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.