I want to add blown-in cellulose insulation to the attic of our 1959 ranch house in central Pennsylvania, where we get moderately cold winters and damp summers. The roof pitch is low. We had a ridge vent put in with a re-roofing 5 years ago, and there are slot vents along the gables at both ends. The roof overhangs the house by about 4 feet, but there are no soffit vents. So far, there doesn't appear to be any water damage in the extreme low ends of the roof, but I'm worried that blowing in the cellulose will trap condensation - Is this not a problem, or should I put in eaves baffles to allow some air movement at the lower ends of the eaves, or should I do something else entirely, like foam? Thanks in advance for your advice!

- Paul

Good question, unfortunately I don't really have a definitive answer. You already have decent air flow it seems. I can give you a scenario, although it was with fiberglass and not cellulose. My parents house, they also had vents like you do, but they stuffed batts in where the soffit vents were, they started to get mold because the humid air had nowhere to go. Seems like if you don't have any mold up there now and it's been there a while, that you might be ok whatever you do, I am assuming though that you have fiberglass up there already, if not, that theory doesn't really hold weight.

Not sure if it would work for you, but I plan to use a system that I saw on the buildingscience website, check out the minneapolis profile for how they did their roof structure, you might be able to glean some good info from that, certainly all the documents they reference have great info too, that is if you can understand it all... which is half my problem.

Thanks for the reply; I'll have to spend some time looking through that buildingscience.com site. I saw the plan for the Minneapolis house, but our situation is a little different - no soffit vents, and no attic rooms. Anyway, I think I've decided that I should go ahead and put up baffles - it won't make things worse, and not doing it could potentially trap moisture in the roof decking. I'm just not looking forward to crawling out to the low ends of the roof any more then necessary! If this job could wait a few years, I could have my son do it for me, but he hasn't got the crawling part down yet.

Read & understand this before proceeding:

http://www.buildingscienceconsulting.com/resources/4-Understanding_Attic_Ventilation.pdf

In high wind areas the addition of soffit vents & baffles leads to wind-washing and redistribution of open-blow insulation. Addition of soffit vents in wildfire prone areas makes the house more prone to burning (sucking flame & ember into to the attic in what might have been a short-term brush-fire flash.) In hurricane prone areas soffit vents contribute to roof deck failures. (Aren't you glad you don't have 'em? :-) )

Vented attics are self-drying, but they contribute to air infiltration within the conditioned space unless the attic floor/conditioned space is VERY well sealed (which can be hard to do as a retrofit.) Air infiltration into the attic from conditioned space is the primary cause of attic condensation, but adding air-permeable insulation to the attic floor can make it worse by lowering the wintertime temp of the attic. You need to reliably halt, not just slow conditioned air migration into the attic to really guarantee it.

Depending on your ceiling/attic floor leakage rates, you may get as much efficiency benefit out of sealing the entire roof deck and gable ends with 2-6" of half-pound foam as adding a foot of cellulose to the attic floor. But splitting the R-value between the foam & cellulose (even if it's roof deck for one, attic floor for the other) is sound as long as the ratio is right for keeping the dew point within the foam layer (which limits the amount you can do in cellulose.)

If your attic is completely uninsulated, sealing the attic floor with 2" of clo$ed-cell 2lb foam creates an both an air-barrier and a class-II vapor retarder on the warm side of the layup, which is ideal. (You may have issues with air-sealing around combustion flues though.) If you have any recessed lighting cans sticking up there, that presents another issue that needs to be resolved before insulating, ( but it's a HUGE air & heat leak.) But if can get it vapor & air sealed on the attic floor you can then heap it up with much-cheaper cellulose as high as you'd like for a super R-value.

Air migration into the attic via studwalls is also common, even in batt-insulated exterior walls (even walls framed with firestops.) Without ripping the walls apart and spraying foam, it can be reduced considerably with dense-packed cellulose (applied by snaking a tube into the wall cavities, from either the inside or outside of the wall. The 2-hole method isn't dense enough to block infiltration, even though it's denser than open-blow in attics.) If the exterior walls have the ubiquitous 1950s R11 lo-density fiberglass batts it makes retrofitting with blown wall cavity insulation a bit more complicated, but not impossible (and usually still worthwhile, from both an R-value and air-infiltration point of view.)

I'm not really trying to complicate the project, but want you to consider all the angles. For sure baffles will reduce the likelihood of mold getting going on that area, but foaming it would be better, yet still no guarantee until/unless you have a demonstrably tight interface between the conditioned space and attic. (And don't forget the attic door!)

For houses that vintage air infiltration tends to be the single largest heat loss factor, which is why even thin foam retrofits make a difference. If the house has a full basement or crawlspace it's worth sealing & insulating the band joist & sill with foam, since that typically accounts for a large share of the air leakage (more than all the doors & windows put together.) Once you've sealed all cavity, plumbing & electrical penetration paths to the attic,and sealed the basement from air coming in, it almost doesn't matter how leaky the doors & windows are (except in high winds), 'cuz you've then nailed the biggest infiltration paths, which is a large fraction of the total heat loss (25-40%, typically.) Then your mid-winter air won't be uncomfortably dry, and your attic won't have condensation issues.