A Heated Response

Thank you, MojoMan, for posing the following questions in a comment on my post entitled "Free Heat". I enjoy and appreciate the opportunity to write about sustainability.

How will you keep all that heat out in the summer?

• I have designed the addition with no glazing in the roof. The sun must come in through glazing on the south wall. The roof will be insulated with R-38 insulation.
  
• I've calculated the eaves (including guttering) to shade the entire wall at mid-day during the middle of summer but allow maximum sun in the winter. I live at approximately 35 degrees north latitude; calculating the sun's elevation and azimuth at any time of year is not difficult.
  
• The prevailing wind is from the southwest and I've designed a door on the west wall and vents in the roof. I should be able to passively vent the room without using electric fans.
  
• Some type of vine (pole beans, perhaps) planted in front of the glazing and encouraged to grow to the top of the guttering will provide shade. This will block morning and afternoon sun that will come in under the eaves. Another common option is shade cloth on the outside of the glazing but I prefer the vines.
  

This past summer I built a gable room over our deck on the north side of the house and built functional shutters on the east and west windows. These passive features kept the house cooler than the previous summer. I expect the addition of the solar room (aka greenhouse) to keep the existing house cooler because it will shade the south wall and the roof design will shade the existing roof.


How well is your existing home insulated?

• R19 walls - Net R13.7
  
• R30 ceiling - Net R25.5
  
• R11 floor - Net R9.9
  
• R1.75 windows
  
• R5 doors

As I designed the addition, I measured walls, windows, doors, etc and calculated net R-values. For example, the walls have R-19 insulation between studs set on 24 inch centers. The studs have an R-value of 6.88. Exterior siding, interior wall material and air films have other R-values. I was pessimistic when making calculations and estimates to make sure I didn't under-size the solar room and solar closet. (Heres's one resource that lists the R-values of various materials: http://coloradoenergy.org/procorner/stuff/r-values.htm)

Just 'wondering': Is the solar closet really worth the space and cost? Could a well-insulated massive concrete slab with a dark finish do almost as much for less?

The short answer is that water filled barrels in a closet are less expensive and more effective than concrete. Also, the closet gives me other design options such as a solar hot water heater. The closet can get to a temperature of 130 degrees and I can vent the solar room to make it comfortable when I'm using it.

Here's a longer answer. I don't think there are rules that can be applied to all situations. What works for me -- given my existing house, elevation, latitude, weather patterns, trees, slope of land and other factors -- may not work for others. I want a greenhouse to raise plants for enjoyment, beauty and food. I want a solar room to enjoy the sun on cold windy winter days and to watch birds, rabbits, hares and other animals at the feeders. The addition easily lends itself to passive heating and cooling. The inclusion of a solar closet takes a reasonable amount of space at a minimal cost. The closet takes about 120 square feet and, since it's inside the solar room, is minimally two additional walls, barrels and water.

For a cost comparison let's assume a cubic yard of concrete cost's $65. A cubic yard of water contains slightly over 200 gallons and will fill four barrels at $14 each. 200 gallons of water costs $3 (one and one-half cents per gallon). Thus, I'm comparing $65 of concrete against $59 of water-filled barrels. Here's the catch -- water stores three times the heat of an equal volume of concrete and convection currents in water enables it to accept heat more quickly.

"Water is less expensive than masonry, and stores about three times more heat by volume, with a lower thermal resistance." (http://www.ece.villanova.edu/~nick/solar/solar.html)

I estimated the addition -- 480 square feet -- would cost me about $4,000, excluding interior furnishing. Thus far, I'm within that estimate. The addition of the solar closet is well under $1,000. Assuming it saves me $300 in propane each year, it should pay for itself in 3 or 4 years. Doing the labor myself and searching for the right materials at the right price has kept the cost down. For example, the barrels cost $14 each -- no tax --and are recycled soy sauce barrels. I've seen similar barrels advertised on the web for as high as $150! (I can't imagine who pays that much.) Also, the glazing is new double-paned tempered glass and cost $30 each. 167 square feet of glass cost $300.

This only scratches the surface of this issue. If you've read this far, I'm amazed!