Can too much soffit and ridge ventilation actually hurt a house if the house isn’t air sealed well
I know it’s dogma in building science that unless you are using spray foam or conditioning attic spaces, there has to be continuous soffit to ridge ventilation for all roofs in a house
and I am sure if the floor of the attic is air sealed and all roofs in the house that don’t have attics(shed roofs, cathedral ceilings etc) have air tight drywall, the air flows from soffit to ridge and is an effective and proven way to have a durable building.
but what if that’s not the case. What if a house doesn’t have air tight drywall and has soffit to ridge ventilation and no attic. Or what if the floor Of the attic is very difficult to air seal bc of the way it was constructed or just the labor and cost
it seems in that situation too much ventilation can absolutely draw outside air into your house, making the whole house much less energy efficient and comfortable.
all thoughts are appreciated!
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The dogma does not stem from energy efficiency but durability of your roof. If you don't have continuous ventilation path you risk rotting your roof out. generally the energy penalty will be less cost long term than a new roof or other potential damage stemming from a failed roof. there is also the issue of ice damming but that is less of a reason for a vented roof
Thanks for the response! I agree with you that it’s for roof durability. It just seems to me that the energy penalty is greatly underestimated and over-ventilation is a real thing.
And having that much air movement in a house brings enormous moisture with it as well, as air leaks obviously can move enormous amount of water vapor
Mike,
"And having that much air movement in a house brings enormous moisture with it as well"
It can in hot, humid climates, but in cold ones the outside air has a lot less moisture in it than the air inside, so once it is introduced the interior RH will go down. You see that in poorly sealed houses in cold climates, which often need humidification.
Mike,
As feyr_design said: the primary purpose of roof venting is avoiding moisture damage. In a poorly sealed house roof venting can increase the stack effect - meaning more air entering the house at the base, and exiting into the attic. That's why the recommendation is to have more vents at the soffits than at the peak, which slightly pressurizes the attic, reducing exfiltration.
Thanks Malcolm for the good response, as usual.
I have so much soffit ventilation due to many cathedral ceilings and shed roofs (no attic in these areas). I just wonder since my house is very drafty, if the soffits can be a case of over-ventilation, especially since there is no way the drywall is air tight, like in new construction.
All of the draftiness will likely go away when I get new siding in a couple of months, which combined with thorough exterior air sealing, will hopefully solve my problem, but having so many soffits without airtight drywall still
Worries me
Mike,
Much of my comment concerned attics, and thinking about it again you may well be right that excessive ventilation causes more problems that we think, especially in cathedral ceilings. Both Martin and Joe. Lstuburek repeatedly made the point that roof ventilation is a cure for problems that shouldn't exist. There's probably more to that than I acknowledge.
Malcolm --
"Both Martin and Joe Lstuburek repeatedly made the point that roof ventilation is a cure for problems that shouldn't exist."
So what do they recommend instead? Exterior insulation? Spray foam on the underside? "Shouldn't exist" is pretty strong language.
DC,
https://www.greenbuildingadvisor.com/article/all-about-attic-venting
They suggest venting should not be the primary strategy to reduce moisture in roofs. Reducing the moisture load is the priority.
Thanks Malcolm.
I realize that article was written in 2013 and a lot has been learned since then, but I have to take exception to this section:
Here are the four most common reasons people suggest to explain the practice of venting attics:
* To reduce the chance of moisture build-up in the attic or condensation on the underside of the roof sheathing.
* To make roofing shingles last longer.
* To lower cooling bills during the summer.
* To reduce the chance of ice dams.
Now, I'm old enough that as a child I was taught that roof venting was necessary for those reasons, particularly the second. But since then I've learned that it's really only about moisture management.
And yes, you can control moisture by making sure there are no condensing surfaces, but usually it's simpler to vent it away.
DC,
I understand where they are coming from - complaining that ventilation shouldn't be used as a cure for poor air-sealing to vapour-migration dues, but I agree with you. If nothing else ventilation adds robustness to assemblies that may not always be as effective at preventing moisture as they are when first built.
What do you think of Mike’s suggestion that in poorly air-sealed houses excessive roof ventilation may lead to increased moisture load in the roof?
Yeah I am definitely torn about what to do going forward with the soffits when the new siding gets put in
Exterior air sealing alone without also air sealing from the inside may lead to warm air from the living space migrating through the exterior walls and condensing on the sheathing causing moisture issues in cold months.
That's what vapor barriers are for.
The most common air barrier, drywall, is very vapor open. In cold climates you need to have something else to keep vapor out of the wall.
Thanks DC. is your point here that I need to add some other vapor barrier besides sheetrock? As mentioned to Josh, My hands are somewhat tied at this point bc it’s not new construction.
Sheetrock is not a vapor barrier.
There are primers you can apply that are vapor barriers, but for some reason they don't seem to stay on the market long.
DC most of the moisture in the building assembly it seems comes from air movement and what I have now is the Sheetrock to block the air movement and the latex paint to function as a vapor retarder. Not sure of what other vapor or air barrier I can add at this point
Thanks Josh. I have Sheetrock now and have tried to seal all the openings like light fixtures, outlets, baseboard gaps etc. if this were new construction I might use an interior vapor retarder but my hands are tight somewhat at this point
You can find opinions on both sides of the debate. Some saying painted air sealed drywall can act like enough of a vapor barrier that you won't have moisture issues because the amount of moisture carried in vapor is so much smaller than the moisture carried in air. Then you will hear those on DC's side that feel fully dedicated vapor barriers are the way. As you said in your case it's not new construction and ripping apart your interior walls isn't an option so I think carefully sealing any and all air leaks from the inside seems like a prudent approach. Although I am no expert, just a DIYer with an interest in building science.
Josh,
I agree. The recent migration of the primary air-barrier to the exterior as more people use the sheathing or a self-adhered WRB for that has made us forget the necessity of a secondary air-barrier at the interior, which has the function of stopping moist interior air cycling into and out of the wall cavities, and having that air reach the cold sheathing. An interior vapour-retarder may not stop that.
Thanks Josh. I appreciate the feedback
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it seems like Lstiburek experiments with vapor diffusion vents in cold climates is a pretty good indicator that not venting is not a solution. Of course we should try and provide as little interior vapor from entering an accumulating in the space, but even when this is achieved (for example in their test houses) there is still an unacceptable accumulation of moisture at the roof sheathing in zones higher than 3 (and that is with a vapor diffusion vent. just closing off your ventillation because you have a gut feeling it is making it worse is not a good strategy.
freyer_deisgn, I think everyone would agree that ventilation is important. The question is to what degree. Once you have soffit vents everywhere the air isn’t only going to go under the roof deck. It can be pulled into the house as well, especially in houses that are poorly air sealed and leaking a lot of air in general at the top. If the drywall is perfectly airtight it will not get into the house, but that’s almost never the case in older houses
I don't think everyone would agree that ventillation is important, and in fact is not my argument. It is more that your concerns about ventilation are in the opposite direction. Lstiburek actually argues against ventilation as a strategy and instead conditioning the roof deck in his article "shakespeare does roofs" https://buildingscience.com/documents/building-science-insights/bsi-141-shakespeare-does-roofs
My point is more that the current code ventillation requirements are in place with the assumption of energy loss from the house to ensure that the ventilated roof actually removes moisture. My point is more that closing vents in an existing structure that uses a vented roof strategy without another way to prevent moisture accumulation at the sheathing is a recipe for a rotten roof. If you stick with the current code you will probably be fine, and they in fact have ratios you should stick to. Here is the code if your interested:
R806.2 Minimum Vent Area
Diagram
The minimum net free ventilating area shall be 1/150 of the area of the vented space.
Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided both of the following conditions are met:
In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.
Thanks for the info freyr_design. I am much less worried about the attic (which can be relatively easily air sealed) than I am about cathedral ceilings and shed roofs
My point was when I said we can all agree ventilation is good, I was referring To my setup. A house that has existing attic, shed roofs and cathedral ceilings with continuous soffit ventilation. Since it’s not a conditioned roof deck, we can all agree ventilation is good
And I wasn’t arguing to close all
The soffit vents. What I am arguing is that for the cathedral
Ceiling and shed roofs there may be an amount of ventilation necessary to remove mositure (which you agree with) but more than that amount may start to have deleterious effects
[replying to #19]
Malcolm: "What do you think of Mike’s suggestion that in poorly air-sealed houses excessive roof ventilation may lead to increased moisture load in the roof?"
I can't think of any scenario where more ventilation leads to more moisture. I can buy that more ventilation would lead to more infiltration losses from the top floor, but I can't picture it happening that the extra ventilation brings more moisture in from the house below than it lets out through the vents.
I'd tend to think the opposite: the more infiltration you have from below, the more ventilation you need to control vapor levels.
I can also see there being a point beyond which more ventilation has no impact on infiltration. Infiltration is driven largely by temperature differences, once the vent space gets to ambient outdoor temperature more ventilation isn't going to make it any colder. I don't see there being a Bernoulli effect where the ventilation air moving perpendicular to the openings in the ceiling creates a vacuum that sucks air out of the house.
DC, I’m not an expert, just a homeowner trying to improve the house but I don’t see why more ventilation couldn’t increase moisture. The cold air from the ventilation can mix with humid indoor air and increase moisture in all parts of the house. None of that matters if everything is all air sealed well, but if you are constantly getting air infiltration from every soffit, that’s a lot of air movement throughout the entire house.
Mike,
Exhausting more air through ventilation at the peak may increase the amount of air that enters the house near the bottom through the stack effect, but I don't know what mechanism would make the air brought into the roof through ventilation enter the house. The direction of both that ventilation air, and the air from inside the enclosure, is upwards and away. You can see this clearly at poorly sealed attic hatches, which often show discolouration from air-leakage above them, but never below.
Why can’t air enter the soffits and get into the house in the gap between where the drywall meets the framing or the gap where the sheathing meets the framing
It could, but the top of the interior of the house tends to be slightly pressurized due to stack effect, particularly in cold weather. What's more likely is that if there are cracks there the air will flow out.
"The cold air from the ventilation can mix with humid indoor air and increase moisture in all parts of the house."
Almost certainly the cold air is going to have a lower moisture content than the interior air, so any mixing is going to reduce moisture. That's why leaky houses get dry in the winter.
DC, I accept your points and you have more much more experience than I do as I’m just a homeowner with an interest in this.
I do think there’s a difference between idealized versions of what’s happening and real
World experience. And in a leaky house like mine, air is being sucked into the house through almost any gap on the lower levels.
I have one roof soffit on the lower floor, part of a shed roof, that I taped close as an experiment. There was a cabinet in the interior under that soffit that became markedly less cold when the soffit was taped.
I think certain situations are much more complicated than currently accepted explanations
You're looking at the wrong thing. The measure of roof ventilation isn't whether the interior gets cold, it's whether the underside of the roof gets wet.
DC last comment from me and I appreciate your expert feedback. I understand the point of the soffits is to keep the sheathing dry. My point is there may be unintended consequences which I think I am experiencing