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)

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

Building Control Layers

Water Air Thermal Vapor


Air Compartmentalization is Key in Multifamily

Stack, Wind, and Mechanically Induced Airflow

Source: Building Science Corp.

Most moisture flow is by airflow. Need to control the airflow.


Stack Effect in Cold Weather

+ NPP

Air flows in at bottom

Air flows out at top

Air leakage condensation at top stories


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

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 0.23 cfm50/ft2 surface area

1.0 l/s-m2 @75 Pa 0.15 cfm50/ft2 surface area


Balanced Ventilation In Each Dwelling Unit

Central-fan-integrated Supply + Single-point Exhaust HRV/ERV



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


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


Humidity control challenges

1. 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

2. 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


3. More energy efficient cooling equipment often has a higher evaporator coil temperature yielding less moisture removal

• Larger evaporator coil by manufacturer design, or up-sized air handler unit or airflow by installer choice

4. Conventional over-sizing to cover for lack of confidence in building enclosure or conditioning system performance causes short-cycling yielding less moisture removal

Humidity control challenges, cont.


Typical Indoor (humid climate)Tdb RH Tdp

winter 72 40 46spring 75 45 52summer 77 50 57fall 75 45 52

10

20

30

40

50

60

70

80

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

Dew

poin

t Tem

pera

ture

(F)

Monthly Average Outdoor Dewpoint Temperature

Miami

Houston

Cincinnati

Boston

Phoenix

San Francisco


10

20

30

40

50

60

70

80

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

Dew

poi

nt T

empe

ratu

re (

F)

Monthly Average Outdoor Dew Point Temperature

Baltimore

Wash DC

Norfolk

Richmond

San Francisco

Miami


0 10 20 30 40 50 60 70 80 90 100 110 120

0.25 ach infiltration

0.1 ach infiltration with 50 cfm ventilation

Moisture Load (lb water/day)

Moisture load for cooling and dehumidification systemsin humid climates (75 F/55% RH indoor, 75 F outdoor dewpt)

Air exchange

People

Cooking

Dishwashing

Bathing

Clothes washing

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

0

200

400

600

800

1000

1200

80 85 90 95 100 105

Outdoor air temperature (F)

Coo

ling

load

(W)

05001000150020002500300035004000

Coo

ling

load

(B

tu/h

)

Total

Sensible

Latent


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. Don’t run on constant fan when the coil is wet 4. Disable fan overrun after the compressor stops


What is Supplemental Dehumidification?

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


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)


What is a good metric for determining the need?

Hours above 60% relative humidity and

Condensation on windows


Dehumidifier and ventilation duct in interior mechanical closet with louvered door


Ducted dehumidifier in conditioned space with living space control


Dehumidifier process

Fan

Evaporator coil

Supply Air

Return Air

Entering Air Dew Point

Leaving Air

Fan

Condenser coil

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

Supply Air


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

Goals:

Provide year-around relative humidity control in high-performance (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

27

Affordable Comfort Conference 3/28/2012 Baltimore

Central system with modulating hot gas reheat

Fan

Evaporator coil

Supply Air

Return Air

Entering Air Dew Point

Leaving Air

Fan

Modulating hot gas reheat coil

Modulate the hot gas reheat to achieve a space-neutral supply air temperature.

T1 T2

Supply Air

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

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