Is soffit venting necessary with a vented rainscreen?
GJPehl
| Posted in Green Building Techniques on
Hello GBAer’s.
I’ve got an assembly design for a vented rain screen, 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.
Does, can, should this assembly eliminate the need for soffit venting? It seems to me that it would probably actually increase the airflow through the rain screen, although now that I think about it, that might not actually be a benefit.
The building inspector said its up to me.
Thanks in advance for any insight, and thanks again for the great resource!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Gerald, Rain screens are typically 1/2" to 3/4" deep, so it probably wouldn't make up the required area for soffit venting. But if it did work out I've also wondered if connecting the two might not increase the potential drying of the wall assembly.
Here in Canada 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 rain-screen, and one which is vented at the top with soffit vents immediately above. I'll be interested to see what other posters think.
Thanks Malcolm.
The space is 3/4", and it turns out to yield about 13 square feet, and I need about 10, although I've heard many people say more is always better. I don't want to be short-sighted, but the attic has 3-4 inches of spray foam, and we're going for PassivHaus retrofit. Blower door test is next week, and the builder is confident that we'll easily surpass the 1.0 ACH50 requirement- the air barrier assembly was very highly simplified for the purpose of making it easy to execute. My point being simply that we shouldn't have a lot of leakage, rather I would expect the air flow to really be driven by exterior temperature/pressure differences.
I hadn't considered the fire spread issue, I'll be interested in other opinions as well. There's no reason we couldn't isolate the two spaces, venting the rain screen at the top of the wall assembly instead of through the ridge vent, but I'd like to simplify anywhere we can.
Gerald, We know that rain-screens vented at both the top and the bottom perform better, but I wonder whether that improvement is significant compared to the benefits that accrue from adding any rain-screen?
With one only vented at the bottom you get a capillary break, a drainage gap to handle bulk water intrusion, and some drying. Here in the wet PNR 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.
Gerald,
I think that the advantages and disadvantages of all the different approaches have been well described. The issue of fire safety is contentious, but in some areas of North America -- areas where wildfires are common -- vented rainscreens of any kind are controversial.
One final point: If you have a good ceiling air barrier in your house, attic venting doesn't matter very much. For more on this issue, see All About Attic Venting.
Building Science Corporation has a paper on fire hazards within rain screens. Basically it's not an issue.
"...We have addressed the gap many times before with respect to drainage. 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. You do not need 2 inches. Using 2 inches for a rain screen to control hydrostatic pressure is insane. We have been here before (“Hockey Pucks & Hydrostatic Pressure”, ASHRAE Journal, January 2012)...."
"When the air gap is small…less than ¾ inch…the friction from both surfaces bounding the air gap limits the air flow. The boundary layer on both surfaces is an effective fire stop[9]. "
https://buildingscience.com/documents/building-science-insights/bsi-098-great-fire-london#_ftnref9
Re: "I wonder whether that improvement is significant compared to the benefits that accrue from adding any rain-screen?"
An in the field study concluded that venting does offer a significant improvement over doing a rainscreen alone. The drying power of this assembly is much great than a vented system. This study was conducted over a number of years, though only one year is presented in the article "RAINSCREEN WALLS: LONG-TERM PERFORMANCE ANDFIELD MONITORING IN COASTAL BRITISH COLUMBIA" - Hubbs, Finch, Dell
As to venting through the attic... is there any benefit to this approach? 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. There is certainly no benefit to your attic to have the two systems connected.
From 'All About Attic Venting'
"Because your attic is vented, you need to feed a continual stream of moisture towards the attic vents so that the vents have something to do."
We cannot under-overestimate the importance of this point- if the vents have nothing to do, they're liable to start wandering the neighborhood in baggy pants, listening to loud, distasteful 'music', and violating all kinds of nuisance laws.
Lest any GBA readers be confused by Gerald Pehl's most recent comment, I want to emphasize that he is quoting from a list of misconceptions. Out of context, the sentence doesn't make a lot of sense. In context, it does.
Again, here is the link: All About Attic Venting.
Sorry, it was a joke- thank you very much Martin. I'm still reading, but I will respond more fully a bit later. To be more clear, I like the comment because it very well illustrates the kind of dogma that passes for 'knowledge' out there in the world. Over the years I have been inundated with input from sources that simply don't understand what critical thinking is, and so reasoning that _could_ be reasonable, like 'venting is for lowering indoor humidity' goes hand in hand with 'the vents need something to do' which is the opposite of reasoning.
And all of this again makes me _very_ grateful for this resource.
Martin Holliday wrote:
"One final point: If you have a good ceiling air barrier in your house, attic venting doesn't matter very much. For more on this issue, see All About Attic Venting."
Thanks Martin, and apologies for being lazy- I simply can't remember if I had read that article in the past or not, but the reminder and the summary helps a lot.
John Clark wrote:
"Building Science Corporation has a paper on fire hazards within rain screens. Basically it's not an issue..."
Thanks for the summary- I'm looking at the article now.
Tyler LeClear Vachta wrote:
"An in the field study concluded that venting does offer a significant improvement over doing a rainscreen alone..."
Yes, I think I've read the article you reference, and we are definitely venting the top to create a continuous channel from top to bottom, the only question is the in the construction details of the top.
The ideal case is to vent the rainscreen into the soffit because it's the easiest assembly at this point in construction, 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 noticable detail change.
You also wrote:
"There is certainly no benefit to your attic to have the two systems connected."
As my initial question showed, I was operating under the assumption that attic venting itself is much more important than it apparently actually is, according to the article Martin referenced.
The real benefit to connecting the two systems is simplicity of the assembly, and the aesthetics of eliminating the venting channel in the soffit. 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" 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.
John,
I'm not sure the Building Science article is directly addressing the same issue. The small cavity and boundary layers of a rain screen may limit the ability of a fire that originates below (like Grenfell Towers) to spread vertically through a rain-screen cavity. But a fire that starts in an exterior wall, or the interior and broaches the exterior wall, only reaches the gap by destroying the boundary layers, and like all other penetrations of the ceiling plane may need to be fire-stopped to keep it out of the roof space.
Tyler,
Thats a very interesting study. A few things stood out to me that may be relevant to this discussion:
- While Building Science Corp. often makes the point that a very small gap works as a rain-screen, the reality of construction is that they may become closed due to the WRB intruding into them, or wood shrinkage over time.
- A well ventilated rain-screen allows better drying than a poorly ventilated one, but the difference in the moisture measured in the walls wan't significant.
- The study expressly suggests not tying the rain-screen to the attic, as they have found moisture and mold problems in building where this was done. Whether these problems are likely to occur outside the PNW is still an open question
Malcolm wrote:
"- The study expressly suggests not tying the rain-screen to the attic, as they have found moisture and mold problems in building where this was done. Whether these problems are likely to occur outside the PNW is still an open question"
Thanks for that clarification definitely food for thought.
Gerald, 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.
Malcolm, I think you are right. A PassivHaus architect has told me that he likes Minnesota because there are a lot weather extremes- it gets very cold, very dry, very hot, very humid. None of those rivals the individual extremes in the world, but it's quite a mix. That said, we don't get the conditions you describe in the Pacific Northwest.
Malcom,
Your point about a small gap becoming blocked is significant. Textured WRBs that create the smallest gap also have the least room for error. 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.
To the point about measured moisture levels, there are some significant thresholds to be aware of: above 15% wood moisture equivalent (WME) is ideal for fungal growth, and above 20% WME is ideal for rot. During the cold winter months those thresholds aren't as important because the freezing cold inhibits rot and fungus.
Most moisture problems aren't due to a one time waterfall in the wall, they're due to a little moisture spending a long time in the wall. Drying with ventilation is the best way to address that. One aspect of this study that was so 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? Which layers get wet in the process? And what if the next moisture event happens before the wall dries?
And thank you for pointing out the article's caution against venting to the attics. I recalled reading it somewhere once upon a time, but there it was, right under my nose!
Tyler,
It may not be particularly significant, but I believe in the coldest climates, moisture is actively removed from the air by freezing, creating dry ambient air for the wood to dry, which 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! 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.
Tyler, Thanks for clarifying those moisture thresholds. That's very useful.
With a much diligence as I can muster I still end up with some wrinkling in my WRBs. I'd imagine you get some capillary bridging between the building paper (or wrap) and the siding in cavities under 3/4", never mind, as you said, the proprietary products.
The typical advice to not worry about winter sheathing moisture levels doesn't work very well here. The temperatures don't get cold enough. That said, including rain-screens in our construction practices has transformed the resilience of walls here. I simply don't see the problems now that were endemic in any building I went to renovate in the past.