Ventilation without soffits

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Old 08-01-02, 10:08 AM
tcHome
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Ventilation without soffits

I am planning to install a ridge vent, but I am not sure how to handle the intake. I have open eaves, and I really like the look of them (1924 Arts and Crafts Bungalow). Is it acceptable to place vents vertically between the roof supports under the eeaves, or do I need to install soffits?
 
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Old 08-01-02, 11:35 AM
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The exposed rafter extentions on a Arts & Craft house is a signature and it would be a shame to encase them in a soffit. You can provide eave ventilation on your house without taking away from the style of the house.

Rafters are structural, the boards on the side of your house that go between the rafters, aid in dealing with the stress that is applied to rafters, which is torsion stress. What this means is that you just cannot cut away the siding and wood between the rafters to provide for eave ventilation. Furthermore, the rafter is probably notched (birds mouth) that is sitting on the top plate. You have to be careful not to cut the top plate. If you are not familar with the structural elements of a roofing system, I strongly suggest you get a professional to do it for you.

There is adequate space between the rafters, below the roof and above the top plate that you can cut or drill into. I would use a 2 inch hole drill bit and drill 3 holes an inch apart. Staple screening over the holes to keep out insects and then cover with a vent cover. I would do it every other rafter and that should provide you with more than sufficient eave ventilation.
 
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Old 08-01-02, 12:53 PM
tcHome
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Getting it to balance...

It seems like this would make it difficult to achieve a balance between eave and ridge. Should I focus on reducing the amount of ridge vent or maximizing the amount of eave vent?

Can I do this between every rafter instead of every other one?
 
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Old 08-01-02, 03:00 PM
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Since your ridge is only on top and your eave area is on both sides of the ridge, you will probably have more eave vent area than ridge vent area. Furthermore, on an Arts and Craft house, the eaves are usually longer than the ridge. If you were to calculate the vent area for the ridge and the eaves vent area doing every other one, the eave vent area would more than likely be slightly more than the ridge vent area. A two inch diameter hole is approximately 1 1/2 square inches. Doing the eave vents just 3 times gives you more than 1 square foot of venting.

You could do it between every rafter, but instead of putting 3 two inch diameter holes, I would recommend putting only two and 2 to 3 inches apart.
 
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Old 08-01-02, 07:58 PM
tcHome
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Complex "T" shape

The house is actually a sort of T shape with a couple of peaked dormers. Because of that, there are not a lot of places I'll be able to place eave vents. Can I punch round hole low in the outer wall in the crawl space for additional intake?
 

Last edited by tcHome; 08-03-02 at 02:32 PM.
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Old 08-03-02, 01:34 PM
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I'm not sure by what you're describing but I can assume that you are referring to the wall on your dormer, which does not extend to the eaves. If you are referring to the eaves behind the dormer walls, yes, you can drill in that area. Keeping in mind what I said in my previous post. If you are referring to the dormer wall itself, no, you cannot drill in that area.
 
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Old 08-03-02, 02:50 PM
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Dormers go all the way

I'll try to explain all the details:

Total "floor area" is 850 sq. feet. That is the area of the entire area covered by the roof including the crawl spaces that will not be used.

By my calculations, I need about 225 sq Inches of ventilation on the ridges and anther 225 sq. inches at the eaves (for a total of about 450 sq. inches).

That's easy at the ridge, where I will cut opening in several places to create a total of about 15 linear feet of ridge opening.

However, I have a total of only 28 eave spaces (spaces between eaves) to create about 225 sq. inches of ventilation. This is because not all of the eaves reach the outer walls. The ridges of the hose form a "T" and where the slopes intersect, they don't reach the outer wall.

By my calculations, I would need a total of 150 2" diameter holes to get 225 sq. inches of air flow. if I put only two holes per eave, that would leave me with only 84 sq. inches of air flow. That means I need to find a minimum of 141 sq. inches of additional eave-level air flow.

So my question is:
Are there any options for creating more sq. inches of airflow per eave, or is there a way of creating additional airflow at the outerwalls (those that are in the un-used crawl spaces)?

Glenn
 
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Old 08-04-02, 12:31 PM
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The rule with free venting is 1 to 300 with a vapor barrier and 1 to 150 without a vapor barrier. Let's assume you don't have a vapor barrier, which requires twice as much free venting than one with a vapor barrier. To determine the minimum amount of free venting you need, you divide 150 into 850. You require less than 6 square feet of free venting. (850/150= 5.67 sq. ft.) Free venting explicitly implies that half of the venting is high (ridge vents area) and the other half is low (eave vent area). Based on that, your home require less than 3 sq. ft. of eave vent area. 84 sq. inches comes out to 7 sq. ft. of eave vent area. That is more than double your minimum requirement. This does not mean you have to have this amount, it means that you cannot have less than the minimum requirement and it doesn't have to be equal amounts.

Do not provide venting in those walls in the unused crawl spaces. One of the problems with attic ventilation is that it is somewhat misunderstood. What attic ventilation actually does is ventilate your roof. The ridge and eave vents will do that for you.
 
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Old 08-04-02, 05:37 PM
tcHome
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Math Problem

Your math is a little off.

1 square foot is equal to 144 square inches.

1 ft. X 1 ft. = 12 inches X12 inches = 144 sq. inches.

So the 5.67 sq feet you are talking about is actually 816.48 sq. inches. 84 sq. inches will not cut it.

I do understand what roof ventilation is for. It is to keep the roof cold in the winter and to prevent condensation (and thus leaking. The vents in the eaves are in the same crawl space you have suggested I not put holes in the wall. The cold air would be going to the same place. It seems to me that it would provide the same type of intake at the same level as the eave vents you have described.

Forgive my skepticism, but your math error leaves me a little wary.

Are there others out there with second or third opinions?
 

Last edited by tcHome; 06-11-03 at 08:25 AM.
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Old 08-05-02, 06:11 AM
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Lightbulb Vents

I have basically the same type of house you do, same size, same "eave/soffit" design.

I took out every other block on the east and west side and replaced them with 1/4" hardware cloth.

Net opening is 22" x 2" each and there are 12 vents (6 on each side)

There are no vents on the North (no eave to install vents) and I didn't put any on the South because it's the front of the house.

Yes it was probably more than building code requires (my intake vents are just under 4 sq ft total) but the 1/300 is a MINIMUM requirement.

For exhaust I used 2 wind turbines.

My attic is rarely more than 20 degrees above outdoor ambient temps and the roof bakes in the sun all day (very little shade)

Ridge vents can work well BUT beware of the "filter" type (the ones where the air most go through a type of cloth/mesh)
The "filters" tend to be VERY restrictive especially after a few years when they plug up with dirt and dust (the way a A/C filter does)

If you go with a ridge vent make sure it's a "filterless" design such as www.cor-a-vent.com

http://www.cor-a-vent.com/airflow.htm shows the effect of how restrictive a "filter" is
 
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