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Some facts about the Residential Energy Laboratory:    
  • Standard production home (NOT a custom home) with a few modifications on a small urban lot. 
  • Modest size of 1602 ft2, based on outside dimensions, with single floor living area, and conditioned crawl space
  • Wall construction type can generally be described as having 2x6 wood stud walls 24" on-center, filled with wet cellulose and then covered with 2" thick rigid foam.  This construction approach for walls was compared by Building Science Corp. with other advanced wall construction types, and this approach was tied with two other approaches for the best wall design based on five, equally rated criteria: thermal control, durability, build-ability, cost, and material use (Building Science Wall Discussion).

Building Envelope

  • Loose cellulose (density spec. of 2.12 lb/ft3) blown over attic floor, with thickness of about 19" giving RUS* = 60 h-ft2-F/Btu (same units used for all R values in the section, but noted simply as "RUS").   Since the thermal  conductance U=1/R, then UUS = 0.0167 btu /h-ft2-F.  Raised heel trusses used for full height insulation near edge of roof.  Ventilation baffles used for soffit ventilation with deep insulation levels. (Insulation not at full depth in picture below, but ventilation baffles and raised heel visible.)
Attic insulation, baffles, and heel
  • 2x6 wood stud wall 24" on-center covered with OSB (oriented strand board) sealed completely on sides, top. and bottom of every stud cavity and all penetrations with expanding foam, and filled to 5.5" thickness with sprayed-in wet cellulose (nominal density = 3.08 lb/ft3).  Then the OSB is covered on the outside with 2" XPS (extruded polystyrene Styrofoam) rigid foam, then Tyvek air barrier, and then concrete-fiber siding.  Interior wall height is 8', and walls are covered in Sheetrock.  This construction gives a wall with a thermal resistance value (including Sheetrock, sheathing, siding, and short-circuiting by studs) of about RUS = 27.9, or thermal conductance of UUS = 0.0359, assuming a framing factor (fraction of wall area filled with studs, excluding open areas) of 19%

Sealing around stud box including bottom plate   Sealed pentrations through top plate   Stud cavity insulated with wet cellulose
  • Infiltration reduction boxes around all electrical boxes on exterior walls, filled with fiberglass insulation after sealing electrical wiring feedthroughs
                                Infiltration reduction electrical box                           Closeup of electrical boxes for reducting infiltration    
  • Conditioned crawl space with 1.5" to 2" of closed cell foam sprayed on inside of the rim joists that are about 12" high to provide an air barrier and insulation, covered with about 6" thick RUS = 19 fiberglass insulation to the inside of the closed cell foam, plus 2" of XPS rigid foam (RUS = 10) on the outside of the rim joists, for a total thermal resistance of about RSI = 34, or UUS = 0.029
  • 3' high crawl space walls below the rim joists insulated on the inside with RUS = 19 fiberglass bats (continuous), and extending out on crawl space floor 1'.  Crawl space walls extend 6" above ground level on the outside, with the remainder 2.5' below ground.  The use of fiberglass batts on crawl space walls can lead to moisture problems, and is not recommended by Building Science Corp., Building Science report on high-R value foundations although the REL is located in a high mountain desert with very low humidities.         
  • 2" thick x 8" high XPS rigid foam on inside footings with RUS = 10.  Gravel was filled in for the crawl space floor to the top of the rigid foam.  The RUS = 19 fiberglass bats mentioned above overlap this rigid foam since the batts extend out onto the crawl space floor. 
Rigid Foam Insulation along Footers
  • 1" thick XPS rigid foam (RUS = 5) between ground and gravel (8" of coarse, round gravel) that makes up crawl space floor .  Fiber-reinforced 12-mil thick plastic covers the gravel.  Since the gravel is insulated from the ground by the foam, and since the HRV pulls air from the first floor of the house into the crawl space, the gravel becomes a significant contributor to the thermal storage for the passive solar heat, weighing about 109,000 lbs., or 55 tons.   (Pictures below show the rigid foam before gravel was added on top.)  
                   Rigid Foam Insulation under Crawl SpaceRigid Foam under Crawl Space wide view  

Windows and Shades
  • Triple-pane, low-emissivity windows with high solar gain (SHGC = 0.49, with thermal conductivity UUS = 0.31) on south, west, and some of the east side, and low solar gain (SHGC = 0.28, with UUS = 0.29) on north and some of east side.  These thermal conductivities correspond to thermal resistance values for the high solar gain windows of RUS = 3.2, and for the low solar gain of RUS = 3.4.
  • Double-cell cellular shades (most "room darkening" with foil liners) with side seals for all windows, and all are closed at night, with thermal resistance measured to be about RUS = 2.0 for the room darkening shades and RUS = 1.2 for the light-filtering shades.  Thus, the combined window and shade R-value (night-time only) for the high solar gain windows and room darkening shades is RUS = 5.2, and for the low solar gain windows is RUS = 5.4.  The air flow sealing at the sides of the shades is accomplished using a labyrinth-type seal as shown in the left hand figure below.  This image is looking almost vertically upward from the bottom of the window.  An end view of the penetrating piece that has been removed from the side of the window is shown in the right hand figure below.  The Z-shaped piece of flexible plastic seals against the inner set of cells in the cellular shade.   
Labyrinth Seal on the Sides of the Cellular ShadesPiece that Penetrates into the Side of the Shade to Effect the Seal

Exterior Doors
  • Two doors to the outside, with storm doors, one combination estimated to be USI = 0.35 (RUS = 2.9), and the other to be USI = 0.30 (RUS = 3.3).  
HVAC (heating, ventilation, air conditioning)
  • High-efficiency (97.5%), natural gas fired, modulating, condensing furnace with ECM fan motor.  The maximum firing rate is 60,000 Btu/h.  "Modulating" means that the furnace first fires at 70% of the full rate for 30 to 45 s.  Then it reduces to the minimum firing rate of 35%.  The firing rate is automatically adjusted to meet demand, increasing gradually to maximum firing rate (100%) if the thermostat is not satisfied within a defined time.  
  • Heat recovery ventilation (HRV) system (to recover thermal energy from exhaust air and transfer it to incoming fresh air) rated at about 75% heat recovery, and using about 26 W continuously on low speed, and 70 W continuously on high speed with ECM fan motors
  • Natural-gas fired, 20,000 Btu/h input, direct-vent fireplace.  "Direct-vent" means that the combustion system is sealed, with combustion air coming from the outside, and combustion products vented to the outside.  
  • Motion sensor bathroom fans with ECM fan motors
  • Kitchen vent fan with ECM motor 
  • Energy Star rated ceiling fans throughout, with no air conditioning 
Solar Energy Systems
  • Yearly average total solar radiation on a flat plate collector aimed due south and tilted at an angle equal to the latitude is 1.87 kBtu/ft2/day (5.9 kWh/m2/day in the more commonly used metric units) at this location, so plentiful solar energy to work with
  • 3.15-kW (DC rating) photovoltaic solar system tied to grid with net use metering.  System includes 14 panels rated at 225 W each.  Mounted on garage/workshop at 26.6 tilt angle from horizontal, and 22 east of due south.  DC to AC conversion specified by manufacturer to be about 83.5%.  Detailed performance documented at Energy Use Details.  
Solar Photovoltaic System
  • Small solar hot water preheater with 25 ft2 absorber area mounted at 36.9 tilt angle from horizontal compared to latitude of 38.6, 50-gallon stainless steel solar storage tank, followed by natural-gas fired tankless water heater with variable input firing range from 11,000 to 199,000 Btu/h..  
      Solar Hot Water Collection Panel     Solar Hot Water Tank and Pump      Tankless Water Heater
  • Passive solar heating using high solar gain windows on south, west, and east sides contributes about 40% of the heating energy requirement.  A roof overhang of 24" on south side limits solar gain during warm weather.  Gravel that makes up the floor of the crawl space, and which is insulated from the ground, may contribute to the thermal storage of solar heat, and weighs about 109,000 lbs. or 55 tons. (Pictures below show south side.)
Passive Solar Heating
Roof Overhang

Appliances and Lighting
  • High-efficiency refrigerator (343 kWh/yr. is official spec., and measured value is same within measurement uncertainty)
  • High-efficiency, front-loading, clothes washing machine (due to low water use)  
  • Electric clothes dryer with exhaust from center of house through conditioned crawl space, so some heat recovery during cooling season
  • Fluorescent lights throughout
  • Low-flow shower heads
  • All drains are through conditioned crawl space, so some thermal energy recovery before water leaves house
Infiltration Measurements
  • Blower door test used to measure air infiltration into house (before addition of storm doors) with negative pressure differential of 0.2" of water, with resulting flow of 850 CFM corresponding to 2.45 air changes per hour (ACH 0.2" or more commonly specified as ACH50Pa which is equivalent) that corresponds to a "natural" ventilation rate of about 0.15 ACH
Other "Green" Features

There are various other features about the house in keeping with an eco-friendly design, but these are outside the scope of this discussion.  

*RUS = h -ft2-F/Btu
UUS = Btu/h -ft2-F


House Orientation - The house was oriented with the long dimension mostly north-south, but with the southern exposure 22 east of south.  This orientation was chosen not as an optimum orientation, but rather due to the street layout in the subdivision, and the standard floor plans that have the long dimension of the houses running at right angles to the street.  The side of the street was chosen so that the solar panels could be mounted on the back of the house and garage, and would be hidden from the street view since there were no other solar panels in the neighborhood at the time of construction.  Also the porch on the front of the house would block much of the winter sun if the house had the front facing south.  By serendipity, solar PV panels operate at best efficiency at this location when pointed 11 east of south according to the computer model PVWatts v2.0, and the orientation 22 east of south is predicted to provide 0.2% better power than a due south orientation!  The effects of the tilt and azimuthal angles on collection efficiency are discussed in more detail in the section REL photovoltaic system.   

Ceiling Height - The ceilings are 9 ft. high throughout the house, with no cathedral or raised ceilings.  

Summer Cooling - The house is located in a high mountain valley in Colorado at about 7100' elevation.  It has Energy Star rated ceiling fans in every room, but no air conditioning.  July has the highest average temperatures, with an average daily high of 83F, and an average daily low of 48F.  By opening the windows at night and the closing the windows in the morning, cool air is sealed inside the house, and the house remains comfortable during the day.  The highest temperature inside the house observed during the past summer was about 76F or 77F. 

Window Details - If calculations are performed for heat loss for this house, it is necessary to provide some details for the windows.  There are 15 windows in the house, with 14 being double-hung and one being fixed.  Double-hung windows are, next to sliders, the worst design for air infiltration.  However, the houses in this neighborhood are required to have single-hung or double-hung windows, and the windows chosen for this house were not available as single-hung windows.  Since the house is not air conditioned, operable windows are necessary for comfort, and, of course, fire codes require some operable windows in a house.

Thirteen of the windows have rough openings of 41" x 59", and all of these are filled with double-hung, triple-pane windows.  There is also a double-hung, triple-pane window with a rough opening of 36" x 36". Finally, there is a fixed triple-pane window in a rough opening 48" x 48".  There are four of the 
41" x 59" windows on the north side, and all are low solar gain (SHGC = 0.28, with UUS = 0.29).  Three more of these 41" x 59" windows are located on the east side, two with low SHGC, and one with high solar gain (SHGC = 0.49, UUS = 0.31).  The one 36" x 36" window is also on the east side, and it is high solar gain.  There are four of the 41: x 59" windows on the south side, with three shaded by the 24" roof overhang, while one is on a porch that, combined with the roof overhang, gives a total overhang of 8.6'.  (One of these was an "extra" window beyond the standard floor plan added to take advantage of the high solar insolation available in this area.)  On the south side, there is also a "full-lite" door with double-pane windows with the glass measuring 21" wide and 5.24' tall, and a full-lite storm storm with a single-pane glass window that overlaps the window in the main door.  The SHGC for the combination is estimated to be 0.25, and USI is estimated at 0.30.  Finally on the west side are two of the 41" x 59" windows, both with high solar gain, and the 48" x 48" fixed window with high solar gain.  

All of these windows are fitted on the inside with double-cell cellular shades that have side seals.  These shades are closed at night, and opened during the day, except during the summer the shades are used to block direct solar radiation that would overheat the house.  

Solar Domestic Hot Water System - Using a tankless water heater downstream of the solar storage tank allows the solar hot water pumps to operate any time that the solar thermal panels are significantly hotter than the water in the storage tank.  If the solar storage tank were heated using auxiliary heat to a delivery temperature of approximately 125F, then the solar hot water pumps would only run when the solar thermal panels were in excess of the 125F.  Using the separate solar storage tank upstream of the tankless heater allows the solar collectors to operate more often and at higher efficiency, since the collection efficiency drops with increasing water-inlet temperatures.  A disadvantage of this approach is that at low hot water flows, the tankless water heater apparently cannot run at low enough heat input, so it does not turn on, and the delivered water is colder than desired.