Roof Venting

Static Vents VS Ridge Vent:

I live in Colorado @ 9000’. Snow fall average in a year is 120". We purchased our current home 18+ months ago & have managed to partially retrofit the roof venting system through replacing the soffit venting, adding rafter baffles & insulating the attic space. The roof is a 4/12 asphalt shingle.
Attic square feet: 1650
Currently Installed…
Soffit venting: 991" square inches/6.88 sq feet net free vent.
6, 10" static vents: 306" sq in/2.12 sq feet free vent.
(Each static vent is screened & information suggests reducing the free venting by half) So that only gives me 1.05 sq feet of venting on the house through the static vents.
To get a 60/40 (soffit/ridge) balance I would need to install an additional 10, 10" static vents.
To get a 50/50 (soffit/ridge) balance I would need to install an additional 14, 10" static vents.

These two options are unacceptable.

Questions:

I can get the acceptable balance with a ridge vent & eliminate the static vents. I am concerned with snow covering the ridge vent for weeks at at time.
Will a ridge vent vent with snow covering it?
What ridge vent would you recommend?

Recommendations: ??

Without seeing pictures I am going to say, close the ridge, eliminate static vents and install gable vents with a gable vent fan in one side.

gaf.com/roofing/residential/ … -vent.aspx

Without seeing pictures I am going to say, close the ridge, eliminate static vents and install gable vents with a gable vent fan in one side.

Good suggestion.

How would the gable vents at both ends of the house, with one having a fan then work in conjunction with the soffit venting?
Removing the soffit vent’s would be a very difficult job.

Thanks

Leave the soffit vent; you need both intake and exhaust. While not the ideal configuration the gable fan will act as a forced exhaust. The gable without the fan and the soffit vent will act as intake when the fan is on. When the fan is not on then both gable vents will act as passive exhausts. As I stated, it’s not the “optimal” configuration but if you’re really concerned about snow load on a lower pitch roof it’s an excellent option imo.

Back in the 70’s, after builders started superinsulating and they started to get moisture problems, a company decided to build a couple of test houses to see which systems worked best. They built identical houses and inserted sensors.

The houses with continuous ridge vents, in conjunction with continuous soffit vents worked best. Surprisingly, they found that gable vents were detrimental. Due to the positive and negative forces applied by wind direction, they could effectively draw in large amounts of moisture and/or snow. Of course, that ruined the insulation. They also found that the gable vents tended to vent only the top sections of the attic, even if there were soffit vents in wind conditions.

Also, they found that gable vents short circuited the flow process from the soffits. Air would exit the slantbacks or ridgevent and pull in cooler air from the gable vents.

The recommendation was to block all gable vents.

Interesting: Do you have the information or the link on that study you mentioned. Sure would be nice to have additional data to confirm the best way to ventilate a home in snow country.
My house has adequate soffit venting & only 1/3 of the required turtle/static vents. So my dilemma remains: will a ridge vent vent when covered with snow??
I would prefer to install the ridge vent because that is the way the system was designed to work.
I spoke with a national roofing company spokesman who was in a meeting with 20 other rep’s and a GAF rep which were all asked the question: will a ridge vent vent with 20" of snow? there was no uniform agreement between the participants. My guess is it won’t! The snow will eventually melt around the vent and it will start to work again but that cannot be advertised as continuous venting! The rep that called me back recommended put some kind of heating strip in the vent itself to keep it clear - how bogus.
Anyway - if anyone can offer a solid solution that permanently fixes this particular problem then they will solve the ridge venting issue for millions of snow country homes.

In your area, you want a cold attic & roof, meaning no warm/humid air from the living space below rising up through holes in the top plates, ceiling light cans, bathroom fans, etc. Because if it does as soon as it hits that really cold air, it’ll condense. The fact that you live in a very dry climate will help as opposed to a cold & damp climate.

So I’m not sure how much venting you’re going to need at 9000’ with snow on the roof. Your attic will be filled with cold, dense air and it’ll be cold outside. So there will be a minimal delta-T between the two. Ridge vents require a larger delta-T to work.

When the Sun comes out, it’ll start melting the snow and slowly warm the roof. With a roof full of snow, it’ll be awhile before your attic temp begins to rise. It may not much at all. If some of the snow melts and begins to run down the roof, it’ll freeze again as the Sun sets. All depends on your daytime/nighttime temp swings and cloud cover.

In the Summer, you’ll have more delta-T, hot air in the attic will rise to the ridge and pull in cooler air at the soffit. Once the attic cools down in the evening, your delta-T will drop and so will this heat-powered venting.

High altitude, cold, dry climates are a whole different animal than most of us live at. You may find some helpful info on Joe Lstiburek’s site Building Science.

I do not work in in an area of heavy snow as our climate is virtually mild.

The first thing that came to mind after hearing the argument against the use of contiuous ridge vent…is that the homeowner can either close up the gable vent, ridge vents, or replace thier Af-50/RVO-38 vents with “high-hat” vents…the kind that are typically usef for flat roofs.

I forget the nomenclature of the vent, but it looks like an RVO-38 metal vent…but only higher. This will allow the attic to vent through and past the potential 20" snow load.

Delta T, High Hat vents,

All sounds good. As I stated in the original post; soffit venting is adequate, static/turtle vents need to be tripled in order to have the correct ratio.
Should I cut another 12 holes in my roof???
A ridge vent, regardless of who manufactures it will not vent when covered with snow,
Static gable vents require me to cover the turtle vents,
Adding a gable fan might be the answer but the fan size need to be matched exactly to the available intake/soffit size in order not to draw air form inside the house,

I appreciate the input & will try again later,

Yes…adding more vents will work also. As for the powered vents…not too comfortable going out the gables, better if the went out of the roof deck.

[quote=“capt1857”]Delta T, High Hat vents,

All sounds good. As I stated in the original post; soffit venting is adequate, static/turtle vents need to be tripled in order to have the correct ratio.
Should I cut another 12 holes in my roof???
A ridge vent, regardless of who manufactures it will not vent when covered with snow,
Static gable vents require me to cover the turtle vents,
Adding a gable fan might be the answer but the fan size need to be matched exactly to the available intake/soffit size in order not to draw air form inside the house,

I appreciate the input & will try again later,[/quote]

NO. Soffit venting is “normally” more but can only account for up to 50% of Venting required. The KEY to the venting is that 50% of what is required is minimum for roof top venting regardless of the amount in the soffit. It does not suggest a ratio.

Typically, Venting should be 1:300. 1:600 roof top (or more) 1:600 soffit (or more)

1650 attic space = 6 AF-50 Vents Minimum. I might suggest a little more, but not Alot more.

FYI, AF-50 refers to the net free air flow. Af-50 are the smaller ones, medium is AF-75 and large vents are af-90. So, assuming you have the smallest, you have enough venting.

Reece Jorgensen
Jorgensenroofing.ca