Is an Unvented Conditioned Attic a Good Way to Save Energy?
RELATED ARTICLES Ted Storm is in the midst of a construction project and at a point where he needs to make a decision about attic insulation. The unvented roof consists of standing-seam metal over synthetic felt and oriented strand board (OSB) sheathing. Both HVAC equipment and air ducts will be located in the attic of this house in Climate Zone 4.Three insulation contractors are unanimous in their recommendation to spray a 6-in. layer of open-cell foam on the bottom of the roof deck.While the advice is consistent, Storm is having second thoughts about his decision not to vent the roof.“I’m beginning (maybe a bit too late) to read about unvented roofs and I’m wondering if this is really the best option,” Storm writes in this Q&A post at GreenBuildingAdvisor. “If it isn’t ideal, are we too late to take another route, given our current progress?”That’s the subject of this week’s Q&A Spotlight. First, that’s not enough insulationArmando Cobo is the first to point out that 6 in. of open-cell foam is skimpy, even in the moderate temperatures of Climate Zone 4. The prescriptive requirements of the International Energy Conservation Code call for at least R-38 — and that, Cobo says, means at least 11 in. of open-cell foam. Are Solar-Powered Attic Ventilators Green?Creating a Conditioned AtticPrevent Ice Dams with Air Sealing and Insulation”Green Basics: Vented or Unvented Attic?It’s OK to Skimp On Insulation, Icynene says CONSTRUCTION DETAILS Our expert’s opinionHere’s how GBA technical director Peter Yost sees it:First, let’s get some terms and concepts defined, such as venting attics, venting roof framing assemblies, and venting roofs. I first wrote and then Joe Lstiburek edited this resource on the BSC website. Take a look for clarification on the three types of roof venting discussed below.Attic venting was invented (pardon the word choice) for cold climates, long before we really did much of any sort of air sealing or insulating at the ceiling plane, before we stuck HVAC equipment in attics, and before we had three different venting approaches to attics. Here is how I see it:We vent attic spaces because it is the cheapest way to introduce a lot of drying potential to the entire roof assembly.When we found the need to condition/use that triangular attic space, the next cheapest way to introduce air flow and drying potential to the roof frame and sheathing was to vent the underside of the roof sheathing, ideally with soffit vents that communicate with ridge vents.Finally, when we decided that you could build roofs just like walls, we do those with no venting at all, all the time. But unfortunately, most roofs need a more forgiving moisture management approach than walls. So, just as we now back-vent wall claddings to increase the drying potential of wall assemblies, we quite often need to back-vent (air space topside of the structural roof sheathing) roof claddings to increase the drying potential of roof assemblies. It’s just more expensive to introduce back-venting to “unvented” roof assemblies (“hot” roofs), particularly when the roof cladding is the most common and least expensive one, three-tab asphalt shingles.I am convinced that in all but the driest climates, all wall and roof assemblies need some sort of free-draining vent space. And the drying accomplished by the venting has to be done for free, by the sun. No attic fans — they represent a mechanical solution to a design problem and more often than not, depressurize homes (see Are Solar-Powered Attic Ventilators Green?).In Ted Storm’s situation, I would suggest that Ted has to maintain drying potential to the interior, stick with an airtight, vapor-permeable cavity insulation (at least thick enough for IRC 2006), and if possible, vent the underside of his roof sheathing with soffit to ridge venting. If that is simply not feasible, Ted will have to make doubly sure that his roof cladding and flashing details are bombproof because his only drying potential is small and slow in either direction. Would powered ventilation help?To David Gregory, a debate that pits vented attics against unvented attics seems “too polarized.”“It’s clear that having an attic that is only vented when the wind happens to be blowing is a liability — still hot in summer (though admittedly unclear net energy effect), but can pull conditioned air out of a not-well-air-sealed house,” he writes, “and is clearly a liability in winter (for energy and condensation issues).”What about installing two fans of equal capacity, he writes, one pulling air in and one exhausting air, and both connected to electrically driven dampers? The equipment would run only when necessary.“Moisture could be handled separately by a dehumidifier (but it seems that the only real moisture issue is from bathroom fans vented to the attic! Or other ceiling penetrations.) So the question should be the cost of a setup to vent when beneficial, vs. spending the money elsewhere. Any references? With costs?”The Rose/Gordon report suggests that ventilation in a mixed climate is a trade-off between summer savings and winter energy losses, Gregory says, while it’s a net energy loser in cold climates. Could you then surmise that in sunny, hot climates, ventilation would be a net winner?David Gregory’s proposed solutions carry a huge energy penalty, says Holladay, and are “wildly energy-intensive.”“It uses a lot of energy to solve an issue that probably isn’t even a problem (that is, regulating the temperature and RH of a volume of air that is OUTSIDE your home’s thermal envelope),” Holladay says. “Plenty of researchers have done a cost/benefit analysis on powered attic ventilators — and I’ll ignore your suggestion of running a dehumidifer outside of your home’s thermal envelope for the time being — and they all conclude that it takes more energy to run a powered attic ventilator than any conceivable energy savings.” Debating vented vs. non-vented atticsAlthough Cobo isn’t convinced that all attics should be unvented, conditioned spaces, he points to research by building scientists Bill Rose and Jeffrey Gordon suggesting the benefits of building that way are many. In wet, cold coastal climates, for example, Rose and Gordon found that attic ventilation results in higher sheathing moisture content, not less, and that ventilation makes the attic colder and wetter.In warm, humid climates, outside air is more humid than inside air, so attic venting tends to increase rather than decrease attic moisture levels.The difference in attic temperatures between a vented and unvented attic with R-30 insulation at the ceiling translates into “minuscule savings,” the pair said, adding, “No savings here have ever been measured. Usually there is a penalty with venting because venting causes greater air pulsing across the ceiling.” Building Plans for the Thermal Bypass Checklist“Your insulation contractors are planning to install only R-20 to R-22,” GBA senior editor Martin Holladay adds. “That’s not much. In fact, it doesn’t even meet the minimum code requirements (2006 IRC) for Climate Zone 4, which calls for at least R-38.“If you install less than the code minimum insulation,” Holladay says, “you will pay higher energy bills for as long as you own the building. Many builders prefer to exceed code minimum requirements.” Condensation is a potential problemHolladay writes that one advantage of vapor-permeable open-cell foam is that it would allow the sheathing to dry toward the interior. “In your climate, wintertime moisture accumulation due to diffusion from the interior shouldn’t be a problem,” he wrote. “However, there is always the possibility of a small roof leak, and drying to the interior is a (small) virtue.”Cobo isn’t so sure.He says he’s seen condensation under roof decking in Kansas and Missouri, both of which are in Climate Zone 4. He says Topeka, Kansas, and St. Louis, Missouri, can have colder average temperatures than New York City and Washington, D.C., and the same as Newark, Providence and Boston, all of which are in Climate Zone 5.A better way of insulating the roof would have been to add between 1 in. and 1 1/2 in. of rigid foam on top of the roof sheathing, along with the 5 1/2 in. of open-cell foam under the roof deck, to avoid condensation, Cobo says.