Building Science & Envelope for Passive House · Building Science definition 4 Heat and moisture...
22
Building Science & Envelope for Passive House Katy Hollbacher, P.E. | beyondefficiency.org March 14, 2010 Hosted by: Passive House California Berkeley, California
Transcript of Building Science & Envelope for Passive House · Building Science definition 4 Heat and moisture...
Building Science & Envelope for Passive House
Katy Hollbacher, P.E. | beyondefficiency.org March 14, 2010
Hosted by: Passive House California Berkeley, California
What is Building Science?
2
The study of thermal energy and moisture dynamics in buildings Thermal energy: energy from vibration of particles Heat: flow of thermal energy (i.e. energy in transfer)
Driving Forces on a Building
humidity
rain
heat
wind
Stack Effect*
water
radon
Pressure
Heat
Moisture Fans
www.energystar.gov
3
Building Science definition
4
Heat and moisture interactions with building Flow throughout a building Transfer between the inside and outside of a
building Chimney effect Conduction Vapor diffusion…
Moisture Water in any form: solid, liquid, gaseous Ice, liquid water, water vapor
Building Science Ignored:
5
Building Science Ignored:
6
Heat Flow Dynamics
7
Conduction Warmer body
transferring heat to colder body by direct contact
Convection Heat carried by moving
fluid (eg air, water) Radiation
Radiant heat emitted from surface of warmer body, transferred in a straight line through space to colder body
What’s a Btu?
8
Technically: amount of energy required to raise the temperature of 1lb of water by 1ºF
Practically: 1 cord of wood = 20 million Btu
neilsontreefarms.com
100W light bulb = 3 million Btu (annual use if on 24hrs/day)
365 pints beer = .34 million Btu
What R-value really means
9
How much energy does it take to maintain 70º inside when it’s 40º outside?
Newton’s Law of Cooling: Q = A • ∆T / R Heat flow = surface area • difference in temperature
between each side • heat transfer coefficient (1/R)
If R-value of assembly is 13.5: Q= A • 30ºF / (13.5 ºF-ft2-hr/Btu)
= 2.22 Btu/hr per sq.ft of wall
= 6,222 Btu/hr for Building B
(walls only, see next slides) 40º 70º
Quiz:
10
Choose the most energy-efficient home (each is 1600sq.ft): Building A: Exceeds Title 24 by 0% Building B: Exceeds Title 24 by 15%
Title 24 Compliance Margin
11
Modeled annual energy use (kBtu/ft2/yr) Building A: 81.9 Building B: 82.7
But Building A exceeds Title 24 by <1%, & Building B exceeds it by 17%
?????????????????
The models:
12
2 stories, 1600sq.ft (20'x40') Roof= 800 sq.ft Slab= 800 sq.ft Wall perimeter= 120 ft Wall area= 2400 sq.ft (wall ht= 20')
TOTAL ENVELOPE AREA= 4000sq.ft
1 story, 1600sq.ft (15'x40'+ 20'x20'+ 15'x40') Roof= 1600 sq.ft Slab= 1600 sq.ft Wall perimeter= 280 ft Wall area= 2800 sq.ft (wall ht= 10')
TOTAL ENVELOPE AREA= 6000sq.ft
Building A
Building B
50% More surface area
T24 Energy Budget: 82.5 kBtu/ft2/yr
T24 Energy Budget: 99.7 kBtu/ft2/yr
The Reality of Title 24:
Compliance tool only Allowable budgets & TDV Relative vs. absolute energy performance
Ready to get serious about optimizing energy use? Look at actual (ie absolute) energy use
Go beyond the compliance margin
13
Air Leakage
25% or more of heat loss from homes is due to air infiltration
Air infiltration can reduce a wall's rated insulative value from R-13 to an effective R-5
14 energystar.gov
Why Air Tightness?
Energy Health
“Naturally” leaky = “Randomly” leaky
Build Tight, Ventilate Right
Safety Durability
What besides heat does air contain?
Water vapor
15
“Carpet filter effect”
Why Air Tightness?
“It is well established that convection, not diffusion, is the major vehicle of moisture transport out of homes.”
- Daniel Friedman, Building Forensics Investigation & Analysis Expert
www.inspectapedia.com
16
Air Tightness Detailing
17
Building Science Corp.
“Breathable”
18
“I don’t want my home to be too airtight; then it’s not breathable and is unhealthy”
Air tightness vs. vapor permeability Two very different things; be careful not to mix
them up Educate your clients
Vapor Permeability
19
Vapor permeability coefficient: measure of the ability of water vapor to pass through a material Technically: amount of water vapor that permeates
a given area and thickness of material in a given amount of time, under given vapor pressure difference
Q = P•A•∆p / d Vapor flow = permeability coefficient of material •
surface area • vapor pressure difference / thickness
Permeability: it’s all relative
20
“Vapor barrier” ≤ 0.1 perm Rubber, polyethylene, glass, aluminum foil…
“Vapor retarder” (semipermeable) > 0.1 perm and ≤ 10.0 perm Oil-based paint, 1” or thicker EPS, plywood & OSB, 1” or
thinner XPS, most latex paints… All of the above are now appropriately classified as
“vapor retarders”, see next slide Vapor permeable
> 10 perm Unpainted drywall, unfaced batt insulation, cellulose,
synthetic stucco, housewrap…
Vapor Retarder Classifications#
21
Class I: ≤ 0.1 perm Class II: > 0.1 perm and ≤ 1.0
perm Class III: > 1.0 perm and ≤ 10.0
perm Code references:
2007 Supplement to the 2006 International Residential Code (IRC) R202 Vapor retarder Class N110.2.5 Vapor retarders
