Attic Ventilation

Original roof ventilation goes back to the 1940’s and 50’s when insulation started being used in homes. This is when the original 1/300 ratio of ventilation was established (more on this later), after extreme moisture problems in attics. In the 1980’s when homes started to be closed up tighter in order to conserve energy, the building code was put into place with the 1/300 ratio for homes.Rotted Crown

The 1/300 ratio means that for every 300 square feet of horizontally projected roof area there should be 1 square foot of ventilation, split between top and bottom for maximum efficiency. In other words, for every 300 square feet of living area, not including basements or second or third levels of a home, it would be required to have .5 square feet of ventilation at the top of the roof, known as the ridge vent or pots, and .5 square feet of ventilation at the bottom of the roof, known as an eave or soffit.This ratio is increased to 1/150 when using cross venting, meaning vents at gable ends, or if no vapor barrier exists between the living area and the attic. Over venting at the ridge is a common problem with today’s ridge vents.  It is better to over ventilate the soffit than the ridge vent and is thus stated in building codes.

These ratios are based on studies that were done in the 1940’s when roofs were built much simpler, had smaller pitches, and thus much less area. Furthermore, these are the minimal requirements for a home and in most cases inspectors do not measure or check these calculations. The 1/300 ratio is more of a guideline and most builders only check it insofar as making sure that they have a fair number of vents while still looking aesthetically pleasing, as well as putting as much ridge vent in as possible. Unfortunately when contractors use pot vents because they are not aesthetically pleasing they tend to put fewer vents in.

The Wisconsin building code recommends that in order to follow the 1/300 ratio, between 50% or better of roof ventilation be at the low end of the roof with the remainder being at the top. Therefore it is better to have more ventilation at the soffit or eave then at the ridge.

According to Air Vent Inc. (accepted authority and Supplier), your attic temperature can be as much as 20 to 30 degrees hotter than the outside temperature; however, your humidity should be within 10 percent of the outside humidity. While this is what one research company says there is no set value for what the difference in temperature or humidity should be. In general you want your attic to closely match the outside temperature because if it does not you have a much higher likelihood of creating ice dams in the winter and scorching your shingles in the summer.

In the winter without proper ventilation the heat from your home will heat your attic which in turn can melt the snow from the bottom side creating water that runs down your roof underneath the snow. Once that water reaches the eave or soffit assuming you have an overhang there is no heat from the home and the water will freeze there generally starting in the gutter and backing up to wherever your house starts. The ice can climb vertically and the shingles then do not do anything to stop the water from entering your home.

In the summer improperly ventilated attics can reach extremely high temperatures causing your shingles to basically cook from both sides. The sun beating down on the shingles outside and the attic heat cooking them from the inside. This causes your shingles to deteriorate much faster and can also void your shingle warranty.

If your putting a new roof on or having aluminum soffit and fascia installed this is the time to make sure the ventilation is proper. Ask you contractor if they did the calculations before installing there products or call us and we can make sure it was done right.


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