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How to Vent a Rainscreen

Should a rainscreen be integrated with soffit vents or vented only on the bottom?

Posted on Sep 18 2017 by Scott Gibson

A vented rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. — 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 GBAGreenBuildingAdvisor.com.

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 barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., 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 capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. 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 claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. 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 barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material.] 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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. 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 ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. 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 effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. to drive air movement.

For a look at how I handled the rainscreen on my own home, see Images #2 and #3 below.


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Image Credits:

  1. Fine Homebuilding

1.
Sep 19, 2017 8:29 AM ET

Edited Sep 19, 2017 8:31 AM ET.

Vented Cavity Depth
by Jeff Stetter

One aspect of the cavity depth discussion that often seems overlooked is the necessary depth of solid wood that is required for attachment of a variety of common siding materials. This is specific to when furring is being placed over rigid insulation and not directly over structural sheathing. Many siding materials require 1" embedment of fasteners at 16" o.c.. This also often precludes advanced framing techniques of 24"oc framing as well as the more shallow cavity depth. In summary, if you want a siding warranty, 5/4x wood furring is often required for proper siding attachment thus providing a 1" vented cavity.


2.
Sep 19, 2017 8:35 AM ET

Diagram doesn't seem to make sense.
by Jeff Stetter

I also notice that the "Rainscreen Vented at Top" doesn't really make any sense. Doesn't the horizontal 1x3 at the top, block any ventilation from happening since the screen is basically wrapping the 1x3?


3.
Sep 19, 2017 9:33 AM ET

Response to Jeff Stetter
by Martin Holladay

Jeff,
The siding is fastened to the furring strips. The horizontal 1x3 at the top of the wall is nailed on the exterior side of the furring strips -- not in the same plane, but proud of the furring strips. This horizontal 1x3 brings the horizontal frieze board to a plane that is well proud of the siding -- the horizontal 1x3 packs out the frieze board. The result is a gap between the bottom of the frieze board (probably a 5/4x4 or a 5/4x6) and the siding. This gap is the vent for the top of the rainscreen cavity.


4.
Sep 20, 2017 10:18 AM ET

Ice damming concerns
by Scott Paulson

My concern for venting into the soffit would be in a cold snowy climate. I think there is a potential issue for the wall acting as a solar air heater and then sending all that air directly to the underside of the roof. Thus starting a cycle of snow melt and freezing that could lead to ice damming.

I would say this is even a concern with having the soffit intake vent near the wall rain-screen vent if they were separated. I always suggest moving the soffit vent as far away from the wall as possible.

I would be interested to here others opinions.


5.
Sep 20, 2017 11:10 AM ET

Even greater separation from
by Jon R

Even greater separation from wall heat would be achieved with vents under the eave shingles (eg, SmartVent) and none in the soffit. This also allows extending some insulation into the overhangs.

https://buildingscience.com/documents/insights/bsi-046-dam-ice-dam


6.
Sep 20, 2017 11:43 AM ET

rain screen direct to the attic
by Trevor Lambert

I am building a house with a 1.5" rain screen, clad with Hardie Panel (the 4x10 sheets, not lap siding). Before reading this article, I was actually considering having the soffit abut on the siding such that the rain screen basically connects directly to the attic space. I gather this is a stupid idea? I'm in southwest Ontario, moderately cold winters, humid summers but not particularly rainy.


7.
Sep 20, 2017 2:26 PM ET

Trevor
by Malcolm Taylor

Our code here in BC has a section devoted to rain screens which prohibits connecting the two. I don't know if the Ontario code has adopted the same standards. You need to look at the requirements for fire-stops between assemblies in concealed spaces to see if it is allowed.


8.
Sep 21, 2017 4:37 AM ET

Response to Trevor Lambert (Comment #6)
by Martin Holladay

Trevor,
Q. "I was actually considering having the soffit abut on the siding such that the rainscreen basically connects directly to the attic space. I gather this is a stupid idea?"

A. The point of this article is to discuss the pros and cons of the approach you mention. GBA readers can consider the arguments of the experts interviewed for the article and can draw their own conclusions.

Personally, I think it makes more sense to vent the rainscreen gap at the top of the wall rather than direct the air into the soffit or the attic. But I think that in the vast majority of cases, connecting the rainscreen gap to the attic doesn't cause any problems.


9.
Sep 21, 2017 5:29 AM ET

Edited Sep 21, 2017 5:43 AM ET.

Lstiburek Ice Dam Warning - Snow Prone Regions
by Andrew Bater

Joe Lstiburek's advice on solar radiation with dark claddings altered the construction methodology in our home. (Figure 9 in the link that Jon R. posted.) Like Trevor above, we knew we were going to utilize Hardie panels with a rain screen assembly.

It's a passive solar SIP home and our original plan was to truncate the roof SIPs at the wall edges. Soffit vents in field built overhangs would have then ventilated the cold roof. Joe's advice changed that direction.

Our wall assembly was built with Cor-A-Vent strips beneath the Hardie panels. Those strips were placed at the future batten locations so as to minimize nail marks. Cor-A-Vent top and bottom provide air flow. (In hindsight I do like the screen detail as shown at the top of this article somewhat better than what we did. It provides less hidden surface area for boring insects to get into the blocking behind the top frieze board.)

Our roof SIPs extend past the wall edges to the gutter line. Our cold roof above was ventilated at fascia edge, more Cor-A-Vent.

Under the category of "it's anecdotal data", here are some south wall temp pictures I captured midday yesterday. It was 82 degrees outside. (They display here bottom to top.)

S-Wall_Bot_Temp.jpg S-Wall_Mid_Temp.jpg S-Wall_U-Mid_Temp.jpg S-Wall_Top_Temp.jpg S-Wall_Vent-Fascia_Temp.jpg


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