This information will help DIYers avoid what could become costly mistakes. If instead you are engaged in the process of selecting a contractor, then this is information to help you knowledgeably discuss your project and understand the bids that are submitted for your consideration.
What is Proper Attic Ventilation?
If you donít know what proper attic ventilation is, how can you sort the wheat from the chaff in discussions with potential contractors or the floor staff at your local big box store?
If your project is to re-roof your home and a potential contractor preparing a quote doesnít begin the process by examining the ventilation provisions for your attic, he is probably not a good choice and you should send him away.
There is some confusion about necessity and how much attic ventilation is correct. Letís first address necessity:
Attics that are not vented properly cause heat and moisture to remain trapped in the attic and create problems like wood rot, mildew and mold not only in the attic, but throughout the home. In extreme cases there can even be deterioration of your homeís structure.
Attic ventilation has such an effect on roof coverings that all major shingle manufacturers void their warranties if their shingles are installed over improperly ventilated attics.
How much Attic Ventilation is needed?
Letís start with the simplest answer. It is an attic for which the total of inlet and exhaust Net Free Area (NFA) in square feet is equal to 1/300 of the open attic floor area in square feet. Unless your building codes call for the newer ratio of 1:150 which results in double the amount of NFA. Letís say your attic floor area is 30 ft. by 50 ft. That equals 1,500 sq. ft. Then, 1500/300 yields 5 square feet of total NFA or 2.5 SF each of inlet and exhaust. These values are minimums and may be exceeded by any amount. But, if the inlet and exhaust areas are not equal, then the inlet area must be the larger of the two.
Net Free Area
Net Free Area (NFA) is the amount of opening available through a vent cover. It is not the total area of the vent covers. In fact, a value of 50% , or less, of the are of the vent cover area is typical. This product information page from Lomanco, Inc. demonstrates this point. An 8-foot long section of 2.75-inch wide continuous soffit vent cover has a total area of 264 sq. inches. The NFA of that product is 72 sq. inches, or slightly more than 25%.
Watch out for these installation shortcuts
Improper openings behind vent covers
Without inspecting the openings behind rectangular vent covers, you cannot realistically estimate the inlet vent area. This picture is an example of why that is so. Note that the vent holes that are cut in the eave soffit donít even begin to approximate openings in the cover. Those undersized holes significantly reduce the NFA of the inlet vents.
The image to the right shows how to NOT install loose fill insulation. These are mistakes that a DIYer might make. The most obvious mistake is that the insulation covers the eaves and leaves no space for ventilating air to enter through eave soffit vents.
There should be baffles installed to keep the insulation from covering the vents We will ignore the fact that the most efficient way to install loose fill insulation is by using a blower system.
One last point to make here: Using unfaced fiberglass roll or batt insulation is the simplest way for a DIYer to supplement the amount of insulation in the attic. When laying the fiberglass insulation on top of insulation already between the ceiling joists, install the new layer perpendicular to the direction of the joists.
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