These are two of the most dramatic changes to industry standards in the last 15 years. Most contractors use ice and water shield now. Not as many give enough attention to proper ventilation. The irony is that if you take care of the ventilation issues the ice and water shield becomes far less crucial. Ice and water shield is more of a bandaid to get you through. Ventilation is the surgery to fix it.
Ice and water shield creates a water resistant barrier on the roof deck, under the shingles. It's placed at gutter lines (run high enough to get at least 2 feet into heated attic space), in valleys, around chimneys, and up side walls. If you have ice damming, it is very good at keeping water from getting to the roof deck or beyond. The problem with ice damming is that during the freeze and thaw cycle, eventually water can back up under shingles. And this is where ventilation becomes relevant. If you have proper ventilation, keeping your attic colder in winter (which is actually a good thing) this cuts down on this cycle and reduces the likelihood of getting backed up water and leaking.
As important of a product as ridge vent is, you can't simply install a ridge vent and say it's fixed. The most important consideration is your attics ability to complete the circuit from intake to exhaust, equally inhaling and exhaling from each respective end. Intake vents are near the gutter line. Exhaust are near the peak. A common problem with ventilation is blockage above the outside walls (commonly from insulation). You may have a top of the line ridge vent running the full length of every ridge on your house but if your intake is blocked, you're not getting good ventilation. This needs to be inspected thoroughly on every job and remedied if inadequate venting is taking place.
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