Ridge vents covered with snow

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  #1  
Old 04-27-15, 03:29 PM
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Ridge vents covered with snow

I live in Massachusetts and where we just recently suffered through a really brutal winter. We had bad ice dams due lack of air sealing between the living space and the attic, poor attic insulation, and poor attic ventilation.

I already have plans to fix the 1st 2 problems, but am looking for advice regarding ventilation. I already know that I need to increase my soffit intake vents according to the 1/300 venting rules. In addition, I should close off the gable vents so that air exiting at the ridge is pulled from the soffits and washes the underside of the sheathing with fresh cold air.

My problem is with the ridge vent. My roofer says that my current ridge vent is low quality and recommended replacing it with a Cobra Snow Country Advanced ridge vent. Personally, I don't understand how a ridge vent, no matter how "advanced", could possibly be effective when covered in more than a foot of snow. My understanding is that it works best when wind blows across the ridge, creating a negative pressure differential (like an airplane wing) that "sucks" air out of the attic. My roof pitch is only 5/12, which is not steep enough to keep snow off of the ridge vent if we get a storm with 12+ inches (this is common during our winters). Also, the amount of snow that needs to melt in order to uncover the ridge vent might be enough to cause an ice dam, at which point, it's already too late.

I did some research and am leaning toward the following Made In Canada solution (I have seen these at a Ski Resort in NH):

Product information for the Slope Roof Model #301 Roof Ventilator

You can also get an adapter custom fitted to your roof pitch so it can be installed directly on the ridge.

The idea is that you use this instead of a ridge vent (which is removed or capped off), with the benefit that the opening will NEVER be covered with snow (if this does happen, you can install a height extender). I understand that you lose the benefit of ventilation even distributed across the ridge within each rafter bay, but hopefully this is outweighed by the benefits, one of which is an enhanced chimney effect due to the height.

Does anyone have any experience with these? One negative I can think of is that it may look ugly, but if it solves the problem, I'm not sure how much I care.
 
  #2  
Old 04-27-15, 05:20 PM
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First we need to identify the problem/s. Ice dams are a result of
#1. Air leakage into the attic from the house. It transports moisture and a lot of heat. Link on air sealing below plus an article on attics.
#2. Insulation, insulation and more insulation, especially out near those soffits. 5/12 doesn't give you a lot of space to work, but you need baffles down by those soffits to maintain the air flow and protect the end of your insulation from "wind washing" which greatly reduces the r-value down there.
#3. And here comes the good news, if you do a good job with #1 and #2 you barely need ventilation.

Now, those gable vents are just fine, that short circuit BS was just the roofing industry's was to hype their ridge vents. You do need good soffit venting, but the incoming air is "pushed" in from the outside and not pulled from the exiting warm air. Technically, the cold air pushes in the soffits and pressurizes the attic that in turn forces the warm air out. Warm air does not rise by itself.

In addition, those gable vents don't get blocked by snow, at least we hope so. As long as the wind hasn't currently been blowing snow and rain in the gables, leave them right there.

More good news, improve the soffit venting as you said it is lacking and your existing ridge vent is just fine. No need for the cupola to protect from heavy snow.

I'll add the two links and let you catch up, but I can explain all in full detail if needed.
http://www.efficiencyvermont.com/ste...ide_062507.pdf
All About Attic Venting | GreenBuildingAdvisor.com

That should save you some money
Bud
 
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Old 04-30-15, 02:25 PM
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More information:

My gable vents are pretty small. Probably 1 to 1.5 sq. ft., with louvres and screen, that is probably MUCH less than 1 sq. ft. (144 sq. in.) NFA per vent (I have one on each side of the house).

My soffits are rather shallow. I measure about 4" from the j-channel on the siding to the aluminum fascia cover. The house is 36' long, which gives me a total of 12 sq. ft. of soffit each on the front and back of the house (24 sq. ft total). Even using an aluminum soffit panel with the highest advertised NFA of any manufacturer I can find (Truline HP Soffit | Quality Edge), that is still only about 540 sq. in. of total soffit vent area including both front and back. Many ridge vents have a NFA of 18 sq. in. per linear foot, which would be 648 sq. in. total. Isn't it bad to have more ridge vent NFA than soffit NFA? I have read that this can depressurize the attic, sucking conditioned air out of the house. Maybe I don't have to worry about this with good air sealing but still...

If I capped the ridge vent and use cupola-style maximum ventilator(s), the total NFA at the ridge will be lower, but probably still adequate according to the maximum ventilator manufacturer.

I also have a power vent near the ridge on the back of the house, roughly in the middle, which is in bad shape and needs to be removed or replaced. Since this is useless when covered by snow, maybe I should just leave the ridge vent where it is, and replace the power vent with a maximum (cupola) vent. This way I'll get extra ventilation when the roof is covered with snow. The manufacturer claims this is bad because of short circuiting between the ridge vent and the maximum vent, but on the other hand, you're saying that the short-circuit theory is a load of BS.

Another question related to soffit baffles... If I want to do blown in cellulose, I need to provide an air channel from the soffit and block insulation from getting into the soffit. The following product looks interesting:
AccuVent - AccuBlock - Roof Ventilation Solutions
I actually bought one of these to test out how difficult it would be to install with my shallow eaves and low 5/12 roof, and let's say it's nearly impossible to reach the top-plate in order to put in staples. I was thinking that instead, since I plan to have my roofer rip-off and redo the soffit, I can just shove the Accuvents into the soffits from the attic ahead of time. Then, when he opens up the soffit from the outside, he can staple the Accuvent to the OUTSIDE edge of the top plate. Does this make sense?
 
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Old 04-30-15, 03:33 PM
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The vent you linked to would be just fine.
As for the short circuit claim, warm air exits the upper regions because there is a positive pressure across the entire upper attic area. Air entering the soffits is independent of where it exits other than the balance between high and low vent areas will adjust those positive and negative pressures to equalize the infiltration with the exfiltration.

Lower vent area is important, but the penalty is not as drastic as they suggest. In a balanced condition, there is already a negative pressure at the attic floor and combined with a positive pressure at the ceiling below it drives warm air into the attic. Air sealing is the primary solution along with insulation. Ventilation is low on the solution list and having more high vent area than low creates a small percentage increase in leakage. A 60% low with a 40 % high split reduces the pressure across the attic plane by about 5% vs a 50/50 balance. But 5% of what. If the leakage is low that isn't a lot. Yours would go the other way, but that is a number I have used before.

They make strip vents in both metal and plastic that would fit your 4" space. By cutting 2" slots between each rafter, leaving some material to hold the soffit in place, and covering it with a strip vent you would have rather good low venting. Just did that on my rear build out.

The cupola you are considering isn't a bad idea, added height for better venting pressures and snow protection, but soffit venting is a priority. If you had no soffit venting the infiltrating air would have to come in the gable vents or the ridge, not good. That's an extreme.

How confident are you in the air seal between the house and the attic. major contributors are chimney and plumbing paths along wit attic hatch and recessed lights and electrical boxes.

Bud
 
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Old 04-30-15, 09:14 PM
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Regarding air sealing, it is pretty bad right now. I have pull down stairs, a whole house fan, AC duct penetrations, 2 bathroom fan/light/heater combo units, assorted non-recessed lighting fixtures, a plumbing vent, and bypasses around inner wall top plates.

The state runs a program (Mass Save) sponsored by the utility companies where they will provide no cost air sealing of all-of-the-above and subsidized insulation up to R-38. I already had an energy audit and got a list of participating contractors. However, I'm going to want to do some extra things which make things more complicated.

1. My bath fans are pretty old, so I want to get new units and re-do the ducting with insulated duct instead of the dryer vent duct I have now.

2. Central AC is old too (25+ years), and on its last legs. I want redo that as well and tighten up the insulation around the ducts and air exchanger. I think this will be more difficult if I wait until after blown-in is added.

3. Half of the attic has a floor with R-19 batts underneath. The floor needs to get ripped up before adding insulation. I may want to build up a storage platform as well.

4. With the new AC, and given that whole house fans are notoriously difficult to air seal, I'm thinking about removing the whole house fan entirely.

5. I want to improve the soffit vents and add Accuvents baffles from the outside before they add insulation. Otherwise, they might just shove a fiberglass batt between the rafters as a wind blocker, and I'm not crazy about that idea.
 
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Old 04-30-15, 09:39 PM
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I really hate when incentive programs run into conflict with better improvements. I have seen nasty old insulation buried because they were not allowed to remove it as it was part of the original audit. But that would be another very hot blog.

If the subsidized amount is reasonable I would talk to both contractors to get an idea as to exactly what they are going to do. Like you say, no sense going backwards having to remove what they do to get things the way you want. They may be reasonable and allow you to prepare the attic the way you want it before they do their work.

For instance, they should be air sealing under that attic floor, so perhaps you removing it would help.

But here is one of the problems. How much air sealing is a good job. Too often they use the blower door leakage test number to calculate a target for sealing. That target isn't the best job, just enough to stay below needing added ventilation. If they seal too much, then they have to install some form of ventilation. But rarely is that an issue for someone who can walk and talk. Understanding ventilation and when to run fans isn't complicated. But the target approach allows them to skip the difficult areas and just seal the low hanging fruit. That leaves the rest for you now buried in added insulation. Mass may be different so see if you can get a detailed list of what is going to be sealed.

Bud
 
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Old 04-30-15, 10:08 PM
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The audit did not include a blower door test. I'm not sure if the contractor will do that in order to get a target like you say.

For what it's worth, the auditor did say that they would build sealed boxes around the pull-down stairs, whole house fan, bathroom fans, and seal electrical boxes, wiring penetrations, and top plates with canned spray foam. He also said the floor would have to be removed and suggested either I do it myself, or try to make arrangements with the contractor to itemize the "extras" separately. I haven't talked to any of the contractors yet, but most on the list seem like they are just insulation contractors. A few do more general-contracting type stuff, and at least one of these has very good reviews on Angie's list. However, I'm a little apprehensive about scaring them off with my list of "extras".

I'm an engineer (semiconductor design) and have had many instances where, after my barrage of questions during an estimate, I don't hear from the contractor again.
 
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Old 05-01-15, 12:04 AM
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Yes, you definitely need to be careful not to scare them off. If they sense you are going to be a picky customer they may just skip over you and take the next. Walk softly but disguise your calculator.

If there was no blower door test, then it technically wasn't an energy audit. The boxes they planned are necessary to hold back the new insulation. Since they mentioned removing the existing attic floor, that is good, although I have lost a lot of confidence in the can foam. It shrinks, releases, and degrades with time. Duct mastic is sticky and messy to work with, but brushed over a seam it will seal it forever. Can foam needs a crack to get into to stand a chance of holding.

If you followed some of the references in that "all about ventilation" and some of Martin's quotes he extracted, he makes the point that the ventilation isn't really that important. But air sealing is a win win from several perspectives, energy, indoor air quality, moisture, and comfort. And it helps with those ice dams.

An elevated walkway for storage and access is a benefit to them as well as you. I included a well built one on my last large building and it proved to be incredibly convenient.

Bud
 
 

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