The Question
The hygroscopic properties of wood — its ability to absorb moisture from the surrounding air — have been intuitively understood by humans for thousands of years. However, it has only been in the last century that we have learned how to quantify this capability, and even more recently that architects and building science researchers have begun to investigate its effects on the climate of indoor spaces.
Just as thermal mass can mitigate diurnal temperature swings by absorbing heat during the day and releasing it at night, hygroscopic materials such as wood can smooth out short-term variations in humidity by absorbing water vapor when the air is wet and releasing it when the air becomes dry. This moisture buffering ability means that wood can be used as a passive strategy for maintaining human comfort inside buildings — a strategy that does not require the use of energy-intensive equipment such as fans or air conditioning units.
However, wood is seldom left exposed once construction is complete. Typically, it covered in or treated with some other kind of material, such as paint, oil, or varnish. These materials, known as finishes, alter the physical properties of the wood to which they are applied, including its hygroscopic capabilities. How do finishes affect wood's to absorb and release water vapor?
Just as thermal mass can mitigate diurnal temperature swings by absorbing heat during the day and releasing it at night, hygroscopic materials such as wood can smooth out short-term variations in humidity by absorbing water vapor when the air is wet and releasing it when the air becomes dry. This moisture buffering ability means that wood can be used as a passive strategy for maintaining human comfort inside buildings — a strategy that does not require the use of energy-intensive equipment such as fans or air conditioning units.
However, wood is seldom left exposed once construction is complete. Typically, it covered in or treated with some other kind of material, such as paint, oil, or varnish. These materials, known as finishes, alter the physical properties of the wood to which they are applied, including its hygroscopic capabilities. How do finishes affect wood's to absorb and release water vapor?
The Hypothesis
Finishes can be applied for their aesthetic effects, altering the color, brightness, luster, or reflectivity of the wood underneath. More often, though, they are said to protect the wood from physical or cosmetic damage. Different types of finishes offer varying levels of protection from:
Protecting wood from moisture damage, while a necessary and worthwhile objective, would seem to be at odds with the goal of using wood for moisture buffering. Indeed, one might even question whether a piece of wood so protected would be left with any moisture buffering capacity at all.
Fortunately, not all finishes protect from moisture intrusion equally, and few, if any, can block it completely. It is logical to assume that those finishes designed to be most resistant to moisture will have the greatest impact on the moisture buffering capabilities of wood. Shellac, for example, is quite permeable and can be expected to preserve a significant amount of the wood's ability to absorb and release water vapor. Spar varnish, on the other hand, is used for sealing wooden boats and can be expected to negate most, if not all, of the underlying wood's moisture buffering potential.
- Mechanical damage, such as dents and scratches.
- Chemical damage from spills and skin oils, which can permanently stain the wood.
- Radiative damage from heat and sunlight, which can darken or fade the wood.
- Biological damage from bacteria and fungi, which can cause the wood to rot.
- Moisture damage from contact with both water vapor and liquid water, either of which can cause wood to swell and crack.
Protecting wood from moisture damage, while a necessary and worthwhile objective, would seem to be at odds with the goal of using wood for moisture buffering. Indeed, one might even question whether a piece of wood so protected would be left with any moisture buffering capacity at all.
Fortunately, not all finishes protect from moisture intrusion equally, and few, if any, can block it completely. It is logical to assume that those finishes designed to be most resistant to moisture will have the greatest impact on the moisture buffering capabilities of wood. Shellac, for example, is quite permeable and can be expected to preserve a significant amount of the wood's ability to absorb and release water vapor. Spar varnish, on the other hand, is used for sealing wooden boats and can be expected to negate most, if not all, of the underlying wood's moisture buffering potential.
The Experiment
Tests will be conducted on samples of southern yellow pine, a class of softwood species that are cheap, readily available, and already
common in construction. The moisture buffering capabilities of these woods is expected to become increasingly important as mass timber products gain greater acceptance among architects and building professionals. All samples will be cut to equivalent lengths and sanded to minimize surface effects.
Due to restrictions placed on facility use as a result of the COVID-19 pandemic, the range of finishes to be tested will be limited to those can be easily and safely applied with a brush or rag. Coatings that require the use of a spray gun, UV curing process, ventilation equipment, or special breathing apparatus will not be considered at this time. However, the design of the experimental procedure does not preclude the investigation of such finishes at a later date.
The tests will be conducted within a cubical box consisting of rigid foam insulation surrounded by a plywood shell. Rigid foam insulation is effective both as thermal insulation and as a moisture barrier; joints between adjacent pieces will be sealed with flashing tape in order to prevent vapor exfiltration during the experiment. Water vapor will be introduced into the interior environment through a tube passing through the side of the box, the other end of which is connected to a humidifier.
common in construction. The moisture buffering capabilities of these woods is expected to become increasingly important as mass timber products gain greater acceptance among architects and building professionals. All samples will be cut to equivalent lengths and sanded to minimize surface effects.
Due to restrictions placed on facility use as a result of the COVID-19 pandemic, the range of finishes to be tested will be limited to those can be easily and safely applied with a brush or rag. Coatings that require the use of a spray gun, UV curing process, ventilation equipment, or special breathing apparatus will not be considered at this time. However, the design of the experimental procedure does not preclude the investigation of such finishes at a later date.
The tests will be conducted within a cubical box consisting of rigid foam insulation surrounded by a plywood shell. Rigid foam insulation is effective both as thermal insulation and as a moisture barrier; joints between adjacent pieces will be sealed with flashing tape in order to prevent vapor exfiltration during the experiment. Water vapor will be introduced into the interior environment through a tube passing through the side of the box, the other end of which is connected to a humidifier.
Two sensors will be attached to the underside of the lid of the box. A DHT11 temperature and humidity sensor will be used to monitor the ambient conditions inside the box. The wood samples will be suspended from a 1kg strain gauge load cell, which will record changes in the mass of the sample as it absorbs moisture from the air. Readings from these two sensors will be collected by an Arduino Uno microcontroller and transmitted to a nearby laptop for storage and analysis.
The Arduino is also responsible for maintaining a consistent humidity level within the box for the duration of the experiment. The microcontroller will be able to turn the humidifier on and off by means of a relay that the humidifier will be plugged into. When the DH11 sensor detects that the relative humidity inside the box has risen above 85%, the Arduino will send a signal to the relay to cut power to the humidifier in order to prevent over-humidification of the box. When the relative humidity inside the box falls below 80%, another signal will be sent to restore power to the humidifier.
The Arduino is also responsible for maintaining a consistent humidity level within the box for the duration of the experiment. The microcontroller will be able to turn the humidifier on and off by means of a relay that the humidifier will be plugged into. When the DH11 sensor detects that the relative humidity inside the box has risen above 85%, the Arduino will send a signal to the relay to cut power to the humidifier in order to prevent over-humidification of the box. When the relative humidity inside the box falls below 80%, another signal will be sent to restore power to the humidifier.
Each test will begin by suspending a dried sample from the load cell. After the lid has been replaced and the box sealed, the humidifier will begin to pump water vapor into the box. Over the next several hours, the sample will absorb moisture from the humid air inside the box. That absorbed moisture will increase the mass of the sample, hopefully causing the readings from the load cell to increase over time. However, the load cell may not be sensitive enough to detect the change in mass due to moisture absorption, in which case another method of determining the moisture content of the sample will have to be devised.
Presentations
Proposal |
Background |
Construction |
Methodology |
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