Air and Vapour Control
Air and Vapour Control
Contents
1 General
Vapour retarders are used to retard the flow of moisture vapour from the interior of a building into the roof system. Moisture vapour flows outward by vapour diffusion and air leakage.
2 Vapour Diffusion and Condensation
A VAPOUR is the gaseous form of any substance, but it commonly refers to those gases, which exist as liquids or solids under normal temperatures and pressures. To fully understand the behaviour of vapours, a simple model should be considered: a sealed container of air with water at the bottom. The liquid molecules are almost as close together as with solid matter but they are not held in a rigid pattern. These molecules are in a constant state of thermal agitation due to its kinetic energy. (This motion is easily evidenced by placing a drop of dye or ink into a glass of water. It will diffuse relatively quickly into a uniform mixture). Some of the molecules near the water will break free (evaporate) due to this motion and diffuse within the air forming water vapour. The vapour molecules are still in a constant state of motion and will collide randomly with the container's surface, with each other, and with the remaining water. These collisions create a pressure on the surfaces known as VAPOUR PRESSURE; the greater the number of collisions, the greater the pressure.
If more heat energy is added to the container, more vapour molecules break free from the water to diffuse in the air. The air is said to be saturated when the number of molecules escaping the water equals the number colliding back into the water. In this state, a point of equilibrium has been reached and the air can hold no more vapour unless additional heat energy is added. Similarly, if the container is cooled, the thermal activity decreases, less vapour molecules break free than return to the liquid, and vapour molecules begin to condense into droplets on the sides of the container (condensation). Therefore, it can be stated that warm air can hold more moisture vapour than cold air.
The pressure exerted on the container by the air/vapour mixture is the sum of the air pressure and moisture temperature and the relative humidity. Temperature has the most effect as the pressure increases exponentially with raises in temperature and linearly with increases in relative humidity. When differences in vapour pressure exist on either side of a material, moisture vapour molecules will attempt to “flow” through the material and equalize the pressures. The moisture vapour molecules will flow at a rate dependant upon the pressure differential, the thickness of the material, and the material's permeance.
PERMEANCE is the measure of a material's ability to permit the flow of vapour. Permeance (SI) is measured in ng / (Pa*s*m2), or nanograms per second for one square meter and one Pascal pressure difference. Permeance (ASTM E96) is also measured in grains of water (700 grains of water = 1 pound of water vapour) per hour for one square foot area and one inch of mercury (1" of mercury = 0.491 psi).
To fully understand how vapour behaves, RELATIVE HUMIDITY (RH) and CONDENSATION must be examined. When the water and air/moisture vapour mixture reach a point of equilibrium and the air is said to be saturated, as discussed earlier, the air is said to have a relative humidity of 100% for those temperature and pressure conditions. Humidity is a term used to describe the amount of moisture vapour in the air. Relative humidity is the ratio of the amount of moisture vapour actually present in the air to that amount which would be present if the air were saturated at that temperature. Relative humidity is expressed as a percentage, and is calculated as follows:
| RH= | mass of vapour present | x 100 |
| mass of vapour if saturated |
If a quantity of unsaturated air is kept at a constant temperature and the moisture vapour is changed, the relative humidity will increase as moisture vapour is added and will decrease as moisture vapour content is kept constant. If the temperature is changed, the relative humidity will decrease as the temperature increases and will increase as it is cooled.
As the unsaturated sample is cooled, the point of saturation may be reached as the molecules lose the ability to remain separate and droplets will form. This is known as the DEW-POINT TEMPERATURE; i.e., the temperature at which a sample of humid air cools from its original temperature, becomes saturated, and condensation occurs.
In buildings located in moderate to colder climates, the conditions exist for problems with vapour diffusion and condensation. The building's interior is kept warm and humid while the exterior is colder and drier. This creates a vapour pressure differential and, as discussed earlier, the moisture vapour will attempt to flow through the material which separates them: the roof and walls. Since heat and vapour flow tends to be upwards, the roof is the “benefactor” of most of the pressure. By insulating the roof, the problem of condensation on the roof deck surface has been eliminated, but not the pressure differential. The vapour pressure now may result in moisture vapour flow up into the insulation. As the moisture vapour travels closer to the exterior, the vapour cools and the dew-point temperature may be reached and condensation within the roof system can occur. Since the roof membrane is traditionally placed above the insulation the moisture is trapped within the system and can cause a roof failure and / or structural problems within the building. Severe condensation is a serious problem.
To calculate the relationships between relative humidity, vapour pressure, and temperature, a PSYCHROMETRIC CHART should be consulted. This chart will reveal the relevant vapour pressure and the temperature at which a given sample of mixed air / moisture will condense. Given this information and the design temperature and humidity for the building's location, the design authority can determine the vapour pressure differential and dew-point temperature, and determine the requirement for a vapour retarder, insulation value, etc.
3 Air Leakage
Although vapour diffusion may be a factor, the most common cause of moisture vapour condensing in a roof system is air leakage. Air leakage occurs through poorly sealed joints and through material imperfections, such as cracks and punctures. The major differences between air leakage and diffusion are the quantity involved and the causes, but the result is the same: moisture vapour condensing within the system. With air leakage, the quantity of water vapour is likely to be much greater than that with diffusion. Air leakage occurs due to differences in air pressure. As with vapour pressure, air tends to flow from areas of high pressure to areas of low pressure.
Air pressure differences resulting from temperature differences between inside air and outside air can sometimes result in a stack effect, similar to a chimney. This effect will cause infiltration of air at lower levels and exfiltration at higher levels: most notably, the roof. Wind will also add to the problem, resulting in infiltration on the windward side and exfiltration on the leeward side. Mechanical systems may also pressurize the building, causing exfiltration. All three factors add to the stack effect and create a large outflow of air trying to escape through the roof. The air pressure differences result in warm humid air flowing outward into the walls and roofs causing the moisture vapour to condense and leading to possible roof and other failures.
4 Causes of Roof Failure
Vapour that is allowed to flow upward through a roof system, whether through diffusion or leakage, may result in roof failure. Some of the common problems posed by moisture vapour are:
- condensation within the insulation, which can reduce or eliminate most insulation's the thermal resistance. The thermal conductivity of water is approximately twenty times that of air. Freeze-thaw cycling of the moisture may break down the insulation.
- absorbed moisture, which can contribute to the decay of organic roofing materials and insulation.
- trapped moisture, which can freeze and cause delamination of the membrane.
- condensation trapped within voids, which can expand with the sun's heat and create blisters.
- condensation which collects on the underside of the insulation and flows through joints or cracks in the roof deck, which can then leak onto suspended ceilings or into the roof assembly.
5 Vapour Retarder Materials
The purpose of a vapour retarder is to retard the flow of moisture vapour from the interior of the building into the roof system. The decision to use a vapour retarder is up to the design authority, as it relates to the design and occupancy of the building. The design authority should perform the necessary psychrometric calculations to determine the requirement for, and the permeance rating of, the vapour retarder.
The vapour retarder systems most commonly available are:
- bituminous vapour retarders
- laminated kraft paper vapour retarders
- vinyl film vapour retarders.
The key to any vapour retarder is in its continuity. The vapour retarder must be sealed at all laps, penetrations, etc., to ensure continuity.
Bituminous vapour retarder systems are commonly used and are usually either:
- two plies No. 15 felt, mopped
- an asphalt-coated base sheet with mopped foil laminate.
- proprietary systems offered by flexible membrane manufacturers.
Laminated kraft paper vapour retarders are usually composed of glass fibre-reinforced kraft paper. Factory Mutual may approve kraft paper laminated with fire-retardant adhesive for use in Class 1 construction. Asphalt is also used with this product to attach the laminated sheets to steel roof decks and to attach insulation to the vapour retarder. Kraft paper laminated with asphalt adhesive must be applied with a fire-resistant adhesive that meets Factory Mutual requirements for Class 1 construction.
The claimed principal advantage for kraft paper retarders is that they possess greater strength than common felts due to glass fibre reinforcement.
Vinyl-film vapour retarders are composed of polyvinyl chloride (PVC) sheets and compatible asphalt-based or rubber-based adhesives. The vinyl sheets serve as the vapour retarder. The cold adhesive is used to adhere the vinyl sheets to the base layer of insulation and the upper layer of insulation to the vinyl sheets.
Vinyl-film vapour retarders were developed to meet standards established by Factory Mutual for Class 1 construction on steel roof decks. However, Factory Mutual tests materials only for their wind uplift and fire resistant properties; a material's performance as a vapour retarder is not tested. The use of vinyl-film vapour retarders should be restricted to steel roof decks.
The design authority should not consider using sheet polyethylene as a vapour retarder in roofing assemblies utilizing hot asphalt.