[Bud9051]

For decades the roofing industry has used the SCT to argue that gable vents will short circuit the desired attic ventilation when left in place with a new ridge vent. They were wrong when they (or someone) put together that whopper and it is still wrong today. But it is not the fault of people who repeat all or parts of it as after all it comes from what we would expect to be a credible source.

This link from Fine Homebuilding: https://www.finehomebuilding.com/2012/01/19/close-off-gable-vents
does a reasonable job of explaining why the SCT is not to be worried about. Although there are still some errors time is working to spread the message.

If anyone has questions I will be glad to address them.
Bud

 

[chandler48]

Like thinking, here. I believe gable venting is second class as compared to ridge venting because ridge venting takes the hottest of the air out totally. I don't think closing up of gable vents is necessary. In fact, it compliments the ridge. The one thing with gable venting in older houses was there was no soffit venting, such as in Capes. That is where the problem exists. Soffit venting is necessary, IMO, for either to work well.

 

[MTN REMODEL LLC]

QUESTION:

I can't believe it, but some nice production building in southern NV does not have soffit venting and ridge venting only. Hot countrty with HVAC in attics!!!!!

I have no idea how it ever passed code. Generally the production homes are about 20-25 years old.

My question three fold:

1) I am totally unfamiliar with stucco finish....is it much more difficult/costly to soffit vent.

2) Would gable vents, or powered gable vents perhaps blowing in or out, be of any assistance....even minnimal

3)While certainly a violation of fire codes, would ceiling venting in the garage, coupled with garage intake venting or garage doors cracked open, yield much/any better attic venting (coupled with the ridge vents.)

Or an ideas also...

TIA

 

[chandler48]

I can't imagine ridge venting without soffit venting. Where would the makeup air come from? We worked on a house last week where we needed to send a vent out the gable as opposed to a non leaking roof. I saw 4 gable vents, and definitely needed to miss them. Up in the attic, there was no light from the gable vents. They were DECORATIVE Luckily she had adequate soffit and ridge venting.

My experience with anything powered in an attic, is an early death of the motor or the thermostat, and avoid them at all costs.

Always take your venting to the atmosphere.

 

[ront02769]

If you have soffit and ridge venting, you don’t need gable vents....and they can be just for show. And yeah, old capes in New England had gable vents each end, period. And the make up air came from one end and went out the other. Fast forward and re-roof a cape and put in ridge vents, you will likely also want to add some soffit vents. Ron

 

[chandler48]

Capes don't have soffits, so it takes a special type of add on vent on the roof edge to accomplish soffit venting. I believe their gable venting is adequate, and ridges can compliment it.

 

[MTN REMODEL LLC]

I can't imagine ridge venting without soffit venting. I couldn't either....but I can sure prove it.:wink2: Where would the makeup air come from? We worked on a house last week where we needed to send a vent out the gable as opposed to a non leaking roof. I saw 4 gable vents, and definitely needed to miss them. Up in the attic, there was no light from the gable vents. They were DECORATIVE Luckily she had adequate soffit and ridge venting.

My experience with anything powered in an attic, is an early death of the motor or the thermostat, and avoid them at all costs.

Always take your venting to the atmosphere.

Any ideas:

QUESTION:

I can't believe it, but some nice production building in southern NV does not have soffit venting and ridge venting only. Hot countrty with HVAC in attics!!!!!

I have no idea how it ever passed code. Generally the production homes are about 20-25 years old.

My question three fold:

1) I am totally unfamiliar with stucco finish....is it much more difficult/costly to soffit vent.

2) Would gable vents, or powered gable vents perhaps blowing in or out, be of any assistance....even minnimal

3)While certainly a violation of fire codes, would ceiling venting in the garage, coupled with garage intake venting or garage doors cracked open, yield much/any better attic venting (coupled with the ridge vents.)

Or an ideas also...

TIA

 

[Bud9051]

1) I am totally unfamiliar with stucco finish....is it much more difficult/costly to soffit vent.
Not at all familiar with stucco anywhere on a house so cannot comment.

2) Would gable vents, or powered gable vents perhaps blowing in or out, be of any assistance....even minimal
Powered venting is a lot more powerful than passive venting. The disadvantage to power venting is the resulting air flow through the ceiling plane due to huge pressure differences.

3)While certainly a violation of fire codes, would ceiling venting in the garage, coupled with garage intake venting or garage doors cracked open, yield much/any better attic venting (coupled with the ridge vents.)
Natural venting is directly proportional to the height difference between high and low vents, so yes it would help, but yes it would be a violation of fire codes (as I understand)

Or an ideas also...
Power venting requires an almost perfect air barrier to prevent loss of conditioned air. Passive attic ventilation is primarily for moisture removal and to prevent ice dams, not designed to cool that space. Insulation protects the house from heat. In hot climates there are other measures. Ventilation is primarily for colder climates.
TIA

Bud

 

[Bud9051]

Some fundamentals:
1. The natural air pressure (no fan no wind) that moves air through our attics is the same as the stack effect that moves air through our homes. A simplified equation fot that pressure, be it in the attic or in our homes is
P = 0.0067 x delta h x delta t.
h is in feet and is the difference between low and high vents
t is in degreed F and is the difference between inside and outside temperatures.
And the answer is in Pascals, a very small pressure.

If either "h" or "t" is zero there is zero air pressure to move any air.

2. The explanation for ventilation often uses the "air follows the path of least resistance" phrase. Yes and no. When two paths enjoy the same air pressure the phrase is correct. But if two paths have a different pressure across them, that phrase should not be used. In the case of gable vents vs ridge vents they are at different heights so experience different pressures.

3. As for keeping or removing a gable vent when a ridge vent is installed, both are in the upper portion of the attic so both are typically experiencing a positive pressure (with respect to outside) pushing air out.

4. The proper explanation for winter attic air flow is, the cooler (heavier) outside air pushes in through the soffits or lower vents, pressurizes the attic, which then pushes the warm air out the ridge or upper vents. There is no such thing as hot air rises, exits and pulls in the cooler air. That is just warm and fuzzy wording that has been used for decades because they really didn't know or want to explain why the air moves.

I'll see if this posts or if the captcha gets me.

Bud

 

[Old Thomas]

I have gables in 3 directions so I have gable wall vents and not roof vents or ridge vents. Those vents and soffit vents have been working for 40 years. I don’t like roof penetrations.

 

[3onthetree]

There are two issues that no one seems to delineate between when talking about mutiple styles of venting:
- stack effect which does not take wind into consideration
- wind aerodynamics on the structure

Considering just stack effect and the exhausting of hotter air, multiple versions of high vents shouldn't cause a negative effect in where the air is exhausted. Although perfectly calm days occur, not addressing the wind is equivalent to controlled lab experiments instead of what actually happens with all variables on the street.

When considering wind then, depending on where the vents are, areas of low pressure created by wind traveling over the ridge or side walls can cause the air to exit/enter where it was not intended, although air may still be exhausted as a whole. If you consider a prevailing wind, simple aerodynamics can predict how the air would behave with multiple vent styles. If you also consider direction and strength of wind around the structure can change instantly, affected by topography, trees, or adjacent buildings, it is harder to predict how the air behaves, so logically a worst-case scenario should be accounted for. The worst-case scenario is pulling rain and snow into a vent.

I am not sure there was a conspiracy by the roofing industry to create the "short circuit theory," as I can't see what they had to gain. I can also not speak for individual people on what they are referring to when giving recommendations on not mixing styles of high venting. But what I am sure of is that not all areas get snow, but everyone gets rain and wind at some point.

 

[Bud9051]

Passive ventilation represents the minimum air flow that an attic will experience when the wind is not blowing. Adding in the wind will in almost all cases greatly increase the air exchange. As for controlled laboratory testing the guidelines provided for vent area based upon attic floor footprint were developed long before laboratory testing and have remained unchanged over the decades. Essentially the building industry has been the testing laboratory which has included the wind with the conclusion that it seems to work.

Wind with all of the variables you mention is indeed part of the ventilation process but building have changed and now need far less air flow than 40 years ago. Better insulation protects the house from attic heat and better air sealing protects the attic from moisture. And both help to protect the roof from ice dams.

Yes, rain and snow need to be considered, but difficult to include that in the codes.

Trying to address just the all too often cited SCT as a reason to eliminate gable vents when installing a ridge vent. That warm and fuzzy language has cost home owners many millions of dollars paying for work and materials that were not necessary.

As for who created the SCT and why I can only guess. The group that has promoted it the most, the roofing industry, might benefit from more ridge vent area once the gable area is eliminated. Pure speculation.

To add to your list od considerations, the pitch and resulting height of the ridge are major components of the resulting air flow. Also, multiple height vents can result in a high soffit vent acting as an exhaust vent, not good when dealing with ice dams.

Thanks for the reply.

Bud

 

[Nealtw]

Talking about blowing snow, we built a house for a farmer in an area that often has large snow drifts. He had his box vents adapted with flapper doors and a lever inside that would open and close them and put the same number back and front of the house.

He connected front and back sets together with a rod and they were both half open.

When the wind and snow blows from either direction it will push one side closed and open the other side.