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 one of the professionals that has contributed extensively to our net-Zero Energy design. For more on moisture control please see our Moisture Control pattern page.

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 Air Exchange
    • Dew point and condensation drainage in the Earth Tubes
    • Desicant?? wheel in the ERU exchanges the latent energy

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 B's concept:

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 Air Exchange

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

  • Dew point and condensation drainage in the Earth Tubes
  • Desicant?? wheel in the ERU exchanges the latent energy


Integration Design

Integrating all these components does require significant thought. Some of the controls will be manual (even behavioral) and some will be automated. The key integration areas are:

  • Optimizing the Earth Tube / ERU - Air Exchanger. Automatically determining when ERU should run and when the whole system should be by passed because the outdoor ambient temperature / humidity is better than taking it through the earth tubes.
  • Need a behavioral based system to determine the days / hours that windows can be opened so that we don't have windows opened and the ERU and/or running at the same time.
  • Need to integrate the air distribution system to accommodate air from any source.
  • Need to automate the moisture control with the all other components.

Natural Ventilation Design - The natural ventilation is expected to be able to assist in about 50 - 90 days per year during the spring and fall months. Clearly in the months of extreme weather, the natural ventilation would make no contribution. The natural ventilation is provided through the ability to open the buildings windows. 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 Hybrid Ventilation 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
  • ZED Factory

To Do's

  • Laurie's data
  • Resources
  • Change === on all pages (Peggy)
  • Short Video
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