Armin Rudd - Building Science Principals for Managing Moisture in Tight Buildings

Building Science Principles for
Managing Moisture in Tight Buildings
for Arlington County Workshop on
Balancing Moisture, Ventilation and Energy Efficiency
by Armin Rudd, ABT Systems, LLC (www.abtsystems.us)

Building Control Layers





Water
Air
Thermal
Vapor

Arlington County Workshop, 2013-12-11
Balancing Moisture, Ventilation, and Energy Efficiency

2

Air Compartmentalization is Key in Multifamily
Most moisture flow is by airflow. Need to control the airflow.
Stack, Wind, and Mechanically Induced Airflow

Source: Building Science Corp.

3

Stack Effect in Cold Weather

Air leakage condensation at top stories

+
NPP

Air flows
in at
bottom


Air flows
out at
top

4

Controlling Stack Effect by Compartmentalization of Floors,
Elevator Shafts, Corridors, Stairs, Chutes, and Units


Elevator Vestibule

Floor to Ceiling Critical Sealing
Unit-to-Unit and Unit-to-Corridor


Fully adhered air barrier drainage plane and insulation

Maximum Air Leakage Targets
Air Barrier Metrics
Material

0.02 l/s-m2 @75 Pa

Assembly

0.20 l/s-m2 @75 Pa

Enclosure

2.0 l/s-m2 @75 Pa

0.30 cfm50/ft2 surface area

1.5 l/s-m2 @75 Pa


1.0 l/s-m2 @75 Pa



8

Balanced Ventilation In Each Dwelling Unit
Central-fan-integrated Supply
+ Single-point Exhaust

HRV/ERV



9

System engineering trade-offs


Start with high-performance building enclosure


Improves the more permanent features of a home which has
longer-term sustainability benefits




Bulk water management, low loss/gain glass, controlled air change,
ducts inside conditioned space, pressure balancing

Allows for reduced cooling system size



Helps pay for the enclosure improvements
More compact duct system lowers cost and helps get the ducts
inside



Makes overall building performance more predictable


Gives confidence for right-sizing equipment






No short-cycling: Better moisture removal, Higher average
efficiency, Better spatial mixing

Controlled ventilation instead of random infiltration

Results in decreased energy consumption along with
increased occupant comfort


10

Humidity control goals



Comfort, and Indoor Air Quality




Control indoor humidity year-around, just like we do
temperature

Durability and customer satisfaction


Reduce builder risk and warranty/service costs


11

Humidity control challenges
1.

2.

In humid cooling climates, there will always be times of the
year when there is little sensible cooling load to create
thermostat demand but humidity remains high
• Cooling systems that modify fan speed and temperature
set point based on humidity can help but are still limited
in how much they can over-cool
More energy efficient homes have less sensible heat gain to
drive thermostat demand but latent gain remains mostly the
same
• Low heat gain windows
• Ducts in conditioned space
• More, and better-installed, insulation
• Less heat gain from appliances and lighting


12

Humidity control challenges, cont.

3.

4.

More energy efficient cooling equipment often has a higher
evaporator coil temperature yielding less moisture removal
•
Larger evaporator coil by manufacturer design, or upsized air handler unit or airflow by installer choice
Conventional over-sizing to cover for lack of confidence in
building enclosure or conditioning system performance
causes short-cycling yielding less moisture removal


13

Monthly Average Outdoor Dewpoint Temperature
80

Dewpoint Temperature (F)

70
60
Miami

50

Houston
Cincinnati

40

Boston
Phoenix

30

San Francisco

20
10
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

winter
spring
summer
fall


Typical Indoor (humid climate)
Tdb
RH
Tdp
72
40
46
75
45
52
77
50
57
75
45
52

Monthly Average Outdoor Dew Point Temperature

Dew point Temperature (F)

80
70
Baltimore

60

Wash DC
Norfolk

50

Richmond
San Francisco

40

Miami

30
20
10
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec


Moisture load for cooling and dehumidification systems
in humid climates (75 F/55% RH indoor, 75 F outdoor dewpt)
Moisture Load (lb water/day)
0

10

20

30

40

50

60

70

80

90 100 110 120
Air exchange
People

0.25 ach infiltration

Cooking
Dishwashing
Bathing
Clothes washing

0.1 ach infiltration
with 50 cfm
ventilation

Floor mopping
Building envelope
New const drying

Source for Cooking through New construction drying: Natural Resources Canada


Internal Moisture Gains
from Boualem Ouazia, NRC-CAN, Humidity Control in Houses ERV
Technology

24 lb/day

Cooling load (W)

1200

4000
3500
3000
2500
2000
1500
1000
500
0

Total

1000
800

Latent

600
400

Sensible

200
0
80

85

90

95

100

Outdoor air temperature (F)


105

Cooling load
(Btu/h)

Cooling Load for:
50 cfm OA, Tdb,in=75, Tdp,in=55, Tdp,out=75

Conventional Cooling System Dehumidification
Enhancements
1.

Lower Airflow (costs more but increases moisture removal)
400 cfm/ton normal cooling in non-humid climate
350 cfm/ton for normal cooling in humid climate
300 cfm/ton (down to 250 cfm/ton) for extra dehumidification

2.

Overcooling
Limit any overcooling to 2 oF below setpoint to avoid comfort
complaints

3.
4.

Don’t run on constant fan when the coil is wet
Disable fan overrun after the compressor stops


19

What is Supplemental Dehumidification?

Moisture removal, supplemental to the cooling
system, when there is no need for cooling.


20

When is it needed?





Mostly when the house is floating between cooling
and heating setpoints
Spring/Fall swing seasons and summer shoulder
months
Summer nights and rainy periods
Sensitive to internal moisture generation too


High occupant density
 Lack of local exhaust use in kitchens and baths
 Cooking habits (open boiling water)


21

What is a good metric for determining the need?
Hours above 60% relative humidity
and
Condensation on windows


22

Dehumidifier and ventilation duct in interior
mechanical closet with louvered door


Ducted dehumidifier in conditioned space
with living space control


Dehumidifier process
Evaporator
coil

Condenser
coil

Fan

Supply Air
Entering Air
Dew Point

Return Air
Supply Air

Leaving Air

Fan
Dehumidifiers add the heat of condensation, compressor heat,
and fan heat to the space. (Supply air is typically 105 to 115 F)


25

What about making the existing cooling or heat pump
equipment also do the supplemental dehumidification?
Goals:






Provide year-around relative humidity control in highperformance (low-sensible gain) dwellings
Without over-cooling the space
At lower installed cost than the same efficiency heating and
cooling system with an additional high efficiency
dehumidifier
By making standard DX cooling equipment switchable
between normal cooling and dehumidification-only using
condenser reheat


26

Central system with modulating hot gas reheat
Evaporator
coil

Modulating
hot gas
reheat coil

Fan

Supply Air
Entering Air
Dew Point

Return Air
Supply Air
Leaving Air
Fan
Modulate the hot gas reheat to achieve a space-neutral
supply air temperature.

Affordable Comfort Conference
3/28/2012 Baltimore

T1

T2
27

Armin Rudd - Building Science Principals for Managing Moisture in Tight Buildings

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