Achieving Net Zero Energy at Conventional Cost

Copyright © 2017 Sterner Design, LLC
IOWA NEST RESIDENCE
Achieving Net Zero Energy
At Conventional Cost

Carl S. Sterner, Assoc. AIA, LEED AP
Owner, Sterner Design LLC
Director, Sol Design + Consulting

20 April 2017

Topics
1.  How early analysis informed design

! 5 studies that set the design on track
2.  Monitoring

! how are we doing?
3.  Designing for a conventional budget

! 3 strategies / lessons learned
4.  Building science

! thermal + air + vapor barriers

Project requirements
•  ~2400 s.f. residence
•  Net Zero Energy
•  Underground / of the land
•  Avoid conventional AC if possible
•  Do all of this on a conventional budget

The site

The design

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Pre-design & Schematic Analysis
5 questions
that informed design

Can performance 
inspire design?
The question behind them all:

1. 
What passive design strategies
are most important?

Sensitivity analysis

+
! STEP-BY-STEP PROCESS @ WWW.IOWANEST.COM
How was this done?

Overlaying cost = “most bang for the buck”

Building form??
Earth berming??
Space layout / thermal zoning??
Peak Loads!
What I would do diﬀerently:

A study I should’ve done:
(Glazing ratio + bldg
form with optimal
shading)

2. 
Can we avoid mechanical cooling?

6 factors
that eﬀect
human
comfort

Strategy: Earth Berming ! air temp + radiant temp

Strategy: Thermal Mass ! air temp + radiant temp

Strategy: Shading ! air temp + radiant temp

Strategy: Natural Ventilation + Ceiling Fans ! air movement

Comfort
analysis (PMV)

Design case
without natural
ventilation =
uncomfortable

Not possible to get into range without dehumidiﬁcation …

… but comfort is personal. Clients are willing to try for no A/C.

Strategy: Plan for the Future ! add dehumidiﬁcation / cooling

How the analysis thus far 
informed the design …

Construction: ICFs + exposed mass

Performance Concept: Insulated mass + operable envelope

Language: Planar vs Volumetric
vs

3. 
Which design option is best? 
(And why?)

OPTION 1: REVEAL
OPTION 2: BEACON
EUI
EUI
Concepts

OPTION 3: GLASS PAVILIONS
OPTION 4: TERRACE
EUI
EUI
Concepts

OPTION 5: ROLLING HILLS
Daylight Legend Energy Legend
EUI
Concepts

Concept Comparison

Envelope area vs EUI
Glazed area vs cooling
Glazed area vs EUI
Envelope area vs heating
What’s driving energy performance?

Design response

4. 
What’s the best combination of
strategies?

! STEP-BY-STEP
PROCESS @
WWW.IOWANEST.COM

Sefaira analysis was used to test m
and discover the best path toward
Response Curves were used to fur
parameters, based on the latest ve
the Sustainable Design Narrative fo
The complete Sefaira analysis can
apps.sefaira.com/concept/project/
single?id=30986&cid=94270&did=Copyright © 2017 Sterner Design, LLC

NET ZERDRAFT SET - NOT FOR CONSTRUCTION
NEST RESIDENCE // Fremont, IA
TERNER DESIGN // 1555 Pullan Ave., Cincinnati, OH 45223
Copyright © 2015 Sterner Design. All rights reserved.
What are the optimal envelope properties?

We’ve come a long way …
Energy Footprint (kBTU)
Peak Space Heating Demand (ton)
Energy Footprint (kBTU)
Peak Space Heating Demand (ton)
Code Baseline
As Designed

5. 
Is the glass productive?

Productive glass + productive mass
Typical conﬁguration =
Portions of the glass are unproductive
This design =
100% of the glass is “productive”
as a passive solar collector

(Recognize that angle??)

Direct sunlight analysis for shading optimization

Is there enough daylight?

PRE-DESIGN: Passive strategies • Net Zero Energy feasibility CONCEPT: Comparison • Correlation
SCHEMATIC / DEVELOPMENT: Envelope optimization • Shading design (direct sunlight) • Daylighting reﬁnements
Summary of studies

Monitoring
The proof is in the data …

Data to date (January – April 2017)
! LIVE DATA @ WWW.IOWANEST.COM/MONITOR

! LIVE DATA @ WWW.IOWANEST.COM/MONITOR
Remarkable stability,
even though some insulation + air sealing still remains (as of April)

Designing for Budget
3 strategies
(AKA hard lessons)

Cost comparison vs conventional house in Iowa

But if you remove the green roof & custom corner lift-slide …

FINISHES ! DESIGN
MECHANICAL SYSTEMS
! INTEGRATIVE DESIGN
ENCLOSURE
! DESIGN + MATERIALS
FOUNDATION ! DESIGN
PLUMBING ! DESIGN
Where did the savings come from?

1. 
Keep the form simple. 
 
(DESIGN)

The original schematic design

The original schematic design
= 2X over budget

What
drove the
cost?

1.  # of corners
2.  # of baths +
plumbing
length
3. shades as
“carved”
elements

A lot of complexity here
= a lot of cost

Back to the drawing board …

The revised schematic design

Fewer corners + simpler plumbing + shading as additive

% cost Original Alternative A Alternative B A Small
Square footage (gross) 3,520 3,173 3,192 2,833
Sq. ft. concrete wall 19% 4,586 3,750 3,660 3,500
Sq. ft. glazing 10% 550 600 590 550
Sq. ft. green roof 10% 1,640 880 1,672 840
Sq. ft. hollow core precast 7% 2,432 2,293 2,432 2,033
Lin. ft. tunnel 5% 64 56 48 48
Sq. ft. exterior cladding 2% 3,022 3,683 2,496 2,200
Estimated cost (JH) $729,400
Estimated cost (CS) $547,592 $501,896 $524,041 $458,245
Less envelope area = less cost

Less envelope area = better performance

ENCLOSURE
! DESIGN + MATERIALS
FOUNDATION ! DESIGN
So: we have 2x the insulation and 3x the airtightness of a
standard house, but spent only marginally more on the enclosure

2. 
Think about labor. 
 
(MATERIALS)

Structure
Insulation
Air barrier
ICFs = 3-in-1 solution: structure + insulation + air barrier

The concrete forms a continuous air barrier

A crew of 2 just installed
•  The formwork for the concrete structure
•  R-45 continuous wall insulation
•  A high-quality air barrier

= signiﬁcant labor cost savings

3. 
Use eﬃciency to reduce 
HVAC costs. 
 
(INTEGRATIVE DESIGN)

Integrative Design in theory:
Better performance at LESS cost

How it worked here:
Tiny heating load ! smaller, cheaper heating system
Good passive design ! no A/C

Designing the Enclosure
Thermal, air, and vapor control

1. 
Thermal control

Continuous insulation
R-45 walls
R-60 roof

Window detail:
Excellent windows (Zola) +
Overinsulation
! SEE THE DETAILS
@ WWW.IOWANEST.COM

2. 
Inﬁltration control 
(air barrier)

Air barrier =
ICF concrete core +
AB membrane over
roof sheathing + 
good details

Window detail:
Siga tapes on interior 
from window to plywood
buck, and from buck
to ICF
! SEE THE DETAILS
@ WWW.IOWANEST.COM

4. 
Vapor / moisture control.

Why is this hard? 
1. High insulation = cold surfaces 
2. Cold surfaces + moisture = condensation

= rot + mold

= "

AND, in the Midwest …

Summer =
Cold inside,
Humid outside !
Vapor drive
inward

Winter =
Cold outside,
Humid inside !
Vapor drive
outward

The solution:
•  Insulate potential condensation surfaces
•  Use air barriers to keep moisture from getting
into assemblies (most moisture travels via air
movement, not diﬀusion)
•  Don’t trap moisture in / let assemblies dry

Strategies:
•  Breathable assemblies
•  Insulate sheathing"
(keep above dewpoint)
•  Ceiling air barrier

To be continued …
! STAY UP-TO-DATE @ WWW.IOWANEST.COM

Thank you!
carl@sternerdesign.com
www.sternerdesign.com
www.iowanest.com

Achieving Net Zero Energy at Conventional Cost

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