Henri Fennell’s Advice on Cathedral Ceilings

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Henri Fennell’s Advice on Cathedral Ceilings

It’s all about the air barrier

Posted on Apr 20 2018 by Martin Holladay
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I first interviewed Henri Fennell, the celebrated spray foam consultant from North Thetford, Vermont, about twenty years ago. Back then, Joe Lstiburek, the founding principal at Building Science Corporation, called Henri “the foam god.” While Fennell’s former company, Foam-Tech, closed up shop years ago, Fennell hasn’t retired. He now works as a consultant, specializing in problems related to spray foam insulation or air leakage.

At the recent Better Buildings By Design conference in Burlington, Vermont, Fennell gave a well-attended presentation called “Theory and Practice for Attics and Cathedral Roof Slopes in Cold Climates.” Fennell’s presentation was based on a paper he authored called “Attic and Roof Slope Ventilation Design.” I highly recommend Fennell’s paper.

I won’t try to recreate Fennell’s paper, since it is available online. Instead, I’ll give a flavor of his presentation by sharing pithy advice that Fennell gave the audience.

Decades of spray foam experience

Fennell told the Burlington audience, “My first spray foam project was in 1971.” Fennell shared a large number of PowerPoint slides illustrating his experience tracking down air leaks.

Instead of insulation, this rafter bay has a heating duct.
[Photo credit: Henri Fennell]

Here are some nuggets that Fennell shared in Burlington:

“I’ve seen everything. I’ve even seen heat ducts located in the roof slopes.”

“Icicles are a symptom of roof melt. Icicles are almost always accompanied by ice dams.”

“In an attic, an air leak [through the ceiling] can heat up the entire attic. In a cathedral ceiling, an air leak will only cause problems in a single rafter bay.”

“I remember one job where the air sealing depended on the polyethylene vapor barrier, sealed with tape. When the blower door test started, you could hear the tape unzipping.”

First, check the air barrier

“The number-one cause of roof slope and attic problems is an air barrier problem. The last item on the list is ventilation. Take care of everything else — including air barrier issues and inadequate insulation issues — before you take care of ventilation.”

“If you increase the ventilation without addressing the air barrier, you increase the leakage.”

“If batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. has no air barrier on the interior, it’s just a filter. I spend half my life making fiberglass work.”

“Vented roofs require a perfect air barrier. Use QA [quality assurance] testing if you vent.”

R-value matters less than airtightness

“When it comes to addressing inadequate insulation, just use code-minimum R-values and you should be all right. It’s all about the air barrier, frankly. Make sure that you locate the air barrier immediately against the insulation.”

“A big chimney in an attic can cause warming. If a chimney is warming the attic enough to cause ice dams, the chimney should be insulated in the attic so you need less ventilation.”

[Photo credit: Henri Fennell]

Complicated roof shapes

“If you are depending on ventilation, complex roof designs don’t work — roofs with skylights, dormers, valleys, or hips.”

“At buildings near ski areas, every time it snows, the temperature in the attic rises dramatically, because the ridge vents are blocked by snow. If you want a vented roof in snow country, ideally you have a continuous cupola vent. You can site-build them.”

“In one attic, we found an eight foot by eight foot chase that started three stories below and went all the way up to the attic. It was just covered by fiberglass batts. It was a death trap. It’s like they were hunting big game in the attic.”

Martin Holladay’s previous blog: “Exterior Insulation for an Ugly Brick Building.”

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Image Credits:

  1. Henri Fennell

1.
Apr 20, 2018 9:39 AM ET

Perfect
by Randy Williams

Perfect article for what I've been dealing with this past winter. Thanks Martin.


2.
Apr 20, 2018 11:59 AM ET

In a nutshell
by Malcolm Taylor

“The number-one cause of roof slope and attic problems is an air barrier problem. The last item on the list is ventilation. Take care of everything else — including air barrier issues and inadequate insulation issues — before you take care of ventilation.”

“If you increase the ventilation without addressing the air barrier, you increase the leakage.”

These two, although logical, are somehow counter-intuitive. I've spent a lot of time in futile efforts to fix roofs because I didn't take this advice on-board.


3.
Apr 20, 2018 12:32 PM ET

Are these concepts counterintuitive?
by Martin Holladay

Malcolm,
I know what you're trying to say -- but these concepts are only counterintuitive to those who have been misled by false explanations of "why you need to vent."

Once we understand the fallacies behind conventional ideas concerning attic ventilation and cathedral ceiling ventilation, and once we understand why air barriers matter, then Henri Fennell's points cease to be counterintuitive.

More on these issues here:

All About Attic Venting

Prevent Ice Dams With Air Sealing and Insulation


4.
Apr 20, 2018 2:28 PM ET

Martin
by Malcolm Taylor

Yes you are right. Counter-intuitive is the wrong way to describe it. Against what was commonly believed, and for too long incorporated into my thinking is probably more accurate.


5.
Apr 20, 2018 4:56 PM ET

Knowledge bordering on wisdom from Fennell here...
by Andy Kosick

I have come into a couple of attic moisture/ice dam jobs now that had made a few revolutions through roofer-builder-insulator and it has made it clear to me that the air barrier is what none of them understood; up to and including bath fans not ducted out of the attic. Education may be the most important task in home performance today.


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