DIY How To

Is Your Roof Ventilation System Balanced for Maximum Efficiency?

No matter how high the quality of your roofing materials, they will fail rather quickly if the substrate begins to rot. Excess heat and especially condensation trapped inside the attic will wreak havoc on plywood and other sheathing materials. Proper ventilation is crucial.

I learned this first hand many years ago when finishing the attic in my first home. I was turning the attic into a spare bedroom, so the insulation had to go in between the rafters, right up against the roof.

How to ensure your roof is ventilated

A Learning Experience

Today I'm the owner at Findley Roofing and ans expert in my field, however back when I was renovating my first home I was planning a very different career and studying for my law degree. I had a good knowledge of roofing from growing up around the construction industry and help was at hand in the form of my father. Despite this without knowing any better, I insulated the roof without installing a ridge vent and soffit vents. Fortunately, I did not drywall the ceiling right away. Within days, my fiberglass insulation was sopping wet!

The reason? Moisture from inside the house was condensing on the bottom of the plywood and was trapped there by the insulation. If I had not discovered the situation, I'm sure my roof would have been soft and beginning to rot in just a few years.

The Solution

A little research revealed the answer to this problem: a venting system. First, I installed a ridge vent along the length of the roof. There are several different kinds available. The key is to make sure air will flow freely from the bays formed by the rafters right up and out the top.

Ridge vent

The next step was to remove the insulation, and install Styrofoam baffles the length of the roof. These are inexpensive and easy to install. Just staple them to the bottom of the roof in each bay.

Baffles

Again, the key is to insure air flow the length of the roof. The baffles must extend from the soffit all the way to the ridge vent. That way, air is drawn in the bottom, rises as it is heated by the roof, and carries moisture up and out the ridge.

If you have open attic space above the insulation or behind a knee wall, then the baffles just need to extend past the insulation as shown in the picture.

The final part of the system is to create a way for air to enter at the bottom. In most cases, soffit vents will be needed. Again, there are various styles. The most common styles are 6" x 12" rectangles or a narrow 3" vent that runs the length of the soffit.

Soffit Vent Rectangle

If your house has vinyl soffits you can add vented pieces to allow air in.

Soffit Vent Vinyl

The most common mistake is that insulation blocks the air from entering the baffles. For this to work properly, you must make sure the bottom of your air baffles extend past the insulation in your soffit so that air can flow freely.

But Is It Balanced?

What do I mean by that? Well, let me make the point by describing an extreme example.

Let's say you have installed your ridge vent along the length of the roof. For this story, let's say it's 30 feet long.

Now, if you only installed one small soffit vent on each side of the house, can you see why we would call it unbalanced?

Those two vents could never supply enough air to keep up with the amount being drawn out the top. The whole system would be bottlenecked and would not work correctly.

How to Balance Your System

An easy way to make sure your roof venting system is balanced is to use this simple formula:

For every square inch of ridge vent, install 1 square inch of venting in the soffit.

The vents should be spread evenly along the eaves of your roof. This will ensure that there will be adequate airflow to get the job done.

The other question is how large does the whole system need to be? Is it necessary to vent along the length of the roof? Many building codes address the issue this way:

One square foot of venting is required for every 300 square feet of attic floor space. Again, the venting would be divided as half intake and half exhaust.

Of course, if the insulation is not up against the roof, the baffles I mentioned earlier are not necessary. Ventilation is always important, though, in any situation.

Hopefully these tips will be helpful for you on your next building project.

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