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Community and Q&A

Venting a Divided Attic

hiyouall | Posted in GBA Pro Help on

Hi!  New build: single story; slab foundation; 38 x 98 sq ft; 6:12 pitch; 9 ft
walls through out; gable in front as porch; zone 2.  Pretty much a long rectangle.
2/3’s of house has 2 HVAC units in attic so it will be insulated as conditioned  under the roof with closed cell foam – 4 in.  This area is separated with a wall and door  in which the wall will also be foamed like the opposing attic wall. Entire space is for the HVAC.  Then the next area is right above the garage.  This is a 3 car/work bench/storage/man cave/tinkering area (980 sq ft).  Framer did not put vented soffits and roofer did not put a ridge vent, in any part of the roof or soffits.
Question is what should we do?  We are debating if we should put in soffit vents, baffles, whirly birds or ridge vent on this side (garage) of house only OR use close cell foam on this area too?  Interested in best functionality and not esthetics. Also at least one garage door will be opened and closed twice daily.  Please advise what would be best for long term and the house!!!

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  1. Expert Member
    NICK KEENAN | | #1

    First, just a little terminology so we're on the same page. "Conditioned" means actively supplied with heating and cooling. "Insulated" means, well, insulated. It's OK to have interior spaces that are not conditioned if they aren't occupied, but if they're inside the building envelope they need to be insulated. You can also have spaces that are under the roof but not inside the building envelope, like vented attics or open crawlspaces, they're going to be unconditioned and uninsulated.

    I think the question you're asking is where the edge of the building envelope -- the division between insulated and uninsulated -- should be. This is an important question, and it's something that people get wrong a shocking amount of the time. The building envelope has to be continuous to do any good -- Joe Lstiburek likes to say you should be able to look at a plan of the house and draw the building envelope without ever lifting your pencil off the paper.

    It sounds like the garage is outside of the building envelope? So the wall between the garage and the rest of the house should be insulated and air sealed. In which case the space above the garage should also be uninsulated and unconditioned. The roof above that space should be vented at the soffit and the ridge. It's really not a big deal to add venting to an existing roof.

    1. hiyouall | | #2

      Thank you! Yes the garage is literally outside the building envelope both ground level and attic level. What is better wind turbines or ridge vent? And is there a way of determining how many vents in the soffit in an area of 30' 4" x 40' ?

      1. Expert Member
        NICK KEENAN | | #3

        I'm old enough that when I was learning about roofs people believed that the reason ventilated roofs lasted longer was that heat destroyed roofs. It turns out that it's moisture that destroys roofs and that ventilating keeps the moisture from building up under the roof. The moisture comes from the occupied parts of the house when warm, humid interior air leaks through the insulation . Adding mechanical ventilation to the attic -- either as turbine vents or electric fans -- tends to make the problem worse, as the air that it draws in tends to come from the interior of the house. It also causes the interior to have increased air leakage which isn't good.

        If you have a ridge vent and a soffit vent that are equal in size, 100% of your ventilation air should be coming in through the soffits, which is what you want. The action of the sun warming the attic and warm air escaping through the ridge provides all the ventilation you need. There is a formula for how large the vents should be but I don't remember it off-hand, I'm sure some of the posters here can rattle it off or a Google search will find it for you.

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