Considering Water Concentration
May 31, 2008 at 5:39 pm | Posted in Design | Leave a commentTags: roof, valley, water concentration
One aspect of design that gets overlooked with amazing frequency is something I call water concentration. Water concentration is a representation of how much rain water will pour off the edge of a roof. The amazing part is the number of times designers will create a condition of great water concentration, then put a door under it. This makes it impossible to enter or exit the house without getting soaked. Sure you can put gutters on, but if the water concentration is too high, the gutters clog quickly or are overwhelmed.
To properly define water concentration, lets take a simple shed roof first. If the shed roof measures 10′ long horizontally, then the water concentration is 10ft. Basically, look at how many square feet of roof footprint (slope doesn’t count) empty through how many linear feet of eave edge. I’ve seen many cases where the combination of dormers and valleys create water concentrations of 300ft or more.
Another example is an L shaped house with two intersecting gable roofs. On the interior of the L, the roofs meet in a valley. Lets assume the two legs of the house are 24 feet wide, 12f t each side of the ridge (again measuring horizontally only). Clearly there is going to be quite a bit of rain water draining out of that valley. We can figure out how much by drawing an imaginary line from the bottom of the valley straight uphill up both adjoining roof planes. Then we calculate how much area is contained by these lines and the ridges, in this case 12ft x 12ft = 144 sqft. This drains off the roof at one point that we’re going to call 1 ft wide, the bottom of the valley. This gives a concentration of 144 sqft of roof area feeding 1ft of eave, 144sqft/1ft = 144 ft.
Not surprisingly, this is a bad place for a door. When rain is falling pretty hard, say 1 inch per hour, that means the amount of water gathered by the roof uphill of the valley will be 1in x 144ft. Convert this to gallons and you get almost 90 gal/hr or 1.5 gal/min. That is plenty of water to make for some real splashing below.
Great Ideas Done Badly – Passive Solar on the Wrong Site
May 31, 2008 at 2:34 pm | Posted in Design | Leave a commentTags: Design, Heating, passive solar, site selection, Solar
I heard about a new house being built in a northern US city that was going to be fully passive solar. Check the glossary page for the difference between active solar and passive solar. This is a great approach to limiting energy consumption because it eliminates one of the largest energy consumers in the house and replaces it with the sun. What could be better?
In this case, there is a real problem. The house is a 2 story cape cod style, probably about 20′ tall. Immediately to the East-Southeast of this small property is a building that is 40+ ft tall. The new house won’t see any meaningful sunlight at all until 10:00 or 11:00am. Additionally, the long dimension of the building is oriented almost north-south, meaning that the smallest side of the house will be exposed to the sun.
With any solar heating system, the larger your solar collector is and the smaller your surface area of the building is, the better off you are. Ideally, the largest wall you have faces south and the rest of the walls are as small as you can make them.
My fear for this home owner is that their new house will be yet another good idea, implemented badly and will be too hot in the summer and will cause the home owner to do something inefficient, like add portable electric heaters, in the winter. In the end, I’m really skeptical that a passive solar house can be built on this lot at all. And, even if it is possible, it will require the design to be so driven by the passive solar aspect that there won’t be any flexibility left to, say, make the house not ugly, or allow some flexibility in how the house is used.
Houses last a long time, or at least they should, and will be used in different ways over time. Starter houses get used as small family houses that become large families, and maybe get sold to a retired couple. By trying to do the right thing in the wrong way, this house may be doomed by not being flexible enough to meet the needs of the people who live in it.
I can certainly see how an active solar house can be done here, and if there is a backup heat source that only gets used for a couple weeks a year, the home owners will be happier, the house will be renovated less, and the energy savings are much more likely to be realized.
Unfortunately, too many people see energy efficient design as a religion, rather than a technical discipline.
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