Michael Thornton posted a blog on his "Photo of the Day . . .that reminded me of another problem associated with the venting of attic spaces.  Most older gable roof style homes have gable vents (which under most circumstances are adequate).  Older homes may or may not have soffit vents. Current standards require soffit vents in conjunction with roof vents or continuous ridge vents, and the amounts of each is a specified amount based on the size of the space being vented.  Typically, by today's standards (with exceptions):  The total net free ventilating area shall not be less than 1 to 150 of the area of the space ventilated (2006, IRC).  Simply put this just means that if you have a 1500 square foot attic you will need 10 sq ft of venting, ½ of which will be low (at the soffits) and ½ of which will be high (at the ridge).

     Roofing manufacturer's warranties often times are "voided" if the space under the roof is not ventilated or inadequately ventilated.  For example Certainteed's warranty states:  "protection does not extend to any shingles applied to non-ventilated or inadequately ventilated roof deck systems" (with exceptions of course).  The point is that these spaces need to be well ventilated.  Ridge vents in conjunction with soffit vents provide the best means of venting these spaces because a "Venturi effect" is created with this type of venting that improves the air circulation.  For more information on the Venturi Effect visit:  http://en.wikipedia.org/wiki/Venturi_effect.  (There are some newer means of insulating roofs that eliminates the need for venting---and we are likely to see changes in the shingle manufacturer warranties as a result----but that is the topic of another blog) 

     Problems arise when there are combinations of different venting systems that can result in a "short-circuiting" of function.  Installing gable vents, or leaving old ones installed, reduces the likelihood that all areas of the space will have adequate air circulation and can result in air moving directly from the gable vent to the soffit vent (much like the example that Michael talked about with the power vent being located right next to the gable vent in his Photo of the Day . . .).  Every home must be evaluated on a case by case basis because there are many variables.  The inspector will be able to tell by his or her visual inspection whether there are problems or are likely to be problems with the way the space is ventilated.  New construction can be more difficult to assess because of a lack of "history", but even so there are things to look for. Here are a couple of pictures of ridge vents.  The first one is from the 70's (re-installed on the newer roof) and the second one is a more modern style and is capped with shingles.

 Charles Buell

www.buellinspections.com