so I have a 110yo house in portland oregon. It had no soffit vents or insulation when we bought the house. I installed soffit vents this summer in the areas where the soffits led directly into the large attic space. I am planning to blow in cellulose rather soon.

There are portions of the house where the interior drywall is screwed directly to the roof rafters (or are they trusses) (the diagonal supports that the roof sheeting is nailed to). I.E. there is no attic and no insulation in these spaces. To add to the problem, these rafters are true 2x4's so there isn't much room in there for insulation.

Hopefully you can visulize what I am talking about. In essence, I need to get insulation on top of the sloping ceilings upstairs and there is only 4" of space for insulation and the only access I have to these areas is a 4" tall opening in the attic.

Finally, the question.. since I don't have access, foam baffles are not going to work and the thought of cramming foam board insulation down these areas (with all the roofing nails exposed) and keeping a 1" space above the foam board for ventilation is nightmarish (not to mention an unimpressive r-value).
Someone suggested I put perforated pvc pipes in each "bay" and connect them to soffit vents and blow in cellulose around them.

Antoher friend said "it's a 110yo house so its "venting" all over the place already, not to mention the wood framing in the attic has had 100+ years to dry out, just blow the insulation down the bays, don't put soffit vents in these areas, and forget about it".

Any suggestions?

Thanks

 

If you want to insulate the cathedral ceiling, take it down install the baffles and insulate.

Attic ventilation is a misnomer. What venting does in your attic is bypass the low moisture vapor permability of roofing materials. It does it through two processes known as free venting and Equilibrium Relative Humidity (ErH%). Installing a perforated PVC pipes will not accomplish adequate free venting. In fact it will act as a conduit for condensation to drip down so it can spread the wood rot instead of confining it.

 

ummm, you kind of lost me there. What do you mean it is a misnomer? You need to bring in fresh air to the attic and have a means for it to circulate out. This process is called venting, is it not?

So what will likely happen if I fill the cathedral ceiling with cellulose? I could always try and leave a little gap when I blow it in but can't really see how I could do that effectively.

There are no vents currently installed, or is the correct term now "low moisture vapor permability bypasses" so what is the harm.

Dumb it down for me if you can. Although I do have a masters degree in stream geomorphology, I don't install insulation that much.

Thanks a bunch, I hope you can handle a litlle friendly chiding.

 

Insulation or vapor barriers do not stop either heat or moisture flow, they slow it down.

Consider there cars on a highway and they are travelling at the same speed. There will be no congestion as a result. Let's say the number of lanes on the highway can allow for a hundred cars per minute to access the highway without causing congestion, provided these cars maintain the same speed as the rest of the cars on the highway. At the end of this highway there are several multilane exit ramps.

The cars represent the grains of moisture in heat. The speed represents the rate of flow. The cars accessing the highway represent the constant introduction of moisture. The exit ramps represent adequate free venting that allow the moisture to escape from the structure.

Roofing materials like asphalt shingles have a very low Perm Rating. To illustrate it, it would only allow 5 cars per minute to exit the highway. Because of the number of cars accessing the highway verses the number exiting per minute, it is just a matter of time when the cars speed comes to a standstill. This represents 100% saturation.

Adequate free venting relies on a process known as Equilibrium Relative Humidity (ErH%). This states that an object of lower humidity will absorb humidity from an object of higher humidity and not vice-versa, until the humidity levels in both objects are equal. What this explicitly implies is that a constant flow of cold air from outside is required to remove the humidity from the structure. So along with the constant flow of moisture in order to create balance or if your prefer avoid a moisture problem, there must be a constant introduction of cold air from outside, equal to or more than the amount required to absorb the amount of moisture introduced per minute.

To illustrate this is adequate free venting is a five lane highway that has five or more lanes at its exit ramps. Inadequate free venting is 5 lane highway into a 4 lane exit ramp. This situation posses a low probability that a moisture will occur. 5 lanes into a 3 lane exit ramp, increases the probability to where moisture problems will occur like ice dams, mold and mildew problems, stuctural failure will take some time. In your intended application is the equivalent to a 5 lane highway exiting into a 1 lane ramp. Not only is a moisture problem assured under this application, structural failure is highly likely.

I do realize this is an oversimplification of the process involved and I have deliberately omitted other important factors. Though these other factors would further support my position, I believe it is sufficient for this situation.

As an Energy Conservationist my objective is to promote conservation practices. We have a Cardinal rule and that is "The Health and Well-Being of the occupants of the dwelling SUPERCEDES any energy conservation measure." This rule came about because in the beginning in our efforts to conserve we made mistakes. These mistakes resulted in structural failure and making these structures uninhabitable. Your application exemplifies just one of these mistakes. Take my advice, "Do it right or don't do it at all!"