Question on Number of Baffles to be Installed

Old 04-07-14, 12:01 PM
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Question on Number of Baffles to be Installed

I recently had some blown-in insulation added to my attic (50 year old house in Texas). Right now, I have baffles (double baffles) just where the soffit vents are located - about every 3 or 4 bays. Should baffles be installed in EVERY rafter bay or is it acceptable to install them just above the soffit vents?

In a perfect world, I'm sure installing more would likely be the best option (more ventilation) but curious to get advice on whether installing them only in the soffit vent bays is acceptable.

Thanks in advance for your time.
Old 04-07-14, 02:35 PM
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The rule of thumb (based on very old and simplistic analysis) is one sq. ft. of net free air flow per 150 sq. ft of attic floor if you have no vapor diffusion retarder. This should be divided equally at the eave and ridge if possible. If you have a vapor retarder then one sq. ft. Per 300 sq. ft. of area.

You should find the net free area associated with your eave vents and determine the sq. inches of open space on the baffles and see how close you are to the suggested minimum. Also, do you have a ridge vent and do you know what brand so that you may be able to track the outflow of air.

Keep in mind that these are VERY basic numbers and that having excess is certainly in your favor.
Old 04-07-14, 03:07 PM
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An important question is, do you have any equipment and or ducts up there? If your ac and ducts are in the attic then better ventilation is important. If no ducts or ac equipment, then the most important parts are air sealing and insulation. Of course the air sealing is easier before the new insulation is installed .

If you have no equipment and sufficient insulation, like 2 feet (just guessing), then the ventilation is less of a factor. Venting a hot attic in a hot climate is not as effective as we would like. At best, any natural ventilation will have little benefit over plenty of insulation. Up in snow country we go with every rafter bay to prevent snow melt and the resulting ice dams.


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