Author Topic: Hot Attic  (Read 2159 times)

Rust

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Hot Attic
« on: June 24, 2013, 07:35:43 AM »
I live in Virginia where it gets nice and hot during the summer.  I just moved into my current home last August.  The house is 30 years old but the roof is newer.  When the new roof was put on they installed a ridge vent.

The attic is really hot though and I couldn't help but notice that the soffits don't have vents. 

I'm going to install soffit venting but I know that you don't want too much venting as it would reduce your effectiveness of my insulation.  Also, I'm not really sure how much insulation exists in the attic as there is plywood covering the entire attic floor.

Does anyone know the formula for how much venting I need?  (I've also got two gables that are open would they need to be closed off for the proper function of the soffit/ridge vent system?)

Thanks for the help.

George_PA

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Re: Hot Attic
« Reply #1 on: June 24, 2013, 10:56:32 AM »
from my understanding you absolutely need to have some type of ventilation in your attic or the plywood holding up the actual the rooftop will start to rot;

a lot of the houses here in PA have both soffit venting with gables in place; the best way to get rid of the hot attic air is to have an attic fan suck it back outside

If you have the insulation sealed really well, I don't think too much ventilation is a problem even if you have a lot of ventilation;

instead of a specific formula, you want to just make sure the insulation below the vented area is sealed really well; you may need pull up plywood to double check; ideally you want a complete airtight seal everywhere (the area between the attic floor and the house below); also during the cold winters or extreme in the hot summers, you can go up there with a thermal camera and look for airleaks to help seal it up;

For soffit venting, you can put up something called "attic vent chutes" to leave enough space for venting but it will also allow you to pile the insulation up higher along the edges of the roof rafters




Greg

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Re: Hot Attic
« Reply #2 on: June 24, 2013, 01:58:03 PM »
Ideally you want 1" continuous air space under the roof sheathing and above any insulation.  Modern truss packages use blocking between "rafters" with (3) 2" screened holes, every other rafter bay (24" o.c.), so if you use (3) 1" venting plugs (little aluminum grills that fit a 1" hole) every bay you would be fine.  But they can't be covered by insulation.

The 2009 IRC specifies that the free venting area be 1/150 of the vented area, with a reduction to 1/300 if 50-80% of the venting is in the upper part of the area to vented (i.e. ridge vent), and the balance is at the eaves.  My roofing supplier says they recommend 100 sq.in. vent per 300 sq. ft. of attic.

You can keep your gable end vents, they used to think it allowed the venting to "short circuit" but that's no longer the case.  Hope this helps.

Rust

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Re: Hot Attic
« Reply #3 on: June 24, 2013, 02:53:10 PM »
It does,

Thank you.