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PASSIVE HOUSE AND PASSIVE SOLAR:
A COMPARISON OF TWO APPROACHES TO LOW-ENERGY HEATING
Matthew B. Hogan
Department of Architecture
University of Oregon
Eugene, OR 97403
Alison G. Kwok
Department of Architecture
University of Oregon
Eugene, OR 97403
ABSTRACT
This paper examines two approaches to low-energy heating:
passive solar heating and Passive House. The design of a
single family house, herein referred to as the ‘Learning
House,’ was conceived of as a Passive House. The
Learning House design was modeled in DesignBuilder in
order to determine its annual energy performance and winter
design week comfort performance. The design of the
Learning House was then modified so that it performed as a
passive solar house. This modified design was modeled in
DesignBuilder to determine its annual energy performance
and winter design week comfort performance; the results
were compared to the results of the Passive House model.
The passive solar model was determined to have a lower
annual electricity demand, while the Passive House model
maintained a steadier indoor air temperature; however, the
indoor air temperature of the passive solar model during the
winter design week fluctuated far less than anticipated.
1. INTRODUCTION
This research offers a comparison between two approaches
to low-energy heating in a building: passive solar heating
and Passive House. A passive solar heating system, as
described by Edward Mazria, consists of “south-facing
glass... for solar collection, and thermal mass for heat
absorption, storage and distribution.” (1) For this reason,
passive solar heating is often dubbed the “mass and glass”
approach. Passive House, by contrast, is a design concept
which uses superinsulation as a means of reducing the size
of the HVAC system to an absolute minimum. Passive
House, therefore, is an “active” system, as it relies on a
mechanical element – the heat recovery ventilator – to
balance internal and solar heat gains. (2) Despite its name,
Passive House has more in common with the
superinsulation movement of the 1970’s than with passive
solar design. Alex Wilson of BuildingGreen.com describes
the intent behind the superinsulation movement and its clear
distinction from passive solar design: “Not long after
passive solar began picking up steam, along came the
competing idea of superinsulation… Superinsulation
proponents sought to create a simpler solution with small
window areas, large quantities of insulation, and simple
geometries.” (3)
1.1 Passive House
A Passive House relies on a superinsulated, airtight
envelope which reduces its heating demand to an absolute
minimum. In many cases, the heating demand is so small
that it can be met by utilizing internal gains (heat from
occupants, appliances, and lighting) and solar gains. In
order to maintain good indoor air quality, every Passive
House must be equipped with a heat recovery ventilator, a
device which constantly ventilates the living space while
recovering heating energy produced inside the envelope.
Typically, a Passive House is designed to be compact in
shape, so as to minimize surface area relative to volume.
The focus of the Passive House approach is minimizing heat
loss through the building envelope.
1.2 Passive Solar
A passive solar building relies primarily on two
components: south facing glass and thermal mass. While
the envelope must be well-insulated to minimize heat loss,
the focus is on optimizing solar heat gain in the winter.
Solar gains are stored in the building’s thermal mass during
the day and radiated into the living space at night. In order
to maximize the potential for solar gains, passive solar
buildings often have a long east-west axis to maximize
southern exposure.
The most basic of solar heating systems is the direct gain
system, in which solar radiation is used to directly heat a
living space. In The Passive Solar Energy Book, Edward
Mazria describes the direct gain system: “In this approach,
there is an expanse of south-facing glass and enough
thermal mass… for heat absorption and storage. South-
facing glass… is exposed to the maximum amount of solar
energy in winter…” (4) When we use the term passive solar
in this paper, we are specifically referring to the direct gain
system.
Though passive solar and Passive House are clearly two
distinct approaches to low energy heating, several
similarities exist between the two. Both approaches often
require supplemental heating to maintain occupant comfort
on the coldest days of the year. Additionally, both
approaches utilize solar heat gains; however, how and to
what degree the two approaches do so is unique to each
approach.
Table 1 offers a side-by-side comparison of Passive House
and passive solar.
TABLE 1: PASSIVE HOUSE & PASSIVE SOLAR
COMPARED
PASSIVE HOUSE PASSIVE SOLAR
Superinsulated Well-insulated
Airtight Not necessarily airtight
Small glazing area Large south glazing area
No significant thermal mass Significant thermal mass
Heat recovery ventilator HRV optional
Compact shape Elongated east-west axis
Constant temperature (68°F) Daily temperature swings
Perhaps one of the most notable differences between
Passive House and passive solar is expressed by the bottom
row in the above chart. Passive Houses maintain a constant
indoor temperature of 68°F (20°C), while passive solar
buildings are designed for a range of temperatures; daily
temperature swings are expected.
This paper focuses on the design of the Learning House,
which is the unrealized design of a small single-family
house in Eugene, Oregon for which the authors have access
to schematic drawings. Designed as a Passive House, the
Learning House has 1320 square feet of living space on two
floors. The goal of this paper is to compare the energy use
and indoor temperature variation of the Learning House to
that of a passive solar house of an identical treated floor area
and volume.
2. THE PROBLEM & HYPOTHESIS
The performance variables with which this study is
interested are temperature variation during the winter design
week, which is related to occupant comfort, and annual
energy use, which corresponds to annual carbon production.
This research is concerned with the following questions:
1. Can a passive solar house in Eugene, Oregon maintain
an average indoor temperature of 68°F (20°C) during
the winter design week while using less energy than a
Passive House of an identical program, living area, and
volume?
2. Can the indoor temperature swings of the passive solar
house during the winter design week be kept to less
than 10°F above and below 68°F (20°C)?
The hypothesis of this study states that a passive solar house
in Eugene, Oregon can maintain an average indoor
temperature of 68°F (20°C) during the winter design week
while using the same amount of energy as an otherwise
identical Passive House.
Additionally, the hypothesis of this study states that the
passive solar house will experience indoor temperature
swings within 10°F above and below the average indoor
temperature.
Supplemental electric resistance heat is assumed in both the
Passive House and passive solar models. The thermostat in
the Passive House model will be set to maintain 68°F
(20°C), while the thermostat in the passive solar model will
be set back to 58°F (14°C) during periods of the day when
the house is assumed to be unoccupied. This will allow the
authors to determine whether or not the additional thermal
mass in the passive solar model is able to moderate the
temperature swings.
3. METHODOLOGY
3.1 Model the Learning House in the Passive House
Planning Package (PHPP)
We will begin the study by modeling the Learning House
design in the PHPP to confirm that it performs to the
Passive House standard. The PHPP is the Passive House
Institute’s proprietary energy modeling software for use in
the design and verification of Passive House buildings.
After modeling the design of the Learning House in the
PHPP, minor design changes will be made as necessary so
that the house performs to the Passive House standard.
3.2 Model the Learning House in DesignBuilder and
Simulate Annual Energy Performance and Winter
Design Week Comfort Performance
The Learning House will be modeled in DesignBuilder and
simulations for a typical year and the winter design week
will be performed. The results of these simulations are for
comparison to the simulation results of the passive solar
model in the fourth phase.
3.3 Passive Heating Analysis and Redesign of the Learning
House for Passive Solar Heating
Using passive heating design calculations as outlined in
Mechanical and Electrical Equipment for Buildings, the
Learning House design will be modified for passive solar
heating. (5) These calculations will provide general
recommendations on envelope U-values and the ratio of
mass-to-glass area.
1. Determine the solar savings fraction (SSF) of the
Learning House
2. Increase area of south glazing as necessary to improve
the SSF
3. Determine required area of exposed thermal mass
4. Redesign house for passive solar heating based on
calculated areas of south glazing and thermal mass
5. Perform building load coefficient (BLC) calculation
and overall heat loss calculation to determine whether
envelope U-values are sufficient for passive solar
heating in Eugene’s climate
3.4 Model the Passive Solar Design in DesignBuilder and
Simulate Annual Energy Performance & Winter Design
Week Comfort Performance
The Learning House model will be modified for passive
solar as determined in the previous step. Simulations for a
typical year and the winter design week will be performed.
Results of these simulations will be compared to the results
of the Passive House model simulations in phase two.
3.5 Means of Analysis
The results of the simulations as described above will be
compared to determine 1) whether or not the passive solar
model has a lower annual electricity demand than the
Passive House model and 2) whether or not the passive solar
model experiences indoor temperature swings within 10°F
above and below 68°F (20°C) during the winter design
week, as stipulated in the hypothesis.
4. RESULTS
4.1 Passive House Planning Package Simulation Results
Minor adjustments to the Learning House design, such as
the use of higher quality windows and the addition of a solar
hot water system, were made so that the house would indeed
meet the performance requirements of the Passive House
standard. According to the PHPP, the total annual
electricity use for the Learning House was calculated to be
13.3 kBtu/ft2
yr.
4.2 DesignBuilder Simulation Results – Passive House
Once it was determined that the design did indeed perform
to the Passive House standard, the Learning House was
modeled in DesignBuilder in order to simulate annual
energy performance and winter design week comfort
performance. According to DesignBuilder, the total annual
electricity use for the Learning House was calculated to be
10.9 kBtu/ft2
yr, corresponding to an annual CO2 production
of 6336.30 lbs. It should be noted that the difference
between the electricity use as simulated by the two software
programs is 20%.
Other performance data of note is the annual electricity use
for electric lighting and the annual heating from solar gains.
The annual electricity use for electric lighting was
calculated to be 4589.04 kBtu/yr. Annual heating from
solar gains was calculated to be 6645.37 kBtu/yr.
In the winter design week simulation, the house maintained
the Passive House comfort temperature of 68°F (20°C) for
the majority of the time. However, on two occasions, the
indoor air temperature increased by several degrees; this is
most likely attributed to significant solar gains through the
south and west windows in the late afternoon on two clear
days.
Fig. 1: Passive House Model in DesignBuilder.
4.3 Passive Heating Analysis and Redesign Results
The design for the Passive House was then modified to
operate as a passive solar house. This was achieved by first
analyzing the design of the Passive House. The ratio of
south glazing to floor area was determined to be 9%,
corresponding to a solar savings fraction (SSF) of 32% for
Salem, Oregon (the nearest location for which data was
available). This 32% solar savings fraction is rather low
according to Mechanical and Electrical Equipment for
Buildings, which gives a range of 37% (low) to 59% (high)
for passive solar buildings with superior performance glass.
(6) The fact that the solar savings fraction is so low
provides further evidence that a Passive House is distinct
from a passive solar house; a Passive House has a much
lower solar savings fraction than a passive solar house.
Therefore, the south glazing area of the Learning House
must be increased to perform as a passive solar house. A
target solar savings fraction of 50% was chosen as a starting
point for the passive solar design.
In order to achieve a solar savings fraction of 50%, the area
of south glazing must be increased to 19% of the floor area,
or 271.7 ft2. At a solar savings fraction of 50%, the passive
solar house would require masonry surface area equivalent
to 3.7 times the south glazing area (according to design data
in Mechanical and Electrical Equipment for Buildings), or
1005 ft2. (7) As an alternative, a water wall would require
much less surface area (0.5 times the south glazing area, or
136 ft2). (8)
The area of south glazing was increased to 267 ft2 by
expanding the width of the window bank at the center of the
south wall and adding 18 inch high clerestories above all
south facing windows. Thermal mass was then added to the
building by changing the material of the floor system and
kitchen wall to concrete. The resulting area of exposed
thermal mass is 1094 ft2. Though the floor system of the
passive solar house differs from the Passive House, the U-
value of the two assemblies was kept constant.
Next, a building heat loss calculation and building load
coefficient calculation1 for the passive solar house were
performed. See Table 2 and the calculations that follow.
UAns envelope = 79.4 Btu/hr °F (see Table 2)
UAventilation = ACH × volume × 0.018 Btu/ft3 °F
ACH = (ventilation rate × 60 min/hr)/volume
= (82 ft3/min × 60 min/hr)/12747 ft
3 = 0.39 ACH
UAventilation = 0.39 ACH × 12747 ft3 × 0.018 Btu/ft
3 °F
= 89.5 Btu/h °F
TABLE 2: U-VALUES AND AREAS
COMPONENT U-VALUE
(Btu/hr ft2 °F )
AREA
(ft2)
UxA
(Btu/hr° F)
Roof 0.0173 1344.3 23.3
NS* Opaque Wall 0.0127 1420.3 18.0
NS Windows 0.11 100.2 11.0
NS Doors 0.14 18.9 2.6
Floor 0.0232 1056 24.5
Total UAns envelope - - 79.4
*Non-south
Since the passive solar house will be equipped with a heat
recovery ventilator with an efficiency of 92%, it was
estimated that the heat recovery element would recover all
but 8% of potential ventilation losses. Therefore:
UAventilation (with heat recovery) = 0.08 × 89.5 Btu/h °F
= 7.16 Btu/h °F
BLC = 24 hr/day(UAns envelope + UAventilation)
= 24 hr/day(79.4 Btu/h °F + 7.16 Btu/h °F)
= 2077.4 Btu/DD
Overall Rate of Heat Loss = BLC/Floor Area
= 2077.4 Btu/DD/1320 ft2
= 1.57 Btu/DD ft2
According to data in Mechanical and Electrical Equipment
for Buildings, the maximum overall rate of heat loss for a
passive solar building in a climate with 3000-5000 HDD65
is 5.6 Btu/DD ft2. (9) Therefore, the efficiency of the
existing envelope is more than adequate. In fact, the
efficiency of the existing envelope could be significantly
reduced and the passive solar house would operate
effectively. In other words, the U-value of a Passive House
is significantly lower than that required for passive solar,
further creating a distinction between the two approaches.
Fig. 2: Passive Solar Model in DesignBuilder.
4.4 DesignBuilder Simulation Results – Passive Solar
The passive solar design was then modeled in
DesignBuilder for comparison to the Passive House model.
According to DesignBuilder, the total annual electricity use
for the passive solar house was calculated to be 9.8 kBtu/ft2
yr, corresponding to an annual CO2 production of 5739.97
lbs.
Other performance data of note is the annual electricity use
for electric lighting and the annual heating from solar gains.
The annual electricity use for electric lighting was
calculated to be 4498.99 kBtu/yr. Annual heating from
solar gains was calculated to be 9835.51 kBtu/yr.
While the indoor temperature in the Passive House model
spiked due to solar gains through the south and west
windows on two clear days during the winter design week,
the indoor temperature swing in the passive solar model on
the same days was moderated by the additional thermal
mass.
In the winter design week simulation, the indoor air
temperature of the passive solar house varied from 65-70°F
(18-21°C). While the thermostat was set back to 58°F
(14°C) during periods when the house was assumed to be
unoccupied, the indoor air temperature never dropped below
65°F (18°C).
5. ANALYSIS
Table 3 presents a side-by-side comparison of the
simulation results for the two models. As expressed by the
first row of data in the chart on the next page, the annual
electricity demand of the passive solar house is less than
that of the Passive House. According to the simulation
results, a passive solar house in Eugene can operate at a
lower annual electricity demand than a Passive House,
provided the occupants set back the thermostat and allow
the thermal mass to moderate daily temperature swings.
Two factors directly attribute to the lower electricity
demand of the passive solar model. First, the annual
electricity demand for electric light is lower in the passive
solar model, as the larger south glazing area results in a
Fig. 3: Passive House Model Comfort Performance for Winter Design Week
Fig. 4: Passive Solar Model Comfort Performance for Winter Design Week
greater amount of daylight. Second, the passive solar model
uses 48% more energy from solar gains for heating than
does the Passive House, thereby reducing heating electricity
use.
The Passive House does experience a steadier indoor air
temperature than does the passive solar house; however, the
indoor air temperature variation of the passive solar house
during the winter design week is much smaller than
hypothesized; the indoor air temperature varies by only 5°F
during the day. This can be attributed to the large amount of
thermal mass in the house, which prevents the temperature
from dropping uncomfortably low when the thermostat is
set back to 58°F (14°C).
TABLE 3: SIMULATION RESULTS COMPARED
PERFORMANCE
DATA
PASSIVE
HOUSE
PASSIVE
SOLAR
Annual Electricity
Demand
10.9 kBtu/ft2
yr 9.8 kBtu/ft2
yr
Annual CO2
Production
6336.30 lbs 5739.97 lbs
Annual Electric
Lighting Demand
4589.04
kBtu/yr
4498.99
kBtu/yr
Annual Heating from
Solar Gains
6645.37
kBtu/yr
9835.51
kBtu/yr
6. CONCLUSION
The hypothesis of the study was proven to be correct, as the
passive solar model has a lower annual electricity demand
than the Passive House model while experiencing
temperature swings well within 10°F of 68°F (20°C). In
fact, the passive solar house performed much better than
anticipated in this respect; the thermal mass moderated the
indoor air temperature so that it never dropped below 65°F
(18°C) during the winter design week.
The results of this study offer promise for passive solar as a
heating strategy in cloudy climates such as the Pacific
Northwest; though there is little access to direct sunlight
during the winter months, a considerable amount of diffuse
solar radiation is still available; further, the thermal mass is
able to greatly moderate temperature swings. If occupants
are willing to accept small daily temperature swings, passive
solar can be a viable, low-energy option.
It should also be reiterated that the occupants of a passive
solar house must set back the thermostat and allow the
thermal mass to moderate the temperatures swings if the
house is to perform as intended. The Passive House, on the
other hand, is able to operate at an incredibly high level of
efficiency without adjustment of the thermostat. In other
words, the passive solar house requires a higher level of
occupant input in order to perform as intended.
As stated in the introduction, Passive House and passive
solar are clearly two distinct approaches to low-energy
heating; this was further illustrated by the low solar savings
fraction and relatively small amount of solar gain in the
Passive House model. While each of the two approaches is
distinct from the other, each is highly effective in reducing
the amount of energy required for heating without
compromising occupant comfort.
7. ACKNOWLEDGEMENTS
We would like to thank University of Oregon Professor
Emeritus John Reynolds for his invaluable insight and
assistance during this research project.
8. REFERENCES
(1) Straube, John. “The Passive House (Passivhaus)
Standard: A comparison to other cold climate low-
energy houses.” Insight, 025, October 2009. Retrieved
from <www.buildingscience.com>
(2) Mazria, Edward. The Passive Solar Energy Book.
Emmaus: Rodale Press, 1979.
(3) Wilson, Alex. “Passive House Arrives in North
America: Could It Revolutionize the Way We Build?”
Environmental Building News. Retrieved from
<www.buildinggreen.com>
(4) Mazria, Edward.
(5) Grondzik, Walter; et al. Mechanical and Electrical
Equipment for Buildings: 11th
edition. Hoboken: John
Wiley & Sons, Inc., 2010.
(6) Grondzik, Walter; et al.
(7) Grondzik, Walter; et al.
(8) Grondzik, Walter; et al.
(9) Grondzik, Walter; et al.
8. END NOTES
(1) The building load coefficient is a measure of the
building’s heat loss per degree day (Btu/DD).
