Ducts need to be inside the insulation not in the middle or on top of it.
Ducts need to be inside the insulation not in the middle or on top of it.

  This past winter, I was called in on a high-end, gut-rehab project in Syracuse, N.Y. The contractor did a thorough, beautiful job except for two details he didn’t fully understand. One was painfully obvious: Ice buildup on the rear elevation and huge icicles told the story. The other is invisible unless you know what to look for — or until it becomes an issue.

The Problem

On most of the project, the undersides of the roof sheathing had been sprayed with half-pound foam when the original flat ceilings were removed and cathedral ceilings created. The roofs in those areas developed ice only where two roof planes converged, creating a funnel effect and ice. In the problem area, the original ceiling remained in place and the old insulation was “sort of” improved — there were multiple recessed lights; many penetrations for plumbing and wiring; a laughable access hatch; and a furnace and ductwork in the attic.

To demonstrate the problem, I set up a blower door and showed the builder and the homeowner how much warm air was leaking into the attic.

The Paradox

Low-density foam on the underside of the roof amd im the walls keeps the roof cold, preventing snow from melting and creating ice when it hits the cold lower edges of the roof.
Low-density foam on the underside of the roof amd im the walls keeps the roof cold, preventing snow from melting and creating ice when it hits the cold lower edges of the roof.

Startling, but true, the natural air leakage for the house was less than half the minimum recommended fresh air ventilation level for that home. The remodeler did such good work everywhere except the one attic area that the house actually needed more fresh air.

Inadequate ventilation is an indoor air quality problem. The house was too tight but, at the same time, part of it was too leaky.

The Solutions

The fix for the heat loss and ice dams in this case is to foam the underside of the roof and bring the attic space inside the building envelope. That way, air sealing the attic floor is irrelevant and the HVAC is where it belongs — inside the conditioned space.

The solution to the air tightness is mechanical ventilation. The best practice is to make the house as tight as possible and then ventilate it mechanically so the occupants get the appropriate amount of fresh air from a healthy source. The remodeler followed best practice and didn’t even know it.

The Take-Home Message

Avoid problems instead of reacting to them. The homeowners and their contractor raised a common question: “How come nobody explained this to us?” There wasn’t anybody whose job it was to do so. The insulator thought the attic was what “everybody” does. The HVAC guy often puts equipment in attics. The architect likes the dormers. The code guy did his job. Make it part of your business plan to consult with someone who specializes in building performance and can knowledgeably review what you plan to do and then check the project from a building performance perspective when it’s done. HERS Raters are a good fit for this important role. Tell ’em Ed sent you.

—Ed Voytovich, a principal at Building Efficiency Resources has been involved in repairing, improving, remodeling, or building homes since 1972.

READ MORE: Click here to see a spray polyurethane foam case study focusing on an insulation retrofit in a 100-year-old house.