ASHRAE Journal - October 2014 - 45
COLUMN BUILDING SCIENCES
FIGURE 1 Roof Assembly. Note the continuity of the water control layer, air control layer, vapor control layer and the thermal control layer. The "lookout" framing for the overhang
is embedded in the rigid insulation. You want a higher R-value roof than this? Easy. Add another layer of rigid insulation. Or two layers. Don't like the cellulose in the cavities? Use
fiberglass. Or use low density or high density foam. Whatever.
system with an ultra-efficient house will not get you to net
zero even there. D.C. has humidity and AC. My hometown
of Toronto? With a real winter? Not a chance. Sacramento,
Calif., pretty easy. Not tough to be net zero in California.
Kinda explains some of the energy politics. It is easy to be
an advocate when your climate is easy.
So what does "ultra-efficient" mean anyway?
Different things to different people. I will give you my
take. Some folks will say no way "not efficient enough."
Others will say "are you crazy that is way over the top."
The good news is fundamentally the overall approach
works the same for both types of folks. This will make
sense later.
‡We built an ultra-efficient house with 5 kW of PV for our daughter. The house was 2,800 ft2
(260 m2) in Concord, Mass. It did not get to net zero. Ultra-efficient for the house then-five years
ago-is not as ultra-efficient as we would do today. Upping the efficiency to "my metric" of ultraefficiency (on the next column of this page) and a 7.5 kW PV system would have got the house to
net zero-with a "conserving lifestyle." Note the "conserving lifestyle" comment.
§Passive House folks relax. Yes, I know you are "ultra" ultra. The approach illustrated will get you to
your metrics as well. Read on.
Here is the list of metrics for the enclosure and equipment for an ultra-efficient house in a cold climate like
Boston or Chicago or Toronto (as defined by me§):
* R-5 windows;
* R-10 slab insulation;
* R-20 basement insulation;
* R-40 wall insulation;
* R-60 roof insulation;
* Airtightness of 1.5 ach at 50 Pa or less;
* Heat recovery on ventilation;
* LED lighting everywhere;
* AFUE 95% - sealed combustion condensing for furnaces, water heaters and boilers;
* SEER 18 or more for AC;
* HSPF 10 or more for heating; and
* Appliances from the top 10% of Energy Star.
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Westford,
Mass. Visit www.buildingscience.com.
O CT O B E R 2 0 1 4
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ASHRAE Journal - October 2014
Table of Contents for the Digital Edition of ASHRAE Journal - October 2014
Contents
ASHRAE Journal - October 2014 - Cover1
ASHRAE Journal - October 2014 - Cover2
ASHRAE Journal - October 2014 - 1
ASHRAE Journal - October 2014 - 2
ASHRAE Journal - October 2014 - Contents
ASHRAE Journal - October 2014 - 4
ASHRAE Journal - October 2014 - 5
ASHRAE Journal - October 2014 - 6
ASHRAE Journal - October 2014 - 7
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ASHRAE Journal - October 2014 - HR1
ASHRAE Journal - October 2014 - HR2
ASHRAE Journal - October 2014 - HR3
ASHRAE Journal - October 2014 - HR4
ASHRAE Journal - October 2014 - HR5
ASHRAE Journal - October 2014 - HR6
ASHRAE Journal - October 2014 - HR7
ASHRAE Journal - October 2014 - HR8
ASHRAE Journal - October 2014 - HR9
ASHRAE Journal - October 2014 - HR10
ASHRAE Journal - October 2014 - HR11
ASHRAE Journal - October 2014 - HR12
ASHRAE Journal - October 2014 - HR13
ASHRAE Journal - October 2014 - HR14
ASHRAE Journal - October 2014 - HR15
ASHRAE Journal - October 2014 - HR16
ASHRAE Journal - October 2014 - HR17
ASHRAE Journal - October 2014 - HR18
ASHRAE Journal - October 2014 - HR19
ASHRAE Journal - October 2014 - HR20
ASHRAE Journal - October 2014 - HR21
ASHRAE Journal - October 2014 - HR22
ASHRAE Journal - October 2014 - HR23
ASHRAE Journal - October 2014 - HR24
ASHRAE Journal - October 2014 - HR25
ASHRAE Journal - October 2014 - HR26
ASHRAE Journal - October 2014 - HR27
ASHRAE Journal - October 2014 - HR28
ASHRAE Journal - October 2014 - HR29
ASHRAE Journal - October 2014 - HR30
ASHRAE Journal - October 2014 - HR31
ASHRAE Journal - October 2014 - HR32
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ASHRAE Journal - October 2014 - Cover3
ASHRAE Journal - October 2014 - Cover4
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