roof venting

[Matt Lovern]

Need some thoughts on roof venting for the following situation.

Home is in Alaska with snow fall regularly exceeding 3 ft per season and not unheard of to get up to 2 feet in one 24 hour period.

Roof is very low pitch, dont know the actual number but high point in attic is 3.5 feet and the spread from eave/soffit vent to ridge is 20 ft for a total distance eave/soffit vent to eave/soffit vent of 40 ft. The length from gable end to gable end is 44 ft.

I will be blowing in insulation to increase r-factor by 12 on top of original bat insulation of 6 inches.

House has a ridge vent, gable vents and eave/soffit vents. When I add the additional insulation it will be above the bottom of the gables vents so I was planing on building a box inside the attic around the gables vents open at the top thus keeping the air flow unimpeded(or at least not as impeded if I did nothing or if I blocked them completely) from the gables.

I have added durovent baffles from soffit openings to well above the height of where the blown insulation will be. 10 feet of baffle in between each rafter.

There is a lot of conflicting info out there regarding the use of gable vents in conjunction with an eave and ridge vent system, thus my question.

Should I block the gable vents in this situation?

I am inclined to keep them flowing as described with the boxing around the gable inside the attic because of the possibility that snow may for some unknowable time completely block the ridge vent.

[KEVIN WOOD]

If you have a ridge vent and soffit vents close up the gables. Make sure that you balance the amount for the soffit to the ridge at 50/50.
You will need to know the flow for the ridge vent design installed and may need more durovent baffles to balance correctly.

[John Overholser]

Originally Posted by kwood

If you have a ridge vent and soffit vents close up the gables. Make sure that you balance the amount for the soffit to the ridge at 50/50.
You will need to know the flow for the ridge vent design installed and may need more durovent baffles to balance correctly.

I'm a new inspector, so forgive me if I sound ignorant...but why wouldn't it be better to have as much ventilation as possible? I appreciate the knowledge.

[Robert Ernst]

I would block the gable vents. It's better to have to cold air washing the bottom of the sheathing. Since you have been adding more insulation you will also need less ventilation, you have less air in the attic. Like mentioned 50/50 is common way of doing it. You can have more at the bottom than the top but not the other way around.

[Matt Lovern]

Thanks for the replies, as usual it leads to another question or two.

on the 50/50 Im not clear as to what is being measured to get to 50/50.

In this situation soffits are vented in between all rafters on both sides of the house so there are a total of 44 2ft openings. All 44 have 10' of durovent baffling stapled to the underside of the decking.

The ridge vent runs aprox 40ft and looks to be a honeycomb design as to how the air escapes via the ridge opening. Without being able to measure flow of ridge verses flow of soffit vents I can only guess but it looks like there is far more flow at the soffits vents than there is at the ridge.

Also to be clear, even though there will be times when the ridge vent is completely blocked with snow for multiple days it is still better to block the gable vents completely?

[KEVIN WOOD]

And that is a good thing. If you are concerned about the ridge getting filled with snow then move the gable vent up to the top. You will still get a good flow and will not have to rely on the ridge all the time for the attic ventilation in the winter.

[Christopher Currins]

Originally Posted by joverholser

I'm a new inspector, so forgive me if I sound ignorant...but why wouldn't it be better to have as much ventilation as possible? I appreciate the knowledge.

The gable vents can become intake vents and prevent air from entering properly at intake vents at roofline (soffit vents).


Here's a really good course.
The Attic: Three Components, One System from CertainTeed

[Robert Ernst]

The ridge vent can still vent some heat and air even with snow on them. The heat is going to come from inside the home mostly. So with the added insulation that is limited so in general the bottom of the roof will stay cooler. As long as you have the same or more venting at the soffit your alright. Try blocking the gables and then check the attic in the winter for heat and air flow. If needed you can put a door on the gables that hinge at the bottom and open them when the snow is blocking the ridge. By hinging at the bottom you prevent the wind from pushing the insulation out of the way.

[Matt Lovern]

Ok, thanks much gentlemen, I will go with blocking the gables with doors and keep an eye on the flow with the snow which will be here any day.

[Dennis R. Goudreau]

http://www.buildingscience.com/docum...f-venting/view

[Roy D. Cooke, Sr]

If you have more ( stronger ) Exhaust in the roof then air coming in from the soffits then frequently you draw conditioned air from the Home Warm moist air in Winter is not a good thing to have , Also summer cold air from the home Cost more money to cool the home .
So more out at the top then comes in at the bottom is not a good thing .
More roof vents, Cyclone or power vents frequently discharge to much air.
Less discharge is some times better .
Seal the home from the attic close all holes from home to attic Gasket and insulated the Access door add weight if necessary

Roy

[David C. Macy]

What is the deal with the excessive roof top vents??

[Michael Larson]

Originally Posted by dmacy

What is the deal with the excessive roof top vents??

It might be needed like that to get enough ventilation on a Hip roof.

Not very attractive though.

1 sq. ft. for every 300 sq. ft of attic floor space.

[Christopher Currins]

Originally Posted by dmacy

What is the deal with the excessive roof top vents??

2 for 1 sale.

Installing a ridge vent would have been more affective, cheaper, less future leakage problems, and a lot more attractive from the street.

[Marcel R. Cyr]

http://www.airvent.com/pdf/literature/PAVbooklet.pdf