Add fan for circulation WITHIN attic?

I've come across a situation that seems unlike any I've seen discussed here, but this is probably common in 30-40 year old houses. This is an approx 20x80 sq ft raised ranch (a basement and 1 floor of living space) with a hip roof. The attic has loose fill fiber, about 4", but it's pretty uneven and dusty (30 years will do that). In addition, insulation has settled deep into the bathroom and kitchen walls, so that if the vanity cabinet in the bathroom is removed, the bathroom wall is open to the attic. There's a ridge vent running along the middle 2/3 of the length of the ridge, and 3-4 passive vents halfway down the roof. While adding some batts seems an obvious way to increase the efficiency of the roof, there are two problems. - the hip roof overhangs about 3 ft in the front of the house, and this is the only place where there are 2 soffit vents - with the lack of soffit vents along the other 3 sides, air may short circuit between the ridge and passive vents
I was wondering if, after adding another 8" of batts, it makes sense to have a small (6 or 8") fan going at low speed, at around half the height of the attic. This could circulate the air enough that moisture (and heat in the summer) would circulate with the outside air through the vents.
Also wondering if laying some Tyvek before laying down the batts where the roof is open to the walls of the living space would be advisable.
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Google for R values for your area then you might be able to make some decisions.
I just added R-30 of blown to a 1977 home. A/c bill dropped in half the next month. According to the FEDS, My home needs, either R-30, R-38 or R-43. I elected to go toward the top and not the bottom. New homes here come with R-30.
Circulation of the air in the attic seems like a dumb idea to me. You have a ridge vent, so as long as you have enough sq inches of supply air the ridge vent should do the rest for ya.
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SQLit wrote:

Depending on the conditions and source, anywhere from R-38 to R-49.

Seeing a show I have 4-5" of very old and patchy fill, adding R-25 or more would probably make a big difference; but the devil's in the details.

Well, there are no soffit on 3 sides, so the fan was to utilize the combination of ridge and the 2 existing soffit vents better. However the roof has come through for two years as it is, so perhaps it is best to leave well alone.
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I suspect this would just kill any natural ventilation which would occur.

Plastic maybe. Not Tyvek. Tyvek allows moisture to pass into the insulation.
Bob
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There are rigid foam baffles you staple to the roof deck at the overhang that are designed for adding more insulation without stoping the airflow in from the soffit vents, cheap and easy to instal.
A fan will do nothing, forget it.
R values are minimums for code. 4 of what you have + 8 will be apx R 42. Optimal anywhere cooling or heating is needed as a major cost yearly is R 60+. Insulation settles, I put in R 100 a few years ago, now it is apx R-80. Once you are doing the work the exrtra insulation is cheap. I am zone 5 to - 20f.
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m Ransley wrote:

The problem is that there are only 2 1x2 sq ft soffit vents in the front, and no vents on the other 3 sides. The roof has no overhang, so there isn't any space to retrofit soffit vents.

Was this fiberglass or cellulose? I was thinking of batts, then after reading some, I was leaning towards cellulose and Home Depot's rental blower. Only worried that the moisture from cracks, etc. in the old walsl and ceiling would saturate the cellulose.
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Bob wrote:

Very little of the living space is open to the roof, so I could add plastic only there, and the rest of the space doesn't have any vapor barrier. Since it was finished 30-35 years ago, there are imperfections due to remodels, repairs in the intervening years, and so instead of sealing the interior, I was thinking of providing a way to escape for the moisture which will make it's way into the walls and roof.
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Nexus7 wrote:

Let's not do some Mickey Mouse something with a fan. It is time to get back to basics. You need to get that insulation back to norms. Walls need to be filled and the attic needs to be corrected. I am not sure how I would approach your specific situation, but I would want it up to current standards if possible.
Same goes for venting. It appears you need improved venting. I can't see those "3-4 passive vents halfway down the roof." but they sound like they should be closed up as they are likely short circuiting the proper air flow. It also sounds like you need a lot more effective soffit venting.
While you may be able to do all or some of this yourself, I am going to suggest a local professional. They often have learned tricks over the years to allow them to do a really good job much faster than you can and often can do a better job and maybe offer a better price on the actual materials used.
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Joseph Meehan wrote:

The roof has no overhang on 3 sides, so no soffit vents can be added. I can clear up the two at the front, but something else would be needed for the other 3 sides. On the other hand, the roof components are in good shape, so whatever is there isn't too bad to begin with. The suggestion fo the fan was to mitigate the short-circuiting of the ridge and passive vents.

They're suggesting new batts all around and that's it. Given that the roof made it all these years, maybe that is the best course of action.
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I just added lots of fiberglass batts, and it made a huge difference in my house. The more the better, I'm sure. I did plug the tops of some walls that opened into the attic with plastic bags stuffed with scraps of insulation.
I have thought about adding plastic or something to the underside of the rafters in my attic. It would go from low in the attic (but open at the bottom) to the top, thus forming a natural convection path to the ridge vent which would be fed by sun heat on the roof. There would be guaranteed air flow under the roof deck in the areas where this is done.
Opinions?
Bob
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If you can't conveniently vent the eaves/soffits (because there aren't any) and you're not willing to give up on vents entirely, which is what I'd do, then you do need to mechanically drive outside air into the the attic somewhere.
Is there some reason why one can't lay PVC drain-tile around the perimeter, and use a high-volume fan to force air from any convenient source through it? Would one have to run a grounding-wire inside the pipe, or is that just when you're using it for dust-collection?
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Nexus7 wrote:

Are all the rafter bays vented on the one side or are just soffit vents in some of them? Adding more would help.
I don't believe you have told us how many square feet of venting you have as soffit and likewise of gable end vents as well as the total number of square feet of attic floor area. Knowing the area of the world where the home is located would also be helpful.

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