Moisture Control

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What is It?

It is the techniques used to control moisture in a building. It includes dew points in the envelope systems, water vapor infiltration, as well as, the indoor relative humidity. Controlling moisture needs to be considered in the design of many of the building components including:

  • Also known as: indoor humidity, water vapor control, ???.

Why is it Important?

The moisture control is important to a building's sustainability because it:

  • Affects the life of building materials (e.g. high moisture can cause material rot in wood)
  • High moisture levels can produce mold which can present human health concerns
  • High indoor humidity levels can cause discomfort for the occupants of the building and results in high energy usage by trying to cool the building to make people more comfortable.

When to Use It?

It is most appropriate to use hybrid ventilation systems when:

  • Using super-insulated building envelope designs...because the these often result in humidity issues.
  • The building is in an climate that has high humidity levels and significant temperature differentials between the outdoor and indoor temperatures.
  • While easier to do in new construction, it is possible to do this in renovations to existing buildings.

Green Garage Use of Hybrid Ventilation System

Sustainability Goals

The sustainability goals for the Moisture Control are:

  • Maintains the indoor relative humidity at 45% +/- 15%
  • Eliminate dew points/moisture build up in the building envelop
  • Ensure a healthy indoor environment
  • The moisture system(s) is simple to maintain, adapt and control.

Strategy and Conceptual Design

Moisture Control Strategy

We have attempted to control moisture in and through every component of the ventilation system. The two main areas are the dew point in the envelop systems and the control of humidity, especially in the summer. We are planning on using a pressure-based strategy to control the humidity. It is an approach that has come from the indepth experience of two of the professionals that contributed extensively to our net-Zero Energy design.

The major elements of our moisture control strategy at the Green Garage are:

  • Pressure-based Moisture Control System
  • Super-insulated Building Envelope - Wall system
    • Weep tubes in the wall system
    • Cellulose insulation will absorb and then dissipate the moisture ...like a living organism
  • Earth Tubes - ERU - Geothermal Air Exchange system
    • Dew point and condensation drainage in the Earth Tubes
    • Desiccant wheel in the ERU exchanges the moisture in the air
    • Geothermal System - heat pump acts as a dehumidifier.

Again we are trying to work with the natural systems off the earth... gravity, and high pressure to low pressure movement create the  ???


Pressure-based Moisture Control System

This is from Ken Bzynski??, KENCO's detailed experience in solving moisture problems. It is done by having a little more air enter the building than leaves the building, thus creating a slightly higher pressure in the building than outside the building.with the intake fans and dampers. This in essence squeezes the moisture out of the building.

Summer Setting

In the summer when outdoor humidity is high, the internal pressure of the building is higher than the outside pressure...causing the water in the air and walls to be squeezed out of the building. All air infiltration will be out of the building. So the only humidity and moisture to address is that in the air exchange and the that caused by internal sources (e.g. humans, plants). occupants of the building would not be able to sense this increased pressure since it would be only a small increment higher than the outdoor pressure.

Winter Setting

In the winter when the outdoor humidity is low, the internal pressure of the building would be maintained slightly less than the outside pressure...causing a very small amount of dry air to enter the building. This would help offset the moisture gains are that created by internal sources (e.g. humans, plants).


???The attached chart show the relative contributions of each component.???

Super-insulated Building Envelope - Wall system

The Super-insulated Building Envelope - Wall system has two significant moisture control components:

  • Weep holes in the wall system
  • Cellulose insulation will absorb and then dissipate the moisture ...like a living organism

The weep holes at the base of the brick wall in the interior air gap will allow any of the condensated water to flow to the outside of the building.

The 8 inches of cellulose insulation will absorb moisture when the inside humidity is high and release it when it is low. It will have a mold inhibitor to ensure that the mold is not a concern.

Earth Tubes - ERU - Geothermal Air Exchange

The Earth Tubes - ERU - Geothermal Air Exchange sub-system has three significant moisture control components:

  • Condensation Drainage - a slit in the bottom of the tube for it's entire length. It would drain through one-way geo-tex material and into 6" of course aggregate.
  • Pressurized tubes - the fan would be placed at the intake manifold and push the air through the tube. This would cause a higher pressure in the tube which will force moisture out of the air, water out of the tube and prevent radon from entering the tube.
  • Desiccant Wheel - in the ERU exchanges the latent energy (i.e. water vapor) between the outbound and inbound air. In the summer the moisture would move from the higher humidity inbound air to the lower humidity outbound air via the desiccant wheel. In the winter the moisture would move in the opposite direction, moving from the higher humidity outbound air to the lower humidity inbound air via the desiccant wheel.
  • Geo-thermal System is last resort to remove moisture from the system. It would do this via the heat pump / compressor acts as a dehumidifier using the forced air component of the geothermal system.


Integration Design

Integrating all these moisture control components does require significant design thought. To the extent needed, all the controls will be will be automated. The key integration areas are:

  • Optimizing the Earth Tube / ERU - Air Exchanger - Geothermal. Automatically determining either the ERU or geothermal should run and when the whole system should be by passed because the outdoor ambient humidity is a better choice than taking it through the system.
  • Need to integrate the air distribution system to accommodate air from any source.
  • Need to automate the moisture control with the all other components.
  • The controls will need to deal with the Natural Ventilation design - The natural ventilation is expected to be able to assist i about 50 - 90 days per year during the spring and fall months. Clearly in the months of extreme high and low humidity, the natural ventilation most likely not be used because of the corresponding low and high temperatures. But one could imagine a humid spring or fall day when the temperature is appropriate but the humidity is not. This would require a behavior aspect to the cotrols to keep the windows closed during these times. More information is available on our Natural Ventilation pattern page.


Moisture Modeling

We used WUFI to model the relative humidity and dew point performance of the wall system. (Capture a picture of it. use the end results...showing low moisture build up.)

The results of WUFI did cause us to add more rigid insulation (an additional 2 inches) between the bricks and cellulose to reduce moisture build up in the center of the wall.

Link to WUFI site to get software.


Supporting Science

The detailed thermal calculations are shown in pages included here. We thank Laurie Catey for her great contributions to our understanding of the how to work with the natural systems through a better understanding the science that describes these .

Proposed Materials / Suppliers

  • The material for the earth tubes can be found on the Earth Tubes page.
  • Manufacturers of the Energy Recovery Unit that we considered during this phase was SEMCO FV Series ERV.
  • The natural ventilation materials can be found on the Natural Ventilation page.
  • The moisture control materials can be found on the Moisture Control page.

Development Story

The Moisture Control System - Development Story page contains many images and videos documenting the process used at the Green Garage to design, build and operate our Hybrid Ventilation System.

Related Internal Links

Resources

  • Passive Haus Institute - moisture
  • WUFI
  • Building science

To Do's

  • Laurie's data
  • Resources
  • WUFI
  • Short Video
  • Image of the earth-tube - ERU - geothermal subsystem
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