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Q&A Spotlight

How to Vent a Rainscreen

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

This rainscreen is vented at the top. A GBA reader wonders whether it's possible, or advisable, to connect the top of the rainscreen vent with the soffit vents.
Image Credit: Fine Homebuilding

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…

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20 Comments

  1. Jeff Stetter | | #1

    Vented Cavity Depth
    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.

    1. Richard Stern | | #14

      I would also be interested to hear comments on the experience of using different types of siding attached to furring strips at 16" o.c. (or other). It seems obvious that stiffer siding is desirable so that the unsupported spans don't flex or be prone to bending with the occasional side loading. Are siding manufacturers who previously assumed attachment directly to sheathing now offering products specifically for the vented cavity installation? Aluminum better than vinyl? Composites even better in this aspect?

      1. Expert Member
        Malcolm Taylor | | #15

        User...957,

        I'm not aware of any manufacturer that has altered their products for installation on rain-screen furring, or of any problems using vinyl, or the various composites. Most have altered their installation guides to include mounting on furring, with the only noticeable possible impediment being the depth of solid material need to provide purchase for fasteners.

      2. Brendan Albano | | #16

        Many manufacturers have a special brochure or instruction manual for installing their products on furring strips.

        For example, here are James Hardie's instructions: https://www.jameshardiepros.com/getattachment/f580da6b-a3b5-4962-8548-5671e761034a/19-jh-over-advanced-framing-or-continuous-insulation.pdf

        At this point, a manufacturer who doesn't offer clear instructions for installing their products on battens over a rainscreen gap seems like they are falling behind the times.

        1. Richard Stern | | #17

          Many thanks for the feedback and link to installation instructions. I'm in Edmonton and could not find any supplier of perforated J channel, but finally found Coravent SV-5 at Convoy Supply.

  2. Jeff Stetter | | #2

    Diagram doesn't seem to make sense.
    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. GBA Editor
    Martin Holladay | | #3

    Response to Jeff Stetter
    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. Scott Paulson | | #4

    Ice damming concerns
    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. Jon R | | #5

    Even greater separation from
    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. Trevor Lambert | | #6

    rain screen direct to the attic
    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. Expert Member
    Malcolm Taylor | | #7

    Trevor
    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. GBA Editor
    Martin Holladay | | #8

    Response to Trevor Lambert (Comment #6)
    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. Andrew Bater | | #9

    Lstiburek Ice Dam Warning - Snow Prone Regions
    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.)

  10. E247 | | #10

    Sorry to comment on an old
    Sorry to comment on an old article, but I have a question that doesn't seem to be addressed by the article. What about venting the rainscreen into the eave if the attic is unvented? There are vents in the soffit but the attic is sealed. Would there be a problem venting the rainscreen into the soffit in this application?

  11. GBA Editor
    Martin Holladay | | #11

    Response to E247
    E247,
    An unvented attic is like an upside-down wine bottle. If you connect something to the mouth of an upside-down wine bottle, it isn't vented. It's sealed at the top.

    If you want the advantages of top-side venting -- namely, air flow and ventilation drying -- you need to vent to the exterior, not to an unvented attic.

  12. E247 | | #12

    I understand that the attic
    I understand that the attic is unvented. I was only asking if the vents in the soffit could be used at the top vent for the wall's rainscreen.

    For example, if the roof assembly was similar to the picture below where the attic is unvented. If soffit vents were installed and the rainscreen for the wall was vented into the soffit, would there be any issues (so air could enter the bottom of the wall assembly, go into the eave and then exit through the vents in the soffit)

  13. GBA Editor
    Martin Holladay | | #13

    Response to E247
    E247,
    As long as local codes permit, there is no reason that your rainscreen gap couldn't be vented into the soffit area.

    If the soffit is ventilated -- that is, if your soffit material allows air flow -- the air movement through the rainscreen gap would be possible with this approach.

  14. Richard Stern | | #18

    It would be nice if someone point me to section details (that I can't seem to find again in GBA, I've seen it somewhere) of the bottom of the rainscreen assembly with best practices for integrating the WRB, exterior mineral insulation, furring strips, and drip edge above foundation. My question here may already be answered in that discussion as to whether or not a drip edge metal flashing should be shingled in with the WRB, or be placed on top of the furring strips and be a drip edge for the cladding. My initial thought was that the former would conceal the bottom edge of the whole assembly (which includes Cor-A-Vent between furring strips), and channel any water originating from either WRB or rainscreen cavity.

  15. Hugh Weisman | | #19

    My project on Martha's Vineyard will have two wide sliding door assemblies on the first floor, one 16' wide, the other 12. The height of the rain screen below the doors is only 12". For a variety of detailing issues, I'm finding it difficult to provide a vent gap at the top of this portion of the rainscreeen. From reading this discussion, it sound's like, given the short height of the affected portion of the rainscreen, omitting top venting below the door sill will have a negligible effect on performance. Am I correct?

    1. Expert Member
      Malcolm Taylor | | #20

      Hugh,

      All residential buildings here in Coastal BC have to include rain-screens. Very few are top-vented, and in what is a quite demanding climate, they seem to work fine without it.

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