Ice Dams and Attic Ventilation Pt. 2

So here's my follow up on my post on ice dams and some corrective measures.  First of all, correcting what is causing the ice dam should be your primary concern.  If your house is of recent vintage, you're probably going to find it rather easy to correct.  Modern houses (20-30 years old) typically have large overhanging soffits.  Sometimes the house was set up correctly for ventilation and insulation was installed in a manner that blocked the free flow of air into the attic from the soffits.

In the diagram above, you can see how the intake air enters.  It then rises to the roof vents; ridge vents, gable vents or the standard old metal "static" vents and out of the attic.  Sometimes overzealous homeowners or insulation contractors block the flow.  To correct this, vent baffles are available.

These foam vents help to keep the insulation from pushing against the underside of the roof sheathing and blocking the air flow.  But......

Yo must verify that the soffits have vents in them in the first place.  A newer house will most likely have vinyl soffit venting installed, but you never know what's behind it.  In a wood house, you can simply look at the wood soffits and see if there are vent.  Another way is to go in the attic and turn off the lights.  If there are vents you'll see some light coming from the soffits; the more the better.  If there is no light and no visual evidence of soffit vents on the exterior, then you'll probably need to install some.  See below for a few photos of soffit vents.

There are some houses that are extremely difficult to ventilate due to design.  There is always a solution, it may not be perfect and it may not be easy to find a contractor who knows how to do it.  I'll talk about the more difficult vernting solutions in the next blog.

A classic photo of the dreaded ice dam

I had the opportunity to take a photo of a textbook case of ice damming the other day.

So you've got big icicles, and if things are really bad, water is entering the interior near the ceiling.  This is why.  When the temperature hovers around freezing, during the day the attic warms and melts the snow on the roof.  It travels as water to the eaves, where it re-freezes.  As the cycle is repeated, it eventually builds an   "ice dam".
What happens next is that water is trapped behind it and backs up under the shingles causing the leaks.  Leaking may enter the soffit and run down the exterior wall, and in the worst case scenario, the inside of the house.
How to fix it?  Ice guard can be used when a new roof is installed.  It's a rubber membrane that is placed at the lower edges of the roof instead of the familiar tar or felt paper.  It seals around nails and typically is folded over the fascia board, so if water backs up under the shingles, it simply travels over the top of the ice guard to the gutter/fascia.
The other way is to keep the roof cold through proper venting.  The colder the roof/attic space,  it becomes less likely that heat will melt the snow.
More on corrective action and some of the problems that are encountered when doing them in my next blog.