House ventilation, air movement in the attic, in the crawlspace and even through our living space is
something that we spend a great amount of time, effort and money trying to manage, and for good
reason. The uncontrolled movement of unconditioned air brings with it extremes, whether it be moist,
or dry air. Both of which can have adverse effects not only on our homes but also affecting our health
Crawl space ventilation is definitely controversial; I’d like to provide you with two schools of thought,
one being the Ontario Building Code (OBC) version, and the second being the building science version.
The OBC states that in an unconditioned (no heat) crawl space we need to incorporate not less than 1.1
square feet of venting for every 538 square feet of floor space. The venting should be such that it will
allow for cross ventilation or “uniformly distributed on opposite sides”. Understanding a little about air
movement and temperature leads us to the common sense, or building science end of things.
Late August, temperature outside in central and southern Ontario can reach 80 degrees F. (30 degrees
C) the relative humidity is hovering around 80% if the relative humidity (RH) reaches 100% that is call
the dew point which means the air can no longer hold any more moisture and we have a change of
state from vapour to condensation.
The temperature in our basements this time of year (August) is always considerably cooler. A
crawlspace is the same. In fact these areas usually maintain a temperature of about 65 degrees F (18
degrees C) all summer long. Building sciences and just simple physics has proven that warm air always
travels to cold. We also know that for every one degree in temperature drop our RH rises by two
percent. With this knowledge it is easy to calculate that if it is 80 degrees outside and a RH of 80% and
we know that warm air is going to move to cold air. A crawl space with open vents will readily pull the
moisture laden air into this cooler space (65 degrees). We have a 15 degree differential in temperature
which means the RH is going to increase 30%. That means we have surpassed the dew point and we
have now just induced moisture in the way of condensation into our crawl space by having open vents.
(As a sidebar if you have an earth or rock floor a complete covering with a 6mm polyethylene (plastic) is
Attic spaces on the other hand work a little differently than the crawl spaces, the ratio for venting here
is 1 square foot of venting for every 300 square feet of insulated ceiling space, this assuming your roof
pitch is greater than four in twelve (each of the square roof vents equals about half of a square foot).
The building code will have you believe it is important that this ratio is split up properly between soffit
(overhanging portion of your roof) and rooftop venting. The OBC’s intent is to ensure air movement will
migrate any heat loss that passes from house to attic space out of the attic as quickly as possible. Warm
air that is trapped in this space in the winter months can contribute to ice damming that can cause
insulation, and roof sheathing damage as well as interior water damage.
One of the most common causes for the restrictive ventilation in the attic is when the soffit venting
becomes plugged with the attic insulation. Venting in this space is a two part process if we do not have
an area for the air to come in the flow of air becomes restrictive. The end result will be an attic space
that is changed from an atmospheric pressure to negative or vacuum type pressure. The results of this
are inducing air leakage into the attic from any penetrations that exist in the upper level ceiling.
Examples of this would be light fixtures, fans, even hard wired smoke detectors. With such a negative
pressure build up within an attic it is also conceivable that with the assistance of the natural “stack
effect” (the movement of heated air as it rises from the lowest level to the upper level of the home).
Warm moist air originating in the hollow masonry of a block foundation will travel up the wall cavities
to eventually end up in the attic space. Looking outside of the OBC rules, common sense would suggest
that our attic ratio for venting should look more like 75% venting in our soffits and 25% in the roof line
thereby ensuring that the air movement through the roof is unable to create a negative impact on the
air pressure. Dealing with problems at the source is always a better route, and a good step toward this
is to ensure that the top of your foundation block is 100% sealed air tight.