C. Clark is preparing to move from a dry region to Lady’s Island, South Carolina, an area with a warm, humid climate that is the mirror opposite of the climate in Clark’s former home. Clark is highly allergic to mold, and that has him thinking about ventilation, insulation, and his HVAC system.
“I have many questions about HVAC, insulation and ventilation, but will just start with a few,” he writes in a Q&A post at GreenBuildingAdvisor. “If we decide on an exhaust-only system for a new-build home, does also trying to button up the envelope tight work with that type of ventilation?”
He expects to insulate the attic floor with blown-in fiberglass because using spray foam would be too expensive. If he does not insulate at the roof, should he install a radiant barrier to keep attic temperatures down?
“I am very concerned about building in a humid climate [Climate Zone 3], because we are very allergic to mold (bed-ridden allergic for me), so preventing condensation and the mold that comes with it are our primary concern, even over energy efficiency,” he writes. “I read the cellulose insulation can trap moisture, which steered me away from it and to fiberglass. If any condensation build up can happen in attic, I don’t want cellulose to be a mold culture.”
He anticipates that ductwork will be located in the attic because his builder has warned that building chases for them inside the building envelope would be too expensive. As a result, he will be looking for the best way to insulate the ducts.
Those are among the issues for this Q&A Spotlight.
Will exhaust-only ventilation work?
One of Clark’s first questions is whether exhaust-only ventilation can be effective. He asks, “Does trying to button up the envelope tight work with that type of ventilation system?” GBA senior editor Martin Holladay assures him that sealing air leaks always makes sense.
“It’s always important to try to make your envelope as airtight as possible,” Holladay says. “Striving for airtightness pays many dividends: lower energy bills, increased comfort, reduced chances of moisture problems in walls and ceilings, and better control of ventilation systems.”
Even when a builder makes a new house as tight as possible, there are typically enough cracks and gaps to supply replacement air when an exhaust-only ventilation system is installed. That’s because exhaust-only systems don’t move that much air — 60 to 120 cubic feet per minute. It would be different if the house were tight enough to meet the Passivhaus standard of 0.6 air changes per hour at a pressure difference of 50 pascals. In that case, he adds, passive air inlets might be needed.
Another consideration, however, is Clark’s choice of fiberglass insulation in the attic, says Dana Dorsett. Cellulose would be a better choice.
“If the air leakage supplying the ventilation air is coming through the ceiling, blown fiberglass can become a potential indoor air quality problem due to airborne glass particulates,” Dorsett says. “Cellulose is a generally better choice for open-blown attic insulation for several reasons, including that one. Fiberglass is somewhat translucent to infrared radiation coming off a hot roof deck, absorbing the energy an inch or so below the top surface. This makes the temperature an inch into the fiberglass layer hotter than the attic air, so you’re insulating against a slightly higher temperature with slightly less insulation.
“Cellulose is opaque to IR,” Dorsett continues, “and the warmest level will be at the surface, close to the attic air temperature. Cellulose is also more air retardant, limiting the volume of air moving through any leaks in the ceiling plane more effectively.”
As Clark’s question about whether he needs a radiant barrier, the answer is no — as long as he has figured out how to move the attic ductwork indoors. Instead, Holladay tells him, install a generous layer of insulation on the attic floor and take the money he would have spent on a radiant barrier to buy more insulation. “You’ll get more bang for your buck that way,” he adds.
Keep the ducts out of the attic
One troubling detail of Clark’s plan is to keep heating and air conditioning ducts in the attic because of the presumed cost of keeping them inside the thermal envelope of the house, as his builder has warned.
But roof trusses with horizontal chases for utilities “are not a huge cost adder,” Dorsett tells him, and mechanical systems also might be ductless.
“Putting the mechanicals in the attic is (almost) universally a mistake,” he says, “making it harder to air-seal, and adding to both the heating and cooling loads.”
This is a caution Holladay repeats. “It pains me to hear of a homeowner designing a new house with ducts in the attic,” he adds. “Now is the time when you can fix this problem inexpensively — before it is too late! This detail really, really matters.”
Among the options Holladay suggests are a crawl-space foundation, a basement foundation, 9-foot ceilings with duct chases, open-web floor trusses that can accommodate ducts, or, as Dorsett had suggested, ductless minisplits for heating and cooling.
“If your builder doesn’t understand this basic issue,” he says, “It may be time to look for a different builder.”
What about the added humidity?
Jon R isn’t so sure an exhaust-only ventilation strategy is a good idea, given the high humidity in the area and the likelihood that Clark will be using air conditioning in warm weather.
“You want to avoid mold but are proposing to pull warm, humid air into your walls and cool it down (when the AC is on)?” he asks. “When the AC is on, maintain the house at a slight positive pressure. All incoming ventilation air should be run through a filter (not the walls). Use a dehumidifier and humidistat at all times to keep the interior dry.”
Holladay doesn’t think it’s going to be a problem. “Unlike Jon,” he writes, “I don’t think that an exhaust-only ventilation system will cause problems, as long as your house has a well-designed heating and cooling system. The use of any ventilation system should be minimized during hot, humid weather, of course — you want just enough ventilation to be comfortable and avoid odors.”
Nor does Holladay think a dehumidifier and humidistat must run at all times. In most cases, he says, the air conditioner should keep the interior comfortable. In humid weather and swing seasons, when the air conditioner doesn’t run as much, a dehumidifier may be necessary. But not all the time.
Insulating the ducts
In Clark’s original scenario, with ducts in an unconditioned attic, the question is how to insulate them effectively. Can flexible ducts be insulated with spray foam? Clark asks. Would he be better off with rigid ducts?
There are many types of insulated flex duct, Holladay replies. Clark should chose ducts that are insulated to R-8, not R-4 or R-6. Seams should be carefully sealed, and the ducts should be installed carefully with short, straight runs that are well supported.
“In an attic with trusses, it’s usually impossible to install the flex duct on top of the drywall,” Holladay tells him. “The flex duct ends up suspended from hangers, because the duct runs go 90 degrees to the trusses. That makes it difficult to insulate the ducts with spray foam.
“If you can install some type of rigid foam support under the flex ducts that are 90 degrees to the trusses, that will make it easier to install the spray foam,” he continues. “If you can do that, and if you have a spray foam installer who is willing to add enough R-value to the ducts to approach the thickness of your attic floor insulation, you’ll have an excellent installation.”
Of course, he adds, all of this may be such a hassle it will get Clark to wonder why he didn’t just keep the ducts indoors in the first place.
The vapor barrier question
Clark wonders also about the need for a vapor barrier in a hot, humid climate. Should one, he asks, be installed directly behind the drywall?
No, Holladay says, that would be a bad idea, especially in that climate and especially if he plans on running an air conditioner. The reason? A vapor barrier becomes a condensing surface for moisture. If the polyethylene is located behind the drywall, that’s where condensation can collect, especially when the polyethylene is cooled by the air conditioning system.
Lady’s Island is fairly well balanced between heating and cooling, adds Dorsett. Whether Clark frames the house with 2x4s or 2x6s, if the walls have R-20 insulation, a true vapor barrier anywhere in the wall would be on the “wrong” side of the stackup for much of the year.
“Using plywood or OSB sheathing as the exterior vapor retarder (typically 1-5 perms, depending on moisture content) and standard latex paint on airtight wallboard (3-5 perms, typically), the house would be able to dry at reasonable rates seasonally in either direction, without letting too much moisture in via vapor diffusion, Dorsett says “The only time it needs more than that is if it has masonry, brick, or stucco on the exterior, which is a special case.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost has to add:
First things first: As usual, Martin Holladay and the devoted crew have provided sound advice on mechanical ventilation and vapor barrier questions. Now, my key points:
For any home in any climate to be considered high-performance, a continuous air barrier is key, and is as important as, if not more important than, a continuous thermal barrier. No matter what mechanical ventilation system and duct location you choose, make sure that your builder knows how to align and achieve a continuous air barrier from rim to ridge. This is key to both energy efficiency and IAQ.
And if I had serious air quality questions because of particular health concerns, I would insist on HVAC duct and equipment configuration in this order:
- Best approach: All ducts and equipment in conditioned space. Making room for equipment and ducts in conditioned space is a design issue, and if a builder insists that this approach is “too expensive,” the builder is deciding for you what your indoor air quality priorities are. The best way to ensure balanced supply and return is to include all equipment inside the conditioned space of the home, period. Here’s the best resource on that issue.
- Next-best approach: A “cathedralized attic” for HVAC ducts and equipment. If you can’t find room for ducts in the space below the attic-ceiling line, then move your air and thermal barrier up to the roofline. This places all HVAC stuff in a semi-conditioned space. With this approach, you will not be introducing IAQ-related pressure problems caused by ducts leaking outside of the conditioned space boundary. There’s more on this approach here and here.
- Next-next-best approach: In a dry climate, consider “attic duct-burying.” If better options can’t be used, follow this approach for HVAC ducts and equipment in unconditioned attics. Note that this approach is not recommended in humid or marine climates because of the risk of condensation on ducts during the summer.
And, in any of these configurations, continuous air and thermal barriers are much more important and effective than attic radiant barriers. That said, the effectiveness of radiant barriers in hot climates has been well documented, and can be seen in full context here.