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Attic ventilation advice - how many soffit vents?

I need to improve ventillation in my attic. Condensation is evident in the attic on the inside of the roof. I am planning to do so using soffit vents. But here\are some relevant facts and complications:

I live in Wisconsin. We have had ice dam problems 2 years in a row.

We use the attic for storage and the attic has a floor in it. The cavity below it accommodats 6-7" of insullation. We need to replace it because it has gotten wet due to an ice dam

Question no. 1: Which insullation would provide best R value per inch while being a bit moisture resistant? I don't believe that moisture will be 100% eliminated

Since we can only get an R value of about 20 from floor insullation we would like to put r-19 insullation loosely attached to the roof rafters. THis would allow air flow, but I think it would also mean that we would need 1 soffit vent per 'compartment'. There are 11 such compartments on each side of the roof. Some contractors say we would only need 3-4 vents per side of roof (23' long). But with the strips of insulation (even if loosely attached and with air space between inside roof surface and insullation bats, woulnn't only the ones ('compartments') with the soffit vents be ventilated. The rest might still suffer from condensation, no?

There is a roof ridge vent running alon the entire length of the roof and 2 small windows in the attic

Total attic area is about 400 sq. ft.

Any advice would be greatly appreciated. If I am not asking the right question, what should it be? Every contractor seems yto have a different opinion on what to do.


Harold Lemel

Re: Attic ventilation advice - how many soffit vents?

since your attic is unfinished and is ventilated i would consider anything above the floor exterior space. by that i mean there shouldn't be more than 20 degrees difference between outside temp and the attic temp. when there is that big of a difference that's when condensation forms so i wouldn't want to trap heat between the floor and roof with insulation on both sides. if you are having condensation problems then i would deffinately add more ventilation.

Re: Attic ventilation advice - how many soffit vents?

Harold,here's what I see.
First thing is you certainly don't have near enough attic insulation for where you live. We increased ours to R40 and made a huge difference. This might partly explain the ice damns another problem might also be from warm air from the inside leaking into the attic. When you say you have condensation on the underside of the roof sounds like there is warm air reaching up into attic. I learned a lot around this forum regarding this issue. I went up into attic with a couple of cans of spray foam to seal all the gaps around the vent stack,electrical wires and boxes and the bath exhaust fans as well along the perimeter where the top of the interior walls are in the attic. After all those were sealed the increased insulation was applied. This made a big difference.

If you were to apply insulation to the rafters then I would have to agree with needing a venting for each rafter right up to the ridge vent. You could get the styrofaom baffles and run them from the soffit right up to the ridge and fill the rater space with insulation. Make sure to put insuation on top of the wall plates at the point where the rafters sit on them.
You would also need to cover the gable ends of the attic with insulation and make sure no other vents or windows are open in this space for all this to be effective.

As for which insulation would give you the biggest bang for your buck , closed cell spray foam would be the #1 , open cell spray foam #2 , cellulose #3 , fiberglass would be last.

Timothy Miller
Re: Attic ventilation advice - how many soffit vents?

Howdy, if you have no soffit vents add some norm is one 6 by12" vent for every 600 sqft of attic space,if my memory is correct.
Coss ventilation is good so one on each side of the attic would be a great start. Check to see if you are having warm air enter the attic from ceiling light fixtures that are not insullated and or where wiring or pipes run into the attic. If you have a humidifier consider lowering the amount of humidity in the house.

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