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Q&A Spotlight

How to Insulate a Flat Roof

An architect looks for the best way to insulate a flat roof on a contemporary house — at a price his clients can accept

Is a flat roof the problem? An Atlanta architect is looking for the best way to insulate the flat roofs he often includes in his designs for contemporary houses. Although this photo doesn't depict the house in question, it illustrates some of the challenges faced by flat-roofed buildings.
Image Credit: James Morgan

Most of the houses that Atlanta architect Scott West designs are contemporary, and they typically come with flat roofs. Construction often consists of 12-in. deep I-joists or open-web 2×4 trusses capped with oriented strand board (OSB) sheathing. Roofs are unvented, and the use of recessed can lights is probably unavoidable.

The most common options for insulating these roofs, West writes in a post at GreenBuildingAdvisor’s Q&A forum, would include:

  • Over the roof sheathing, add an additional layer of OSB installed on tapered 2x furring strips to slope the surface of the roof to drains, and then insulate with R-30 fiberglass batts between the rafters. “Not particularly smart or green in any way,” he says, “and requires extensive air-sealing, but as with most things, the most-common way is most affordable.”
  • Place 4 in. of XPS rigid insulation on top of the deck and 4 in. of closed-cell foam on the underside of the deck for a total R-value of about 44.
  • Place 4 in. of XPS rigid insulation on top of the decking, 2 in. of closed cell foam under the deck as an air barrier and then add R-30 batts for a total R-value of about 62.
  • Skip the XPS above the deck and use sloped decking as in the first option, then use 2 in. of closed-cell foam and fill out the remaining 10 in. of the roof cavity with dense-packed cellulose for a total R-value of about 47.

Among West’s questions are:

  • What option provides the best R-value for the money?
  • Does the use of closed-cell spray foam under the roof sheathing make it harder to track leaks in the future?

“Any help in assessing these options and especially in comparing real-world costs wold be greatly appreciated,” West writes. That’s the topic for this Q&A Spotlight.

First, lose the can lights

What jumps out at GBA senior editor Martin Holladay first is West’s use of recessed lighting in the ceiling below the roof deck.

“It’s too bad that you (as an architect) don’t have the ability — either the persuasive power or the judgment — to insist that an insulated ceiling have zero recessed can lights,” Holladay says. “If the can lights are there, it clearly makes no sense whatsoever to depend on fluffy insulation (either fiberglass or cellulose) sitting on top of the ceiling drywall. All of those options should be taken off the table.”

What’s left, he adds, are using spray-foam insulation on the underside of the roof sheathing or rigid foam on top of the roof sheathing. Holladay informed West that these options were discussed in a recent GBA article, Insulating Low-Slope Residential Roofs.

West is aware of the energy shortcomings of recessed lighting fixtures, but they seem to come with the territory, he tells Holladay.

“I try to limit the number of recessed cans as much as possible and use many fewer than most architects I know,” he says, “but the reality is that people that want a modern home tend to also value having wall art and they want to light it appropriately and there’s no other decent/affordable way to do that except with recessed can products.”

Besides, he adds, most of the scenarios he proposes include spray-foam insulation on the underside of the roof sheathing, an air and thermal barrier that would not be compromised by the light fixtures.

Combine blown fiber insulation with rigid insulation

“In Atlanta even an inch or two of exterior rigid polyiso or 3 in. of rigid high-density rigid rock wool panels would be enough dew point control to allow you to blow the joists full of cellulose,” Dana Dorsett replies. “Alternatively, 2 in. of closed cell foam on the underside of the roof deck would be sufficiently protective to fill the rest with blown fiberglass.”

Only one of West’s four options makes sense to Dana Dorsett, and that’s a slightly modified No. 4.

Instead of using rigid extruded polystyrene insulation, however, Dorsett suggests polyisocyanurate, a greener choice, or 2 in. of closed-cell polyurethane foam with the rest of the roof cavity filled with cellulose. Spray foam roofing could even be used on the top of the roof deck as long as it was finished with a UV-resistant coating. That, Dorsett says, would some money in upfront costs and eventual roofing replacement costs. “Mopping on new UV-resistant goop every 15-25 years is a lot easier and cheaper than re-roofing with membranes or composite asphalt solutions,” he says. (For more information on the use of spray foam as roofing, see Spraying Polyurethane Foam Over an Existing Roof and Roofing With Foam.)

Dorsett would avoid all of the other options, either because they use batt insulation or because of the global warming potential of the XPS foam.

And then there’s this whole flat roof thing

To James Morgan, the central question is why use a flat roof in the first place. It may be nothing more than architectural artifice.

“In 1933 a group of European architects took a cruise around the Mediterranean and decided the whitewashed flat-roofed parapet-walled village buildings they saw there should be the model for a ‘rational’ style of architecture applicable anywhere, in any climate and in any culture, whatever local indigenous, traditional building wisdom may tell us to the contrary,” Morgan writes. “We have suffered the consequences ever since.

“Which is my way of saying, why not put a ‘real’ roof on your projects?” he continues. Morgan suggests a roof with “climate-appropriate overhangs and roof pitches designed to actually and reliably protect the vulnerable structure below. Oh, and by the way, there are many better ways to light artwork than ceiling cans. As any decent contemporary art gallery can attest.”

“Does holding up a few bad examples justify maligning an entire genre of architecture?” West replies. “One can find examples of bad design and lack of appropriate maintenance across all periods and styles. I’m sure I could send you dozens of photographs of the same issues with traditional homes. Once you go over one-story in height, most normally sized overhangs don’t provide that much protection to the lower floors anyway. How often does the rain fall perfectly vertically?”

Our expert’s opinion

We have comments from two experts, GBA technical director Peter Yost and also Brett Dillon, managing director of IBS Advisors of San Antonio, Texas.

First, Peter Yost:

I hate to draw this line in the sand, but I have just seen too many air leaks and related moisture problems with can lights in unvented roof assemblies: if you must have them, install them in a dropped soffit or ceiling so that the cans and all wiring are NOT within the roof assembly proper.

Atlanta is a real challenge: significant moisture drive during both heating and cooling seasons, and what I recall as an average of 54 inches of rain annually. Better get those unvented roof assemblies details right.

I asked Brett Dillon to weigh in on this question for three reasons: he knows his building science cold, he works in lots of different climates, and — being from the Southwest — he deals with flat roofs all the time.

Here’s Brett:

Here’s what I have from an architect friend in the Atlanta area, including construction cost to frame the roof:

  • Built-up roof, $6-$11 per s.f.
  • Single-ply membrane, $7-$11 per s.f.
  • Closed-cell spray polyurethane roofing foam, 1.5 to 2 in. thick, $6-$12 per s.f.
  • Standing seam metal roof, $9-$15 per s.f.
  • Vegetated roof system with growing medium and root barrier, $110-$120 per s.f.

These roofing materials are approved for a 1/4:12 (2%) slope. Asphalt roll roofing isn’t supposed to go on anything with less than 1:12 slope, so I didn’t include it.

I’ll be honest — I think low-slope roofs are a really bad idea in the Southeast where we get huge amounts of rain at once, especially Atlanta. I’ve worked on projects that have used the built-up roof, single-ply membrane, and the vegetated roof systems. They all had failures within four years.

The vegetated roof system had to be re-engineered during design to support the weight of saturated growing medium plus plants and furniture. That added to the cost, plus it had open-cell spray foam under the deck, tapered polyisocyanurate foam board above the deck, a single-ply membrane, a root barrier, growing medium and furniture platforms, and plants. It still leaked over time.

Nothing fixes a flat roof like making it a 2:12 slope!


  1. Bill Rose | | #1

    flat roof

    There's a lot of good material in this post for comment. Let me just mention that the flat roof as a motif certainly predates 1933. Gropius and Le Corbusier in the 20s sought to capture the machine age by using "industrial" style roofs. It became a unifying motif of the International Style at Mies van der Rohe's Weissenhof Exhibit in Stuttgart in 1927. Richard Pommer from Vassar College wrote several good papers on the cultural context of flat roofs. In Weimar Germany the roof style issue became politically charged, as the conservatives, who favored the heimatschutz (fatherland-style with steep pitches) found the Weissenhof examples to belong more in "a suburb of Jerusalem".

  2. Deleted | | #2


  3. DrLuc | | #3

    In what world is polyiso 'greener' than EPS? (Purely R-value per inch? That's pretty simplistic. Journalists, please verify the claims of your sources!) If EPS bothers you, look at graphite-EPS. From a sustainability perspective, first avoid spray-applied urethane if at all possible (kills any chance at adaptation/reuse), then Control Your Dew-point! (This has to be region-and-climate-specific.) Nothing is less sustainable than a building that has to be torn down because of mold problems. Most hybrid systems (foam board plus blown-in) serve adequately; it's the holes and gaps (e.g. recessed cans, mechanical systems, or just plain poorly-punchlisted construction) that kill you. And it's worth saying that a well detailed flat roof, particularly protected with ballast and/or a green roof assembly, can last longer than a pitched roof, and thus justify its extra cost.
    P.S. Listen to Yost. He's the best at this stuff.

    1. Tyler Keniston | | #4

      I'm curious if you have more to share on EPS vs Polyiso. You seem eager to denounce something but offer no meat to your criticism. What's the story. Share the 'world' we should become familiar with.

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