Attic ventilation connected to rain screen *with no overhang roof design* in a cold climate?
I am distraught about building a home with no overhangs, I am aware this is far from ideal. Our architect said “in the end it all comes down to looks” when I originally inquired about the practicality of not having overhangs, asking about keeping water away from the structure and foundation… the house is somewhat poorly sited, because it is fully exposed and at the top of a hill in an area with ~ 30” of rain and 130” of snow annually.
We are now mid-construction and the roof is up. It looks nice and modern, but I started researching on buildingscience.com and realized we were misguided during design. The builder is good but they are still working on changing the attic venting. Original plans included 50% ridge vents and 50% gable end vents but I did not like the look of the gable vents and prefer ridge exhaust and continuous soffit intake… problem is: we have no soffits.
The siding and roof will both be exposed fastener (ribbed) steel – black. I know it is going to get very hot, full sun exposure. It’s a 10:12 gabled roof. Climate zone 6. The rain screen behind the siding is bottom vented, I believe the builder is currently working to add top venting which was not in the architect’s design. I am worried about ice damming and attic moisture issues. We require 576 sq in of NFA (min) so 288 for the ridge and 144 sq in on each side of the roof. I thought we want ideally more intake NFA than exhaust to avoid depressurizing the attic but I think if the vent is the full length it would be plenty (~ 40 ft but there are trusses in there).
It sounds like the idea is to ventilate up behind siding through the unconditioned attic (ridge exhaust). I found one related post on GBA, is this a bad idea to connect the rain screen cavity and the attic? I don’t know the fire risk is high where we are, but it is rural and surrounded by forest.
So, if we have to go back to gable vents… I’m not sure how easy that is because the roof is done as are the gable ends. Seems like we should have figured this out a long time ago, in case it makes more sense to keep the original venting. In that case, I’m not sure if we would just leave the top of the siding unvented but that does not seem ideal with black metal.
Most importantly, the attic will be well sealed aside from a mandatory hatch (R-50). There will be ceiling penetrations though, I guess they have to individually seal them. And a steel chimney. Attic floor insulation will be R80 mineral wool or cellulose (TBC) and they are adding ‘moore vents’ but I couldn’t find info, I think they want to add cardboard baffles.
In short: if you were forced to not have any roof overhang, how would you ventilate a big unconditioned attic? With all black steel siding and roofing.
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Replies
Izzza,
Unless your code precludes it (as it does here in BC), I would not worry much about the fire risk of connecting the rain-screen to the roof unless it was deeper that 3/4". The vent channel should be too narrow to sustain flames. If wildfire is a threat, then wherever the intakes end up being located should be protected by a mesh screen fine enough to prevent embers entering.
The advice against connecting the two vent spaces comes primarily from two things:
- It is very unlikely the narrow wall cavity will supply sufficient intake air to the roof.
- The wall cavity is there to remove moist air. You don't want to exhaust it into your roof system, where you are trying to do the same thing.
Given where you are in the construction process, I would suggest one of two approaches:
- Blocking the wall cavities a few inches shy of the roof, and introducing intake air at the top of the wall, using a similar detail to whatever your builder was proposing for venting the wall cavity there.
- Adding gable vents. Simple, unconditioned attics do quite well with just gable and ridge venting. Check with your inspector though before going that route.
Thank you Malcolm,
I will ask the builder. It seems gable vents could have been good enough then but unfortunately since the builder did not discuss the detail with us until this point I am not sure what can be done. I simply did not like how they look and wanted to know if there is another option, had someone simply told me about the complexity - we could have just kept the gable vents. Communication at its finest 😉 They did not build the gable vents and the steel is already up, so the location they would need to be on each end (centred) is blocked by trusses now. I am not sure if they can simply cut a hole and put a vent over a piece of wood blocking the middle, I assume it needs to be framed.
Either way, it still seems good to fully ventilate the rain screen by opening the top. However, it is unclear how this is supposed to happen with no soffits/overhang to protect from rainwater entering into the cavity from above.
I understand your advice is to ensure the two cavities are disconnected, with wall exhaust venting opening up a few inches below the roof intake. This way although some heat will transfer, cold outdoor air will mix and provide better intake for the attic? That might be what they are planning, I want to know if it is creating any issues with ice damming.
Funny to go to all this trouble and end up throwing on standard gutters instead of something concealed. I foresee them getting destroyed in a storm at some point. Renting equipment to repair them will certainly be fun!
I have designed and/or built many houses with no overhangs with no problems; the key is to make sure the WRB is 100% watertight before any roof or wall cladding is installed.
I agree with Malcolm; separate your wall vent and roof vent. I often use Coravent products for this kind of thing.
Thanks Michael, that is nice to hear! So you have done this type of roof venting with no overhangs? I struggled to find any examples of what this could look like which helps me understand it, do you have any visuals? No problem if not.
Well, I sure hope our builder ensured it is watertight… I was a bit disturbed when I saw we are using Tyvek WRB and OSB because I thought these were low quality. Do you think this is adequate if installation is flawless? Perhaps the bad reputation is based on less informed builders who use Tyvek over untaped OSB as an all-in-one solution?
Oh, we have 4” of rigid foam between the well-taped OSB and the WRB on top. So water seems to have a long way to go to reach the OSB…
I have designed the details on two houses with metal siding and essentially no overhangs, but those were ten years ago when I worked elsewhere so I don't have drawings. I'm sure we used our standard detail of venting the rain screen into the roof vent. That approach is safer with metal siding than with a reservoir cladding.
You probably have conventional rafter vents? Those don't work well with no overhang if you want the eaves to be waterproof. The best approach is a sarking membrane system where the WRB on the walls is continuous with the WRB at the roof, typically a membrane directly over the rafters or trusses, with 2x furring over that to create the vent space and sheathing over that.
Back when I was a carpenter on Nantucket 20+ years ago we often did minimal overhangs, venting the roof with a small gap behind a frieze board.
There are venting drip edges. In fact I think we used them on one of the metal siding projects.
Thanks, I will have to ask my builder. I’m sure they do not need my help, I think they are good and I know I am an overly ‘engaged’ client, but I both lack trust and also find this really interesting 😂
Our project manager is newer to residential building, when I asked what kind of vents they are using instead of the gable vents he did not know! I asked if it was a vented drip edge which I saw online, he was unaware and said it comes from the siding manufacturer, vague, and could not explain anything beyond him and another guy “doing some calculations” which might be enough for a normal person but I want to know exactly what calculations they are doing… He mentioned the ridge venting being about 2/3 of total NFA required by code and this concerned me but he could have misspoken.
Here is a corner of the roof during installation last week. I have no idea where the vents are going or if they are rafter vents… this is what keeps me up at night and I want some answers!
Do you happen to have a mockup/3D rendering showing what the roof to wall transition is going to look like? I'm having a hard time envisioning it. Are you going to just put a facia board and gutter?
Hello fellow Canadian 👋 I wish I had this in 3D! I’ve been dying to understand many of the details but without good visuals/3D I find it impossible…
Here is the original detail with my markups, though the builder might have changed other details that I am not aware of. This was also pre-intake venting being part of the picture, they are planning cardboard baffles to keep the insulation in place. Cardboard seems odd but I guess they know what they are doing. I wanted integrated/hidden gutters but I think that ship has sailed. Does this visual help?
Izzza,
With the roofing ending virtually flush with the cladding, that intersection is very vulnerable to bulk water intrusions. I would steer away from trying to add intake vents on the walls, and revisit your decisions to eliminate the venting on the gable ends, even it it means backtracking a bit.
Thanks Malcolm, that is why I was worried. I will see if the builder is planning to do something different than what you see in the visual, otherwise I will ask if they can keep the gable vents as originally designed. I am not sure why they decided to forego the gable vents without my approval.
So, what would you do with the top of the siding/rain screen? Would you open it for ventilation and if so, how does water stay out? Or would you keep it closed and just let the wall assembly roast under the black steel?
Izzza,
In your situation I think the benefits of top venting the rain-screen is outweighed by the risk of water intrusion. Make sure the cavity has a clear (but screened) opening at the bottom and be done. Hot walls and hot attics can sometimes transfer their heat to the house inside, but from a resilience perspective they are great at dealing with any moisture that occurs. That's in large part why you see so much more damage on the north faces of walls and roofs.
I have owned two houses that experienced ice damming and icicles. One of the houses regularly had icicles the full height of the first floor. Neither house had soffit vents, one because it was 1812 timber frame with no opportunity for venting, and the other had the soffits covered in several layers of materials. Both houses were cured of their problems by careful air sealing and insulating. Both houses had a small amount of gable venting and one had some ridge vents. My point is that attic venting is overrated. The requirements are a holdover from the way houses used to be built. I wouldn't sweat it if you air seal and insulate well.
Thanks Malcolm, sound advice! Learning about inward solar vapour drive freaked me out - would you be worried about the OSB degrading or mold developing over time in the walls without opening the top of the rain screen cavity for ventilation? I am worried if water gets behind the metal and then gets heated/evaporated that it will get trapped in the wall assembly. I think we are using Intello Plus inside under the drywall so maybe that will be fine?
Seabornman - nice to hear! Those icicles sound terrifying, glad you lived to tell the tale 😉 I wish someone had mentioned this to me before we unknowingly paid multiple people (at our builder) to spend their time trying to figure out how to get rid of the gable vents. It seems water intrusion is a much bigger concern than attic moisture.
They are going to looove me when I ask if we can throw away whatever they’ve been working on behind the scenes and go back to the gable vents.
I don’t know if it will be possible because there is a vertical piece of wood right in the middle of the gable ends now 😭 See image below… do you think they can cut into the steel and make the gable vent without making the rest of the steel look like crap? I don’t know how they are going to reach - we had to put the roof on with a crane. The scaffolding does not go that high. I guess they could to it from inside the attic. I don’t know who made the final call to exclude the gable vents but this is such a nightmare. As soon as everyone is back to work in the New Year I’ll get on this.
Thanks for the feedback, this was really helpful! Glad I signed up for GBA. I have ~ 10,000 more questions to ask on here.
Izzza,
I think your walls are fine. Lack of a top opening means they are not Ventilated Cavities, but they are Vented. So there is still a path for moist air to be removed, and as you say, the interior is also vapour permeable. We have rain-screens mandated here in Coastal BC. Hardly anyone vents the top, and they work well.
I've got the same situation in my attic - two studs right in the middle of the gable. The vent sits right over it.
I can rest easy tonight! That is good to know we can put a vent right over the stud. I was imagining having to cut it and frame some opening like a window.
Your place looks like a cozy cottagey cabin, nice 😉
The builder said something about clients not having control over certain elements or it can void Tarion coverage, so I’ll see if they might want to reconsider the venting based on my evolving preferences…
GBA is amazing, it would take like 10 years of back and forths to get this same info otherwise. Thank you for taking the time to help!
I'm just a hobbyist but here is my two cents.
Is the orange layer in that diagram the air-gap for the rain screen? In other words, will there be 3/4" furring strips run vertically at 16" intervals from the bottom of the wall to the very top (meaning that even your sub facia has a rain screen gap behind it)?
If so, then I am guessing they would just stop the insulation and sheathing a little short so that there is a gap into the attic. Alternatively (and probably better), they might cut channels or holes in the top of the insulation and sheathing so that there are air channels from the rain screen gap into the attic but there is also still some support for the edge of the roof sheathing.
That actually seems like it is not too bad of a plan all things considered.
Re the insulation baffles in the attic. They do make foam baffles as well. Like you, I'm not a fan of the idea of cardboard in that application. They are cheap either way so they may be happy to switch to foam if you ask. Here is one example.
https://www.owenscorning.com/en-us/roofing/components/products/raft-r-mate
Thanks, I’ll ask about the foam baffles. They might be thinking cardboard because they are a green builder but I’ll ask about the pros/cons.
Yes, you’ve marked the air cavity. So it could easily be vented into the attic without dealing with outside air or creating an opening, I’m just not sure about the warm air heating the underside of the roof sheathing. Also not sure if it will provide enough air flow to the attic.