Passive Heating

Passive Heating

• Uses the energy from the sun to keep occupants comfortable without the use of mechanical systems

Passive Heating

• Direct Solar Gain• Trombe Wall and Attached Sunspace

Direct Solar Gain

• Direct gain is the heat from the sun being collected and contained in an occupied space

• Simplest and least costly way of passively heating a building with the sun

• Avoided in hot sunny climates

Direct Solar Gain

• Retained by the building’s thermal mass• Avoided with reflective materials

Direct Solar Gain

• Typically more is desired in the winter and less/none is desired in the summer

• More desirable in the morning and less/none in the late afternoon

Glazing for Solar Gain

• Area/placement are extremely important• Typically for most latitudes it is optimized by

large areas of equator-facing glazing– Major living spaces are exposed directly behind

this• Several glazing types may be appropriate on

different faces

Avoiding Losses

• Large areas of glazing will readily let heat out at night

• Escapes through direct heat transfer, or long wave radiation passes through the glazing as interior materials re-radiate their heat

• Provide some for of nighttime blockage to minimize conduction/convection losses

Surface Color and Cool Roofs

• Light colored surfaces bounce light around within the space, distributing it over a number of surfaces

• Dark colored materials absorb most of the incident energy as soon as it strikes– Both can be useful depending

• Surface of a dark roof can easily get 75° hotter than the surface of a white roof

Surface Color/Cool Roof

• One common measure of the materials ability to reflect sunlight is called albedo– Light colored roofs have “high albedo”

• The most common measurement is Solar Reflectance Index (SRI), takes into account reflectance and emissivity– 0 is the least heat reflective standard black paint– 100 is the most heat reflective standard white paint

• Cool Roof has an SRI above 78 for flat roofs and 29 for steep roofs

• Emissivity is the ability of the roof surface to radiate absorbed heat. Measured on a scale of 0 to 1, where 1 is the most emissive (doesn’t retain as much heat). Low emissivity may benefit buildings in colder climates by retaining heat and reducing the heating load. To prevent the heat island effect, high emissivity is preferable.

SRI Values

Trombe Wall and Attached Sunspace

• Trombe wall – system for indirect solar heat gain– Not extremely common but good for thermal

mass, solar gain, glazing properties– Consists of a dark colored wall with high thermal

mass that faces the sun– Has glazing in the front to leave a small air space– Glazing traps solar radiation like a small

greenhouse

Trombe Wall and Attached Sunspace

Trombe Walls

• Thermal storage walls• Named after French inventor Felix Trombe• Typically 8-16” thick masonry wall painted in a

dark heat absorbing color (can take up to 10 hrs to heat)

• Faced with a single or double layer of glass– Spaced 1-6” away from the wall to create a small

airspace– Prevents the escape of radiant heat from the wall– This further heats the wall

Trombe Walls

• Can also set it up to heat internal air

Avoiding Losses

• Use a low-E glazing to prevent heat from re-radiating out through the glass

Adapting to Day & Season

• Ideally the glazing will have exterior insulating shutters for nighttime use

Sunspaces

• Also called conservatries• Heat spaces through radiation and convection• Difference with the Trombe wall is this is

habitable space• Primarily used for indirect solar heat gain• Typically have more glazing area than floor

area• Consider operable windows at the top and

bottom

Sunspaces

Solar Chimney

• Passive ventilation