Venting roof ridge in Zone 7 under 8 feet of snow (430 lb/sqf ground snow load)?
I’d like to build a roof with a vented over-roof, over the top of our unvented cathedral ceiling. We’ll have continuous soffit vents. The remaining piece of the puzzle is the ridge vent.
We can’t have a conventional ridge vent because it will plug up in no time – it’s covered in as much as 8 feet of snow. We can’t have a roof monitor or cupola due to height restrictions. It’s a cathedral ceiling, so nowhere to put a gable vent.
Our home is post-and-beam construction, leaving us with the ability to have a continuously open space beneath the ridge that is sealed from the elements above the ridge.
Here’s the question: Can we vent at the end of the ridge, using continuous vents facing down in the eave portion of the ridge? Has anyone ever done this? Any reasons this would not work?
First thing that comes to mind, there’s no induced negative pressure since the ridge is sealed at the top and under a lot of snow. Warm moist air will rise, but will the warm air find it’s way to the opening at the end of the ridge before warming up the roof decking?
We need about 800 square inches of soffit venting. We have a 5-foot eave at the ridge, so we can certainly create a big enough vent, although that will need to be protected against fire brands / embers since we live in an area that has frequent “severe” wildfire hazards. There are WUI-rated soffit vents, and we could us those.
I forgot to mention that the we can only vent from one end of the ridge. The home is built on a steep lot on the side of a mountain. This means we have a multi-story home with multiple roofs (due to height variance restrictions), with one end of the roof ridge pointing away from the mountain, and the other end sealed against the adjacent home level closest to the mountain.
If we have a 65 degree F interior and a 0 degree F exterior, and 1300 sqf of roof (one level), we need to exhaust about 2200 BTU per hour from the roof, and that requires about 28 CFM. There’s some air resistance/head from the stud bays and the fire-rated continuous soffit vents, but at a very low air speed (0.63 feet per minute), the air resistance is negligible. A 50 CFM exhaust fan might do the job and will use about 18 kW a month – not passive, but not a bank breaker.
One of the three roofs has a chimney. We could build an 800 square inch vent next to the chimney without violating the height variance. The top of the chimney is protected and free of snow. We might use that as a vent for at least that one level. Can we passively vent the roof through a single point (800 square inches) that is somewhat central to the roof ridge and protected from snow? Not quite a roof monitor or cupola, but it can generate some negative pressure. If it matters, our wind speed for code purposes is 135 mph.
Eric
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
This may not meet your height requirements, but a solution in heavy snow areas is a "Boston Ridge." Not much info on the web, but the idea is simple enough. There is an old FHB article that describes it here -
https://www.finehomebuilding.com/1990/11/01/framing-a-cold-roof
A more modern application in standing seam can be seen here -
https://www.youtube.com/watch?v=Ap4FdaCyjfw
John, thank you - a gold mine of framing details in that FH article.
The writer is in Sun Valley, ID where the snow loads at the resorts are comparable to our location (200 lb/sqf), and the snow loads in town are comparable to the towns here (Truckee, CA @ +100 lb/sqf). An excellent proxy!
And it makes sense. A continuous protected vent. Simple enough.
I also like the FH article comment that 2x4 sleepers on edge work, but 2x4 sleepers on their side didn't (in the builders experience). Nice attachment detail for the sleepers.
Thanks again - Boston Vent in the Sierras is in the #1 design spot. Hopefully it'll pass aesthetic muster with my wife.
Eric
Eric, the IRC roof venting requirements are famously based on guesses made in the 1940s but they seem to work so we have stuck with them. There are two options: provide roughly balanced ventilation at eave and ridge, and you only need the vent area to equal 1/300 of the floor area. If you can't get roughly equal ventilation at eave and ridge, the IRC requires 1/150 of the floor area to be vented. In practice it's better to have balanced ventilation, but if you can find a way to get 1/150 of your floor area in venting, you will be code-compliant, and will likely have a durable structure, if not as resilient as it could be.
Michael, does the IRC allow for venting ONLY at the soffits without venting higher up (ridge) on a sloped roof (at 1/150)?
Yes, check out R806.1--they only require vents that are protected from rain and snow and provide "cross-ventilation," though they don't define exactly what they mean by that. https://codes.iccsafe.org/content/IRC2015/chapter-8-roof-ceiling-construction#IRC2015_Pt03_Ch08_SecR806
Your CFM calculation seems to assume that the air you are exhausting is at indoor air temperature. But the goal is to avoid that. If, for example, you allow a 15 F rise above outdoor ambient, you would need 135 CFM.
To minimize the power consumption, you'd want a fan carefully matched to the pressure required, which might take iteration: buy a fan, measure the static pressure, and then if necessary buy a new fan.
The biggest fan energy savings opportunity might be a variable speed fan with the speed based on a temperature sensor, adjusted to keep the roof just below freezing, but not any colder than necessary. And turn off the fan if the outside air gets above freezing.
Thanks Charlie. I did make lots of assumptions. I calculated the amount of heat transferred from the interior to the vented roof. The vented roof is at 0F on a cold day, and the interior of the home is at a balmy 65F. I used the calculation:
BTU/hr = surface area (in sqf) * temperature difference (in deg F) / R-value
You can find a calculator here: https://rimstar.org/renewnrg/heat_transfer_loss_calculations.htm
As for the CFM, I assumed that it takes about 130 CFM to vent 10,000 BTU. This is a pretty broad stroke estimate, where it takes 100 CFM to vent a 10,000 BTU cook top (dry air), and 130 CFM for a condensing heating device (moist air).
That's how I rather crudely got to a ballpark of 28 CFM without doing any fancier calculations.
If you think my assumptions and calculations are inappropriate, let me know!
I did think about sensors, but up on the roof in the middle of winter, I prefer simple. I really don't want to be replacing a failed sensor in the middle of winter (although I could put in a manual bypass switch on the sensor to force the fan on). At 18 kWh per month, that's $5/mo for 5 months of the year (turn it on beginning of December, turn it off end of April) - or whenever ambient temps no longer drop below 32F. Hmmm, if the sensor isn't too expensive, and there's a sensor bypass... OK, you've convinced me that a sensor is viable!
~ Eric
The 130 CFM/(10 kBTU/h) rule is for a furnace with a 70 F delta T. But you don't want to let the air in that channel get to 70 F above the outdoor temperature! The formula I recommend is Q (in BTU/h) = 1.08 (delta T) (CFM), so for example 1.08*15 F*135 CFM = 2200 BTU/h
Thank you! 135 CFM it is. I'm not an HVAC guy.
What's the thinking for a 15F temperature rise above ambient?
I think Lstiburek noted that ice damming seems to happen when ambient temps are between 22F and 32F, which might be when attic temps tend to be above 32F (?). This would argue a (delta T) of 10F. Adding a 50% safety factor for a (delta T) of 15F could make sense.
I'm just want to understand if the 15F is arbitrary, or if it's based in practice or science.
Thanks again!
Oh, the 15 is arbitary. If you wanted to prevent any melting at 26 outdoor temp, you'd need to limit the rise to 5 F, so you'd need 3X higher airflow. But at some point it becomes impossible to have enough air flow to avoid any melting. So you are just avoiding so much melting that it becomes excessive.
Charlie, this is where I pulled the 130 CFM/10,000 BTU airflow:
https://www.contractingbusiness.com/archive/article/20865137/practical-standards-to-measure-hvac-system-performance
Although 150 CFM/10,000 BTU might be more appropriate (condensing furnace).
Thank you for giving me another way to think about the fan sizing.
~ Eric
Deleted