Unvented roof – U.S. vs. Europe – Why such VERY different techniques and guidance?
Hi all, first time poster 🙂
I’m navigating my latest rehab project in Zone 4 (Raleigh, NC) and struggling with a specific Cathedral Ceiling scenario…. more details on that later, but first some context.
I have read nearly every GBA article (multiple times) published over the last decade on the subject of moisture management, rot, open/closed cell insulation, vented / un-vented roof assemblies, fiberglass / dense packed cellulose, etc. and must admit when paired with expert opinions available outside GBA, the degree of “certainty” on this general theme is anything but certain!
Enter my latest “pause”… a VERY expert forensic roof inspector routinely sought in litigation for rot issues involving spray foam (both closed and open cell foam) warmed me to NEVER put foam on the underside of roof decking and waxed on at length about the dubious science on the subject.
This gave me pause because I had determined from reading all the GBA guidance that a fully conditioned attic with open cell foam on the underside of the roof deck was a proven and virtually risk free implementation. This expert indicated that his experience with homes in the Carolina’s is that any foam on the underside of the roof deck is a roll of the dice, including those that are fully conditioned… with 90%+ positive outcomes, but that the other 10% experienced deep regret and great expense. His personal “expert” opinion was that no one yet truly understands the physics of moisture migration well enough to be certain of a risk free approach to attic encapsulation and un-vented roof assemblies.
This disconcerting encounter than drove me to research other approaches… Enter Gutex (from Germany) and their line of products design to be “earth friendly” and to “embrace” water vapor absorption / dispersion friendly materials.  After consuming every technical document I could find on the European “findings” of Gutex products I’m now thoroughly perplexed. Why? Because they claim their earth-friendly, vapor friendly materials and membranes solves for the “ping-pong” moisture migration reality that happens based on time of year and micro-climate. Moreover, they aggressively advocate a cellulose, un-vented roof assembly so long s it’s paired with their vapor migration membrane called Intello Plus. Again, the opposite of GBA guidance on unvented, dense pack cellulose cathedral ceilings.
Now to my Cathedral Ceiling challenge…
My rafters are only 7.25″ deep (originally 5.5 inches before building them up). The shingles are brand new (installed by the immediate previous owner) and I really don’t have the budget to remove and install Poly ISO on the outside.  Furring out the depth is also not ideal due to a relatively low side wall height (7 feet-8 inches).  My roof is currently vented…
Enter my intrigue with Gutex. The Gutex folks appear staunch advocates of using dense packed cellulose in an un-vented roof assembly so long as their moisture management membrane Intello Plus is used to the inside of a building envelope.
So…Â I am keen to hear from those up on the “science” behind the Gutex approach to un-vented roof assemblies as well as their earth-friendly materials approach to embrace an absorb / disperse vapor solution vs an approach here in the States which seems built more on “make damn sure the moisture can’t get in”, and just in case, be sure to condition the air.
Thanks in advance for input / guidance.
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I'd be interested in knowing what building codes (vs a self interested company) in Europe say about unvented roof insulation designs.
Here's a link to a Dutch building code certification body that has sanctioned the use of Gutex products as a solution to hot, unvented roofs.
https://foursevenfive.com/blog/intello-db-approved-by-dibt-for-use-in-unvented-hot-roof-assemblies/
Brad,
Q. "I had determined from reading all the GBA guidance that a fully conditioned attic with open cell foam on the underside of the roof deck was a proven and virtually risk free implementation."
A. Evidently you missed some key articles that you should have read. Here are the links to two important articles that explain why you are wrong:
"High Humidity in Unvented Conditioned Attics"
"Open-Cell Spray Foam and Damp Roof Sheathing"
Q. "They aggressively advocate a cellulose, unvented roof assembly so long as it’s paired with their vapor migration membrane called Intello Plus. "
A. The U.S. distributors of Intello Plus are making similar arguments in this country, even though their approach violates U.S. building codes. For a thorough discussion of the issue, see this GBA article: "Smart Vapor Retarders for Walls and Roofs."
Most of northern & western Europe have warmer winters than US zone 4A/4B locations, despite (often) having more heating degree days, and the dew point averages in summer are also quite low compared to zone 4A. Take Amsterdam NL for example:
https://weatherspark.com/m/51381/1/Average-Weather-in-January-in-Amsterdam-Netherlands#Sections-Temperature
https://weatherspark.com/m/51381/8/Average-Weather-in-August-in-Amsterdam-Netherlands#Sections-Humidity
Compared to Louisville KY (4A):
https://weatherspark.com/m/15227/1/Average-Weather-in-January-in-Louisville-Kentucky-United-States#Sections-Temperature
https://weatherspark.com/m/15227/8/Average-Weather-in-August-in-Louisville-Kentucky-United-States
The winter temps are comparable, but the shoulder seasons in A'dam are much longer, average annual dew point temps are quite a bit lower. In annual HDD it would be comparable to US zone 5, but it's really nowhere near the same climate.
Another distinction between European & US roof assemblies is that the US paradigm is fairly impermeable asphalt shingles, whereas in Europe the roof decks are essentially top-vented to the outdoors with tile or stone roofing (sometimes thatch in the cool-kids' neighborhoods. )
That makes a roof built using moisture-tolerant vapor permeable fiberboard decking such as Gutex essentially a large vapor diffusion vent. Vapor diffusion ridge vents are now allowed in US zones 1-3 in lieu of attic venting, with some requirements regarding size and vapor permeance, and it's likely that a Gutex clad roof (no OSB or plywood) with inherently vented roofing would still work in other US climate zones. (See: https://www.greenbuildingadvisor.com/article/vapor-diffusion-ports )
But with a #30 felt and asphalt shingle layup on the exterior Gutex won't buy you much, since the roofing is about 0.1 perms. Gutex is moisture tolerant, but would storing moisture, not releasing it to the exterior.
In Raleigh NC the summertime temperatures & humidity mean air conditioning is almost universally used (unlike northern Europe, where it's not often needed for comfort.) A vapor permeable roof deck & roofing could result in condensation at the ceiling during the dog-day weeks of extreme humidity.