We want Rose Home to be as eco-friendly as possible, which means taking advantage of passive solar heating. In late fall through spring, the months when we welcome extra warmth from the sun, a good passive solar design allows as much light as possible to enter the home. During the summer, however, we want to limit the amount of direct solar radiation in order to reduce energy spent on mechanical cooling. One strategy to reconcile these needs is to include roof overhangs (eaves) that block the high summer sun but that allow the winter sun, which remains low in the sky, to warm the home. Overhangs are more effective than blinds or curtains because they block solar energy before it transfers into the building.
Window Orientation Matters for Comfort & Energy Usage
While overhangs block the hot summer sun when it is overhead, they do have limits. For instance, they aren’t good at blocking unwanted sunlight that comes in from the east or west. Because the summer sun is low in the sky early in the morning and in the evening, it can shine directly into any east- and west-facing windows, causing unpleasant glare and overheating. West-facing windows in particular are an energy liability in summer because the extra solar gain occurs in the late afternoon when it is already the hottest part of the day. In winter, east- and west-facing windows aren’t helpful either: because winter sunlight is weak except around mid-day, windows on the east and west sides of a building fail to gather heat when it is needed during the coldest months.
Windows oriented to the north lose the most heat in winter: sunlight does not shine directly on northern exposures and cold winds come from this direction. Sunlight from the north tends to be diffuse and bluer in color, but at least it is consistent throughout the day. As a result, rooms on this side of the home are less prone to the temperature swings that occur with windows to the east and west. Even so, it is generally recommended to have fewer windows on the north sides of the home due to energy losses and the dimmer lighting.
Moral of the story: limiting windows to the east and west improves summer comfort, and limiting windows to the north improves winter comfort.
Soaking up the Sun: Applying Passive Solar to Our Home!
There are several reasons why we decided to prioritize the home’s ability to capture solar heat.
- Where we live, heating needs are much more dominant than cooling needs. Well-shaded homes might need air conditioning for only a few weeks in summer, while the heating season can last over half the year!
- It’s easier and cheaper to cool a room by 10-15 degrees than it is to heat that room to a comfortable level in winter when there is a greater temperature difference between inside and outside.
- Because the house is on wheels, we can always move it into the shade during summer.
Despite our needs for solar heat gain during most of the year, we didn’t want to sacrifice summer comfort too much, especially since the heat could easily cook us out of our bedroom loft! So to maximize year-round comfort, we optimized our design to match the movement of the sun with our needs in different areas of the home throughout the day.
Optimizing Solar Orientation in our Floorplan
Because the house is mobile, its solar orientation is not fixed; however, we wanted to design the longest and tallest side to ideally face south wherever we situate the home. Of all the glass on our house, a whopping half is located on this “solar wall” (even the door will be mostly glass). In winter, the sun will shine directly on these south-facing windows for most of the day, allowing the home to soak up the available heat. In summer, the eaves will block the sun and keep us cool. Throughout the year, these windows will allow warm, uniform sunlight to enter evenly throughout the upstairs and downstairs of the home.
While windows on the south side are ideal, it’s important not to go overboard – large expanses of south-facing glass can overheat the home even in winter, and they lose heat quickly at night. In other words, because glass is a poor insulator, having too many windows (especially windows facing north, east, or west) will increase energy bills. At the same time, we need windows on all sides of the home for balanced light and views. Thus, the trick is designing the home so that the light entering supports the function of each room. For instance, if you watch TV in the family room most evenings, then it would be wise to limit the number of west-facing windows in that room because the sunset glare would interfere with your routine of watching TV. Similarly, if you are an early morning person, lots of east-facing windows in the bedroom and kitchen would suit your needs well (but would be less-appreciated by someone who likes to sleep in).
The challenge is determining the optimal proportion of glass for each side of the house and ensuring that the light from each orientation is best suited to the room and the time of day when that area is used.
Windows to the East, West, and North
The short sides of the house will face east and west. These walls contain enough glass to provide balanced daylighting while keeping energy losses to a minimum. We designated a small window on “east” side of the home, which happens to contain the bathroom. Because the bathroom is highly utilized in the morning, we will appreciate direct light from the east in this area. We did not include an east-facing window in the bedroom, for good reason: we don’t want to be woken up by the sun blazing in! And in summer, the intense morning rays could cause us to wake up hot and sweaty.
On the “west” side of the home (tongue end of the trailer), we have a moderately-sized window in the lounge loft as well as one in our office/kitchen area (a total of ~15% of the home’s glass). The windows are ideally located for the time of day that we intend to these particular areas of the home. Having a west-facing window in the loft will make it a warm cozy place to relax in the later part of the day after we’re done with work. Having a west-facing window in our office nook downstairs is also ideal because it will provide soft light during the morning when we are on the computer (and don’t want direct sunshine coming in). But in the evening, when we’re cooking in the kitchen, we’ll benefit from the rich light and sunset views.
The remaining third of the home’s glass is located on the long back wall, which will ideally be situated to the north. These windows will temper the strong light from the southern exposure and provide views from the kitchen and dining areas, which are used throughout the day. We have a long skinny north-facing window that will provide light without excessive heat to any plants we wish to place atop our cabinetry.
The Take-Away from this Post. Building an energy-efficient home requires attention to the sun’s predictable movement across the sky. Windows oriented south are the most desirable: they provide high-quality lighting, are easy to shade in summer, and will help heat the home on clear winter days. Optimal comfort and daylighting can be achieved by planning from the start so that windows will receive direct sunlight when and where you want it. The intentional solar design is just one more reason why we are so excited to be building our home from the trailer up!