Can you vent the top of a rain screen into the attic?
hondro
| Posted in General Questions on
In designing our new home in a dry area of the Pacific Northwest, my wife and I want to design for fire resistance as much as possible. One point of concern is embers from a wildfire entering through the soffit vents. Our architect has proposed venting the top of the rain screen cavity into the unconditioned attic, and omitting soffit vents. Air would be drawn up behind the rain screen to provide attic ventilation. Are there any concerns with this approach?
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Replies
If I remember from previous discussions on here, the problem may be the boundary layer depth (from the rain screen depth) causing issues, preventing enough air flow up the rain screen to properly vent the attic.
There are some types of soffit vents specifically designed to stop fire spread and not ingest embers, etc. I've seen them for sale but that's the extent of my knowledge.
hondro,
Three issues:
- As Chris said, the typical 1/2" to 3/4" rain screen gap is too narrow to allow the much larger air-flow necessary to vent a roof. You would probably need an 1 1/2" wide cavity to do this, and then you bump into the risk in my next point.
- You don't say where in the PNW you are but BC's building code considers a rain-screen gap a concealed space, and precludes connecting it to the roof for fire spread reasons. There is some debate as to whether the risk would materialize during a fire with the typical gap, but wider ones would be a problem
- Maybe not as important in your dryer climate, but it doesn't make much sense to vent a wall to remove moisture, and then send that air into a roof to help drying.
I wouldn't vent the top at all. The main attributes of rain-screens are they act as capillary breaks, help redistribute moisture, allow better drying though the sheathing, provide a drainage path, and have ventilated channel. All you would be losing is the last one. Here in coastal BC hardly anyone vents the top, and they work well.
Sorry, this doesn't help solve your dilemma as to how to vent the roof.
I've considered venting a rain screen into a vented attic space. My first concern is introducing additional moisture into the attic. Moisture accumulated in the siding could evaporate into the rain screen space and rise in a convection pattern due to sun warming the walls and roof. Of course if there is sufficient ventilation in the attic that may not be an issue, but it seems like a situation best avoided.
Do you truly need to vent the top of the rainscreen? Ideally the top is purposely vented to allow convection pattern drying. However, i have often used "accidental venting" only at the top of the rain screen. (There are often gaps in the siding-trim at the top that provide a mininmal pathway). Some sidings styles have a lot of air flow through them which can minimize the benefit of venting the top of the rain screen. Often the rain screen gap can dry nicely just be venting out through the siding. The rain screen still allows bulk water to flow down, and the space allows moisture to disappate over a large area where it can dry faster even without an intentional top venting space. For instance, Cedar shingle siding has a lot air flow through the siding everywhere.
If you decide to vent the rain screen into the attic I would stress detailing the air barrier to minimize the need for venting. (Good practice regardless). A sealed air barrier minimizes the moisture that the attic venting must eliminate, adding a bit of drying capacity cushion.
You might consider a non-vented roof system as well.
We just installed Hardie soffits with the holes for vents. Since the holes are larger than 1/8", we glued metal insect screen on before installing the soffits. This went pretty quickly given the 200 ft of soffit (but was messy). Labor here is expensive and in the end I think this probably cost as much (due to labor) as installing Vulcan vents. Not sure which I'd go with if doing it again. But I'd do either long before I'd dink around with the rain screen cavity.
Thank you everyone for your helpful thoughts! The house site is NE of Mount Hood in Oregon, just on the dry side of the Cascades. We've had increasing numbers of wildfires over the last several years, so it seems prudent to add fire-resistance to our list of design goals, along with energy efficiency, low embodied carbon, and low toxicity.
To summarize your replies, there are several reasons not to vent the rain screen gap into the attic:
- typical gap size is too narrow to supply enough air flow for sufficient attic venting
- a larger gap size is still not a good idea (and against BC code) because it would increase the chance of a fire spreading from the siding to the attic.
- moisture behind the siding would be introduced into the attic
- there exist soffit vents designed for fire zones; e.g. Vulcan Vents and Brandguard Vents.
Hondo,
It's a small point but I don't think the interdiction on connecting concealed spaces comes from the fear of wildfires, but from those originating inside the house. Fire blocking is required on any opening between the living area and the roof space roof above. Typically that's things like balloon framing, or bulkheads over cabinets.
For a start I'd question the decision to go with a vented roof, especially if wildfire is a concern. But if that's a done deal then there are special soffit vents made for these conditions.
This website from Marin County, California, also has some useful information and details.
https://firesafemarin.org/harden-your-home/fire-resistant-soffits-eaves/
eave vents
https://firesafemarin.org/harden-your-home/fire-resistant-vents/