulking will impact how much air infiltrates or escapes your home.

2. Window square footage. Windows make up 20 percent of the typical home exterior and 40 percent in contemporary construction. Numerous large, uncovered windows can break your energy budget when they’re on a cold north exposure in winter or a sun-saturated west wall in summer.

3. Window coverings. Heat moves towards cold. In the winter it’s drawn to cold glass toescape your home (see illustration above). In the summer outdoor heat moves toward the windows of your air-conditioned home. The white exterior backing on all HunterDouglas proprietary products reflects the sun. Using layers and cellular construction treatments at your windows can increase R-value by1 to nearly 5 points. In extreme climates, that could save more dollars than you would spend to install windows with a higher energy efficiency.

All window coverings help defend your home against summer heat and winter cold. The design, materials and construction of the window coverings discussed here enable them to make a more significant contribution to energy efficiency.

Show Me the Savings

Energy efficient window treatments help you to:

Cut heat loss in winter and increase your home’s cozy comfort as they slow the escape of warm air. Window treatments act as insulation, covering glass to keep warm air inside (see illustration at left).

Cut infiltration of solar heat, the single largest contributor to the workload of your home’s cooling system.

Save money on utilities and save the natural resources used to generate energy.

Save your interior fabrics, furnishings and woods from fading and deterioration.

Energy-efficient window treatments translate into
year-round savings on heating and cooling costs.

The more heat loss and heat gain your window treatments prevent, the more money will be saved in keeping rooms warm in winter and cool in summer. Simple numeric values attributed to product and fabric type will help you gauge the energy efficiency of any Hunter Douglas window fashion.

Winter Comfort: R-Value

A product’s “R-value” is a measurement of its ability to resist heat flow. In a home during winter, R-values indicate the ability of walls, roofs and windows to keep heat inside. The higher its R-value, the more insulation a product provides (i.e., higher is better).

The walls and roof of a well-constructed home typically have an average R-value of 25. The low-e double-glazed windows used in such a home can have an R-value of 3.5. These windows would allow about 7 times more energy to escape per square foot than the rest of the home’s exterior. With windows making up 20% of the exterior surface, this calculates to about 64% of the home’s heat loss — and 64% of its heating energy consumption. Windows truly are energy “holes.”

This is where Hunter Douglas window fashions can have a very positive impact. For example, fully-recessed inside-mounted Duette® Architella® 1 1/4″ opaque shades add 3.56 to the R-value of low-e windows. The shades double the energy efficiency of the R-3.5 windows and reduce heat loss through them by about 55%. This in turn reduces overall home heat loss by approximately 35% — with corresponding decreases in heating energy usage and costs.

2 sets of R-values are shown in the chart below. The first, under the heading “Low-E Double Glazing With Product,” is the combined measurement of R-3.5 low-e double glazing plus the window covering. The second is the contribution of the window covering to the combined R-value, shown under the heading “Amount Added By Product.” This makes product comparisons very easy. 

 Summer Comfort: Solar Heat Gain Coefficient

A product’s “solar heat gain coefficient” (SHGC) is a measurement of its ability to reduce heat gain from the sun. When used with windows, it is the fraction of solar energy striking a window that actually passes through it to heat the room. SHGC is expressed as a number between 0 (none) and 1 (all). The lower a window’s solar heat gain coefficient, the less solar energy it transmits (i.e., lower is better).

The SHGC of a standard double-glazed window is 0.76, meaning 76% of the incident solar energy is transmitted through it. If we add the same Duette Architella shade from the R-value example, the solar heat gain coefficient drops to 0.15 — only 15% of the incident solar energy enters the room to heat it. This greatly reduces the cooling load, saving energy and money.

Why are we not using R-value (or its inverse, the U-factor) as a measure of energy efficiency in summer? R-value relates to the heat flow into the home due to the inside/outside temperature differential. On a 90° day with an inside temperature of 72°, this heat gain is about 5 BTU per square foot of glazing. Depending on the angle of the sun, solar heat gain could be as much as 180 BTU per square foot — 36 times greater! SHGC is clearly the more important measurement in summer.

The SHGC of Hunter Douglas products is shown as measured with standard double glazing, which has an SHGC of 0.76. The SHGC of low-e double glazing varies widely, from 0.70 (high gain) to 0.30 (low gain), making product comparisons less meaningful.