Attic ventilation - new roof.

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  #1  
Old 07-20-20, 08:17 AM
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Attic ventilation - new roof.

I providing an adequate ventilation system for an attic a code requirement?

My attic has 720 sq/in of intake (soffitt) ventilation and about 300 sq/in of exhaust (2 small gable end vents). My ubnderstanding is that a 50/50 split of intake and exhaust is considered adequate. Considering the small size and inefficiency of gable end vents I am no where near that.

My contractor told me that he was going to install a ridge vent - he didn't.
 
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Old 07-20-20, 11:40 AM
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Old 07-25-20, 11:46 AM
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The old rule of thumb for attic ventilation is inlet should be twice as large as discharge. While meeting code is one thing, comfort and economy of operation may also be considered:

Here is another post on subject:

There is no ideal attic temperature. The idea is to reduce attic air temperature with ventilation as close as practical to outside air temperate to reduce room/house AC heat load. It is great way to improve AC performance and reduce operating costs.

With my ventilation fan running attic air temperature is 5F to 7F above outside at 90F. Fan is activated by regular room AC thermostat in attic with set at 85F with 2 degree delta-T.

Here in evening /night outside temperature is typically 80F or below so fan does not run all night. On hot nights in summer may raise it to activate at 90F.

Many old attic ventilation fans did not active until well above 100F. What a waste.

https://www.doityourself.com/forum/ducting-air-circulation-ventilation-systems/619635-attic-temperature.html#post285444
 
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Old 07-25-20, 02:16 PM
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Thanks for the response. It gave me what I needed to confront my contractor. As I read the code exhaust ventilation should be no less than 40% of the required net area ventilation. My exhaust is less than 10%.

The IRC you cited is the same one our building inspector uses. The attic is not code compliant - the contractor "forgot" to install the roof vent. Now the building inspector is involved and is holding up the CO (which is OK by me). This is the same contractor that "forgot" to draw a permit for my garage.
 
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Old 07-25-20, 02:26 PM
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I have not seen the rule of thumb you cite. Everything I looked at on the internet seemed to indicate a 50/50 balance. The IRC allows some flexibility there. This is a new roof structure with new shingles. It must be code compliant. Current code specifies the minimum vent area.and requires a near balance between intake and exhaust vent area.
 
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Old 07-25-20, 08:18 PM
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The main purpose of building codes is to protect public health, safety and general welfare as they relate to the construction and occupancy of buildings and structures. Wikipedia

The codes are not about comfort, convenience or efficiency.

Attic air inlet area size is a basic factor on exhaust fan performance.

My fan amp draw is 78% of rated for the Roton roof fan assembly. It is a direct measure that fan is working well and motor is not overloaded. I would not want an overloaded attic fan motor starting a fire.

 

Last edited by doughess; 07-25-20 at 09:23 PM.
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Old 07-25-20, 08:29 PM
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Passive ventilation and mechanical ventilation are two completely different animals. The op has only passive ventilation.
 
 

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