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What is It?
Moisture Control refers to the techniques used to control moisture in a building. It includes dew points in the envelope system, water vapor infiltration, as well as indoor relative humidity. Controlling moisture needs to be considered in the design of many of the building components including:
- Building Envelope - dew points and vapor infiltration
- Ventilation System - dehumidification
- Earth Tubes - condensation and drainage
- Sustainable Window Design - condensation and vapor infiltration
- Also known as: indoor humidity, water vapor control.
Why is it Important?
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 address moisture control when:
- Using super-insulated building envelope designs because these often result in humidity issues.
- The building is in a 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
The sustainability goals for Moisture Control are:
- Maintain the indoor relative humidity at 45% +/- 15%.
- Eliminate dew points/moisture build up in the building envelope.
- Ensure a healthy indoor environment.
- The moisture system 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 envelope system 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 in-depth 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 holes 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, high pressure to low pressure movement, and the earth's constant low underground temperature.
Pressure-based Moisture Control System
This is from Ken Byczynski, owner of KENCO, an insulation company with detailed experience in solving moisture problems. The system involves 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. It operates using intake fans and dampers. This, in essence, squeezes the moisture out of the building.
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 will be in the air exchange and whatever is 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 slightly higher than the outdoor pressure.
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 that are 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 that 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 5 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 its entire length. It would drain through one-way geo-tex material and into 6" of coarse 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 would 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, which acts as a dehumidifier using the forced air component of the geothermal system.
Integrating all these moisture control components does require significant design thought. To the extent needed, all the controls will be automated. The key integration areas are:
- Optimizing the Earth Tube / ERU - Air Exchanger - Geothermal. An automatic determination needs to be made as to whether either the ERU or geothermal should run and when the whole system should be bypassed 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 for about 50 - 90 days per year, during the spring and fall months. Clearly, in the months of extreme high and low humidity, natural ventilation most likely will 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 behavioral approach to the controls to keep the windows closed during these times. More information is available on our Natural Ventilation pattern page.
We used WUFI to model the relative humidity and dew point performance of the wall system. The WUFI manual indicates that "Oak Ridge National Laboratory (ORNL) and Germany's Fraunhofer Institute of Bauphysics (IBP) have teamed up to bring a powerful new moisture engineering tool to North America. For the first time ever, designers and researchers will have a simple, accurate tool to evaluate the temperature and moisture conditions within any building enclosure over time." (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.
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 natural systems through a better understanding of 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 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
- Passive Haus Institute - winter moisture anayysis look down to the middle of the page.
- WUFI website Click here for the download of WUFI 4.2 Light, the current free version.
- Building Science - Moisture Control - Outstanding resource for deep sustainable building design...with the science to back it up.
- Laurie's data
- Short Video
- any photos???
- Image of the earth-tube - ERU - geothermal subsystem
Peggy edited this page :)