Thread: Attic Ventilation

I was on an inspection today and I was asked a question that is a little past my expertise. There is an attic 712 sq ft and there is only one side of the house that has soffit. They installed the aluminum vented soffit every 4' which has an free space air of 6.5 sq in (left a box laying around) and have (2)-9" exhaust vents existing the top of the roof. The problem that I saw right away is they only have sq inches of incoming air based on what they currently have. The questions were is it OK to open up the entire soffit and make it all vented which would give them 221sq in of air coming in and 123 sq ft of air exiting. Granted it's not the 343 sq in of input and 343 sq ft of output they need, but they could add another vent on the roof to bump it up to 189 sqin. They are willing to pretty much cut out the entire wooden soffit between each joist and place an aluminum vented section and remove the solid.

The 2nd question was since all the airflow is/will be coming in one side of the attic only, will this be sufficient to circulate the air or will the air just come up the baffle and out the top.

Thanks,

HP

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Without getting into all the mathematical numbers, it's best to have continuous low and and high end ventilation for optimal attic ventilation. A continuous soffit vent would be best with an equal amount of open ventilation space at the peak of the roof. Soffit vents every 4 feet is spotty and could definitely be improved.

thanks. It is a common practice to open up all the wood soffit and make it a continous vent? Also, on the opposite side of the attic, there is no soffit available. It's the fire wall for the rest of teh house (2 story).

Continuous soffit ventilation is always a good idea and relatively easy to install.

But if this is an older building, they should take a look at the attic space and check the sheathing before worrying about whether it needs more venting.

A roof that faces the sun might need more venting than one tilted away. A well insulated attic needs more venting. We need more info, year built, evidence of a problem, etc.

Originally Posted by howard peters

. . .

The questions were is it OK to open up the entire soffit and make it all vented which would give them 221sq in of air coming in and 123 sq ft of air exiting. Granted it's not the 343 sq in of input and 343 sq ft of output they need, but they could add another vent on the roof to bump it up to 189 sqin.

343 square feet of attic ventilation is a LOT of attic ventilation. (So is 123 SF.) 343 SF is equivalent to a hole 18'-6" square. I think you meant to say 343 square inches. (Keep in mind that 1 square foot = 144 square inches.)

A house that size should have a minimum of 240 sq inches coming in and a maximum of 240 going out.

Taken from the GAF ELK web site:
Attic Ventilation

Originally Posted by John Kogel

A roof that faces the sun might need more venting than one tilted away

Might be a regional thing but around here it's the opposite... I see far more problems from poor ventilation on the north and east facing roof slopes. Mainly moldy roof decking from soffit vents being blocked with insulation.

In an older built home that has R-30 insulation, soffit vents and a ridge vent should the gable vents be sealed?

I'm a fan of the wind turbines as high as possible on the rear of the house without being seen from the road. I also like low profile roof vents placed about 3' up from the gutters to supplement the soffit vents. I wouldn't close up the gable vents...mores better. The idea I would think is to keep the attic the same temperature as the outside air to save on cooling bills in the summer and to cut down on condensation in the winter.

Originally Posted by James Duffin

I also like low profile roof vents placed about 3' up from the gutters to supplement the soffit vents.

That system would actually 'short-circuit' the ventilation. Air from the soffit vent would exit the low roof vents, nothing would be reaching the higher (hopefully ridge) vents.

Air Vent

Where is "wheaton"?

What state?

Please add your state to your location in your profile. Knowing that may make a difference.

Originally Posted by Jerry Peck

Where is "wheaton"?

What state?

Please add your state to your location in your profile. Knowing that may make a difference.

Wheaton, Illinois, most likely. That poster is long gone. The new poster is Larry in central Flahdah.

Originally Posted by John Kogel

Continuous soffit ventilation is always a good idea and relatively easy to install.

But if this is an older building, they should take a look at the attic space and check the sheathing before worrying about whether it needs more venting.

A roof that faces the sun might need more venting than one tilted away. A well insulated attic needs more venting. We need more info, year built, evidence of a problem, etc.

I would say a well insulated attic would need less venting. I define well insulated at having at least the currnet minimum level of insulation for your location and no major air leaks, ideally no air leaks.

With a well insualted attic you keep the heat in the house which prevents hot spots that melt snow and cause ice dams. It also reduces the amount of moist air can leads to moisture problems. The lower moisutre level reduces the amount of ventalation needed to keeo RH in the proper range.

If an attic has a sealed floor with virtually no leaks and plenty of insualtion venting requirements can be met with 1 SF of vent per 300 sf of attic floor. Most vents have a free area of about 40%. A 1200 sf house would need 4 sf.

Originally Posted by howard peters

The 2nd question was since all the airflow is/will be coming in one side of the attic only, will this be sufficient to circulate the air or will the air just come up the baffle and out the top.
HP

For one thing, if you think about it, the incoming cooler air will cause at least some circulation in the attic. It won't be as effective as incoming vents on all sides, but but heat will still rise and the cooler air will flow to fill the lower space before heading to the exit vents.

In Michigan, we call for 1 SF of clear venting for every 300 SF of insulated floor space. On existing homes where there is not already soffit and ridge venting, we use high and low "mushroom" style roof vents: Where venting is not continuous, the vents should be spaced within two feet of the attic floor and within two feet of the ridge. Gable vents are not considered "clear" venting and only half of their open area can be included in the calculations.

Robert is correct, the minimum area requirement when using a continuous system is a ratio of 1:300, with 60% of ventilation in the soffits and 40% at the ridge. However, if there is no ridge vent installed, then the 40% should be at the mechanical vent/attic fan, passive/static, tubine vents or gable vents. Ridge vents work well with soffit vents, provided the soffits are also not blocked by insulation or stored items and work optimally when there are baffles installed.

I make many recommendations to my clients to install solar attic fans or ventilators.

Originally Posted by Debra Monte

Robert is correct, the minimum area requirement when using a continuous system is a ratio of 1:300, with 60% of ventilation in the soffits and 40% at the ridge. However, if there is no ridge vent installed, then the 40% should be at the mechanical vent/attic fan, passive/static, tubine vents or gable vents.

I am curious where those numbers come from as they do not come from the code (unless they are specific to your area's code)?

I make many recommendations to my clients to install solar attic fans or ventilators.

Those simply depressurize the attic, which sucks the conditioned air out of the living space, which must then be replaced by air from the outdoors (i.e., outside the thermal envelope of the house, which may be a basement, crawlspace, outdoors, etc.).

Originally Posted by Jerry Peck

Those simply depressurize the attic, which sucks the conditioned air out of the living space, which must then be replaced by air from the outdoors (i.e., outside the thermal envelope of the house, which may be a basement, crawlspace, outdoors, etc.).

The idea of an attic fan is to draw air from the soffit or gable vents not the space. In fact that would be the path of least resistance and provides ideal moisture removal.

Originally Posted by Rod Butler

The idea of an attic fan is to draw air from the soffit or gable vents not the space. In fact that would be the path of least resistance and provides ideal moisture removal.

If you depressurize the attic (e.g., by powered vents or turbine vents) air will be drawn into the attic through all openings, not only the path of least resistance, if the pressure in the attic is lower than the pressure on the other side of the openings. If the attic/house interface is not perfectly sealed conditioned air in the living space will exfiltrate to the attic if the pressure in the attic reduced to less than the pressure inside the house at the upper ceiling.

Originally Posted by Bruce Breedlove

If you depressurize the attic (e.g., by powered vents or turbine vents) air will be drawn into the attic through all openings, not only the path of least resistance, if the pressure in the attic is lower than the pressure on the other side of the openings. If the attic/house interface is not perfectly sealed conditioned air in the living space will exfiltrate to the attic if the pressure in the attic reduced to less than the pressure inside the house at the upper ceiling.

Are you trying to make this simple concept complicated?

The only place the air will enter the attic is thru the path of least resistance. Air will only "exfiltrate" if there is a hole. The hole is thru gable and soffit vents.

Originally Posted by Rod Butler

Are you trying to make this simple concept complicated?

Not at all. It is very simple - air flows from high pressure to low pressure. If you reduce the pressure in the attic to below the pressure in the house air will flow from the house to the attic.

Originally Posted by Rod Butler

The only place the air will enter the attic is thru the path of least resistance.

So what you are saying is that air will only flow into the attic through one hole - the hole that happens to be the "path of least resistance"? Someone should tell the air about this because it is taking multiple paths into and out of the attic.

Originally Posted by Rod Butler

Air will only "exfiltrate" if there is a hole. The hole is thru gable and soffit vents.

If you will carefully read what I wrote I said the air will exfiltrate from the house into the attic. Everything is relative; exfiltration from the house is infiltration into the attic.

Originally Posted by Bruce Breedlove

If the attic/house interface is not perfectly sealed conditioned air in the living space will exfiltrate to the attic if the pressure in the attic reduced to less than the pressure inside the house at the upper ceiling.

Originally Posted by Jerry Peck

I am curious where those numbers come from as they do not come from the code (unless they are specific to your area's code)?



Those simply depressurize the attic, which sucks the conditioned air out of the living space, which must then be replaced by air from the outdoors (i.e., outside the thermal envelope of the house, which may be a basement, crawlspace, outdoors, etc.).

MY reference to 40% was to the free vent area of some soffit vents I did calculations on. Based on the overall size of the vent only 40% was free vent area. If you looked at a 8 x 16" vent it totals 128 square inches or .88 sf. Its free vent area is 40% of the 128 square inches or 51 squre inches/0.35 squre feet. Three of these vents are needed to give 1 sf of free vent area.

If you are viewing the house from the ground and you computed it needs 3 sf of soffit vents then you need to count 9 of the 8 x 16 vents.

I realize that this post is a bit out of the time frame of the issue, however, it may be essential to utilize a power vent with a gable vent on both ends of the house -- if possible?

Originally Posted by Scott Ludtke

I realize that this post is a bit out of the time frame of the issue, however, it may be essential to utilize a power vent with a gable vent on both ends of the house -- if possible?

I think you need to get a handle on what venting should be and what is could accomplish. Venting has a minor effect on attic temps. With high summer temps the dew point would be so high the moisture would not be a problem.

With the power vents you will create so much negative pressure it will be sucking conditioned air out of the house. You will drive up utility costs from the fan and ac.

Originally Posted by Robert Hronek

MY reference to 40% was to the free vent area of some soffit vents I did calculations on. Based on the overall size of the vent only 40% was free vent area. If you looked at a 8 x 16" vent it totals 128 square inches or .88 sf. Its free vent area is 40% of the 128 square inches or 51 squre inches/0.35 squre feet. Three of these vents are needed to give 1 sf of free vent area.

If you are viewing the house from the ground and you computed it needs 3 sf of soffit vents then you need to count 9 of the 8 x 16 vents.

That depends on the vent.

If the vent is not marked, one can, with some effort, come up with an approximate net free vent area ... for each vent, each square foot of such vent, or each linear foot of such vent.

The code address some of that with calculating metal louvers at 75% of the overall vent area and wood louvers at 25% of the overall vent area.

Continuous soffits are rated by the factor (they can be found on-line) as number of square inches per linear foot (not sure if they have the net free vent area on their back sides or not, but it would be a good idea). Individual soffit vents (newer ones within the last 10-15 years anyway) will have the net free vent area on them, either molded in or stamped in.

Screen soffit vents should be so identified but may not be. I've taken some screen vents, taped off a 1" x 1" area, counted the number of threads in each direction, measured the thickness of the threads in each direction (the number and thread thickness can vary with direction) and determined the approximate net free vent area that way - that is equivalent to taking all the threads and sliding them to one side and to one end, leaving an open area, which would be the net free vent area.

Now, though, I will get back to the post I quoted and the question I asked:

Originally Posted by Debra Monte

the minimum area requirement when using a continuous system is a ratio of 1:300, with 60% of ventilation in the soffits and 40% at the ridge. However, if there is no ridge vent installed, then the 40% should be at the mechanical vent/attic fan, passive/static, tubine vents or gable vents.

Those are NOT code numbers, so I again ask where those numbers come from.

What you stated (see below) is different than what Debra stated, and thus my question was to Debra.

Originally Posted by Robert Hronek

If an attic has a sealed floor with virtually no leaks and plenty of insualtion venting requirements can be met with 1 SF of vent per 300 sf of attic floor. Most vents have a free area of about 40%.

In my opinion....the goal is to keep the attic at the same conditions as the outside is....like there is no roof installed. If its 100 outside the best you can do is keep the attic at 100 degrees. If its 10 outside then you want the attic to be 10 degrees. If the attic has the proper ventilation the pressure in the attic will be the same as the outside pressure. A gable roof with the gables wide open would be the perfect attic.