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Calculating Venting Needs for an A-Frame Attic

innertubechamp | Posted in General Questions on

Hello there! I have an A-frame house on the northeast coast. We are in the midst of insulating the cathedral ceilings with a vented system, using site-made foam baffles to vent from the soffits to the attic space. (Sketch attached for reference.)

The attic space is only about 127 sqft, and the living space below is obviously significantly larger. I understand how to calculate the Net Free Area required for venting (one sqft of venting space for every 300 sqft of attic space). But for this particular structure, do I calculate this based on the square footage of the attic floor OR the living space floor, since this is where the venting system starts?

When I calculate based on the attic square footage, the NFA just seems so small, and I don’t want to under-ventilate the space. Many thanks in advance!

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  1. Expert Member
    Malcolm Taylor | | #1


    Your roof extends over the whole floor area, and that's what you need to calculate your required venting. The only way the attic plays into this is you can use gable vents as your top side venting if you would like.

  2. plumb_bob | | #2

    You have a bit of a hybrid system with both vented rafters and an attic. Rafters should be vented to 1:150, attics to 1:300. This may mean increasing the size of the openings at the soffits and ridge/gable.

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