What do you call a “good” ventilation job. Is it to code, which was based on incomplete knowledge from 45-50 years ago, or is it way beyond code? “Good” can mean al ot of things. Some people think American cars are “good” while others think a Toyota is “good”; can millions and millions of Toyota owners be wrong??? It’s going to surpass GM as the largest vehicle maker in the next year or so.
I have directed people to a picture of a research test site where shingles are installed on the same slope with vented and unvented roof areas…very little to no difference appears in the shingles over the vented and unvented sections but a big difference shows between fiberglass and organic (paper felt) based shingles. No one seems to want to believe their eyes!! It’s not the venting.
When people quote anecdotal stories on a technical subject, I firstly as a scientist have to say there are no controls (different shingles, different colours, different slopes, different orientation, different houses) or scientific rigidity!!! Show me a longterm study with shingles from the same batch installed on the same roof with a segmented/divided attic, half with code venting and half with none. Then I will really have a good look at the results.
As an example: In terms of venting of attics in cool/cold climates for moisture removal, a 10 year old study by the engineering dept at U. of Alberta found that in 4 matched pairs of houses in Edmonton (very cold) and Vancouver (quite temperate/mild), some houses with code venting had more attic moisture than houses with no venting and vice versa!!! Code venting was not the determinant for a dry attic. An observation that came out of the study (and I have seen repeated by others) was that in coastal areas, venting actually adds moisture to the attics so the wood is already storing moisture when the cooler weather occurs…the wood has no/little “absorption” capacity and condensation must form on the interior sheathing surface.