Shutter BoysUpdated
The short version: windows lose a lot of a home’s heat, and anything that covers them helps a little. But nobody has yet published a rated energy figure for louvered interior shutters, ours or anyone’s. If someone quotes you an R-value or a percentage saving for plantation shutters, ask where it came from.
Why windows matter
Cornell Cooperative Extension puts it simply: “In a typical single family home, 25% to 35% of the heat is lost through windows”[1]. It also notes that a double-glazed window loses about half the heat of a single-glazed one[1]. So the first question is always the glass: if yours is single-glazed or the seals have failed, no covering will make up for it.
What the research measured
One of the main public studies of window coverings is a 2013 report by Lawrence Berkeley National Laboratory for the US Department of Energy. It simulated homes in several climates with different “attachments”: blinds, cellular shades, interior panels, storm windows and more[2].
- Interior window panels (inserts that work like an inside storm window) and exterior storm panels “consistently show lowest energy use”[2].
- Cellular shades saved both heating and cooling energy in the better-quality versions modelled[2].
- Horizontal blinds “provide some savings in cooling energy for more reflective materials”[2].
Louvered shutters were left out. In the report’s words, “Several categories of window attachments were not included in this study due to the current lack of reliable simulation models”, and louvered shutters are on that list[2]. The Department of Energy’s window attachments program has since listed louvered shutters for a later phase of its rating work, after cellular shades, films, and exterior products such as awnings[3].
Insulating shutters are a different product
Cornell’s guide gives insulating shutters a typical R-value of 4, and says they “may be as high as R-10”[1]. Those are solid panels, often with a foam core, that close tightly against the window frame. A louvered shutter is built to let air and light through when it’s open, and even closed it has gaps between louvers and around each panel. Treat any R-value borrowed from insulating shutters as not applying.
What a louvered shutter actually does
This section is our reasoning, not a measured result.
- Blocks direct sun before it reaches you. Closed or tilted louvers stop the patch of sunlight on the floor and the sofa, which is most of what makes a sunny room feel hot. Sun that passes the glass has already brought some heat inside, which is why exterior shading does more than any interior covering.
- Holds a layer of still air. Closed louvers slow the air moving across cold glass in winter, the draft you feel near a big window even when it’s sealed. A tight inside mount helps more than an outside mount with gaps at the edges.
- Lets you steer the light. Most shades trade light for shade in one movement: lower them and the room gets darker. Louvers can tilt up to throw daylight at the ceiling while blocking the view of the sky and the hottest part of the sun, so the room stays bright without the glare.
- Wood, not plastic. Wood is a poor conductor of heat, so the room side of a closed wood shutter stays close to room temperature. It doesn’t make the shutter an insulator; it does mean it won’t feel cold to the touch.
How to use them to save energy
- Summer: close or tilt the louvers on sun-facing windows before the sun reaches them, not after the room is hot. West windows matter most in the late afternoon.
- Winter days: open south-facing windows fully when the sun is out; low winter sun is free heat.
- Winter nights: close everything, especially single-glazed and north-facing windows.
- Mount tight: an inside mount with an accurate measurement leaves the smallest gaps. How to measure
Each of our city pages shows that city’s daylight and how high the sun gets at noon in summer and winter, which tells you how much of this applies where you live.
If energy is your main reason
Be honest with yourself about why you’re buying. If the goal is the lowest heating bill, the published evidence points to storm windows or interior panels first, and cellular shades next[2]. Shutters earn their place on other grounds: light control, privacy, no cords, and a covering made of wood that you buy once. Some people pair them: a cellular shade or storm panel for the numbers, a shutter for the room.
Common questions
Do plantation shutters keep heat out in summer?
Closed louvers stop direct sun reaching the room, which cuts glare and the heat you feel near the window. Because they sit inside the glass, some of the sun’s heat is already indoors by the time it hits them, so they do less than an exterior awning or a reflective film. No rating body has published a figure for louvered interior shutters yet.
Are shutters better than cellular shades for energy?
On the published evidence, cellular shades have the stronger case: the Lawrence Berkeley National Laboratory study modelled them and found heating and cooling savings, while louvered shutters were left out of the study for lack of a reliable model. If energy is your main reason for buying, that matters.
What R-value do shutters have?
The R-values you see quoted, up to about R-10, come from insulating shutters: solid panels with an insulating core that seal against the frame. Louvered shutters have gaps between the louvers and around the panels, so those figures don’t apply to them.
Sources
- Energy saving window treatments (rev. 2000), Cornell Cooperative Extension (university extension)
- Energy savings from window attachments (October 2013), Lawrence Berkeley National Laboratory, for the US Department of Energy (government)
- Window attachments (Building Technologies Office), US Department of Energy (government)