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Tuesday, August 27, 2013

How It Works: Passive Cooling

System Purpose.  Provide adequate cooling with minimum power

How It Works Summary.  Keep outside heat outside.  Keep sunlight outside.  Move inside heat to outside.   Minimize inside heat generation.   Move air inside house for evaporative cooling.

System Cost.  $200 for fans.    All other components are shared with other systems.   For example, wall insulation part of heating system.   

System Components

Keep outside heat outside.
Double-stud wall  with sandwiched R40 insulation.  R60 ceiling insulation.  R5 windows.
Keep windows closed while outside temperature is greater than inside.

Keep sunlight outside.
Majority of windows face south enabling effective blockage of sunlight by 2.5 foot overhang May through mid-August (while allowing passive solar heating in winter).    Exterior shade cloth blocks sunlight late August through early September. 
Porch over large east facing window blocks sunlight.
Two small windows face west and one east where overhangs are ineffective.  Interior shades provide some sunlight blockage.
Steel roof reflects sunlight away from attic.

Move inside heat to outside
When outside temperature falls below inside (night), heat rises up stairway to second floor where window fan exhausts heat outside and is displaced by cooler air entering open first floor windows.  This is ineffective on nights above 75F.    Most challenging for us is a series of such nights. 
Concrete slab-on-grade conducts heat into ground.    This is only partially effective since our slab is insulated to retain heat in winter.   Waiting for invention of “adjustable slab insulation”.

Minimize inside heat generation
Remember, inefficient is just another word for heater.  A 60% efficient refrigerator uses 40% of its electricity to heat your home, so you not only pay for electricity you don’t use, but you pay to get rid of the heat it creates.    
All our lights and appliances are very efficient and as such produce very little heat.  LED lights.  Sunfrost fridge in kitchen.   SunDanzer fridge and freezer in cellar.  Total simultaneous electrical draw for lights, freezer and 2 fridges averages 100 watts. 

Move inside air for evaporative cooling, i.e. use fans to wick sweat.

System Maintenance.  Open windows at night (if cooler outside).  Close windows in morning (when warmer outside).   Turn on 2nd floor window fan, if needed.  Hang shade cloth over south facing windows in late August.  Remove in mid-September (or whenever the summer heat finally breaks).   

Report Card.   B    (See blog post for details).

Biggest Challenge.  Humidity.  Since passive cooling reduces temperature without removing moisture, the relative humidity actually rises inside the house.  Interior doors may swell and not close. 

Biggest success factor.  Summer attitude.   While 80F can feel hot (we hit 86F during last summer’s prolonged heat wave), we get used to it by enjoying ourselves outside on the porch.     A cool beverage really helps. 

The Connection.    We wanted a house that varied with the seasons, that says “yes, we are part of this ever-changing Land”.   


  1. Love the post! How did you end up mounting your shade cloth to the bottom of the windows? Seems like it would work better there than at the top like you originally tried (

    1. Thanks Greg. I hung the shade cloth on small screws attached the trim boards between the windows. Hanging it off the gutter actually blocked more sun, but the wind kept blowing the cloth on top of the roof :(

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