House Construction Details
Net Energy Use
Energy Use Details
Costs and Payback for Net-Zero
Infrared Images of REL
Energy Efficient Design
Comparison of PV Systems
R-Value of Cellular Shades
Energy Codes for Windows
Solar PV Raw Data
REL CONSTRUCTION DETAILS
American (English) UnitsSome 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
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).
- 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.)
- 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%
reduction boxes around all electrical boxes on exterior walls, filled
with fiberglass insulation after sealing electrical wiring feedthroughs
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.
- 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
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
(Pictures below show the rigid foam before gravel was added on top.)
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.
HVAC (heating, ventilation, air conditioning)Solar Energy Systems
- 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).
- 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
(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.
- 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
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
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.)
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)
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
Other "Green" Features
- 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
are various other features about the house in keeping with an
eco-friendly design, but these are outside the scope of this
*RUS = h -ft2-F/Btu
UUS = Btu/h -ft2-F
- 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
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.
- 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 83°F, and an average daily low of 48°F. 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 76°F or 77°F. 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.
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
the solar storage tank were heated using auxiliary heat to a
delivery temperature of approximately 125°F,
the solar hot water pumps would only run when the solar thermal panels
were in excess of the 125°F. 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.