Thread: Ice Dams and Mansard Roofs

I have recently completed an inspection on a two-story home with a mansard roof. The mansard is on the front of the house and extends to the floor line of the second-floor. During my inspection, the house had ice dams on the roof, mostly on the mansard section.

The attic is well insulation (six-inch blown cellulose and un-faced batts placed over the blown at a 90% angle); however, the attic has the furnace/A/C unit that provides conditioned air to the second floor. I’m assuming that heat radiating from the furnace, is part of the ice dam problem. The ducts (also in the attic) are circular sheet metal with insulated flexible duct as the outer sleeve. The ducts may be leaking, raising the temperature within the attic.

Since it’s a mansard, the front of the roofing system has no soffits; hence, no place to install vents to ventilate the attic area on that side. Since there are not vents, the insulation has been pushed to the edge of the attic. There is light black discoloration on the underside of the roof deck about four feet up from edge (mansard side).

I understand how the ice dams form. The attic HVAC unit and ducts probably contribute to the problem with heat radiation. The lack of ventilation through the mansard side probably also contributes to the issue.

Would it help if rigid insulation was applied to the roof deck on the mansard side ? The insulation would “shield” the deck from the “warm” attic air and radiation from the furnace.

Yes, but spray foam would be better, just a thin coating, depending on your climate. It would also benefit the HVAC. Better yet, turn it into a conditioned attic space. See BSC website for more information.
One question though, does it have a gas furnace and if so, where does the combustion air come from?

Cheaper short term is sealing air leaks and providing ventilation, but you already knew that.

Its a gas furnace, combustion air comes from the attic. The attic has conventional perforated soffits for ventilation with and a ridge vent at the top. The mansard side has no vents

Rigid insulation applied directly to the roof sheathing can trap moisture and cause decay. Like you said there is evidence of excess moisture already. I live in a cold climate. we are required to apply a rubber membrane(ice shield) to the top side roof sheathing along the eaves and up the valleys. This adheres to the wood and prevents leakage due to ice damming. I also live in a wild fire zone, and we are no longer allowed to vent through the soffit. We use "eyebrow vents", or "through the roof vents" these vents are attached to the roof along the eaves. Perhaps a couple of eyebrow vents will get the air flowing, then install closed cell rigid insulation leaving a 1" air space above.

Poor design having HVAC in the attic. Not the best fix but all the ducts need to be adequately sealed and insulated. An enclosure should be build around the furnace to trap the head and force it to the inside.

The fiberglass batts are likley not adding much R value . Instead of removing them top it off with another layer of cellulose.

Others things to consider fixing are attic bypasses, leaky can lights etc.

It would be a good idea to recommend and energy audit with a blower door test.

In the NBC 2010 for Canada it states that gutters are not required but are recommended. Therefore I would suggest removing the gutters in the area that are subject to ice damming if its not going to harm the exterior to any major extent. Removing the gutters in this area will stop water from backing up and entering the wall. Perhaps having a quick deteachable gutter system would work, that way it can be replaced after the snow has melted in early spring.

There are vents made specially for mansard roofs that install at the bottom of the upper section to allow air circulation.

Most of the old mansard roofs had no provision to vent from the lower or side wall roof to the upper roof and since this was predominantly done to save money to add a second story and prior to most of our current understanding of ventilation and insulation standards that we now take for granted. However, the flashing at the joint of the side and upper roofs should in most cases allow for the ice to dissipate before it penetrates the system unless the builder did not flash this joint.

Ps. i like the newer vent system that are available but don't know if the cost conscience client will want to spent the money.