A vented rainscreen — an air gap behind the siding — has become a standard detail in many new houses. It helps remove moisture that works its way through the siding, and in the process helps siding last longer. It’s the “vented” part of this equation that has Gerald Pehl thinking.
“I’ve got an assembly design for a vented rainscreen, and it will be held continuous to the soffit spaces, which then vent through to the attic ridge vent via conventional vent chutes between the rafters,” Pehl says in a comment posted in the Q&A forum at GBA.
His question, and the start of this Q&A Spotlight, is whether this plan eliminates the need for additional soffit venting. Pehl’s building inspector has left the decision up to him.
Risks in the event of a fire
Malcolm Taylor suggests that connecting the rainscreen and attic ventilation might increase the drying potential of the wall assembly. He points out, however, that a rainscreen is typically 1/2 inch to 3/4 inch deep, a function of what material is used to create it, and that probably isn’t enough to make up the required area for soffit vents. Even if it did, he adds, that detail raises another issue: What happens in the event of a fire?
“Here in Canada,” Taylor writes, “we can’t connect the two to find out. The code precludes it because it creates a concealed air space from which fire can spread up into the roof assembly. Practically, I wonder how much difference there is from a fire spread point of view between a connected rainscreen, and one which is vented at the top with soffit vents immediately above. I’ll be interested to see what other posters think.”
GBA senior editor Martin Holladay notes fire safety is a question that’s been raised before (see, for example, “Can a rainscreen become a chimney for fire?” and “Fire-resistant rainscreen system”). “The issue of fire safety is contentious,” he says, “but in some areas of North America — areas where wildfires are common — vented rainscreens of any kind are controversial.”
He also notes that if the house has an effective ceiling air barrier, attic venting doesn’t matter much (see “All About Attic Venting”).
John Clark cites an article on this topic by Joseph Lstiburek of the Building Science Corporation. In it, he discusses the Grenfell Tower fire in London, exacerbated by rainscreen details that included a large air gap of 2 inches and a combustible outer skin. The bottom line is that air gaps don’t have to be all that big.
“We have addressed the gap many times before with respect to drainage,” Lstiburek wrote. “We want to control hydrostatic pressure and pretty much call it a day. We do not need much of a gap to control hydrostatic pressure…. a continuous 1/32 to 1/16 of an inch is all that you need.”
When the air gap is as big as it was in London, the fire implications are “huge,” but when the air gaps is small, less than 3/4 inch, “the friction from both surfaces bounding the air gap limits the air flow,” Lstiburek writes. “The boundary layer on both surfaces is an effective fire stop.”
In other words, in this assembly fire dangers are a non-issue.
Where the vents should be
According to Taylor, rainscreens that are vented at both the top and the bottom work better, although it’s not clear whether the top vent actually adds much in the way of performance.
“With one only vented at the bottom you get a capillary break, a drainage gap to handle bulk water intrusion, and some drying,” Taylor says. “Here in the wet Pacific Northwest almost all are only vented that way, and they perform very well. In a less damp climate, with a wall assembly that isn’t risky, venting at the top probably doesn’t bring huge benefits.”
Tyler Leclear Vachta cites research in British Columbia on the benefit of vented rainscreens, but he doesn’t necessarily see any added benefit to linking a rainscreen and attic ventilation.
“We most commonly see a small vent space just below the soffit, protected from wind-driven rain by a trim board and protected against insects by back wrapping the rainscreen fabric,” Vachta says. “There is certainly no benefit to your attic to have the two systems connected.”
The connection, Pehl replies, is a matter of convenience as well as appearance.
“The ideal case is to vent the rainscreen into the soffit because it’s the easiest assembly at this point in construction,” he says. “It also supports the aesthetics we want, although venting to the exterior at the top of the cladding as you describe might not even be a noticeable detail change.
“The real benefit to connecting the two systems is simplicity of the assembly, and the aesthetics of eliminating the venting channel in the soffit,” he continues. “To be clear, in the assembly that we are now considering, attic venting, for code and whatever benefit it does provide, is provided through from the bottom of the cladding, through a bug screen at the bottom of the 3/4-inch rainscreen channels, up to the soffit at the top, through the vent chutes into the attic, out the ridge vent at the top of the roof.”
Connecting rainscreen and attic may encourage mold
Taylor notes that while Building Science Corporation has made the point that a very small air gap is effective behind a rainscreen, “the reality of construction is that they may become closed due to the WRB [water-resistive barrier] intruding into them, or wood shrinkage over time.”
He also finds a couple of points worth repeating in the British Columbia study that Vachta has cited. First, a well-ventilated rainscreen works better than a poorly ventilated one. More important, the study “expressly suggests” that connecting the rainscreen to the attic is a lousy idea because of the moisture and mold problems that occur in buildings where this was done.
“Whether these problems are likely to occur outside the Pacific Northwest is still an open question,” Taylor writes. “Things happen in our climate that simply don’t elsewhere. The plywood roof sheathing in a well air-sealed and ventilated attic can grow mold, as can the cladding on the north side of houses. You may be building in more forgiving circumstances than we encounter here.”
Taylor’s remarks about the danger of a small rainscreen gap becoming blocked are well taken, Vachta says.
“Textured WRBs that create the smallest gap also have the least room for error,” he writes. “Rainscreen meshes also compress significantly, both impeding the drainage/ventilation gap and resulting in an uneven work surface. You want to be sure the rainscreen maintains a cavity during installation and the life of your building.”
The real risk to moisture-sensitive materials, he adds, is not from a one-time soaking, but from chronic exposure to small amounts of moisture.
“Drying with ventilation is the best way to address that,” he says. “One aspect of this [British Columbia] study that was unique is that it looked at the walls over a long period of time. There are a lot of studies and testing that just look at how walls perform under a single ASTM test where they have to drain 90% of the liquid moisture poured at the top of the assembly, or identify points where water under pressure comes spraying through the wall.
“But how long does it take the rest of that moisture to get out of the wall?” he continues. “Which layers get wet in the process? And what if the next moisture event happens before the wall dries?”
Are outdoor temperatures a factor?
Ambient temperatures may play a part in whether elevated moisture levels in walls lead to mold growth or rot, Vachta suggests. While a 15% wood moisture equivalent is perfect for fungal growth, he says, a WME of more than 20% is ideal for rot.
“During the cold winter months those thresholds aren’t as important because the freezing cold inhibits rot and fungus,” he says.
Low temperatures also remove moisture from the air, Pehl says, adding that “may be at least as important as the cold temperature in preventing mold/rot. I’ve been told that in the coldest months in Minnesota it’s as dry as the driest desert.”
“Yes, being a Minnesotan myself I can attest to the dry winter air!” Vachta says. “Cold air simply can’t hold as much moisture, true, but does that have a drying effect on the wood? Air/ temperature/ water move from high pressure/ temperature/ concentration to low pressure temperature/ concentration. The moisture in the wood runs the risk of condensing, or frosting. In a roundabout way, I suppose frosting (like freezer-burn) is drying.”
In Taylor’s neck of the woods, temperatures don’t get cold enough to offer that kind of winter protection from mold, but there’s no doubt that rainscreens have been a very significant improvement in building.
“Including rainscreens in our construction practices has transformed the resilience of walls here,” he says. “I simply don’t see the problems now that were endemic in any building I went to renovate in the past.”
Our expert’s opinion
Peter Yost, GBA’s technical director, added this:
Fire safety and vented rainscreens: The Institute for Business and Home Safety (IBHS) recommends the use of 1/8-inch mesh screening, robust enough to keep embers and brands from getting into any vented space on the building exterior. For more information, see “Top 10 Ways to Protect Your Property from Wildfire.”
On a residential project in Portola Valley, California — in a very wildfire-prone area — we vented the rainscreen cladding top and bottom using not only 1/8-inch mesh but also an intumescent paint (painted onto the wire mesh) that would foam and close off the top and bottom openings of the rainscreen when activated by high heat (that is, in the event of a wildfire). We developed this approach based on conversations and advice from Steve Quarles, a leading wildfire expert who is now Chief Scientist for Wildfire and Durabilty at the IBHS Research Center.
Closing off the top of a vented rainscreen wall assembly: I have seen projects taking this approach with the paint peeling off the top course or two of lapped siding because warm, moisture-laden and buoyant air is “stacking up” in the closed-off vented area. (Some folks call it a “vented” assembly when the top is closed and a “ventilated” assembly when top and bottom are open, with the former not supporting convective drying while the latter does.) Whether you close the top to prevent moisture from getting in (wind-driven rain) or open it to let moisture out (solar-driven heating of cladding in hot-humid conditions) may be based on your own experience, climate, and site conditions.
And yes, the depth of the vented space does make a difference. In British Columbia, codes mandate a 10 millimeter (about 3/8-inch) space between the cladding and the rest of the wall, choosing this as a minimum depth to promote 100% free drainage and air movement.
Rainscreen products like HydroGap that create a 1-mm gap between the cladding and the rest of the wall pass the ASTIM drainage test minimum of 90% free drainage. That said, the manufacturer of HydroGap readily admits that a 1-mm gap, regardless of whether it is open on the top and bottom, does not support convective air movement.
Connecting ventilated wall claddings to soffit-to-ridge venting: I don’t believe that any U.S. code addresses this issue but plenty of building code inspectors will have their own perspective or position. I have asked Joe Lstiburek about this, as well as asked a bunch of builders who take this approach, and have not heard any negative reports, if you limit this connection to the movement of outside air and not any air leaking at eaves. In other words, with a continuous air barrier at the eaves, connecting wall and roof venting just gives you a taller, longer column of warm air, and hence more stack effect to drive air movement.
For a look at how I handled the rainscreen on my own home, see Images #2 and #3 below.