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Q&A Spotlight

Fixing a Poorly Insulated Roof

A huge air conditioner and roof heating cables can’t overcome basic design flaws

A litany of problems: Chris Butson annotated this photo of his roof to help explain the problems he's attempting to solve.
Image Credit: Chris Butson

Chris Butson’s Utah home sits at an elevation of 6,000 feet and experiences everything from sub-zero temperatures in the winter to 100-degree summer days. Built in 1994, the house has what Butson believes is an underinsulated roof that contributes to big electric bills and massive ice dams.

“From what I have observed, the roof/attic was not insulated well when built,” Butson writes in a post on GBA’s Q&A column. “This has been partly remedied by blowing in extra insulation into the attic, but most of the cathedral ceilings were framed with 2x12s with probably 8-inch or 9-inch thick batts of fiberglass. In short, the roof has lots of heat gain during the summer and lots of heat loss in winter.”

To compensate, the builder installed a huge air conditioning unit to make summers more comfortable, as well as hundreds of feet of heating cables along the eaves to melt ice dams as they form in winter. Does it work? No. “Not surprisingly,” Butson says, “our electric bill is quite high and we have massive ice dams.”

Butson has done his homework. His solution includes adding rigid foam insulation of some kind on top of the existing roof sheathing to get the R-value up to 49, adding blown-in insulation to the attic, improving attic ventilation, eliminating skylights, and building a conditioned mechanical room in the attic for the furnace and AC equipment.

“Does this plan sound reasonable, or am I missing something fundamental?” he asks. “Why do I never see heating cable in cold regions in Europe? In fact, I have several German friends and colleagues and they don’t even know what I’m talking about. Is my perception correct and if so what is different about the way their houses are built?”

That’s the topic for this Q&A Spotlight.

Air-sealing is key

While Butson’s plan looks reasonable, Steve Knapp writes, the new insulation will do more good if the structure is as airtight as possible. “One thing I would suggest is undertaking a thorough air-sealing exercise,” he says, “in combination with blower door testing.”

Good advice, adds GBA senior editor Martin Holladay.

“In addition to following Steve’s advice — performing air sealing measures — you’ll also need to find a way to block the air intake into the ventilation chutes in your existing cathedral ceilings, and to also block the tops of these ventilation chutes near the ridge,” Holladay adds. “Adding rigid foam above the roof sheathing to supplement the performance of the fluffy insulation between the rafters is a great idea, but that approach will only work if you can exclude exterior air from the ventilation channel between the top of the fiberglass batts and the underside of the roof sheathing.”

After sealing those areas, he says, Butson can (if he wants) build new ventilation channels above the new rigid foam insulation in the roof. These new ventilation channels should connect soffit vents and ridge vents.

Get rid of the heating cables

Heating cables melt accumulating ice along the eaves, preventing ice dams from forming. The cables are a familiar sight on older homes in snow country, but they are evidence of inadequate insulation and air leaks.

Jon R suggests that Butson make sure the cables are plugged into a smart controller so the cables use less electricity than they would with “dumber” controls.

But Holladay would have Butson remove the cables completely.

“Installing heating cables at the eaves of a roof is a terrible idea, so the simplest explanation is that Europeans are smarter than Americans,” he says in answer to Buston’s query about why he’s never seen heating cables on any European roofs. “Seriously, though: most areas of continental Europe don’t get as much snow as New England or the upper Midwest, so ice dams are rare in the Netherlands, Belgium, or the lower elevations of France or Germany. “

In colder parts of Europe like Scandinavia, he adds, building practices are better than they are in the U.S.; most houses in Scandinavia get more insulation and better air barriers than American homes.

The heating cables should be unplugged and removed, he says. “The fact remains that any house with heating cables at the eaves suffers from fundamental design errors that can be avoided during the design and construction phases of any new home,” Holladay says.

Use 2x4s to create an air gap

The roof probably needs ventilation — just not the ventilation that’s currently built into it.

As Holladay explains, ventilation typically includes vents at both the soffits (which admit air) and at the ridge (which allow the air to escape). The air flows above the uppermost layer of insulation and exits the building at the top of the roof, carrying with it moisture that would otherwise accumulate in the insulation or on the bottom surface of the roof sheathing.

These existing air passages will be in the wrong location if Butson adds new rigid foam above the existing roof sheathing, so these are the ventilation channels that should be sealed when the new foam is installed. Butson can add new ventilation channels in the roof when he adds the new rigid foam. One way of accomplishing that, Holladay says, is to install 2x4s on the flat, 24 inches on center, over the rigid foam on the roof. These 2x4s are installed so they’re perpendicular to the ridge, creating ventilation channels 1 1/2 inch deep.

If the roofing requires solid sheathing, Butson can add a layer of OSB or plywood over the 2x4s, followed by underlayment and roofing. If he wants the install screw-down metal roofing, he could install 1×4 or 2×4 purlins over the first layer of 2x4s, and attach the roofing panels to them.

Holladay points out that the ventilation channels above the new rigid foam are optional. That said, the ventilation channels reduce the chance of ice dams in snowy climates, making them advisable in Butson’s case.

Local builders aren’t getting it

Butson says the local builders he’s contacted haven’t been of much help.

“More broadly, I am slowly coming to terms with the fact that I will have to do most of the engineering work for this renovation,” he says. “I have hired an architect who can create plans and construction details for what I want to execute. What I have not been able to find is a builder who seems to understand these issues or possible solutions.”

Butson has spoken to several builders, but they all insist that he will need heating cables.

“I fundamentally disagree with this, but I doubt I will be able to convince them otherwise,” he says, “and even if I do them, I don’t have confidence in their ability to execute the necessary modifications and details.”

For homeowners in search of a builder who is familiar with building science principles, one option, according to Holladay, is to contact a local energy rater certified by RESNET or the Building Performance Institute. To find a local energy rater, Butson can use the search function on one or both of these two websites.

“Call up the energy rater and ask for the names of local contractors who understand energy issues, building performance issues, and building science issues,” Holladay says. “Good luck.”

Our expert’s opinion

GBA technical director Peter Yost had this to add:

Heating cables address the symptom, not the cause. Ice dams need lots of conductive and convective heat loss at the eaves. You can’t ventilate your way out of ice dams either: you can’t move enough cold, dry air to overcome the warm, moist air leaking at the eaves.

While ventilating the new roof deck is a good idea, you don’t necessarily need that ventilation; there are plenty of well-performing, durable “hot” (or unvented) cathedral roof assemblies. They all have continuous air and thermal barriers at the eaves and tops of the eave walls.

A great resource for understanding ice dams is Bill Rose’s Water in Buildings, Chapter 4, which reads in part: “To correct existing ice dams, identify the vagrant heat source and correct it…. Venting is limited in its ability to dilute excess heat — limited by low quantities of flow through vents and a low heat capacity in air. Unfortunately, heat losses, especially stupidly wasteful losses… have no such physical limitations.”

The tougher question sounds like who will do the work and do it well. Continuous air and thermal barriers require workers who respect the building science of hygrothermal flows. The best suggestion I have — besides Martin’s recommendation to ask for advice from BPI-certified professionals — is to contact local and state home builders associations and ask them for a list of their members who have completed the Advanced Green Building: Building Science two-day course. (I was one of the primary authors of this course and it was developed using a ton of content from Green Building Advisor.)


  1. Melanie Paskevich | | #1

    Hot, not cold, right?
    Doesn't Peter mean to say "there are plenty of well-performing, durable "HOT" or unvented cathedral roof assemblies?" He wrote "cold." It's always been my understanding that an unvented roof assembly was a "hot" roof...

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to John Birch
    You're right, of course. Thanks for catching the error. I have corrected it.

    The phrase "cold roof" refers to a vented roof assembly. The phrase "hot roof" refers to an unvented roof assembly.

  3. Walt Ott | | #3

    attic ventilation, cfm of soffit / ridge vent
    There are many references at GBA on the number of inches of NFVA are needed but how does this equate to CFM ? Is the amount of CFM dependant on the roof's slope? Air temperature both inside and out?

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Walt Ott
    Building codes have mandated that vented unconditioned attics be equipped with ventilation openings that meet certain requirements. The standard code formula requires 1 square foot of net free ventilation area (NFVA) for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the lower half of the attic (generally at the soffit) and half near or at the ridge. If a roof has only soffit vents and no ridge vents, most codes require 1 square foot of net free ventilation area (NFVA) for every 150 square feet of attic floor area.

    These code requirements are not based on science. Research shows that the most important factor in preventing moisture problems in attics is to create an airtight ceiling, not to provide ventilation.

    There is no direct correlation between NFVA and CFM of air movement. Research shows that ventilation through soffit vents and ridge vents is almost entirely dependent on wind speed.

    For more information on these issues, see All About Attic Venting.

  5. Walt Ott | | #5

    attic ventilation, cfm of soffit / ridge vent
    Well I guess I should ask the base question. We had ice dams.

    this summer I've sealed and Insulated as much as possible. I may need ventilation( we'll find out this year) there is no soffit, there is almost NO room above the top plate or for several feet inward from there.
    So here my crazy idea: I can vent at the top of the roof and allow air to escape and I can also pull in cold air from the outside and duct it to the bottom of each bay. Would this work ? How much air CFM do I need?

    Thanks again

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Walt Ott
    For advice on ice dams, I suggest that you read these two GBA articles:

    Prevent Ice Dams With Air Sealing and Insulation

    Ice Dam Basics

    If you have only a limited area between the top plates of your exterior walls and the underside of your roof sheathing, it's hard to get enough insulation as well as a vent channel in this critical location. In that case, by far the best solution is to install an adequately thick layer of rigid foam above the existing roof sheathing. For more information on this work, see How to Install Rigid Foam On Top of Roof Sheathing.

    Adding ventilation channels without addressing air leaks and insulation issues is rarely successful. If you end up installing thick rigid foam above your rigid insulation, however, you can install ventilation channels above the rigid foam if you want, followed by a second layer of roof sheathing and new roofing. The work isn't cheap, but it's a robust solution to ice damming problems.

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