Q: I have a question about attic insulation. I was under the impression that attics should remain ventilated and uninsulated to prevent the buildup of ice dams. Can you clarify?

I have a walk-up attic with floorboards. There is insulation under the floorboards, but nowhere else. There are two walls (inverted v-shape) at the two ends of the house, and in between just the slanted roof down to the attic floor. All the wood framing is exposed. There are two circular vents in the roof.

Should I insulate the two side walls, but not the roof? Or the roof, too? Or nothing? –Craig N.

A: You’re actually half right. Ice damming can sometimes be a little difficult to visualize, but the two primary factors that cause it to occur are a lack of insulation and a lack of ventilation.

When snow lies on the roof, heat loss from inside the house will melt the underside of the snow layer. The water from the snow melt runs under the snow layer until it reaches the eaves, where there is no longer any heat loss from the house, and it will refreeze there in what is called an ice dam. As this cycle continues, the ice dam blocks the flow of the water runoff, causing it to back up on the roof and under the shingles. The joint between the wall and ceiling pretty much marks the boundary between heated and unheated space, making that the most common spot where water damage shows up.

Left uncorrected, ice damming can cause all sorts of problems. The moisture in the attic wets the insulation, which reduces its effectiveness and leads to more heat loss and more ice damming. The moisture also contributes to mold growth, and eventually to structural damage to the wood and drywall.

By increasing the amount of insulation between the heated and the unheated spaces, you reduce the amount of heat being lost from the house and into the attic, which reduces the likelihood of an ice dam occurring.

In your particular instance, you want to get as much insulation on top of the ceiling as possible — ideally about R-38. For your house, this might involve removing some of the attic floorboards in order to increase the amount of loose-fill insulation you can install, or simply adding batt insulation over the tops of the boards. So long as you have a sufficient amount of insulation on the attic floor/ceiling, there is no need to insulate the roof or the gable end walls.

The second factor in preventing ice dams is good ventilation. Having an adequate amount of high and low ventilation in the attic creates a passive air flow that helps flush warm air out of the attic. Known as a "cold roof" strategy, it keeps the underside of the roof closer to the ambient outdoor temperature, which again minimizes snow melting. You also mention that you have only two vents in the roof, so you will almost certainly want to add additional ventilation.

Both insulation and especially ventilation are very site-specific, and the requirements and the methods for installation will vary from house to house. I would suggest that you have a licensed insulation contractor come out and inspect the house, make specific recommendations, and get you an estimate for the work. Incidentally, both the insulation and ventilation will also help greatly with keeping the house cooler in the summer, as well as prolonging the life of the roofing.

Q: I read your article about heat cables on a roof being a waste of money plus still having ice buildup. I’m looking into that now because I’ll be getting a knee replacement and won’t have the fun of freezing three to four times every winter to chop that ice off the roof.

I have a manufactured house, which means bad insulation. The pitch of the roof is a problem for ice buildup. Do you have any suggestions? And if I do decide to go with cables, how do I find a dealer that installs them? –Ken D.

A: Unfortunately, in your case you don’t have a lot of options. With manufactured homes, you have a combination of low slopes and relatively low amounts of attic insulation and attic ventilation, all of which can contribute to ice damming situations. In your particular situation, it’s especially important to keep the lower portion of the roof clear of snow.

One advantage to manufactured homes is that they usually have a rather low eave height. So some people have had good luck using a lightweight snow rake to pull the snow off the lower 3-4 feet of the roof before it freezes. Snow rakes are available at a lot of home centers and hardware stores, or online through places like Amazon.com.

If that’s impractical or just something you’d rather not hassle with, then the heat cables are probably your only other viable option. As far as finding an installer, I’d recommend checking with a good local hardware store or a local retailer of electrical supplies and ask them for recommendations of licensed contractors who do heat cable installations. 

Talk to two different installers and get their recommendations and estimates. See how they would install the cables, how they’d be controlled, how effective they think they’d be, and what type of past experience they have with these types of installations. If at all possible, ask for references from a couple of past clients who’ve had heat cable installations done, and give them a call to see how well the installation worked for them.

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