Monday, July 15, 2019

Design - Mean Radiant Temperature and Our Project - Part I - An Outline

In a previous post I made the case for using mean radiant temperature the vehicle for understanding passive solar.  After defining the concept, I explained how it works for three types of structures:  (1) stick-built houses, (2) classic earth sheltered passive solar houses and (3) Annualized GeoSolar (AGS) houses like ours.  Now I would like to use a couple of posts to describe how we are controlling MRT to eliminate conventional heating and air conditioning.  This is an outline; the next post will fill in the details.


I.  THE CHALLENGE
Controlling mean radiant temperature by balancing solar gain, storage capacity and heat gain/loss through the building envelope.

II.  SUMMARY
A.  Our conditioning system is both winter centric and summer centric, hence "Annualized" GeoSolar
B.  Direct solar gain through south-facing windows during winter
C.  Isolated solar gain during the summer when days are longer and sun is more intense
D.  Huge soil-based thermal mass despite limited earth sheltering
E.  Year-round semi-constant comfort level without supplemental heat or HVAC


III. SOLAR GAIN
A.  Solar collector harvests heat from summer sun; conduits distribute it throughout thermal mass (primary source of heat)
B.  Windows collect heat from winter sun (secondary source)
C.  Windows: clear glass when thermal mass is adjacent; frosted glass when mass is distant 
D.  Solar overhangs shade windows in summer

IV.  STORAGE CAPACITY
A.. Control of MRT is assured by having abundant thermal mass
               1.  Concrete:  floor and tall north wall
               2.  Soil (with its moisture and temperature controlled):
                            a.  Beneath floor and immediately behind north wall
                            b.  Beneath R-10 to R-20 "insulation/watershed umbrella"
B.  Interior colors: absorptive when mass is adjacent; reflective when mass is distant 

V.  HEAT GAIN / LOSS
A.  Rice hull insulation at +/-R-4 per inch 
B.  Truss walls 15" thick with minimal thermal bridging at R-60
C.  Truss ceilings 18" thick with minimal thermal bridging at R-73
D.  Swing type fiberglass windows (R-4) inset 10" from exterior wall surface to minimize  wind washing and excessive solar gain in Spring and Fall
E.  Fastidious air sealing of entire exterior envelope, inside and out
F.   Energy recovery ventilator for indoor air quality
G.  Cool roof design:  ventilated space between truss sheathing and roofing sheathing
H.  Frost-protected shallow foundation using insulated concrete forms
I.  Highly reflective steel roofing and siding
J.  House and garage that shield main entry from north and west winter winds
K.  Air lock inside main entry to modulate incoming hot and cold outside air

VI.  OUTCOME
Stable year-round MRT fluctuating between 72 and 78 degrees without supplemental heat or conventional HVAC


V.  PHOTO-VOLTAIC ARRAY
The smallest photo-voltaic array that is practical and net-metering to break even with the electric company; utility costs will be limited to natural gas for the gas range, tankless water heater and clothes dryer plus fees for municipal water and sewer service.

*     *     *     *     *     *     *     *
The next post, Part II, will expand on these topics.

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