Venting for powered attic fan?

[Ifican]

Ok ive turned to you guys for more obscure things in the past hoping to get some input on this one as well. We do not need to debate powered vs non powered and yes i have read a great majority of article on why and how its not good. I am working now to get intake capability to be neutral or positive and thats where you all come in. I will not be able to accomplish this via soffit venting alone. The way the house was built it just doesnt work worth a damn. So i am looking for other ideas. I am considering putting in ridge vents to make up the intake difference but am slightly concerned that with air being pulled from the ridge that it could potentially pull in rain. Looking for insight on other ideas or if anyone has had such experience and hasnt seen an issue in that regard.

 

[Smooky]

Hip, gable, gambrel or flat roof?

 

[Sponsor]

 

[Ifican]

asphalt shingle and not very pitched. The house has been remodeled a few times and the roof is non standard design for sure. I would have to measure to be sure but if i had to guess its either 2/12 or 3/12.

Just realized i didnt answer the question either, its a hybird hip/gable. There is a single gable vent and that is where the exhaust of the powered fan goes.

 

[BadgerBoilerMN]

A powered attic fan does not work! ? Go figure...

 

[Ifican]

Attic fan works exceptionally well. The issue is the structure does not have adequate intake ventilation, even though soffit vents were installed, they were not installed correctly. I am working to fix that now and thus the ask of this thread. Even with inadequate intake the attic was far cooler through this summer than it ever was the previous year. If not installed correctly they are horrible as in my case right now with it pulling the amount of air through the ceiling envelope that its pulling. Though that will be re-mediated when i get adequate intake and again the point of this thread.

 

[Smooky]

Put a power vent in the roof, blowing out and let the air enter at eves and on the gamble end.

 

[Ifican]

I have considered this but this still will not account for the amount of airflow needed. The biggest issue is there is no way to get any significant airflow from the eves.

 

[Dana]

How are you determining that the ventilation is inadequate? Is there mold growing in places on the roof deck or rafters?

Or is the purpose of the attic fan to keep the attic cool (which takes a LOT more ventilation than it takes for keeping the attic dry, the original and primary purpose of attic ventilation.)

 

[Ifican]

It all started a few years ago when i was doing some work in the attic, air just felt stale. Upon investigating realized soffit vents were there in appearance only. I have fixed that where i could however most without tearing off the roof and sheathing to remove blocking that was put in place between rafters essentially blocking airflow, just are never going to work. I did open the soffit where i could and this helped, long and short i installed a powered attic fan and the temp is drastically down, so much so it make a difference in the house. Just need to figure out best way to for better intake. Currently looking at putting in some ridge vents as well as soffit edge vents just not sure how feasible the edge vents are based on the roof being somewhere around 20 yr's old. Still in great shape but not sure how easily those bottom edge shingles are going to comeup to install the vents correctly. One other aspect i do plan on better sealing the ceiling envelope once i get adequate intake.

 

[Dana]

How is stale air in the attic a problem, as long as it's not humid enough to support rot?

 

[Ifican]

The stale air in and of itself isnt a problem the radiated heat even with r-40 insulation is the problem. Once i get proper intake taken care of my not need / use the fan we will see.

 

[Dana]

If you're trying to run the attic cooler to stay more comfortable indoors an attic fan is about the worst way to go about it. Attic ventilation is not about cooling, and it's DEFINITELY not about mitigating radiated heat transfer from the roof deck directly to the insulation. (The breeze outdoors didn't exactly stop the sun from heating up your roof, did it?)

Like I stated before it takes a heluva lot of attic ventilation to have a relevant effect on cooling. Attic ventilation is about keeping it dry. Unless the ceiling is perfectly air-tight an attic exhaust fan even adds to the load of an air conditioning system, since it draws conditioned air out of the house by pulling it up into the attic first.

To address the cooling load issue head on:

If the insulation is fiberglass, it's somewhat translucent to the infra-red radiation, making the temperature an inch or two into the fiberglass layer hotter than the attic air, since it's absorbing the radiated heat from the even hotter roof deck. Overtopping it with even a couple inches of blown cellulose insulation stops the IR radiation at the surface of the insulation, which then runs at about the same temp as the attic air, not the temp of the roof deck. Cellulose is opaque to the radiated heat, so it heats up on the surface, not at some inner depth.

Painting the under side of the roof deck with a low-E "radiant barrier" paint, makes a difference. More expensive but also more effective, perforated aluminized polyester type radiant barrier applied to the underside of the rafters makes a bigger difference. If you do both, the attic air temperature will run cooler. Don't use aluminized mylar or bubble-pack versions, since those are vapor barriers, and can create a moisture problem where none existed previously. The perforated aluminized fabric type radiant barrier run about 5 perms, about as vapor permeable as standard latex paint, which is fine.

If you're due for re-shingling, a high solar reflective index shingle (SRI greater than 50) would also make a dent. If the pitch is 2:12 or lower an even higher SRI would be appropriate.