![]() Insulation is rated in R-value, with the higher the R-value the greater the resistance to the movement of heat. Insulation prevents heat from traveling through surfaces from hot to cold. There are some exceptions such as closed cell foam (which is all three) and open cell foam which is an air barrier and insulation (but not a vapor barrier). Insulation generally isn’t an air barrier or a vapor barrier and air and vapor can move through it freely and easily. If you fail to have vapor barrier, there is less likelihood of moisture issues within the wall structure, but more likelihood of moisture issues inside due to moisture diffusing into the space through the walls. This is why a properly installed vapor barrier is located in a place that allows for drainage on the warm side that will be prone to condensate. If water vapor makes contact with a surface below dew point, it can condense into liquid water, which can then result in nasty biological growth. Over time moisture in the vapor state can work its way through block, plywood, or whatever else is used to sheath the walls and roof unless an appropriate vapor barrier is installed. In Florida, we have many block homes that have no exterior vapor barrier at all other than paint. This is when vapor molecules (in this case, water vapor) can move through a porous material in the direction of high relative humidity to low relative humidity. Water vapor can move through many surfaces through a process called permeation. The leakage rate can be tested using a blower door and a precision manometer can be used to figure out the pressure differential impacting the space and areas within the space. ![]() Others are caused by duct leakage, imbalanced return/supply into a space within a building, and due to ventilation both overall and localized such kitchen hoods and bath fans. Some pressure differences are natural due to stack effects, wind, etc. This air leakiness of a home is impacted by how leaky the home is, and how great the pressure differences are inside to out. This requires testing and planning, but it is the best way to make a home airtight. It is a much better strategy to bring in an appropriate amount of outdoor air from a clean and designed location and temper it through filtration, energy recovery ventilator / heat recovery ventilator, or a dehumidifier as appropriate for the climate. This is true, but you also don’t want the outdoor air entering from musty attics and crawlspaces and across dirty floors. Some people may say that you don’t want a building to be too tight, otherwise you won’t have the proper amount of outdoor exchange.
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