Moisture and Condensation Control looks at how the design of the thermal envelope plays an important part in being able to eliminate moisture movement in various building assemblies. There are three different components that we need to consider when examining the performance of an assembly. These are vapour barriers, air barriers and thermal barrier (insulation).
Before I address these three components, we need to look at how moisture is created and moves across building assemblies.
The basic rule to be kept in mind is to keep the outside out and the inside in. This will eliminate most problems in buildings. There can be some major issues which are caused by air leakage. This can be deteriorating for the building components and create health issues for the building occupants.
Many building performance problems can be traced to air leakage through the building envelope. These problems range from high heating costs and poor temperature control in occupied spaces, to rain penetration and the deterioration various components within a building assembly.
Why does moisture move from inside to outside?
Moisture exists and hence can be moved in two forms. Most air actually all air, has a certain moisture content referred to as relative humidity. Humans need a certain amount of humidity to survive and live comfortably, in heated indoor air to maintain a healthy environment (otherwise we get nosebleeds, etc.). It is important to control moisture in that regard.
So, moisture exists as microscopic parts of the air itself. This moisture can be moved right through the building envelope like ghosts move through solids...), as well as through cracks in the building envelope.
When moisture inside the building escapes through defects in the building envelope it condenses, freezes and causes damage inside of the wall. When there is an excessive amount of moisture present in the building, the amount of moisture exfiltrating the building increases accordingly. Hence the more moisture the higher the potential for damage.
The three main causes of Air Movement within the Building Envelopes are:
Pressure Differential Temperature Differential Wind Air pressure differential is the key force that drives both air leakage and vapor diffusion through the building envelope.Some of the major areas where air leakage can occur are chimneys, bathrooms, electrical outlets and anything relating to mechanical services is a major contributor to air leakage like any opening s such as doors and windows, joints at windows and different assemblies like the joints between the foundation and the floors. The foundation junctions, foundation slabs are all places where moisture can accumulate. This is inclusive of the basements due to pressure differential.
Air leakage can occur at several different places.
What drives pressure differential?
The air pressure differential is driven by, or relies on several key factors and components. If there is no pressure difference between the interior and exterior, there will not be air or vapor movement across the building envelope. That is the case in the summer when the pressure differential in the summer is basically the same inside and outside the house. This is due to windows left open and the exchange of air from outside and inside, therefore any moisture accumulation and vapor does not have time to get into the wall.
In the winter however, when you start to close the windows, the buildup of heat within causes pressure differential. The reasons for this are:
temperature differential (hot inside, cold outside heat always travels from the hot area to the cold ones.) stack effect (warm air rises) wind pressure (windward pressure, leeward suction) fan pressurization (kitchen, bathroom and furnace fans)So, air pressure differences of the building envelope follow the different effects. In the stack effect, when warm air rises to the top, it builds a negative effect on the bottom of the building. Therefore, the negative pressure brings in air from the outside. This is not the same at the top of the building which needs to expel some of the warm air. Therefore, there is a stack effect that can create positive and negative pressure differentials. The higher the building the greater is the stack effect.
Wind can also create stack effect. It basically works the same way and moves the wind horizontally on the windward side of the building. This causes a combination of positive and negative pressure on the outside and inside of the building. When the wind flows over the building it causes a negative pressure on the outside and a positive pressure on the inside of the building.
Fan pressurization creates a situation all around depending on the type of fan. In case of an exhaust fan it would push out the air and create a negative pressure inside the building. The top will bring in pressure from the outside and create a positive pressure. Fan pressurization has the effect of bringing in the outside air but the opposite can create a negative effect.
The stack effect is when warm air rises and the taller the building the more will it be affected by stack effect. The movement of air within the building is determined by no open areas and the number of floors in the building.
At the bottom of the building there will be more negative impact to the bottom of the building. The air does not circulate around. The stack effect does not play much of a role in smaller buildings.
Stack effect is a bigger problem the higher the building. Look at high rises with "problems". Usually, it is evidenced at the top floors of the building. First as there is more driven warm moist air trying to escape through the upper part of walls.
Fan pressurization depending on the direction of air movement from the fan, a house can either be pressurized or suck air in from outside. If there is mechanical equipment to push out air for instance an exhaust fan in the bathroom, then you will be exhausting it and that goes on to create a negative effect and cause infiltration of outside air.
In the case of a supply only, the heat recovery equipment will cover bringing the air but also exhausting the inside air, so it is less of a problem. However, in some cases of a furnace it can cause a positive impact on the building.
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